The role of coarse aerosol particles as a sink of HNO<sub>3</sub> in wintertime pollution events in the Salt Lake Valley

Abstract. Wintertime ammonium nitrate (NH4NO3) pollution events burden urban mountain basins around the globe. In Salt Lake Valley of Utah in United States, such are often driven by formation persistent cold-air pools (PCAPs) that trap emissions near surface for several consecutive days. As a result, secondary pollutants including fine particulate matter less than 2.5 µm diameter (PM2.5), largely form NH4NO3, build up during these and lead to severe haze. part an extensive measurement campaign understand chemical processes underlying PM2.5 formation, 2017 Winter Fine Particulate Study, water-soluble trace gases constituents were continuously monitored using ambient ion monitoring chromatograph (AIM-IC) system at University campus. Gas-phase NH3, HNO3, HCl, SO2 along with NH4+, Na+, K+, Mg2+, Ca2+, NO3-, Cl−, SO42- measured from 21 January February 2017. During two PCAP captured, was dominated NH4NO3. The comparison total (HNO3 + NO3-) NHx (NH3 NH4+) showed excess both events. However, composition analysis snowpack first event revealed concentration deposited NO3- nearly 3 times greater NH4+. Daily snow measurements strong correlation between Ca2+ snowpack. presence non-volatile salts (Na+, Mg2+), which frequently associated coarse-mode dust, also detected AIM-IC events, accounting roughly 5 % mass loading. significant particle area coarse mode indicated size-resolved aerodynamic sizer. Taken together, observations imply atmospheric gas-phase fine-mode may not represent atmosphere, implying potentially role uptake dust. Using : NH4+ ratio observed estimate proportion present mode, we amount NH4NO3 could double absence sink. underestimation indicates incomplete account oxidant production ability particles permanently remove HNO3 influence is discussed information about size distribution.