ACP - Detection of large-scale cloud microphysical changes within a major shipping corridor after implementation of the International Maritime Organization 2020 fuel sulfur regulations

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href="https://cdn.copernicus.org/apps/htmlgenerator/css/htmlgenerator.css?v=1"> <meta name="citation_fulltext_world_readable" content=""> <meta name="citation_publisher" content="Copernicus GmbH"/> <meta name="citation_title" content="Detection of large-scale cloud microphysical changes within a major shipping corridor after implementation of the International Maritime Organization 2020 fuel sulfur regulations"/> <meta name="citation_abstract" content="<p><strong class="journal-contentHeaderColor">Abstract.</strong> New regulations from the International Maritime Organization (IMO) limiting sulfur emissions from the shipping industry are expected to have large benefits in terms of public health but may come with an undesired side effect: acceleration of global warming as the climate-cooling effects of ship pollution on marine clouds are diminished. Previous work has found a substantial decrease in the detection of ship tracks in clouds after the IMO 2020 regulations went into effect, but changes in large-scale cloud properties have been more equivocal. Using a statistical technique that estimates counterfactual fields of what large-scale cloud and radiative properties within an isolated shipping corridor in the southeastern Atlantic would have been in the absence of shipping, we confidently detect a reduction in the magnitude of cloud droplet effective radius decreases within the shipping corridor and find evidence for a reduction in the magnitude of cloud brightening as well. The instantaneous radiative forcing due to aerosol–cloud interactions from the IMO 2020 regulations is estimated as <span class="inline-formula"><i>O</i></span>(1 W m<span class="inline-formula"><sup>−2</sup></span>) within the shipping corridor, lending credence to global estimates of <span class="inline-formula"><i>O</i></span>(0.1 W m<span class="inline-formula"><sup>−2</sup></span>). In addition to their geophysical significance, our results also provide independent evidence for general compliance with the IMO 2020 regulations.</p>"/> <meta name="citation_publication_date" content="2023/07/25"/> <meta name="citation_online_date" content="2023/07/25"/> <meta name="citation_journal_title" content="Atmospheric Chemistry and Physics"/> <meta name="citation_volume" content="23"/> <meta name="citation_issue" content="14"/> <meta name="citation_issn" content="1680-7316"/> <meta name="citation_doi" content="https://doi.org/10.5194/acp-23-8259-2023"/> <meta name="citation_firstpage" content="8259"/> <meta name="citation_lastpage" content="8269"/> <meta name="citation_author" content="Diamond, Michael S."/> <meta name="citation_author_institution" content="Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL 32306, USA"/> <meta name="citation_author_orcid" content="0000-0003-2147-5921"> <meta name="citation_author_email" content="msdiamond@fsu.edu"> <meta name="citation_reference" content="Albrecht, B. 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"> <meta name="citation_funding_source" content="citation_funder=Florida State University;citation_funder_id=100006597;citation_grant_number=N/A (new faculty startup)"> <meta name="citation_pdf_url" content="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023.pdf"/> <meta name="citation_xml_url" content="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023.xml"/> <meta name="fulltext_pdf" content="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023.pdf"/> <meta name="citation_language" content="English"/> <meta name="libraryUrl" content="https://acp.copernicus.org/articles/"/> <meta property="og:image" content="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-avatar-web.png"/> <meta property="og:title" content="Detection of large-scale cloud microphysical changes within a major shipping corridor after implementation of the International Maritime Organization 2020 fuel sulfur regulations"> <meta property="og:description" content="Abstract. New regulations from the International Maritime Organization (IMO) limiting sulfur emissions from the shipping industry are expected to have large benefits in terms of public health but may come with an undesired side effect: acceleration of global warming as the climate-cooling effects of ship pollution on marine clouds are diminished. Previous work has found a substantial decrease in the detection of ship tracks in clouds after the IMO 2020 regulations went into effect, but changes in large-scale cloud properties have been more equivocal. Using a statistical technique that estimates counterfactual fields of what large-scale cloud and radiative properties within an isolated shipping corridor in the southeastern Atlantic would have been in the absence of shipping, we confidently detect a reduction in the magnitude of cloud droplet effective radius decreases within the shipping corridor and find evidence for a reduction in the magnitude of cloud brightening as well. The instantaneous radiative forcing due to aerosol–cloud interactions from the IMO 2020 regulations is estimated as O(1 W m−2) within the shipping corridor, lending credence to global estimates of O(0.1 W m−2). In addition to their geophysical significance, our results also provide independent evidence for general compliance with the IMO 2020 regulations."> <meta property="og:url" content="https://acp.copernicus.org/articles/23/8259/2023/"> <meta property="twitter:image" content="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-avatar-web.png"/> <meta name="twitter:card" content="summary_large_image"> <meta name="twitter:title" content="Detection of large-scale cloud microphysical changes within a major shipping corridor after implementation of the International Maritime Organization 2020 fuel sulfur regulations"> <meta name="twitter:description" content="Abstract. New regulations from the International Maritime Organization (IMO) limiting sulfur emissions from the shipping industry are expected to have large benefits in terms of public health but may come with an undesired side effect: acceleration of global warming as the climate-cooling effects of ship pollution on marine clouds are diminished. Previous work has found a substantial decrease in the detection of ship tracks in clouds after the IMO 2020 regulations went into effect, but changes in large-scale cloud properties have been more equivocal. Using a statistical technique that estimates counterfactual fields of what large-scale cloud and radiative properties within an isolated shipping corridor in the southeastern Atlantic would have been in the absence of shipping, we confidently detect a reduction in the magnitude of cloud droplet effective radius decreases within the shipping corridor and find evidence for a reduction in the magnitude of cloud brightening as well. The instantaneous radiative forcing due to aerosol–cloud interactions from the IMO 2020 regulations is estimated as O(1 W m−2) within the shipping corridor, lending credence to global estimates of O(0.1 W m−2). In addition to their geophysical significance, our results also provide independent evidence for general compliance with the IMO 2020 regulations."> <link rel="icon" href="https://www.atmospheric-chemistry-and-physics.net/favicon.ico" type="image/x-icon"/> <script type="text/javascript" src="https://cdn.copernicus.org/libraries/jquery/1.11.1/ui/jquery-ui.min.js"></script> <script type="text/javascript" src="https://cdn.copernicus.org/libraries/jquery/1.11.1/ui/jquery-ui-slider-pips.js"></script> <script type="text/javascript" src="https://cdn.copernicus.org/libraries/jquery/1.11.1/ui/template_jquery-ui-touch.min.js"></script> <script type="text/javascript" src="https://cdn.copernicus.org/js/respond.js"></script> <script type="text/javascript" src="https://cdn.copernicus.org/libraries/highstock/2.0.4/highstock.js"></script> <script type="text/javascript" src="https://cdn.copernicus.org/apps/htmlgenerator/js/CoPublisher.js"></script> <script type="text/x-mathjax-config"> 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class="container CMSCONTAINER"> <div id="cmsbox_126230" class="cmsbox "> <span class="header-small text-uppercase"> </span> <h1 class="display-4 header-get-function home-header hide-md-on-version2023"> Article   </h1> </div></div> </section> <div id="breadcrumbs" class="breadcrumbs"> <div class="container"> <div class="row align-items-center"> <div class="d-none d-sm-block text-nowrap pageactions"></div>    <div class="justify-content-between col-auto col-md CMSCONTAINER" id="breadcrumbs_content_container"><div id="cmsbox_1088152" class="cmsbox ">  <ol class="breadcrumb"> <li class="breadcrumb-item"><a href="https://acp.copernicus.org/">Articles</a></li><li class="breadcrumb-item"><a href="https://acp.copernicus.org/articles/23/issue14.html">Volume 23, issue 14</a></li><li class="breadcrumb-item active">ACP, 23, 8259–8269, 2023</li> </ol>  </div></div> <div class="col col-md-4 text-right page-search CMSCONTAINER" id="search_content_container"><div id="cmsbox_1088035" class="cmsbox ">   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} #templateSearchResultTerm { font-weight: bold; } #resultsSearchHeader { display: block !important; } #scrolltopmodal { font-size: 3.0em; margin-top: 0 !important; margin-right: 15px; } @-webkit-keyframes animation_dots_breath { 50% { -webkit-transform: scale(1); transform: scale(1); opacity: 1; } 100% { opacity: 0; } } @keyframes animation_dots_breath { 50% { -webkit-transform: scale(1); transform: scale(1); opacity: 1; } 100% { opacity: 0; } } @media (min-width: 768px) and (max-width: 991px) { #templateSearchResultModal .modal-dialog { max-width: 90%; } } </style> <script> if(document.querySelector('meta[name="global_moBaseURL"]').content == "https://meetingorganizer.copernicus.org/") FINDER_URL = document.querySelector('meta[name="global_moBaseURL"]').content.replace('meetingorganizer', 'finder-app')+"search/library.php"; else FINDER_URL = document.querySelector('meta[name="global_moBaseURL"]').content.replace('meetingorganizer', 'finder')+"search/library.php"; SEARCH_INPUT = document.getElementById('search_query_solr'); SEARCH_INPUT_MODAL = document.getElementById('search_query_modal'); searchRunning = false; offset = 20; INITIAL_OFFSET = 20; var MutationObserver = window.MutationObserver || window.WebKitMutationObserver || window.MozMutationObserver; const targetNodeSearchModal = document.getElementById("templateSearchResultModal"); const configSearchModal = { attributes: true, childList: true, subtree: true }; // Callback function to execute when mutations are observed const callbackSearchModal = (mutationList, observer) => { for (const mutation of mutationList) { if (mutation.type === "childList") { // console.log("A child node has been added or removed."); picturesGallery(); } else if (mutation.type === "attributes") { // console.log(`The ${mutation.attributeName} attribute was modified.