title: Increasing importance of deposition of reduced nitrogen in the United States.
abstract: Rapid development of agriculture and fossil fuel combustion greatly increased US reactive nitrogen emissions to the atmosphere in the second half of the 20th century, resulting in excess nitrogen deposition to natural ecosystems. Recent efforts to lower nitrogen oxides emissions have substantially decreased nitrate wet deposition. Levels of wet ammonium deposition, by contrast, have increased in many regions. Together these changes have altered the balance between oxidized and reduced nitrogen deposition. Across most of the United States, wet deposition has transitioned from being nitrate-dominated in the 1980s to ammonium-dominated in recent years. Ammonia has historically not been routinely measured because there are no specific regulatory requirements for its measurement. Recent expansion in ammonia observations, however, along with ongoing measurements of nitric acid and fine particle ammonium and nitrate, permit new insight into the balance of oxidized and reduced nitrogen in the total (wet + dry) US nitrogen deposition budget. Observations from 37 sites reveal that reduced nitrogen contributes, on average, ∼65% of the total inorganic nitrogen deposition budget. Dry deposition of ammonia plays an especially key role in nitrogen deposition, contributing from 19% to 65% in different regions. Future progress toward reducing US nitrogen deposition will be increasingly difficult without a reduction in ammonia emissions.
authors.affiliation: Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523;
authors.forename: Yi
authors.initials: Y
authors.lastname: Li
authors.affiliation: National Park Service, Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, CO 80523;
authors.forename: Bret A
authors.initials: BA
authors.lastname: Schichtel
authors.affiliation: Office of Research and Development, National Risk Management Research Laboratory, US Environmental Protection Agency, Research Triangle Park, NC 27711;
authors.forename: John T
authors.initials: JT
authors.lastname: Walker
authors.affiliation: Office of Research and Development, National Exposure Research Laboratory, US Environmental Protection Agency, Research Triangle Park, NC 27711;
authors.forename: Donna B
authors.initials: DB
authors.lastname: Schwede
authors.affiliation: Office of Research and Development, National Risk Management Research Laboratory, US Environmental Protection Agency, Research Triangle Park, NC 27711;
authors.forename: Xi
authors.initials: X
authors.lastname: Chen
authors.affiliation: National Atmospheric Deposition Program, University of Illinois Urbana-Champaign, Champaign, IL 61820;
authors.forename: Christopher M B
authors.initials: CM
authors.lastname: Lehmann
authors.affiliation: Clean Air Markets Division, US Environmental Protection Agency, Washington, DC 20460.
authors.forename: Melissa A
authors.initials: MA
authors.lastname: Puchalski
authors.affiliation: National Atmospheric Deposition Program, University of Illinois Urbana-Champaign, Champaign, IL 61820;
authors.forename: David A
authors.initials: DA
authors.lastname: Gay
authors.affiliation: Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523; collett@atmos.colostate.edu.
authors.forename: Jeffrey L
authors.initials: JL
authors.lastname: Collett
mesh_terms: D000383:Agriculture / Q000639:trends; D000641:Ammonia / Q000032:analysis* / Q000737:chemistry; D001272:Atmosphere / Q000737:chemistry; D003247:Conservation of Natural Resources; D017753:Ecosystem; D004784:Environmental Monitoring; D004785:Environmental Pollutants / Q000032:analysis* / Q000737:chemistry; D006801:Humans; D009566:Nitrates / Q000032:analysis* / Q000737:chemistry; D017942:Nitric Acid / Q000032:analysis* / Q000737:chemistry; D009584:Nitrogen / Q000032:analysis* / Q000737:chemistry; D009589:Nitrogen Oxides / Q000032:analysis* / Q000737:chemistry; D010084:Oxidation-Reduction; D014481:United States; D001335:Vehicle Emissions / Q000032:analysis
chemical_list: D004785:Environmental Pollutants; D009566:Nitrates; D009589:Nitrogen Oxides; D001335:Vehicle Emissions; D017942:Nitric Acid; D000641:Ammonia; D009584:Nitrogen
keywords: agriculture; ammonia; dry deposition; nitrogen oxides; wet deposition