%AShah, Viral%AShah, Viral%AJaeglé, Lyatt%AJaeglé, Lyatt%AThornton, Joel%AThornton, Joel%ALopez-Hilfiker, Felipe%ALopez-Hilfiker, Felipe%ALee, Ben%ALee, Ben%ASchroder, Jason%ASchroder, Jason%ACampuzano-Jost, Pedro%ACampuzano-Jost, Pedro%AJimenez, Jose%AJimenez, Jose%AGuo, Hongyu%AGuo, Hongyu%ASullivan, Amy%ASullivan, Amy%AWeber, Rodney%AWeber, Rodney%AGreen, Jaime%AGreen, Jaime%AFiddler, Marc%AFiddler, Marc%ABililign, Solomon%ABililign, Solomon%ACampos, Teresa%ACampos, Teresa%AStell, Meghan%AStell, Meghan%AWeinheimer, Andrew%AWeinheimer, Andrew%AMontzka, Denise%AMontzka, Denise%ABrown, Steven%ABrown, Steven%BJournal Name: Proceedings of the National Academy of Sciences; Journal Volume: 115; Journal Issue: 32; Related Information: CHORUS Timestamp: 2019-12-11 16:25:13 %D2018%IProceedings of the National Academy of Sciences %JJournal Name: Proceedings of the National Academy of Sciences; Journal Volume: 115; Journal Issue: 32; Related Information: CHORUS Timestamp: 2019-12-11 16:25:13 %K %MOSTI ID: 10065362 %PMedium: X %TChemical feedbacks weaken the wintertime response of particulate sulfate and nitrate to emissions reductions over the eastern United States %X

Sulfate (SO42-) and nitrate (NO3-) account for half of the fine particulate matter mass over the eastern United States. Their wintertime concentrations have changed little in the past decade despite considerable precursor emissions reductions. The reasons for this have remained unclear because detailed observations to constrain the wintertime gas–particle chemical system have been lacking. We use extensive airborne observations over the eastern United States from the 2015 Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER) campaign; ground-based observations; and the GEOS-Chem chemical transport model to determine the controls on winterSO42-andNO3-. GEOS-Chem reproduces observedSO42-NO3-NH4+particulate concentrations (2.45 μgsm-3) and composition (SO42-: 47%;NO3-: 32%;NH4+: 21%) during WINTER. Only 18% ofSO2emissions were regionally oxidized toSO42-during WINTER, limited by low [H2O2] and [OH]. Relatively acidic fine particulates (pH∼1.3) allow 45% of nitrate to partition to the particle phase. Using GEOS-Chem, we examine the impact of the 58% decrease in winterSO2emissions from 2007 to 2015 and find that the H2O2limitation onSO2oxidation weakened, which increased the fraction ofSO2emissions oxidizing toSO42-. Simultaneously, NOx emissions decreased by 35%, but the modeledNO3-particle fraction increased as fine particle acidity decreased. These feedbacks resulted in a 40% decrease of modeled [SO42-] and no change in [NO3-], as observed. Wintertime [SO42-] and [NO3-] are expected to change slowly between 2015 and 2023, unlessSO2and NOx emissions decrease faster in the future than in the recent past.

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