`); } } }; // Create an observer instance linked to the callback function const observer = new MutationObserver(callbackSearchModal); // Start observing the target node for configured mutations observer.observe(targetNodeSearchModal, configSearchModal); function _addEventListener() { document.getElementById('search_query_solr').addEventListener('keypress', (e) => { if (e.key === 'Enter') _runSearch(); }); document.getElementById('start_site_search_solr').addEventListener('click', (e) => { _runSearch(); e.stopPropagation(); e.stopImmediatePropagation(); return false; }); $('#templateSearchResultModal').scroll(function() { if ($(this).scrollTop()) { $('#scrolltopmodal:hidden').stop(true, true).fadeIn().css("display","inline-block"); } else { $('#scrolltopmodal').stop(true, true).fadeOut(); } }); } function scrollModalTop() { $('#templateSearchResultModal').animate({ scrollTop: 0 }, 'slow'); // $('#templateSearchResultModal').scrollTop(0); } function picturesGallery() { $('body').off('click', '.paperlist-avatar img'); $('body').off('click', '#templateSearchResultContainer .paperlist-avatar img'); searchPaperListAvatar = []; searchPaperListAvatarThumb = []; search_pswpElement = document.querySelectorAll('.pswp')[0]; if (typeof search_gallery != "undefined") { search_gallery = null; } $('body').on('click', '#templateSearchResultContainer .paperlist-avatar img', function (e) { if(searchPaperListAvatarThumb.length === 0 && searchPaperListAvatar.length === 0) { $('#templateSearchResultContainer .paperlist-avatar img').each(function () { var webversion = $(this).attr('data-web'); var width = $(this).attr('data-width'); var height = $(this).attr('data-height'); var caption = $(this).attr('data-caption'); var figure = { src: webversion, w: width, h: height, title: caption }; searchPaperListAvatarThumb.push($(this)[0]); searchPaperListAvatar.push(figure); }); } var target = $(this); var index = $('#templateSearchResultContainer .paperlist-avatar img').index(target); var options = { showHideOpacity:false, bgOpacity:0.8, index:index, spacing:0.15, history: false, focus:false, getThumbBoundsFn: function(index) { var thumbnail = searchPaperListAvatarThumb[index]; var pageYScroll = window.pageYOffset || document.documentElement.scrollTop; var rect = thumbnail.getBoundingClientRect(); 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switch(field_type) { case "text": case "password": case "textarea": case "hidden": elements[i].value = ""; break; case "radio": case "checkbox": if (elements[i].checked) { elements[i].checked = false; } break; case "select-one": case "select-multi": elements[i].selectedIndex = -1; break; default: break; } } } function generateShowMoreButton(offset, term) { var code = '<button aria-label="ShowMore" id="showMore" class="btn btn-success float-right mr-2" data-offset="' + offset + '">Show more</button>'; return code; } function hideModal(id) { $("#"+id).modal('hide'); } function showModal(id) { $("#"+id).modal({}); } function prepareForPhotoSwipe() { searchPaperListAvatar = []; searchPaperListAvatarThumb = []; search_pswpElement = document.querySelectorAll('.pswp')[0]; } function _sendAjax(projectID, term) { let httpRequest = new XMLHttpRequest(); if(searchRunning) { console.log("Search running"); return; } if (!httpRequest) { console.error("Giving up :( Cannot create an XMLHTTP instance"); showError(-1); return false; } // httpRequest.timeout = 20000; // time in milliseconds httpRequest.withCredentials = false; httpRequest.ontimeout = (e) => { showError(-1, "result timeout"); searchRunning = false; }; httpRequest.onreadystatechange = function() { if (httpRequest.readyState === XMLHttpRequest.DONE) { searchRunning = false; if (httpRequest.status === 200) { let rs = JSON.parse(httpRequest.responseText); if(rs) { if(rs.isError) { showError(rs.errorCode, rs.errorMessage); } else { let html = rs.resultHTMLs; $("#modal_search_query").val(rs.term); $("#templateSearchResultTerm").html(rs.term); $("#templateSearchResultNr").html(rs.resultsNr); $("#templateRefineSearch").html(rs.filter); if(rs.filter == false) { console.log('filter empty'); $("#refineSearchModal").removeClass('d-block').addClass('d-none'); } if(rs.resultsNr==1) $("#templateSearchResultNrPlural").hide(); else $("#templateSearchResultNrPlural").show(); if(rs.resultsNr==0) { hideModal('templateSearchLoadingModal'); $("#templateSearchResultContainer").html(""); 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searchRunning = true; } function _runSearch() { var projectID = document.querySelector('meta[name="global_projectID"]').content; var term = _searchTrimInput(SEARCH_INPUT.value); if(term.length > 0) { _sendAjax(projectID, term); } else { showError(2, 'Empty search term') } } function _searchTrimInput(str) { return str.replace(/^\s+|\s+$/gm, ''); } function run() { _addEventListener(); $('#templateSearchInfoBtn, #modalSearchInfoBtn').popover({ sanitize: false, html: true, content: $("#templateSearchInfo").html(), placement: "bottom", template: '<div class="popover" role="tooltip"><div class="arrow"></div><button class="m-1 float-right btn btn-sm btn-danger" id="templateSearchInfoClose"><i class="fas fa-times-circle"></i></button><h3 class="popover-header"></h3><div class="popover-body"></div></div>', title: "Search tips", }); $(document).click(function (e) { let t = $(e.target); let a = t && t.attr("data-toggle")!=="popover" && t.parent().attr("data-toggle")!=="popover"; let b = t && $(".popover").has(t).length===0; if(a && b) { $('#templateSearchInfoBtn').popover('hide'); $('#modalSearchInfoBtn').popover('hide'); } }); $('#templateSearchInfoBtn').on('shown.bs.popover', function () { $("#templateSearchInfoClose").click(function(e){ $('#templateSearchInfoBtn').popover('hide'); e.stopPropagation(); e.stopImmediatePropagation(); return false; }); }) $('#templateSearchResultModal').on('hidden.bs.modal', function(e) { $('body').off('click', '#templateSearchResultContainer .paperlist-avatar img'); var pswpElement = document.querySelectorAll('.pswp')[0]; var gallery = null; var paperListAvatar = []; var paperListAvatarThumb = []; $('.paperlist-avatar img').each(function(){ var webversion = $(this).attr('data-web'); var width = $(this).attr('data-width'); var height = $(this).attr('data-height'); var caption =$(this).attr('data-caption'); var figure = { src:webversion, w:width, h:height, title:caption }; paperListAvatarThumb.push($(this)[0]); paperListAvatar.push(figure); }); $('body').on('click', '.paperlist-avatar img', function (e) { if(paperListAvatarThumb.length === 0 && paperListAvatar.length === 0){ $('.paperlist-avatar img').each(function(){ var webversion = $(this).attr('data-web'); var width = $(this).attr('data-width'); var height = $(this).attr('data-height'); var caption =$(this).attr('data-caption'); var figure = { src:webversion, w:width, h:height, title:caption }; paperListAvatarThumb.push($(this)[0]); paperListAvatar.push(figure); }); } var target = $(this); var index = $('.paperlist-avatar img').index(target); var options = { showHideOpacity:true, bgOpacity:0.8, index:index, spacing:0.15, getThumbBoundsFn: function(index) { var thumbnail = paperListAvatarThumb[index]; var pageYScroll = window.pageYOffset || document.documentElement.scrollTop; var rect = thumbnail.getBoundingClientRect(); return {x:rect.left, y:rect.top + pageYScroll, w:rect.width}; } }; gallery = new PhotoSwipe( pswpElement, PhotoSwipeUI_Default,[paperListAvatar[index]],options); gallery.init(); }); }); $('#templateSearchResultModal').on('hide.bs.modal', function(e) { $("#templateRefineSearch").removeClass('d-block').addClass('d-none'); $("#refineSearchModalHide").removeClass('d-block').addClass('d-none'); $("#refineSearchModal").removeClass('d-none').addClass('d-block'); offset = INITIAL_OFFSET; }) $(document).on("click", "#showMore", function(e){ offset+=INITIAL_OFFSET; runSearchModal() e.stopPropagation(); e.stopImmediatePropagation(); return false; }); $(document).ready(function() { $(document).on("click", "#refineSearchModal", function (e) { $("#templateRefineSearch").removeClass('d-none').addClass('d-block'); $(this).removeClass('d-block').addClass('d-none'); $("#refineSearchModalHide").removeClass('d-none').addClass('d-block'); }); $(document).on("click", "#refineSearchModalHide", function (e) { $("#templateRefineSearch").removeClass('d-block').addClass('d-none'); $(this).removeClass('d-block').addClass('d-none'); $("#refineSearchModal").removeClass('d-none').addClass('d-block'); }); $(document).on("click", "#modal_start_site_search", function (e) { runSearchModal(); e.stopPropagation(); e.stopImmediatePropagation(); return false; }); }); } function runSearchModal() { var projectID = document.querySelector('meta[name="global_projectID"]').content; var queryString = $('#library-filters').serialize(); var term = _searchTrimInput($('#modal_search_query').val()); term+='&'+queryString; if(term.length > 0) { _sendAjax(projectID, term); } else { showError(2, 'Empty search term') } } if(document.getElementById('search_query_solr')) { run(); } </script> </div></div> </div> </div> </div> </div> </header>  <main class="one-column version-2023"> <div id="content" class="container"> <div id="page_content_container" class="CMSCONTAINER row"> <div class="col"> <div class="article"> <div id="top"></div> <div class="row no-gutters header-block mb-1 align-items-end"> <div class="col-12 col-xl-5"> <div class="row d-xl-none mb-3"> <div class="col-12" > <div class="d-none d-lg-block articleBackLink"> <a href="https://acp.copernicus.org/">Articles</a> | <a href="https://acp.copernicus.org/articles/23/issue14.html">Volume 23, issue 14</a> </div> <div class="tab co-angel-left d-md-none"></div> <div class="tab co-angel-right d-md-none"></div> <div class="mobile-citation"> <ul class="tab-navigation no-styling"> <li class="tab1.articlf active"><nobr><a href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023.html">Article</a></nobr></li><li class="tab2.assett"><nobr><a href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-assets.html">Assets</a></nobr></li><li class="tab3.discussioo"><nobr><a href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-discussion.html">Peer review</a></nobr></li><li class="tab450.metrict"><nobr><a href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-metrics.html">Metrics</a></nobr></li><li class="tab500.relationt"><nobr><a href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-relations.html">Related articles</a></nobr></li> </ul> </div> </div> </div> <div class="d-lg-none"> <span class="articleBackLink"><a href="https://acp.copernicus.org/">Articles</a> | <a href="https://acp.copernicus.org/articles/23/issue14.html">Volume 23, issue 14</a> </span> <div class="citation-header" id="citation-content"> <div class="citation-doi">https://doi.org/10.5194/acp-23-8259-2023</div> <div class="citation-copyright">© Author(s) 2023. This work is distributed under <br class="hide-on-mobile hide-on-tablet" />the Creative Commons Attribution 4.0 License.</div> </div> </div> <div class="hide-on-mobile hide-on-tablet"> <div class="citation-header"> <div class="citation-doi">https://doi.org/10.5194/acp-23-8259-2023</div> <div class="citation-copyright">© Author(s) 2023. This work is distributed under <br class="hide-on-mobile hide-on-tablet" />the Creative Commons Attribution 4.0 License.</div> </div> </div> </div> <div class="col-7 d-none d-xl-block"> <div class="text-right articleBackLink"> <a href="https://acp.copernicus.org/">Articles</a> | <a href="https://acp.copernicus.org/articles/23/issue14.html">Volume 23, issue 14</a> </div> <div class="tab co-angel-left d-md-none"></div> <div class="tab co-angel-right d-md-none"></div> <div class="mobile-citation"> <ul class="tab-navigation no-styling"> <li class="tab1.articlf active"><nobr><a href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023.html">Article</a></nobr></li><li class="tab2.assett"><nobr><a href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-assets.html">Assets</a></nobr></li><li class="tab3.discussioo"><nobr><a href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-discussion.html">Peer review</a></nobr></li><li class="tab450.metrict"><nobr><a href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-metrics.html">Metrics</a></nobr></li><li class="tab500.relationt"><nobr><a href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-relations.html">Related articles</a></nobr></li> </ul> </div> </div> </div> <div class="ms-type row no-gutters d-none d-lg-flex mb-1 mt-0 align-items-center"> <div class="col"> <div class="row no-gutters align-items-center"> <div class="col-auto"> <mark>ACP Letters</mark> </div> <div class="col-auto">  | <strong>Highlight paper</strong> </div> <div class="col">  | <a target="_blank" href="https://creativecommons.org/licenses/by/4.0/" rel="license" class="licence-icon-svg"><img src="https://www.atmospheric-chemistry-and-physics.net/licenceSVG_16.svg"></a> </div> </div> </div> <div class="col-auto text-right">25 Jul 2023</div> </div> <div class="ms-type row no-gutters d-lg-none mb-1 align-items-center"> <div class="col-12"> <mark>ACP Letters</mark> | <strong>Highlight paper</strong> | <a target="_blank" href="https://creativecommons.org/licenses/by/4.0/" rel="license" class="licence-icon-svg "><img src="https://www.atmospheric-chemistry-and-physics.net/licenceSVG_16.svg"></a> | <span>25 Jul 2023</span> </div> </div> <a class="article-avatar hide-on-mobile hide-on-tablet" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-avatar-web.png" target="_blank"> <img border="0" src="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-avatar-thumb150.png" data-caption="© Author(s). Distributed under the Creative Commons Attribution 4.0 License." data-web="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-avatar-web.png" data-width="600" data-height="417"> </a> <h1>Detection of large-scale cloud microphysical changes within a major shipping corridor after implementation of the International Maritime Organization 2020 fuel sulfur regulations</h1> <div class="auto-fixed-top-forced article-title"> <div class="grid-container show-on-fixed" style="display: none"> <div class="grid-85 mobile-grid-85 tablet-grid-85 grid-parent"> <span class="d-block hide-on-mobile hide-on-tablet journal-contentHeaderColor">Detection of large-scale cloud microphysical changes within a major shipping corridor after implementation of the International Maritime Organization 2020 fuel sulfur regulations</span> <span class="d-block hide-on-desktop journal-contentHeaderColor">Detection of large-scale cloud microphysical changes within a major shipping corridor after...