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Quantitative thresholds, termed \u201ccritical loads\u201d (CLs), have been developed to estimate the deposition rate above which damage is thought to occur. However, there remains no comprehensive comparison of when, where, and over what time periods individual CLs have been exceeded. We addressed this knowledge gap by combining several published data sources for historical and contemporary deposition, and overlaying these on six CL types from the National Critical Loads Database (NCLDv2.5; terrestrial acidification, aquatic acidification, lichen, nitrate leaching, plant community composition, and forest\u2010tree health) to examine exceedances from 1800 to 2011. We expressed CLs as the minimum, 10th, and 50th percentiles within 12\u2010km grid cells. Minimum CLs were relatively uniform across the country (200\u2013400\u00a0eq\u00b7ha<jats:sup>\u22121<\/jats:sup>\u00b7yr<jats:sup>\u22121<\/jats:sup>), and have been exceeded for decades beginning in the early 20th century. The area exceeding minimum CLs peaked in the 1970s and 1980s, exposing 300,000 to 3\u00a0million km<jats:sup>2<\/jats:sup> (depending on the CL type) to harmful levels of deposition, with a total area exceeded of 5.8\u00a0million km<jats:sup>2<\/jats:sup> (~70% of the conterminous United States). Since then, deposition levels have dropped, especially for S, with modest reductions in exceedance by 2011 for all CL types, totaling 5.2\u00a0million km<jats:sup>2<\/jats:sup> in exceedance. The 10th and 50th percentile CLs followed similar trends, but were not consistently available at the 12\u2010km grid scale. We also examined near\u2010term future deposition and exceedances in 2025 under current air quality regulations, and under various scenarios of climate change and additional nitrogen management controls. Current regulations were projected to reduce exceedances of any CL from 5.2 million km<jats:sup>2<\/jats:sup> in 2011 to 4.8\u00a0million km<jats:sup>2<\/jats:sup> in 2025. None of the additional N management or climate scenarios significantly affected areal exceedances, although exceedance severity declined. In total, it is clear that many CLs have been exceeded for decades, and are likely to remain so in the short term under current policies. Additionally, we suggest many areas for improvement to enhance our understanding of deposition and its effects to support informed decision making.<\/jats:p>","DOI":"10.1002\/eap.1703","type":"journal-article","created":{"date-parts":[[2018,5,1]],"date-time":"2018-05-01T15:09:30Z","timestamp":1525187370000},"page":"978-1002","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":59,"title":["Atmospheric deposition and exceedances of critical loads from 1800\u22122025 for the conterminous United States"],"prefix":"10.1002","volume":"28","author":[{"given":"Christopher M.","family":"Clark","sequence":"first","affiliation":[{"name":"National Center for Environmental Assessment Office of Research and Development U.S. Environmental Protection Agency (8623\u2010P) 1200 Pennsylvania Avenue NW Washington D.C. 20460 USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jennifer","family":"Phelan","sequence":"additional","affiliation":[{"name":"RTI International 3040 East Cornwallis Road, P.O. Box\u00a012194 Research Triangle Park North Carolina 27709 USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Prakash","family":"Doraiswamy","sequence":"additional","affiliation":[{"name":"RTI International 3040 East Cornwallis Road, P.O. Box\u00a012194 Research Triangle Park North Carolina 27709 USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"John","family":"Buckley","sequence":"additional","affiliation":[{"name":"RTI International 3040 East Cornwallis Road, P.O. Box\u00a012194 Research Triangle Park North Carolina 27709 USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"James C.","family":"Cajka","sequence":"additional","affiliation":[{"name":"RTI International 3040 East Cornwallis Road, P.O. Box\u00a012194 Research Triangle Park North Carolina 27709 USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Robin L.","family":"Dennis","sequence":"additional","affiliation":[{"name":"National Exposure Research Laboratory U.S. Environmental Protection Agency Research Triangle Park North Carolina 27709 USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jason","family":"Lynch","sequence":"additional","affiliation":[{"name":"Office of Atmospheric Programs U.S. Environmental Protection Agency 1200 Pennsylvania Ave NW Washington D.C. 20460 USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Christopher G.","family":"Nolte","sequence":"additional","affiliation":[{"name":"National Exposure Research Laboratory U.S. Environmental Protection Agency Research Triangle Park North Carolina 27709 USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Tanya L.","family":"Spero","sequence":"additional","affiliation":[{"name":"National Exposure Research Laboratory U.S. Environmental Protection Agency Research Triangle Park North Carolina 27709 USA"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2018,5]]},"reference":[{"key":"e_1_2_8_2_1","doi-asserted-by":"publisher","DOI":"10.1034\/j.1600-0889.1999.t01-3-00002.x"},{"key":"e_1_2_8_3_1","doi-asserted-by":"publisher","DOI":"10.1016\/S0169-5347(98)01544-4"},{"key":"e_1_2_8_4_1","doi-asserted-by":"publisher","DOI":"10.1890\/1051-0761(2006)016[0433:HNDTDA]2.0.CO;2"},{"key":"e_1_2_8_5_1","doi-asserted-by":"publisher","DOI":"10.1525\/bio.2011.61.8.6"},{"key":"e_1_2_8_6_1","doi-asserted-by":"publisher","DOI":"10.5194\/bg-10-1635-2013"},{"key":"e_1_2_8_7_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.ecolind.2013.10.011"},{"key":"e_1_2_8_8_1","doi-asserted-by":"publisher","DOI":"10.1002\/ecs2.1806"},{"key":"e_1_2_8_9_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.envpol.2010.11.005"},{"key":"e_1_2_8_10_1","volume-title":"Air quality and ecosystem services workshop report: Santa Monica Mountains National Recreation Area, Thousand Oaks, CA \u2013 February 24\u201326, 2015","author":"Blett T. 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