제목: Atmospheric Nitrate Chemistry and Response to Emissions Reductions in the Northeastern US​​​​​​​​​​​

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​​​​​​​​​​​​​​​​​​연사: 김희정 박사 (Brown university)​​​​​​

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장소: 과학관 553호​​

Abstract:

​​The successful implementation of the US Clean Air Act has reduced nitrogen oxides (NOx) emissions, with significant improvements in air quality and acid deposition. However, predicting the impact of primary emission reductions on secondary pollutants such as ozone, particulate matter, and deposition remains challenging, as NOx influences oxidant concentrations and aerosol formation. In particular, particulate nitrate (pNO3) concentrations have not declined significantly during winter, despite overall reductions in NOx emissions. Stable nitrogen and oxygen isotopes of nitrate (δ15N, Δ17O, and δ18O) serve as powerful observational constraints for the understanding of atmospheric nitrate sources and/or chemical processing. Here, we quantified nitrogen and oxygen isotopes in nitrate from a unique 2005-2015 aerosol record in the northeastern US to constrain precursor NOx sources and chemistry changes in response to emission regulations. We observed a significant decreasing trend in δ15N(pNO3) from 2005 to 2015 (-0.5‰ yr-1), which reflects a shift in NOx emission sources driven by emission regulations. Specifically, there is an imprint in the δ15N(pNO3) record of the gradual phase-out of coal for electricity generation and a rise in the use of natural gas. As total NOx emissions decreased (by 53% in the northeastern US over the decade), the shift in oxidation chemistry led to a shorter NOx lifetime (from 54h to 44h (-19%)) and enhanced aerosol formation rate in urban areas since 2005. This explains the continued elevated wintertime nitrate that plagues air quality in the eastern US despite significant NOx decreases. These isotopic constraints using nitrogen and oxygen isotopes greatly enhance our ability to test and predict the effectiveness of emissions reductions on the shift of NOx emission sources and oxidation chemistry.

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