</span> <span>Michael S. Diamond</span> </div> <div class="grid-1 mobile-grid-15 tablet-grid-15 grid-parent text-right"> <a id="scrolltop" class="scrollto" href="https://acp.copernicus.org/articles/23/8259/2023/#top"><i class="co-home"></i> </a> </div> </div> </div> <div class="mb-3 authors-with-affiliations"> <nobr><span class="hover-cursor-pointer journal-contentLinkColor hover-underline" data-toggle="modal" data-target=".author818609">Michael S. Diamond<a href="mailto:msdiamond@fsu.edu"><i class="fal fa-envelope ml-1"></i></a></span></nobr> </div> <div class="modal fade author818609" tabindex="-1" aria-hidden="true"> <div class="modal-dialog modal-dialog-centered modal-dialog-scrollable"> <div class="modal-content"> <div class="modal-header"> <div class="container-fluid p-0"> <h3 class="modal-title">Michael S. Diamond</h3> <div class="row no-gutters"> <div class="col-12">CORRESPONDING AUTHOR</div> <div class="col-12"><a href="mailto:msdiamond@fsu.edu"><i class="fal fa-envelope mr-2"></i>msdiamond@fsu.edu</a></div> </div> <div class="row no-gutters"> <div class="col-12"> <a class="orcid-authors-logo" target="_blank" href="https://orcid.org/0000-0003-2147-5921" data-title="https://orcid.org/0000-0003-2147-5921"><svg class="mr-2" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"><image xlink:href="https://www.atmospheric-chemistry-and-physics.net/orcid_icon.svg" src="https://www.atmospheric-chemistry-and-physics.net/orcid_icon_128x128.png" width="100%" height="100%"></image></svg>https://orcid.org/0000-0003-2147-5921</a> </div> </div> </div> <button type="button" class="close" data-dismiss="modal" aria-label="Close"> <span aria-hidden="true">×</span> </button> </div> <div class="modal-body"> <div class="container-fluid p-0"> <div class="row"> <div class="col-12 mb-3"> Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL 32306, USA </div> </div> </div> </div> </div> </div> </div> <div class="abstract sec" id="abstract"><div class="grid-container no-margin header-element"><span class="grid-100 mobile-grid-100 tablet-grid-100 grid-parent more-less-mobile" data-show="#abstract .co-arrow-open,.abstract-content" data-hide="#abstract .co-arrow-closed,.abstract-mobile-bottom-border"><div class="h1"><span class="section-number"> </span>Abstract<span class="hide-on-desktop hide-on-tablet triangleWrapper"> <i class="co-arrow-closed" style="display:none"></i><i class="co-arrow-open" style="display:inline-block"></i></span></div></span></div> <div class="abstract-content show-no-js"><p id="d1e81">New regulations from the International Maritime Organization (IMO) limiting sulfur emissions from the shipping industry are expected to have large benefits in terms of public health but may come with an undesired side effect: acceleration of global warming as the climate-cooling effects of ship pollution on marine clouds are diminished. Previous work has found a substantial decrease in the detection of ship tracks in clouds after the IMO 2020 regulations went into effect, but changes in large-scale cloud properties have been more equivocal. Using a statistical technique that estimates counterfactual fields of what large-scale cloud and radiative properties within an isolated shipping corridor in the southeastern Atlantic would have been in the absence of shipping, we confidently detect a reduction in the magnitude of cloud droplet effective radius decreases within the shipping corridor and find evidence for a reduction in the magnitude of cloud brightening as well. The instantaneous radiative forcing due to aerosol–cloud interactions from the IMO 2020 regulations is estimated as <span class="inline-formula"><i>O</i></span>(1 W m<span class="inline-formula"><sup>−2</sup></span>) within the shipping corridor, lending credence to global estimates of <span class="inline-formula"><i>O</i></span>(0.1 W m<span class="inline-formula"><sup>−2</sup></span>). In addition to their geophysical significance, our results also provide independent evidence for general compliance with the IMO 2020 regulations.</p></div><span class="abstract-mobile-bottom-border mobile-bottom-border hide-on-desktop hide-on-tablet" style="display:none"></span></div> <div id="oldMobileDownloadBox" class="widget dark-border hide-on-desktop download-and-links"> <div class="legend journal-contentLinkColor">Download & links</div> <div class="content"> <ul class="additional_info no-bullets no-styling"> <li> <a class="triangle" data-toggle=".box-notice" data-duration="300" title="PDF Version (2893 KB)" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023.pdf" > Article (PDF, 2893 KB) </a> </li> </ul> </div> <div class="content"> <ul class="additional_info no-bullets"> <li> <a class="triangle" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-supplement.pdf">Supplement</a> <nobr>(3242 KB)</nobr> </li> </ul> </div> </div> <div id="downloadBoxOneColumn" class="widget dark-border hide-on-desktop download-and-links"> <div class="legend journal-contentLinkColor">Download & links</div> <div class="content"> <ul class="additional_info no-bullets no-styling"> <li><a class="triangle" title="PDF Version (2893 KB)" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023.pdf">Article</a> <nobr>(2893 KB)</nobr> </li> <li> <a class="triangle" title="XML Version" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023.xml">Full-text XML</a> </li> <li> <a class="triangle" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-supplement.pdf">Supplement</a> <nobr>(3242 KB)</nobr> </li> <li><a class="triangle" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023.bib">BibTeX</a></li> <li><a class="triangle" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023.ris">EndNote</a></li> </ul> </div> </div> <div id="share" class="oneColumnShareMobileBox widget dark-border hide-on-desktop"> <div class="legend journal-contentLinkColor">Share</div> <div class="content row m-0 py-1"> <div class="col-auto pl-0"> <a class="share-one-line" href="https://www.mendeley.com/import/?url=https%3A%2F%2Facp.copernicus.org%2Farticles%2F23%2F8259%2F2023%2F" title="Mendeley" target="_blank"> <img src="https://www.atmospheric-chemistry-and-physics.net/mendeley.png" alt="Mendeley"/> </a> </div> <div class="col-auto"> <a class="share-one-line" href="https://www.reddit.com/submit?url=https%3A%2F%2Facp.copernicus.org%2Farticles%2F23%2F8259%2F2023%2F" title="Reddit" target="_blank"> <img src="https://www.atmospheric-chemistry-and-physics.net/reddit.png" alt="Reddit"> </a> </div> <div class="col-auto"> <a class="share-one-line last" href="https://twitter.com/intent/tweet?text=Detection+of+large-scale+cloud+microphysical+changes+within+a+major+shipping+corridor+after+implementation+of+the+International+Maritime+Organization+2020+fuel+sulfur+regulations https%3A%2F%2Facp.copernicus.org%2Farticles%2F23%2F8259%2F2023%2F" title="Twitter" target="_blank"> <img src="https://www.atmospheric-chemistry-and-physics.net/twitter.png" alt="Twitter"/> </a> </div> <div class="col-auto"> <a class="share-one-line" href="https://www.facebook.com/share.php?u=https%3A%2F%2Facp.copernicus.org%2Farticles%2F23%2F8259%2F2023%2F&t=Detection+of+large-scale+cloud+microphysical+changes+within+a+major+shipping+corridor+after+implementation+of+the+International+Maritime+Organization+2020+fuel+sulfur+regulations" title="Facebook" target="_blank"> <img src="https://www.atmospheric-chemistry-and-physics.net/facebook.png" alt="Facebook"/> </a> </div> <div class="col-auto pr-0"> <a class="share-one-line last" href="https://www.linkedin.com/shareArticle?mini=true&url=https%3A%2F%2Facp.copernicus.org%2Farticles%2F23%2F8259%2F2023%2F&title=Detection+of+large-scale+cloud+microphysical+changes+within+a+major+shipping+corridor+after+implementation+of+the+International+Maritime+Organization+2020+fuel+sulfur+regulations" title="LinkedIn" target="_blank"> <img src="https://www.atmospheric-chemistry-and-physics.net/linkedin.png" alt="LinkedIn"> </a> </div> <div class="col pr-0 mobile-native-share"> <a href="#" data-title="Atmospheric Chemistry and Physics" data-text="*Detection of large-scale cloud microphysical changes within a major shipping corridor after implementation of the International Maritime Organization 2020 fuel sulfur regulations* Michael S. Diamond" data-url="https://acp.copernicus.org/articles/23/8259/2023/" class="mobile-native-share share-one-line last"><i class="co-mobile-share display-none"></i></a> </div> </div> </div> <div id="citation-footer" class="sec"> <div class="h1-special journal-contentHeaderColor">How to cite. </div> <div class="citation-footer-content show-no-js"> <p> <div class="citation-footer"> Diamond, M. S.: Detection of large-scale cloud microphysical changes within a major shipping corridor after implementation of the International Maritime Organization 2020 fuel sulfur regulations, Atmos. Chem. Phys., 23, 8259–8269, https://doi.org/10.5194/acp-23-8259-2023, 2023. </div> </p> </div> </div> <div id="article-dates" class="sec"> <div class="article-dates dates-content my-3"> <nobr>Received: 11 May 2023</nobr> – <nobr>Discussion started: 22 May 2023</nobr> – <nobr>Revised: 20 Jun 2023</nobr> – <nobr>Accepted: 29 Jun 2023</nobr> – <nobr>Published: 25 Jul 2023</nobr> </div> </div> <div class="sec intro" id="section1"><div class="grid-container no-margin header-element"><span class="grid-100 mobile-grid-100 tablet-grid-100 grid-parent more-less-mobile" data-hide="#section1 .co-arrow-open,.section1-content" data-show="#section1 .co-arrow-closed,.section1-mobile-bottom-border"><div id="Ch1.S1" class="h1"><span class="label">1</span> Introduction and approach<span class="hide-on-desktop hide-on-tablet triangleWrapper"> <i class="co-arrow-closed"></i><i class="co-arrow-open" style="display:none"></i></span></div></span></div> <div class="section1-content show-no-js hide-on-mobile-soft"><p id="d1e131">Since 1 January 2020, International Maritime Organization (IMO) Marine Environment Protection Committee (MEPC) regulations have limited sulfur in marine fuels from 3.5 % by mass to 0.5 % or required exhaust gas cleaning systems (scrubbers) to achieve an equivalent reduction in sulfur oxide (SO<span class="inline-formula"><sub><i>x</i></sub></span>) pollution (IMO, 2019). These IMO 2020 fuel sulfur regulations and the resulting decrease in sulfate aerosol (airborne particulates) are expected to have large benefits to public health (Partanen et al., 2013; Sofiev et al., 2018; Zhang et al., 2021). They are also expected to have an undesired side effect, however: as sulfate aerosol cools the climate by reflecting sunlight directly and indirectly via changing cloud properties, the IMO 2020 SO<span class="inline-formula"><sub><i>x</i></sub></span> reductions may accelerate global warming.