PNNL

Abstract

Abstract. Mountain basins in Northern Utah, including the Salt Lake Valley (SLV), suffer from wintertime air pollu-tion events associated with stagnant atmospheric conditions. During these events, fine particulate matter concentrations (PM2.5) can exceed national ambient air quality standards. Previous studies in the SLV have found that PM2.5 is primar-ily 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 chem-istry is most active. The 2017 Utah Winter Fine Particu-late Study (UWFPS) provided the first aircraft measurements of detailed chemical composition during wintertime pollu-tion events in the SLV. Coupled with ground-based observa-tions, analyses of day- and nighttime research flights con-firm that PM2.5 during wintertime pollution events is principally composed of NH4NO3, limited by HNO3. Here, ob-servations and box model analyses assess the contribution of N2O5 uptake to nitrate aerosol during pollution events using the NO production rate, N2O5 heterogeneous up-take 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, addi-tional model simulations suggest nocturnal N2O5 uptake pro-duces 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 ac-curate quantification is dependent on modeled dilution, mix-ing processes, and photochemistry.