A074-07
Constraining Elevated Urban Atmospheric Ammonia/Ammonium Sources in New England Utilizing Novel Isotopic Measurements

Wednesday, 9 December 2020: 10:48
Virtual
Wendell W Walters1, Emma Wilcocks2, Bok Baek3, Maddie Karod4, Danielle E. Blum5 and Meredith Galanter Hastings1, (1)Brown University, Department of Earth, Environmental and Planetary Sciences, and Institute at Brown for Environment and Society, Providence, RI, United States, (2)Brown University, Program in Biology, Division of Biology and Medicine, Providence, RI, United States, (3)University of North Carolina at Chapel Hill, Chapel Hill, NC, United States, (4)Brown University, Department of Earth, Environmental, and Planetary Sciences, Providence, United States, (5)Brown Univeristy, Department of Chemistry and Biochemistry, Providence, RI, United States
Abstract:
Atmospheric ammonia (NH3), as the primary inorganic base in the atmosphere, plays an important role in degrading air quality via influencing gas/particle partitioning and contributing to fixed nitrogen deposition. Yet, NH3 has received relatively little focus in urban areas due to these regions' minimal agricultural activity. Here we present NH3 emission modeling in New England (US) and spatiotemporal analysis of concentration measurements of atmospheric NH3 and its secondary product particulate ammonium (pNH4+) from national monitoring networks (AMON and CASTNET) and measurements conducted in Providence, RI, US. Overall, ambient NHx (NH3 + pNH4+) concentrations were significantly elevated in urban-affected regions. However, identifying the causes of elevated NHx in urban regions is a challenging task due to the coexistence of many locally produced sources, including emissions from vehicles, fuel combustion, industrial processes, humans, and long-range transport. The stable nitrogen isotopic composition (δ15N) of NHx is a promising tool to constrain NHx emissions and transport due to recent improvements in methodology, understanding of fractionation processes, and δ15N source characterization studies, particularly the finding of a unique δ15N signature for vehicle emissions. Capitalizing on these recent improvements, we have investigated the sources of elevated urban NHx in Providence, RI, utilizing δ15N measurements that were evaluated in combination with meteorological data and statistical air mass back trajectory analysis. Overall, our analysis indicates that vehicle emissions are an important year-round background source of urban NH3 providing strong support that these emissions are significantly under-predicted in the US national emission inventories. Additionally, temperature-dependent NH3 volatilization emissions and urban stagnation events are found responsible for elevated summertime NH3 concentrations. Long-range transport of pNH4+ was found to play an important role in the seasonal pNH4+ concentration trends in New England and is significantly increased by local urban NH3 emissions. Our work enhances our understanding of NHx dynamics in complex urban environments and may help guide future NH3 emission regulations.
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