</p><p id="d1e152">Shipping effects on clouds were first identified in the mid-1960s in satellite imagery of ship tracks, or curvilinear cloud perturbations following individual ships (Conover, 1966; Twomey et al., 1968). For the same amount of liquid water within a cloud, increasing aerosol increases the cloud droplet number concentration (<span class="inline-formula"><i>N</i><sub>d</sub></span>) and decreases the cloud-top effective radius (<span class="inline-formula"><i>r</i><sub>e</sub></span>), brightening the clouds (Twomey, 1974, 1977). Cloud macrophysical adjustments to this aforementioned Twomey effect have been observed within ship tracks as well and can reinforce the microphysical brightening effect by suppressing drizzle (Albrecht, 1989; Goren and Rosenfeld, 2012) or counteract it by enhancing entrainment (Chen et al., 2012; Coakley and Walsh, 2002; Toll et al., 2019). Understanding how much greenhouse gas warming is masked by these aerosol–cloud interactions from shipping and other forms of pollution is the la<span id="page8260"></span>rgest source of uncertainty in quantifying present-day anthropogenic radiative forcing (Forster et al., 2021).</p><p id="d1e178">Although ship tracks have long served as “natural experiments” for testing hypotheses about aerosol–cloud interactions in cases of clear causality (Christensen et al., 2022), until recently, attempts to observationally assess regional- to global-scale cloud perturbations and forcing from shipping have found negligible (Schreier et al., 2007) or null effects (Peters et al., 2011) due to the small fraction of ships that form easily identifiable tracks and the large background variability in cloud properties. New methods using machine learning have identified many times more ship tracks than has been possible with manual identification (Watson-Parris et al., 2022; Yuan et al., 2022, 2019), and analyses tracking air masses from ship locations have shown that cloud adjustments differ systematically between easily identifiable and “invisible” ship tracks (Manshausen et al., 2022). Using some of these newer methods, it has been shown that ship track occurrence decreased regionally after the introduction of emission control areas around North America and Europe and then globally after the IMO 2020 regulations went into effect (Gryspeerdt et al., 2019; Watson-Parris et al., 2022; Yuan et al., 2022). Large-scale changes in cloud microphysical and macrophysical properties have been more equivocal, however. Yuan et al. (2022) found smaller <span class="inline-formula"><i>N</i><sub>d</sub></span> increases within ship tracks after the IMO 2020 regulations, as expected, but, paradoxically, greater <span class="inline-formula"><i>r</i><sub>e</sub></span> decreases than before and no difference in cloud brightness. Watson-Parris et al. (2022) did not find evidence for a change in global or regional <span class="inline-formula"><i>N</i><sub>d</sub></span> after the IMO 2020 regulations despite the clear decrease in ship tracks, with a possible exception in the southeastern Atlantic.</p><p id="d1e214">In this work, we assess the detectability of large-scale cloud perturbations from the IMO 2020 regulations by revisiting an alternate solution to the limitations of “bottom-up” methods tracking individual ship tracks: a “top-down” statistical approach developed by Diamond et al. (2020), hereafter D20, to identify regional-scale cloud perturbations within a shipping corridor in the southeastern Atlantic Ocean basin. A unique meteorological setup makes that region ideal for estimating causal aerosol effects: near-surface winds blow parallel to the shipping corridor and closely constrain the pollution, which also happens to intersect a major stratocumulus cloud deck. D20 used a universal kriging method (see Zimmerman and Stein, 2010, and references therein) to estimate counterfactual fields of cloud properties and radiation in the absence of the shipping corridor based on the observed spatial statistics of nearby, non-shipping-affected grid boxes. They found significant increases in <span class="inline-formula"><i>N</i><sub>d</sub></span> and cloud albedo (a measure of cloud reflectivity) and decreases in <span class="inline-formula"><i>r</i><sub>e</sub></span> within the stratocumulus deck but estimated that several years' worth of data were needed to detect a clear signal. Thus, it is possible that the effect of the IMO 2020 regulations will have just become detectable using their method.</p><p id="d1e240">Here, we apply an updated version of the D20 universal kriging algorithm to satellite retrievals of <span class="inline-formula"><i>r</i><sub>e</sub></span> and overcast albedo (<span class="inline-formula"><i>A</i><sub>cld</sub></span>; top-of-atmosphere albedo when clouds are present) from the Clouds and the Earth's Radiant Energy System (CERES) Single Scanner Footprint (SSF) product for the Terra satellite (Loeb et al., 2018; Minnis et al., 2011). The reader is referred to Methods in Appendix A for further details about the data, universal kriging algorithm, and significance tests. Although D20 found a substantial decrease in cloud liquid water path within the corridor during the afternoon, no significant cloud macrophysical adjustments were found in the morning. We therefore interpret any changes in <span class="inline-formula"><i>r</i><sub>e</sub></span> and <span class="inline-formula"><i>A</i><sub>cld</sub></span> using the Terra record (observations at <span class="inline-formula">∼</span> 10:30 local time) as being dominated by the Twomey effect. We focus on both the austral spring season (SON; September–October–November), which features the strongest shipping signal (likely due to a combination of favorable meteorology and lower background <span class="inline-formula"><i>N</i><sub>d</sub></span>; Grosvenor et al., 2018), and the annual mean (ANN), which averages a greater number of observations and thus should minimize noise. For a variable <span class="inline-formula"><i>X</i></span>, the “factual” or observed value in the presence of the shipping corridor is referred to as “Ship” (<span class="inline-formula"><i>X</i><sub>Ship</sub></span>), the counterfactual value in the absence of shipping obtained via kriging is referred to as “NoShip” (<span class="inline-formula"><i>X</i><sub>NoShip</sub>)</span>, and the Ship–NoShip difference is signified as <span class="inline-formula">Δ<i>X</i></span> and is interpreted as the effect due to the presence of the shipping corridor.</p></div><span class="section1-mobile-bottom-border mobile-bottom-border hide-on-desktop hide-on-tablet"></span></div> <div class="sec" id="section2"><div class="grid-container no-margin header-element"><span class="grid-100 mobile-grid-100 tablet-grid-100 grid-parent more-less-mobile" data-hide="#section2 .co-arrow-open,.section2-content" data-show="#section2 .co-arrow-closed,.section2-mobile-bottom-border"><div id="Ch1.S2" class="h1"><span class="label">2</span> Results<span class="hide-on-desktop hide-on-tablet triangleWrapper"> <i class="co-arrow-closed"></i><i class="co-arrow-open" style="display:none"></i></span></div></span></div> <div class="section2-content show-no-js hide-on-mobile-soft"><p id="d1e355">An unambiguous decrease in the magnitude of the <span class="inline-formula"><i>r</i><sub>e</sub></span> perturbation within the shipping corridor is evident in the post-regulation (2020–2022) data compared to the pre-2020 climatology (2002–2019) and the immediately preceding 3-year period (2017–2019) during austral spring (Fig. 1). Although several significant grid boxes (observations falling outside the 95 % confidence interval of the counterfactual) remain in the south of the domain, and thus some level of continued shipping influence is detected (as indicated by field significance at the <span class="inline-formula">≪</span> 0.05 level), the microphysical changes are smaller and less clearly tied to the corridor; the signal is completely lost further north. Similar results are found for the annual mean values (Supplement Fig. S1), albeit with a clearer continued effect of shipping in the 2020–2022 data.</p><div class="fig" id="Ch1.F1"><a target="_blank" class="figure-link" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f01-web.png"><img alt="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f01" data-webversion="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f01-web.png" src="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f01-thumb.png" data-width="2067" data-height="1436"></a><div class="caption"><p id="d1e378"><strong class="caption-number">Figure 1</strong>Maps of factual (observed) and counterfactual values and their difference for austral spring cloud-top effective radius for the pre-2020 climatology <strong>(a–c)</strong>, the immediately pre-regulation 3-year period 2017–2019 <strong>(d–f)</strong>, and the immediate post-regulation 3-year period 2020–2022 <strong>(g–i)</strong>. The analysis domain of 18 to 8<span class="inline-formula"><sup>∘</sup></span> S, 13<span class="inline-formula"><sup>∘</sup></span> W to 8<span class="inline-formula"><sup>∘</sup></span> E is outlined in black. Grid points for which the observed values fall outside the 95 % confidence interval obtained via kriging are indicated by white dots, and the corresponding field significance values are reported in <strong>(c)</strong>, <strong>(f)</strong>, and <strong>(i)</strong>.</p></div><p class="downloads"></p></div><p id="d1e433">The shipping perturbation in overcast albedo is less well defined than that in the cloud microphysics, but there is still a clear perturbation in the 2002–2019 climatology and 2017–2019 data that is diminished in the 2020–2022 data in austral spring (Fig. 2). Similar results are found in the annual mean, although the 2020–2022 change is more ambiguous from visual inspection alone (Fig. S2). Lower background <span class="inline-formula"><i>A</i><sub>cld</sub></span> values in 2020–2022, particularly in the annual mean (Fig. S2g), may be related to unusually warm sea surface temperatures (Figs. S3–S4); as dimmer clouds are relatively more susceptible to aerosol perturbations, this effect may partially obscure<span id="page8261"></span> the decrease in cloud brightening from the IMO 2020 regulations.</p><div class="fig" id="Ch1.F2"><a target="_blank" class="figure-link" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f02-web.png"><img alt="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f02" data-webversion="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f02-web.png" src="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f02-thumb.png" data-width="2067" data-height="1436"></a><div class="caption"><p id="d1e450"><strong class="caption-number">Figure 2</strong>As in Fig. 1 but for austral spring overcast albedo.</p></div><p class="downloads"></p></div><p id="d1e459">To assess how anomalous the post-regulation 2020–2022 shipping perturbation values are, we compare them to those from prior 3-year periods by averaging over a core shipping corridor region (see Methods in Appendix A) and, to minimize effects from changing background conditions, also calculate perturbations as relative differences (100 % <span class="inline-formula">⋅</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M31" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi mathvariant="normal">Δ</mi><mi>X</mi><mo>/</mo><msub><mi>X</mi><mi mathvariant="normal">Ship</mi></msub></mrow></math><span><svg xmlns:svg="http://www.w3.org/2000/svg" width="47pt" height="16pt" class="hide-js svg-formula" dspmath="mathimg" md5hash="7701bb432d848f30d5284eb767a8f180"><image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-ie00001.svg" width="100%" height="16pt" src="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-ie00001.png"></image></svg></span></span>). Full results are reported in Table S1 and summarized in Fig. 3. For the austral spring <span class="inline-formula"><i>r</i><sub>e</sub></span> perturbations, 2020–2022 is unprecedentedly weak (Fig. 3a) and does not overlap any prior period's value within their 95 % confidence intervals (Table S1 in the Supplement). The separation between 2020–2022 and any other period's values is not as clear for austral spring <span class="inline-formula"><i>A</i><sub>cld</sub></span> (Fig. 3b), although the 2020–2022 perturbation values are the lowest on record and are the only period for which the effect is not distinguishable from zero at the 95 % confidence level (Table S1). For the annual mean <span class="inline-formula"><i>r</i><sub>e</sub></span> perturbations, the 2020–2022 values are lower than any other period and the difference with the climatological value is much larger than for any other period, although the separation is not as clear as for austral spring (Fig. 3c). While the annual mean <span class="inline-formula"><i>A</i><sub>cld</sub></span> perturbations for 2020–2022 are also the lowest on record, the difference from climatology is not extreme compared to other periods (Fig. 3d).