A074-06
A Paradigm Shift in Sulfate-Nitrate-Ammonium Aerosol Formation in the United States and its Implications for Reactive Nitrogen Deposition

Wednesday, 9 December 2020: 10:45
Virtual
Da Pan1, Denise L Mauzerall2, Katherine Beem Benedict3, Rui Wang1, Levi Golston4, Jeffrey Lee Collett Jr5, Lei Tao1, Kang Sun6,7, Xuehui Guo1, Bret A Schichtel8, Jay M Ham9, Anthony J Prenni10, Melissa Puchalski11, Tomas Mikoviny12, Markus Müller13, Armin Wisthaler14 and Mark A Zondlo1, (1)Princeton University, Civil and Environmental Engineering, Princeton, NJ, United States, (2)Princeton University, Princeton School of Public and International Affairs, Princeton, NJ, United States, (3)University of California Davis, Davis, CA, United States, (4)NASA Ames Research Center, Moffett Field, CA, United States, (5)Colorado State University, Atmospheric Science, Fort Collins, CO, United States, (6)University at Buffalo, RENEW Institute, Buffalo, NY, United States, (7)University at Buffalo, Department of Civil, Structural and Environmental Engineering, Buffalo, NY, United States, (8)National Park Service, Air Resources Division, Denver, CO, United States, (9)Colorado State University, Fort Collins, CO, United States, (10)National Park Service Lakewood, Lakewood, CO, United States, (11)Environmental Protection Agency Washington DC, Washington, DC, United States, (12)University of Oslo, Department of Chemistry, Oslo, Norway, (13)University of Innsbruck, Innsbruck, Austria, (14)University of Oslo, Oslo, Norway
Abstract:
SO2 emissions have decreased by 90% in the US over the last two decades, leading to a paradigm shift in sulfate-nitrate-ammonium (SNA) aerosol formation. To quantify the impact of this paradigm shift, we integrate data from four trace gas and aerosol composition monitoring networks in the US. We find that nitrate has become the dominant component of SNA aerosols, in 2019 accounting for ~50% of SNA aerosol mass. Using the integrated dataset and a thermodynamic model (ISORROPIA II), we find a 30% reduction in total nitrate (TN=HNO3g+NO3-) leads to a larger reduction in PM2.5 concentrations than a 30% reduction in sulfate at 26 (43) sites out of the 48 US sites with collocated trace gas, aerosol, and meteorological observations in 2019 (2019 winter). In the northeastern US, a 30% reduction in total ammonium (TA=NH3g+NH4+) results in an even greater reduction in PM2.5 than a 30% TN reduction. Between 2011 and 2019, the annual mean molar partitioning of NH3g relative to TA has increased from 20 – 75% to 50 – 100% at the 74 sites in the US with collocated trace gas and aerosol observations, whereas the annual mean molar partitioning of HNO3 relative to TN has slightly decreased in the northeastern US. As a result, the importance of dry NH3 deposition is expected to increase significantly. Using IASI satellite NH3 observations, we identified 11 (26) national parks that are within 100 (200) km of an NH3 emission source or a concentration hotspot and are thus subject to potential impacts of the hotspot on their reactive nitrogen (Nr) deposition. As a case study, we quantified the impact of NH3 emission hotspots in Northeastern Colorado on dry NH3 deposition in Rocky Mountain National Park (RMNP) using mobile and open-path eddy covariance flux measurements. We show NH3 emissions from the hotspots are transported to RMNP during upslope events and contributed 80% of dry NH3 deposition in RMNP during the summer of 2016. Our results highlight the importance of Nr in aerosol formation in the current atmospheric chemistry regime and the need to quantify the impacts of NH3 hotspots on downwind ecosystems in the US.
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