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. Mountain basins in Northern Utah, including the Salt LakeValley (SLV), suffer from wintertime air pollution events associated withstagnant atmospheric conditions. During these events, fine particulate matter concentrations (PM 2.5 ) can exceed national ambient air qualitystandards. Previous studies in the SLV have found that PM 2.5 isprimarily composed of ammonium nitrate ( NH4NO3 ), formed from thecondensation of gas-phase ammonia ( NH3 ) and nitric acid ( HNO3 ).Additional studies in several western basins, including the SLV, havesuggested that production of HNO3 from nocturnal heterogeneous N2O5 uptake is the dominant source of NH4NO3 duringwinter. The rate of this process, however, remains poorly quantified, inpart due to limited vertical measurements above the surface, where thischemistry is most active. The 2017 Utah Winter Fine Particulate Study(UWFPS) provided the first aircraft measurements of detailed chemicalcomposition during wintertime pollution events in the SLV. Coupled withground-based observations, analyses of day- and nighttime research flightsconfirm that PM 2.5 during wintertime pollution events is principallycomposed of NH4NO3 , limited by HNO3 . Here, observations andbox model analyses assess the contribution of N2O5 uptake tonitrate aerosol during pollution events using the NO 3 - productionrate, N2O5 heterogeneous uptake coefficient ( γ(N2O5) ), and production yield of ClNO2 ( φ(ClNO2) ), which had medians of 1.6  µ g m −3  h −1 , 0.076,and 0.220, respectively. While fit values of γ(N2O5) maybe biased high by a potential under-measurement in aerosol surface area,other fit quantities are unaffected. Lastly, additional model simulationssuggest nocturnal N2O5 uptake produces between 2.4 and 3.9  µ g m −3 of nitrate per day when considering the possible effects ofdilution. This nocturnal production is sufficient to account for 52 %–85 %of the daily observed surface-level buildup of aerosol nitrate, thoughaccurate quantification is dependent on modeled dilution, mixing processes,and photochemistry.

atmospheric chemistry and physics

On the contribution of nocturnal heterogeneous reactive nitrogen chemistry to particulate matter formation during wintertime pollution events in Northern Utah