</p><div class="fig" id="Ch1.F3"><a target="_blank" class="figure-link" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f03-web.png"><img alt="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f03" data-webversion="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f03-web.png" src="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f03-thumb.png" data-printversion="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f03-high-res.pdf" data-width="2067" data-height="3218"></a><div class="caption"><p id="d1e533"><strong class="caption-number">Figure 3</strong>Probability densities (via Gaussian kernel density estimation) for the Ship–NoShip relative differences within the core shipping corridor for austral spring <span class="inline-formula"><i>r</i><sub>e</sub></span> <strong>(a)</strong>, austral spring <span class="inline-formula"><i>A</i><sub>cld</sub></span> <strong>(b)</strong>, annual mean <span class="inline-formula"><i>r</i><sub>e</sub></span> <strong>(c)</strong>, and annual mean <span class="inline-formula"><i>A</i><sub>cld</sub></span> <strong>(d)</strong>. The 2002–2019 climatology values are shown as gray shading, the 3-year periods prior to the IMO 2020 regulations as colored lines, and the 2020–2022 period as black lines. Solid, dashed, and dotted lines indicate decreasing degrees of field significance.</p></div><p class="downloads"><a class="triangle journal-contentLinkColor figure-download" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f03-high-res.pdf" target="_blank">Download</a></p></div><p id="d1e599">To assess whether a reduction in the shipping effect after the IMO 2020 regulations went into effect is detected at various possible levels of confidence, Table S2 reports different percentiles of the ratio of the 2020–2022 relative differences over the climatology. A decrease in the <span class="inline-formula"><i>r</i><sub>e</sub></span> perturbation is detected at greater than 99 % confidence in the austral spring and at greater than 95 % confidence in the annual mean, whereas decreases in the <span class="inline-formula"><i>A</i><sub>cld</sub></span> perturbation are only significant at the 90 % confidence level in the austral spring and within the interquartile range in the annual mean. We thus conclude that the effect of the IMO 2020 regulations has been clearly detected in the large-scale cloud microphysics and that there is strong evidence for a decrease in cloud brightness, although more years of data may be required for unequivocal detection of changes in overcast albedo.</p></div><span class="section2-mobile-bottom-border mobile-bottom-border hide-on-desktop hide-on-tablet"></span></div> <div class="sec" id="section3"><div class="grid-container no-margin header-element"><span class="grid-100 mobile-grid-100 tablet-grid-100 grid-parent more-less-mobile" data-hide="#section3 .co-arrow-open,.section3-content" data-show="#section3 .co-arrow-closed,.section3-mobile-bottom-border"><div id="Ch1.S3" class="h1"><span class="label">3</span> Discussion<span class="hide-on-desktop hide-on-tablet triangleWrapper"> <i class="co-arrow-closed"></i><i class="co-arrow-open" style="display:none"></i></span></div></span></div> <div class="section3-content show-no-js hide-on-mobile-soft"><div class="sec"><h2 id="Ch1.S3.SS1"><span class="label">3.1</span> Monitoring compliance with IMO regulations</h2> <p id="d1e639">Assessing (non)compliance with the IMO 2020 regulations is of critical importance for ensuring that the intended public health benefits are realized. One assessment method is<span id="page8262"></span> to monitor the sulfur content of the global fuel oil supply. According to data supplied to the IMO MEPC (IMO, 2020, 2021, 2022, 2023), before 2020, the average sulfur mass content of marine fuel oils was <span class="inline-formula">∼</span> 2.5 % and <span class="inline-formula">∼</span> 80 % of the global fuel oil supply exceeded 0.5 %; since 2020, the average sulfur mass content has declined to <span class="inline-formula">∼</span> 1 % and only <span class="inline-formula">∼</span> 20 % of fuel has exceeded 0.5 % (Fig. S5). These values understate compliance, as a “carriage ban” forbids ships from carrying the remaining noncompliant fuel oil unless they have scrubbers installed (IMO, 2018). Geophysical monitoring via cloud changes, as has been shown in Yuan et al. (2022) and Watson-Parris et al. (2022) for ship track occurrence and here for large-scale cloud microphysical properties, offers an independent check to increase confidence that there has been substantial compliance with the IMO 2020 regulations. As our understanding of the cloud effects from shipping aerosol improves, it may become possible to assess regional differences in compliance or even compliance for individual ships, complementing other successful geophysical monitoring programs like those for detecting ozone-depleting substances (Montzka et al., 2018; Park et al., 2021; Rigby et al., 2019).</p> <p id="d1e670">Given the clear detection of cloud microphysical changes in austral spring after the IMO 2020 regulations went into effect, it is reasonable to ask whether advanced statistical methods are necessary for evaluating (some level of) compliance or if simple time series (e.g., Fig. S5 of Watson-Parris et al., 2022) would suffice. From the time series of austral spring Ship and NoShip <span class="inline-formula"><i>r</i><sub>e</sub></span> values averaged over the southeastern Atlantic (Fig. 4), it is evident that the shipping effect before 2020–2022 is of similar magnitude to interannual variability in the background values and that the 2020–2022 <span class="inline-formula"><i>r</i><sub>e</sub></span> values are estimated to be the highest on record even before any IMO 2020 effect is considered. As an estimate of what the 2020–2022 observed value would have been under a scenario of complete noncompliance with the sulfur regulations, the average Ship–NoShip difference from the 2002–2019 climatology is applied to the 2020–2022 NoShip value (“Noncompliance” in Fig. 4). The <span class="inline-formula">+</span>0.1 <span class="inline-formula">µ</span>m difference between the observed (Ship) value and this noncompliance hypothetical is due to compliance with the IMO 2020 regulations. If we had rather based our noncompliance scenario on a persistence forecast of the 2017–2019 value and then observed the value from the “true” noncompliance estimate calculated above, we would erroneously conclude that the IMO 2020 regulations were successfully implemented and led to a <span class="inline-formula">+</span>0.3 <span class="inline-formula">µ</span>m increase in regional <span class="inline-formula"><i>r</i><sub>e</sub></span>. Of course, in this latter scenario, the true value of the difference due to IMO 2020 would have been zero and the apparent effect only an artifact of the changing background. Caution is therefore advised in attempting to interpret time series of large-scale cloud properties without applying a method (like track identification or kriging) that plausibly establishes causality.</p> <div class="fig" id="Ch1.F4"><a target="_blank" class="figure-link" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f04-web.png"><img alt="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f04" data-webversion="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f04-web.png" src="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f04-thumb.png" data-printversion="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f04-high-res.pdf" data-width="2067" data-height="1400"></a><div class="caption"><p id="d1e739"><strong class="caption-number">Figure 4</strong>Time series of observed Ship (black circles) and mean NoShip (blue diamonds) values averaged the southeastern Atlantic analysis domain (18 to 8<span class="inline-formula"><sup>∘</sup></span> S, 13<span class="inline-formula"><sup>∘</sup></span> W to 8<span class="inline-formula"><sup>∘</sup></span> E) for austral spring <span class="inline-formula"><i>r</i><sub>e</sub></span>. Error bars represent 95 % confidence for the NoShip values. A noncompliance scenario in which the IMO 2020 regulations were not enforced and the Ship–NoShip differences in 2020–2022 were the same as for the 2002–2019 climatology is denoted as a dark-red “x”. The dotted red line denotes the estimated effect from compliance with the IMO 2020 regulations, calculated as the difference between the observed Ship value and the hypothetical noncompliance value expected for no change in 2020–2022. The dotted orange line denotes the mistakenly determined effect that would have resulted if the noncompliance scenario were true and observed but a persistence forecast of 2017–2019 were used as the expectation value for no change in 2020–2022.</p></div><p class="downloads"><a class="triangle journal-contentLinkColor figure-download" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f04-high-res.pdf" target="_blank">Download</a></p></div> </div><span id="page8263"></span><div class="sec"><h2 id="Ch1.S3.SS2"><span class="label">3.2</span> Radiative forcing implications</h2> <p id="d1e794">Assuming that the Terra-based <span class="inline-formula"><i>r</i><sub>e</sub></span> and <span class="inline-formula"><i>A</i><sub>cld</sub></span> perturbations are dominated by the Twomey effect as in D20, it is possible to estimate the instantaneous radiative forcing due to aerosol–cloud interactions (IRF<span class="inline-formula"><sub>ACI</sub></span>; Forster et al., 2021) from the IMO 2020 regulations within the shipping corridor (see Methods in Appendix A). Results are shown in Fig. 5 for the 2002–2019 climatology, 2020–2022, and their difference (interpreted as the effect of the IMO 2020 regulations). The Twomey effect estimates are much better constrained for the calculations using <span class="inline-formula"><i>r</i><sub>e</sub></span>, but those using <span class="inline-formula"><i>A</i><sub>cld</sub></span> show consistent results. The IMO 2020 regulations led to a <span class="inline-formula">∼</span> 2 W m<span class="inline-formula"><sup>−2</sup></span> IRF<span class="inline-formula"><sub>ACI</sub></span> within the shipping corridor during austral spring and a <span class="inline-formula">∼</span> 0.5 W m<span class="inline-formula"><sup>−2</sup></span> IRF<span class="inline-formula"><sub>ACI</sub></span> in the annual mean. Applying this <span class="inline-formula">∼</span> 35 %–70 % decline in IRF<span class="inline-formula"><sub>ACI</sub></span> to the <span class="inline-formula">−</span>0.1 to <span class="inline-formula">−</span>0.6 W m<span class="inline-formula"><sup>−2</sup></span> range of forcing due to shipping emissions from climate models (Capaldo et al., 1999; Lauer et al., 2007; Peters et al., 2013; Righi et al., 2011; Sofiev et al., 2018), global forcing values of <span class="inline-formula"><i>O</i></span>(0.1 W m<span class="inline-formula"><sup>−2</sup></span>) due to the IMO 2020 regulations are plausible. The strongest shipping effect in Lauer et al. (2007) represented 40 % of their global ACI; a 70 % reduction from that fraction would represent a forcing of <span class="inline-formula">0.2±0.1</span> W m<span class="inline-formula"><sup>−2</sup></span> based on the currently assessed IRF<span class="inline-formula"><sub>ACI</sub></span> value of <span class="inline-formula">0.7±0.5</span> W m<span class="inline-formula"><sup>−2</sup></span>, or <span class="inline-formula">0.3±0.2</span> W m<span class="inline-formula"><sup>−2</sup></span> including adjustments (Forster et al., 2021)</p> <div class="fig" id="Ch1.F5"><a target="_blank" class="figure-link" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f05-web.png"><img alt="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f05" data-webversion="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f05-web.png" src="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f05-thumb.png" data-printversion="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f05-high-res.pdf" data-width="2067" data-height="1699"></a><div class="caption"><p id="d1e1054"><strong class="caption-number">Figure 5</strong>Probability densities (via Gaussian kernel density estimation) of IRF<span class="inline-formula"><sub>ACI</sub></span> for austral spring <strong>(a)</strong> and the annual mean <strong>(b)</strong> over the core shipping corridor calculated using the changes in <span class="inline-formula"><i>r</i><sub>e</sub></span> (shading) from Eq. (A1) and <span class="inline-formula"><i>A</i><sub>cld</sub></span> (lines) from Eq. (A2) for the 2002–2019 pre-regulation climatology (solid light-blue shading and lines) and 2020–2022 post-regulation period (solid dark-gray shading and lines) due to the presence of the shipping corridor and the 2020–2022 minus climatology difference as an estimate of the effect due to implementation of the IMO 2020 regulations (patterned red shading and lines).