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

Mountain basins in Northern Utah, including the Salt Lake Valley (SLV), suffer from wintertime air pollution events associated with stagnant atmospheric conditions. During these events, fine particulate matter concentrations (PM_2:5) can exceed national ambient air quality standards. Previous studies in the SLV have found that PM_2:5 is primarily composed of ammonium nitrate (NH₄NO₃), formed from the condensation of gas-phase ammonia (NH₃) and nitric acid (HNO₃). Additional studies in several western basins, including the SLV, have suggested that production of HNO₃ from nocturnal heterogeneous N₂O₅ uptake is the dominant source of NH₄NO₃ during winter. The rate of this process, however, remains poorly quantified, in part due to limited vertical measurements above the surface, where this chemistry is most active. The 2017 Utah Winter Fine Particulate Study (UWFPS) provided the first aircraft measurements of detailed chemical composition during wintertime pollution events in the SLV. Coupled with ground-based observations, analyses of day-and nighttime research flights confirm that PM_2:5 during wintertime pollution events is principally composed of NH₄NO₃, limited by HNO₃. Here, observations and box model analyses assess the contribution of N₂O₅ uptake to nitrate aerosol during pollution events using the NO-3 production rate, N₂O₅ heterogeneous uptake coefficient (?.N₂O₅/), and production yield of ClNO2 ('.ClNO₂/), which had medians of 1.6 μg m^-3 h^-1, 0.076, and 0.220, respectively. While fit values of.N₂O₅/may be biased high by a potential under-measurement in aerosol surface area, other fit quantities are unaffected. Lastly, additional model simulations suggest nocturnal N₂O₅ uptake produces between 2.4 and 3.9 μg m-3 of nitrate per day when considering the possible effects of dilution. This nocturnal production is sufficient to account for 52 %-85 % of the daily observed surface-level buildup of aerosol nitrate, though accurate quantification is dependent on modeled dilution, mixing processes, and photochemistry.