</p></div><p class="downloads"><a class="triangle journal-contentLinkColor figure-download" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-f05-high-res.pdf" target="_blank">Download</a></p></div> </div></div><span class="section3-mobile-bottom-border mobile-bottom-border hide-on-desktop hide-on-tablet"></span></div> <div class="sec conclusions" id="section4"><div class="grid-container no-margin header-element"><span class="grid-100 mobile-grid-100 tablet-grid-100 grid-parent more-less-mobile" data-hide="#section4 .co-arrow-open,.section4-content" data-show="#section4 .co-arrow-closed,.section4-mobile-bottom-border"><div id="Ch1.S4" class="h1"><span class="label">4</span> Conclusions<span class="hide-on-desktop hide-on-tablet triangleWrapper"> <i class="co-arrow-closed"></i><i class="co-arrow-open" style="display:none"></i></span></div></span></div> <div class="section4-content show-no-js hide-on-mobile-soft"><p id="d1e1109">There is a detectable change in large-scale cloud microphysical properties and evidence supporting a decrease in cloud brightening within the major southeastern Atlantic shipping corridor after implementation of the IMO 2020 fuel sulfur regulations, resulting in a positive IRF<span class="inline-formula"><sub>ACI</sub></span> within the corridor of <span class="inline-formula"><i>O</i></span>(1 W m<span class="inline-formula"><sup>−2</sup></span>). Although this study did not address potential changes in cloud adjustments from the IMO 2020 regulations, this will be an important area of future work, especially<span id="page8264"></span> as the fuel regulations are expected not only to decrease overall aerosol numbers but also shift them toward smaller sizes and sootier composition (Ault et al., 2010; Lack et al., 2011; Seppälä et al., 2021).</p></div><span class="section4-mobile-bottom-border mobile-bottom-border hide-on-desktop hide-on-tablet"></span></div> <div class="app sec" id="section5"> <div class="grid-container no-margin header-element"><span class="grid-100 mobile-grid-100 tablet-grid-100 grid-parent more-less-mobile" data-hide="#section5 .co-arrow-open,.section5-content" data-show="#section5 .co-arrow-closed,.section5-mobile-bottom-border"><div id="App1.Ch1.S1" class="h1"><span>Appendix A:</span> Methods<span class="hide-on-desktop hide-on-tablet triangleWrapper"> <i class="co-arrow-closed"></i><i class="co-arrow-open" style="display:none"></i></span></div></span></div> <div class="section5-content show-no-js hide-on-mobile-soft"><div class="sec"><h2 id="App1.Ch1.S1.SS1"><span class="label">A1</span> Data</h2> <p id="d1e1159">All cloud, radiation, and meteorological data in this work come from the CERES SSF regional 1<span class="inline-formula"><sup>∘</sup></span> <span class="inline-formula">×</span> 1<span class="inline-formula"><sup>∘</sup></span> (SSF1deg) monthly product based on the CERES instrument from the Terra satellite (CERES Science Team, 2021, 2023; Loeb et al., 2018; Wielicki et al., 1996). Radiative fluxes are temporally interpolated over the diurnal cycle assuming constant cloud and meteorological properties but varying the solar zenith angle (Doelling et al., 2013); our results therefore reflect the diurnal average assuming constant Terra conditions rather than the instantaneous midmorning value, which would be much greater in magnitude, but do not account for any diurnal cloud evolution. Overcast albedo values are calculated as in D20 but with the clear-sky albedo assumed to be 0.1 to avoid issues with missing clear-sky data in the SSF1deg product. The constant clear-sky albedo may cause a high bias in the absolute <span class="inline-formula"><i>A</i><sub>cld</sub></span> values, especially during the southern African biomass burning season (June to October), but this effect should be small given the very overcast conditions and would not strongly affect the observed versus counterfactual differences. The overcast albedo (albedo as seen from space when clouds are present) differs from the cloud albedo (cloud reflectivity) due to the scattering and absorption of sunlight from above-cloud aerosols and gases.</p> <p id="d1e1198">Cloud properties are retrieved from Moderate Resolution Imaging Spectroradiometer (MODIS) measurements using CERES algorithms (CERES Science Team, 2016; Minnis et al., 2011), which have some differences from the standard MODIS products (Platnick et al., 2017). Only daytime cloud retrievals utilizing 3.7 <span class="inline-formula">µ</span>m channel radiances are used in this work. Low cloud fraction is defined for clouds with cloud-top effective pressure values greater than 700 hPa.</p> <p id="d1e1209">Meteorological variables including surface skin temperature (over oceans, the Reynold's sea surface temperature), estimated inversion strength (Wood and Bretherton, 2006), and wind speed are from the NASA Goddard Space Flight Center Global Modeling and Assimilation Office (GMAO) Goddard Earth Observing System (GEOS) version 5.4.1 (CERES Science Team, 2021).</p> <p id="d1e1212">Sulfur dioxide (SO<span class="inline-formula"><sub>2</sub></span>) emissions data from 2010 are from the Emissions Database for Global Atmospheric Research (EDGAR) version 4 (Crippa et al., 2018) and are identical to those used in D20. The EDGAR SO<span class="inline-formula"><sub>2</sub></span> values are only used for identification of the shipping corridor location.</p> </div><div class="sec"><h2 id="App1.Ch1.S1.SS2"><span class="label">A2</span> Shipping corridor identification</h2> <p id="d1e1241">For each latitude between 8 and 18<span class="inline-formula"><sup>∘</sup></span> S, shipping-affected grid boxes are identified as those with the maximum EDGAR SO<span class="inline-formula"><sub>2</sub></span> emission values between 13<span class="inline-formula"><sup>∘</sup></span> W and 8<span class="inline-formula"><sup>∘</sup></span> E as well as the four grid boxes to the west and two to the east. This represents a northward and westward expansion of the shipping corridor definition used in D20 for their subtropical domain and is intended to better center the microphysical effects. Ship tracking via the automatic identification system (AIS) identifies substantial traffic slightly west of where EDGAR places the maximum SO<span class="inline-formula"><sub>2</sub></span> emissions, and there are indications of an additional westward shift in traffic during 2020 (March et al., 2021). As a sensitivity test, the analysis in Fig. 1 was repeated using a shipping corridor mask shifted further west by 2<span class="inline-formula"><sup>∘</sup></span>, but no notable differences were found. The core shipping corridor area used in Figs. 3 and 5 and Tables S1–S2 is defined as the central three grid boxes of the shipping mask for each latitude.</p> </div><div class="sec"><h2 id="App1.Ch1.S1.SS3"><span class="label">A3</span> Universal kriging</h2> <p id="d1e1307">The universal kriging algorithm mostly follows the implementation of D20, using the geoR statistical package (Ribeiro and Diggle, 2018). Universal kriging is a classic geostatistical method (Zimmerman and Stein, 2010) that has been widely employed in the geosciences and other fields (Chilès and Desassis, 2018), in which estimates of unknown<span id="page8265"></span> values at some location are informed by nearby observations of the same variable under the assumption that errors around a mean function are spatially correlated as a function of the distance between locations only (stationarity). In our case, counterfactual values for the shipping-affected grid boxes identified above are estimated using the values of nearby non-shipping-affected grid boxes between 8 and 18<span class="inline-formula"><sup>∘</sup></span> S and 13<span class="inline-formula"><sup>∘</sup></span> W and 8<span class="inline-formula"><sup>∘</sup></span> E. Our mean function takes the form of a multiple linear regression model using as regressors some combination of the surface skin temperature (SST), estimated inversion strength (EIS), and wind speed (WS) from the SSF1deg auxiliary data and latitude (lat), longitude (long), and their squares (lat<span class="inline-formula"><sup>2</sup></span>, long<span class="inline-formula"><sup>2</sup>)</span> and product (lat <span class="inline-formula">⋅</span> long), as determined by whichever combination minimizes the Bayesian information criterion (BIC) to avoid overfitting. Table S1 reports the selected combination of regressors (based on BIC minimization) for each combination of variable (<span class="inline-formula"><i>r</i><sub>e</sub></span> and <span class="inline-formula"><i>A</i><sub>cld</sub>)</span> and time period. A logit transform is applied to the <span class="inline-formula"><i>A</i><sub>cld</sub></span> values before kriging, which was found by D20 to produce more normally distributed errors around the mean function for bounded fields like albedo and cloud fraction. The stationary error term is then estimated by using weighted least squares to fit a parametric (exponential) covariance model to an empirical variogram (a plot of the squared difference between pairs of variables versus their distance). Figures S6–S9 show the binned empirical variograms and fitted variograms (see Zimmerman and Stein, 2010) for austral spring <span class="inline-formula"><i>r</i><sub>e</sub></span> and logit(<span class="inline-formula"><i>A</i><sub>cld</sub>)</span> and annual mean <span class="inline-formula"><i>r</i><sub>e</sub></span> and logit(<span class="inline-formula"><i>A</i><sub>cld</sub>)</span>, respectively. Using the statistical model provided by the kriging process above (Ribeiro and Diggle, 2018), we simulate 5000 realizations of the NoShip counterfactual for each variable–time-period combination.</p> </div><div class="sec"><h2 id="App1.Ch1.S1.SS4"><span class="label">A4</span> Statistical significance testing</h2> <p id="d1e1458">Four distinct tests of statistical significance are used in this work, the first three following D20. Statistical significance for individual shipping-affected grid boxes is assessed as whether the observed Ship value exceeds the 97.5th percentile or falls below the 2.5th percentile of the distribution obtained via kriging for the counterfactual NoShip value for that grid box.</p> <p id="d1e1461">Field significance is assessed by determining whether the number of individually significant grid boxes calculated above is extreme as compared to that which could occur by chance under the null hypothesis that the region is unaffected by shipping; <span class="inline-formula"><i>p</i></span> values (<span class="inline-formula"><i>p</i><sub>field</sub></span>) are calculated as the fraction of the 5000 NoShip simulations that would have a number of individually significant grid boxes greater than or equal to the factual case and are adjusted for multiple testing using a Benjamini–Hochberg adjustment to control the false-discovery rate (Benjamini and Hochberg, 1995; Ventura et al., 2004). When none of the 5000 NoShip simulations produced a number of individually significant grid boxes as or more extreme than the Ship field, <span class="inline-formula"><i>p</i><sub>field</sub></span> is reported as <span class="inline-formula"><i><</i></span> 0.0001 instead of zero in Table S1. All <span class="inline-formula"><i>r</i><sub>e</sub></span> perturbations (except 2020–2020 austral spring) are field significant at a <span class="inline-formula"><i><</i></span> 0.0001 level; the <span class="inline-formula"><i>A</i><sub>cld</sub></span> perturbations have more variation, although all are significant at greater than 90 % confidence (Fig. 3 and Table S1). Interpreting the field significance as a measure of the robustness of the shipping effects, we should therefore have greatest confidence in the <span class="inline-formula"><i>r</i><sub>e</sub></span> results and least (but still a good deal of) confidence in the annual <span class="inline-formula"><i>A</i><sub>cld</sub></span> results.</p> <p id="d1e1552">The range of Ship–NoShip values generated from the 5000 simulated NoShip fields is used to assess whether the magnitude of effects within the core shipping corridor area is statistically distinct from zero at 95 % confidence (Table S1).</p> <p id="d1e1555">Finally, a new test for “detectability” at different confidence interval thresholds is presented in Table S2 and based on the range of possible ratios of 2020–2022 over climatological relative Ship–NoShip values from the 5000 simulated NoShip fields. We adopt significance at 95 % confidence or greater as distinguishing between “detection” for the <span class="inline-formula"><i>r</i><sub>e</sub></span> changes versus “evidence” short of detection for the <span class="inline-formula"><i>A</i><sub>cld</sub></span> changes.</p> </div><div class="sec"><h2 id="App1.Ch1.S1.SS5"><span class="label">A5</span> Twomey effect calculations</h2> <p id="d1e1589">For the <span class="inline-formula"><i>r</i><sub>e</sub></span> perturbations, IRF<span class="inline-formula"><sub>ACI</sub></span> is estimated following Eq. (A1): </p><div class="disp-formula" content-type="numbered" id="App1.Ch1.S1.E1"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M127" display="block" overflow="scroll" dspmath="mathml"><mtable><mlabeledtr><mtd><mtext>(A1)</mtext></mtd><mtd><mrow> <msub> <mi mathvariant="normal">IRF</mi> <mi mathvariant="normal">ACI</mi> </msub> <mo>=</mo> <mo>-</mo> <msub> <mi>F</mi> <mo>⊙</mo> </msub> <msub> <mi>C</mi> <mi mathvariant="normal">low</mi> </msub> <msub> <mi mathvariant="italic">ϕ</mi> <mi mathvariant="normal">atm</mi> </msub> <msub> <mi mathvariant="italic">α</mi> <mi mathvariant="normal">cld</mi> </msub> <mfenced close=")" open="("> <mrow> <mn mathvariant="normal">1</mn> <mo>-</mo> <msub> <mi mathvariant="italic">α</mi> <mi mathvariant="normal">cld</mi> </msub> </mrow> </mfenced> <mfenced close=")" open="("> <mrow> <mo>-</mo> <mi mathvariant="normal">Δ</mi> <msub> <mi>r</mi> <mi mathvariant="normal">e</mi> </msub> <mo>/</mo> <msub> <mi>r</mi> <mrow> <mi mathvariant="normal">e</mi> <mo>,</mo> <mi mathvariant="normal">Ship</mi> </mrow> </msub> </mrow> </mfenced> <mo>,</mo> </mrow></mtd></mlabeledtr></mtable></math><div><svg xmlns:svg="http://www.w3.org/2000/svg" width="416pt" height="16pt" class="hide-js svg-formula" dspmath="mathimg" md5hash="4d39fe6487feca792f825163599ea185"><image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-e_A1.svg" width="100%" height="16pt" src="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-e_A1.png"></image></svg></div></div><p id="d1e1589-3"> where <span class="inline-formula"><i>F</i><sub>⊙</sub></span> is the insolation, <span class="inline-formula"><i>C</i><sub>low</sub></span> is the low cloud fraction, <span class="inline-formula"><i>ϕ</i><sub>atm</sub></span> is a transfer function between changes in overcast and cloud albedo (Diamond et al., 2020; Wood, 2021), and <span class="inline-formula"><i>α</i><sub>cld</sub></span> is the cloud albedo. Based on the values in D20, <span class="inline-formula"><i>ϕ</i><sub>atm</sub></span> is estimated as 0.6 and <span class="inline-formula"><i>α</i><sub>cld</sub></span> as 0.5.</p> <p id="d1e1752">For the <span class="inline-formula"><i>A</i><sub>cld</sub></span> perturbations, IRF<span class="inline-formula"><sub>ACI</sub></span> is estimated following Eq. (A2): </p><div class="disp-formula" content-type="numbered" id="App1.Ch1.S1.E2"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M136" display="block" overflow="scroll" dspmath="mathml"><mtable><mlabeledtr><mtd><mtext>(A2)</mtext></mtd><mtd><mrow> <msub> <mi mathvariant="normal">IRF</mi> <mi mathvariant="normal">ACI</mi> </msub> <mo>=</mo> <mo>-</mo> <msub> <mi>F</mi> <mo>⊙</mo> </msub> <msub> <mi>C</mi> <mi mathvariant="normal">low</mi> </msub> <mi mathvariant="normal">Δ</mi> <msub> <mi>A</mi> <mi mathvariant="normal">cld</mi> </msub> <mo>.</mo> </mrow></mtd></mlabeledtr></mtable></math><div><svg xmlns:svg="http://www.w3.org/2000/svg" width="416pt" height="14pt" class="hide-js svg-formula" dspmath="mathimg" md5hash="f1ed171ef2fd8fd5c66501328a787fdc"><image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-e_A2.svg" width="100%" height="14pt" src="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-e_A2.png"></image></svg></div></div><p id="d1e1752-3"> Equations (A1) and (A2) neglect liquid water path and cloud fraction adjustments to the Twomey effect. The effective radiative forcing due to aerosol–cloud interactions (ERF<span class="inline-formula"><sub>ACI</sub></span>), accounting for cloud adjustments, would be greater in magnitude than calculated here if cloudiness were increased via drizzle suppression and lower if cloudiness were decreased via enhanced entrainment. D20 found that adjustments were small in the morning but substantially offset brightening during the afternoon in austral spring. The apparently small effects in the morning may reflect diurnal competition between precipitation suppression, which maximizes overnight, and entrainment drying, which maximizes during the day (Sandu et al., 2008). Thus, the IRF<span class="inline-formula"><sub>ACI</sub></span> values here are likely larger than ERF<span class="inline-formula"><sub>ACI</sub></span> values would be after accounting for adjustments over the full diurnal cycle, at least in austral spring.</p> </div></div><span class="section5-mobile-bottom-border mobile-bottom-border hide-on-desktop hide-on-tablet"></span></div> <div id="section6" class="sec"><div class="grid-container no-margin header-element"><span class="grid-100 mobile-grid-100 tablet-grid-100 grid-parent more-less-mobile" data-hide="#section6 .co-arrow-open,.section6-content" data-show="#section6 .co-arrow-closed,.section6-mobile-bottom-border"><div class="h1"><span class="section-number"> </span>Code availability<span class="hide-on-desktop hide-on-tablet triangleWrapper"> <i class="co-arrow-closed"></i><i class="co-arrow-open" style="display:none"></i></span></div></span></div> <div class="section6-content show-no-js hide-on-mobile-soft"><p id="d1e1843">Code for processing the data and recreating the analyses in this work is available from GitHub (<span class="uri"><a href="https://github.com/michael-s-diamond/IMO2020" target="_blank">https://github.com/michael-s-diamond/IMO2020</a></span>, last access: 12 June 2023, <a href="https://doi.org/10.5281/zenodo.8165409">https://doi.org/10.5281/zenodo.8165409</a>, Diamond, 2023a). The universal kriging algorithm is implemented in R (R Core Team, 2014) using the geoR package (<span class="uri"><a href="https://CRAN.R-project.org/package=geoR" target="_blank">https://CRAN.R-project.org/package=geoR</a></span>, Ribeiro and Diggle, 2018). Other analyses are performed in Python using the numpy (Harris et al., 2020), cartopy (Met Office, 2010–2015), matplotlib (Hunter, 2007), scipy (Virtanen et al., 2020), statsmodels (<span class="uri"><a href="https://github.com/statsmodels/statsmodels" target="_blank">https://github.com/statsmodels/statsmodels</a></span>, statsmodels, 2023) and xarray (Hoyer and Hamman, 2017) packages.</p></div><span class="section6-mobile-bottom-border mobile-bottom-border hide-on-desktop hide-on-tablet"></span></div> <div id="section7" class="sec"><div class="grid-container no-margin header-element"><span class="grid-100 mobile-grid-100 tablet-grid-100 grid-parent more-less-mobile" data-hide="#section7 .co-arrow-open,.section7-content" data-show="#section7 .co-arrow-closed,.section7-mobile-bottom-border"><div class="h1"><span class="section-number"> </span>Data availability<span class="hide-on-desktop hide-on-tablet triangleWrapper"> <i class="co-arrow-closed"></i><i class="co-arrow-open" style="display:none"></i></span></div></span></div> <div class="section7-content show-no-js hide-on-mobile-soft"><p id="d1e1861">SSF1deg data (<a href="https://doi.org/10.5067/Terra/CERES/SSF1DegMonth_L3.004A">https://doi.org/10.5067/Terra/CERES/SSF1DegMonth_L3.004A</a>, CERES Science Team, 2023) are available from the NASA Langley Research Center CERES ordering tool (<span class="uri"><a href="https://ceres.larc.nasa.gov/data/" target="_blank">https://ceres.larc.nasa.gov/data/</a></span>, NASA Langley Research Center, 2023). EDGAR data (European Commission Joint Research Centre, 2018) are available from the European Commission Joint Research Centre Data Catalogue (<a href="https://doi.org/10.2904/JRC_DATASET_EDGAR">https://doi.org/10.2904/JRC_DATASET_EDGAR</a>). Processed data used in this work are available in a Zenodo repository (<a href="https://doi.org/10.5281/zenodo.7864530">https://doi.org/10.5281/zenodo.7864530</a>, Diamond, 2023b).</p></div><span class="section7-mobile-bottom-border mobile-bottom-border hide-on-desktop hide-on-tablet"></span></div> <div id="section8" class="sec"> <div class="grid-container no-margin header-element"><span class="grid-100 mobile-grid-100 tablet-grid-100 grid-parent more-less-mobile" data-hide="#section8 .co-arrow-open,.section8-content" data-show="#section8 .co-arrow-closed,.section8-mobile-bottom-border"><div class="h1"><span class="section-number"> </span>Supplement<span class="hide-on-desktop hide-on-tablet triangleWrapper"> <i class="co-arrow-closed"></i><i class="co-arrow-open" style="display:none"></i></span></div></span></div> <div class="section8-content show-no-js hide-on-mobile-soft"><p id="d1e1876">The supplement related to this article is available online at: <a href="https://doi.org/10.5194/acp-23-8259-2023-supplement">https://doi.org/10.5194/acp-23-8259-2023-supplement</a>.</p></div><span class="section8-mobile-bottom-border mobile-bottom-border hide-on-desktop hide-on-tablet"></span></div> <div id="section9" class="sec"><div class="grid-container no-margin header-element"><span class="grid-100 mobile-grid-100 tablet-grid-100 grid-parent more-less-mobile" data-hide="#section9 .co-arrow-open,.section9-content" data-show="#section9 .co-arrow-closed,.section9-mobile-bottom-border"><div class="h1"><span class="section-number"> </span>Competing interests<span class="hide-on-desktop hide-on-tablet triangleWrapper"> <i class="co-arrow-closed"></i><i class="co-arrow-open" style="display:none"></i></span></div></span></div> <div class="section9-content show-no-js hide-on-mobile-soft"><p id="d1e1885">The author has declared that there are no competing interests.</p></div><span class="section9-mobile-bottom-border mobile-bottom-border hide-on-desktop hide-on-tablet"></span></div> <div id="section10" class="sec"><div class="grid-container no-margin header-element"><span class="grid-100 mobile-grid-100 tablet-grid-100 grid-parent more-less-mobile" data-hide="#section10 .co-arrow-open,.section10-content" data-show="#section10 .co-arrow-closed,.section10-mobile-bottom-border"><div class="h1"><span class="section-number"> </span>Disclaimer<span class="hide-on-desktop hide-on-tablet triangleWrapper"> <i class="co-arrow-closed"></i><i class="co-arrow-open" style="display:none"></i></span></div></span></div> <div class="section10-content show-no-js hide-on-mobile-soft"><p id="d1e1891">Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.</p></div><span class="section10-mobile-bottom-border mobile-bottom-border hide-on-desktop hide-on-tablet"></span></div> <div class="ack sec" id="section11"> <div class="grid-container no-margin header-element"><span class="grid-100 mobile-grid-100 tablet-grid-100 grid-parent more-less-mobile" data-hide="#section11 .co-arrow-open,.section11-content" data-show="#section11 .co-arrow-closed,.section11-mobile-bottom-border"><div class="h1"><span class="section-number"> </span>Acknowledgements<span class="hide-on-desktop hide-on-tablet triangleWrapper"> <i class="co-arrow-closed"></i><i class="co-arrow-open" style="display:none"></i></span></div></span></div> <div class="section11-content show-no-js hide-on-mobile-soft"><p id="d1e1897">Hannah M. Director merits continued gratitude for her contributions to the original code base and analysis methods. Discussion with Leon Simons provided the impetus for attempting to detect a signal with only 3 years of post-2020 data. Thanks are owed to Clare Singer, Emily de Jong, and an anonymous reviewer for their constructive comments.</p></div><span class="section11-mobile-bottom-border mobile-bottom-border hide-on-desktop hide-on-tablet"></span></div> <div id="section12" class="sec"><div class="grid-container no-margin header-element"><span class="grid-100 mobile-grid-100 tablet-grid-100 grid-parent more-less-mobile" data-hide="#section12 .co-arrow-open,.section12-content" data-show="#section12 .co-arrow-closed,.section12-mobile-bottom-border"><div class="h1"><span class="section-number"> </span>Financial support<span class="hide-on-desktop hide-on-tablet triangleWrapper"> <i class="co-arrow-closed"></i><i class="co-arrow-open" style="display:none"></i></span></div></span></div> <div class="section12-content show-no-js hide-on-mobile-soft"><p id="d1e1902">This research has been supported by the Florida State University (new faculty startup).</p></div><span class="section12-mobile-bottom-border mobile-bottom-border hide-on-desktop hide-on-tablet"></span></div> <div id="section13" class="sec"><div class="grid-container no-margin header-element"><span class="grid-100 mobile-grid-100 tablet-grid-100 grid-parent more-less-mobile" data-hide="#section13 .co-arrow-open,.section13-content" data-show="#section13 .co-arrow-closed,.section13-mobile-bottom-border"><div class="h1"><span class="section-number"> </span>Review statement<span class="hide-on-desktop hide-on-tablet triangleWrapper"> <i class="co-arrow-closed"></i><i class="co-arrow-open" style="display:none"></i></span></div></span></div> <div class="section13-content show-no-js hide-on-mobile-soft"><p id="d1e1908">This paper was edited by Markus Petters and Timothy Garrett and reviewed by Clare Singer and one anonymous referee.</p></div><span class="section13-mobile-bottom-border mobile-bottom-border hide-on-desktop hide-on-tablet"></span></div> <div class="ref-list sec" id="section14"> <div class="grid-container no-margin header-element"><span class="grid-100 mobile-grid-100 tablet-grid-100 grid-parent more-less-mobile" data-hide="#section14 .co-arrow-open,.section14-content" data-show="#section14 .co-arrow-closed,.section14-mobile-bottom-border"><div class="h1"><span class="section-number"> </span>References<span class="hide-on-desktop hide-on-tablet triangleWrapper"> <i class="co-arrow-closed"></i><i class="co-arrow-open" style="display:none"></i></span></div></span></div> <div class="section14-content show-no-js hide-on-mobile-soft"><p class="ref" id="bib1.bib1"><span class="mixed-citation">Albrecht, B. 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href="https://acp.copernicus.org/articles/23/8259/2023/#abstract" class="link_level1 scrollto" data-fixed-element=".auto-fixed-top-forced.article-title">Abstract</a></li> <li class="menuitem_level1 co_function_get_navigation_is_parent co_function_get_navigation_is_closed" id="co_getnavigation_page_about"> <a href="https://acp.copernicus.org/articles/23/8259/2023/#section1" class="link_level1 scrollto" data-fixed-element=".auto-fixed-top-forced.article-title">Introduction and approach</a></li> <li class="menuitem_level1 co_function_get_navigation_is_parent co_function_get_navigation_is_closed" id="co_getnavigation_page_about"> <a href="https://acp.copernicus.org/articles/23/8259/2023/#section2" class="link_level1 scrollto" data-fixed-element=".auto-fixed-top-forced.article-title">Results</a></li> <li class="menuitem_level1 co_function_get_navigation_is_parent co_function_get_navigation_is_closed" id="co_getnavigation_page_about"> <a href="https://acp.copernicus.org/articles/23/8259/2023/#section3" class="link_level1 scrollto" data-fixed-element=".auto-fixed-top-forced.article-title">Discussion</a></li> <li class="menuitem_level1 co_function_get_navigation_is_parent co_function_get_navigation_is_closed" id="co_getnavigation_page_about"> <a href="https://acp.copernicus.org/articles/23/8259/2023/#section4" class="link_level1 scrollto" data-fixed-element=".auto-fixed-top-forced.article-title">Conclusions</a></li> <li class="menuitem_level1 co_function_get_navigation_is_parent co_function_get_navigation_is_closed" id="co_getnavigation_page_about"> <a href="https://acp.copernicus.org/articles/23/8259/2023/#section5" class="link_level1 scrollto" data-fixed-element=".auto-fixed-top-forced.article-title"><span>Appendix A:</span> Methods</a></li> <li class="menuitem_level1 co_function_get_navigation_is_parent co_function_get_navigation_is_closed" id="co_getnavigation_page_about"> <a href="https://acp.copernicus.org/articles/23/8259/2023/#section6" class="link_level1 scrollto" data-fixed-element=".auto-fixed-top-forced.article-title">Code availability</a></li> <li class="menuitem_level1 co_function_get_navigation_is_parent co_function_get_navigation_is_closed" id="co_getnavigation_page_about"> <a href="https://acp.copernicus.org/articles/23/8259/2023/#section7" class="link_level1 scrollto" data-fixed-element=".auto-fixed-top-forced.article-title">Data availability</a></li> <li class="menuitem_level1 co_function_get_navigation_is_parent co_function_get_navigation_is_closed" id="co_getnavigation_page_about"> <a href="https://acp.copernicus.org/articles/23/8259/2023/#section9" class="link_level1 scrollto" data-fixed-element=".auto-fixed-top-forced.article-title">Competing interests</a></li> <li class="menuitem_level1 co_function_get_navigation_is_parent co_function_get_navigation_is_closed" id="co_getnavigation_page_about"> <a 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journal-contentLinkColor">Download</div> <div class="content"> <ul class="additional_info no-bullets no-styling"> <li><a class="triangle" title="PDF Version (2893 KB)" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023.pdf">Article</a> <nobr>(2893 KB)</nobr> </li> <li> <a class="triangle" title="XML Version" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023.xml">Full-text XML</a> </li> </ul> </div> <div class="content"> <ul class="additional_info no-bullets no-styling"> <li> <a class="triangle" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023-supplement.pdf">Supplement</a> <nobr>(3242 KB)</nobr> </li> <li><a class="triangle" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023.bib">BibTeX</a></li> <li><a class="triangle" href="https://acp.copernicus.org/articles/23/8259/2023/acp-23-8259-2023.ris">EndNote</a></li> </ul> </div> </div> <div class="widget dark-border"> <div class="legend journal-contentLinkColor">Executive editor</div> <div class="content hide-js shortSummaryFullOnMSType">In 2020, a new international law was imposed that placed strong restrictions on sulfur emissions from the international shipping industry. In addition to reducing air pollution, an anticipated side effect was reduction of the climate cooling effect that is often associated with "ship-tracks". Aerosol pollutant emissions from ships, when they rise into overlying clouds, lead to higher cloud droplet number concentrations, smaller cloud droplet sizes, and clouds that are more reflective to incoming sunlight, easily seen in satellite imagery as long bright lines downwind of ships. Past studies into whether the new law has led to darker clouds have been equivocal. For this study, the authors used sophisticated statistical techniques to compare cloud droplet size and reflectivity before and after the law was implemented focusing on a shipping corridor in the southeast Atlantic. They found strong evidence that droplet sizes have indeed increased, and that clouds have darkened with a significant local climate warming. Globally, the impact is much smaller, but may still represent an important consideration for assessments of the total summed effect of aerosols on climate. </div> <div style="display: none" class="content show-js shortSummaryShortenOnMSType">In 2020, a new international law was imposed that placed strong restrictions on sulfur emissions...</div> <div class="content"> <a href="#" class="more-less show-js triangle" data-hide=".shortSummaryFullOnMSType" data-show=".shortSummaryShortenOnMSType" data-toggleCaption='Hide'>Read more</a> </div> </div> <div class="widget dark-border"> <div class="legend journal-contentLinkColor">Short summary</div> <div class="content hide-js shortSummaryFull">Fuel sulfur regulations were implemented for ships in 2020 to improve air quality but may also accelerate global warming. We use spatial statistics and satellite retrievals to detect changes in the size of cloud droplets and find evidence for a resulting decrease in cloud brightness within a major shipping corridor after the sulfur limits went into effect. Our results confirm both that the regulations are being followed and that they are having a warming influence via their effect on clouds.</div> <div style="display: none" class="content show-js shortSummaryShorten">Fuel sulfur regulations were implemented for ships in 2020 to improve air quality but may also...</div> <div class="content"> <a href="#" class="more-less show-js triangle" data-hide=".shortSummaryFull" data-show=".shortSummaryShorten" data-toggleCaption='Hide'>Read more</a> </div> </div> <div class="widget dark-border hide-on-mobile hide-on-tablet p-0" id="share"> <div class="legend journal-contentLinkColor">Share</div> <div class="row p-0"> <div class="col-auto pl-0"> <a class="share-one-line" href="https://www.mendeley.com/import/?url=https%3A%2F%2Facp.copernicus.org%2Farticles%2F23%2F8259%2F2023%2F" title="Mendeley" target="_blank"> <img src="https://www.atmospheric-chemistry-and-physics.net/mendeley.png" alt="Mendeley"/> </a> </div> <div class="col-auto"> <a class="share-one-line" href="https://www.reddit.com/submit?url=https%3A%2F%2Facp.copernicus.org%2Farticles%2F23%2F8259%2F2023%2F" title="Reddit" target="_blank"> <img src="https://www.atmospheric-chemistry-and-physics.net/reddit.png" alt="Reddit"> </a> </div> <div class="col-auto"> <a class="share-one-line last" href="https://twitter.com/intent/tweet?text=Detection+of+large-scale+cloud+microphysical+changes+within+a+major+shipping+corridor+after+implementation+of+the+International+Maritime+Organization+2020+fuel+sulfur+regulations https%3A%2F%2Facp.copernicus.org%2Farticles%2F23%2F8259%2F2023%2F" title="Twitter" target="_blank"> <img src="https://www.atmospheric-chemistry-and-physics.net/twitter.png" alt="Twitter"/> </a> </div> <div class="col-auto"> <a class="share-one-line" href="https://www.facebook.com/share.php?u=https%3A%2F%2Facp.copernicus.org%2Farticles%2F23%2F8259%2F2023%2F&t=Detection+of+large-scale+cloud+microphysical+changes+within+a+major+shipping+corridor+after+implementation+of+the+International+Maritime+Organization+2020+fuel+sulfur+regulations" title="Facebook" target="_blank"> <img src="https://www.atmospheric-chemistry-and-physics.net/facebook.png" alt="Facebook"/> </a> </div> <div class="col-auto pr-0"> <a class="share-one-line last" href="https://www.linkedin.com/shareArticle?mini=true&url=https%3A%2F%2Facp.copernicus.org%2Farticles%2F23%2F8259%2F2023%2F&title=Detection+of+large-scale+cloud+microphysical+changes+within+a+major+shipping+corridor+after+implementation+of+the+International+Maritime+Organization+2020+fuel+sulfur+regulations" title="LinkedIn" target="_blank"> <img src="https://www.atmospheric-chemistry-and-physics.net/linkedin.png" alt="LinkedIn"> </a> </div> <div class="col pr-0 mobile-native-share"> <a href="#" data-title="Atmospheric Chemistry and Physics" data-text="*Detection of large-scale cloud microphysical changes within a major shipping corridor after implementation of the International Maritime Organization 2020 fuel sulfur regulations* Michael S. 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ACP - Detection of large-scale cloud microphysical changes within a major shipping corridor after implementation of the International Maritime Organization 2020 fuel sulfur regulations