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. Wintertime ammonium nitrate (NH4NO3) pollution eventsburden urban mountain basins around the globe. In the Salt Lake Valley ofUtah in the United States, such pollution events are often driven by theformation of persistent cold-air pools (PCAPs) that trap emissions near thesurface for several consecutive days. As a result, secondary pollutantsincluding fine particulate matter less than 2.5 µm in diameter(PM2.5), largely in the form of NH4NO3, build up during theseevents and lead to severe haze. As part of an extensive measurement campaignto understand the chemical processes underlying PM2.5 formation, the2017 Utah Winter Fine Particulate Study, water-soluble trace gases andPM2.5 constituents were continuously monitored using the ambient ionmonitoring ion chromatograph (AIM-IC) system at the University of Utahcampus. Gas-phase NH3, HNO3, HCl, and SO2 along withparticulate NH4+, Na+, K+, Mg2+, Ca2+,NO3-, Cl−, and SO42- were measured from 21 Januaryto 21 February 2017. During the two PCAP events captured, the fineparticulate matter was dominated by secondary NH4NO3. Thecomparison of total nitrate (HNO3 + PM2.5 NO3-) andtotal NHx (NH3 + PM2.5 NH4+) showed NHx wasin excess during both pollution events. However, chemical compositionanalysis of the snowpack during the first PCAP event revealed that the totalconcentration of deposited NO3- was nearly 3 times greaterthan that of deposited NH4+. Daily snow composition measurementsshowed a strong correlation between NO3- and Ca2+ in thesnowpack. The presence of non-volatile salts (Na+, Ca2+, andMg2+), which are frequently associated with coarse-mode dust, was alsodetected in PM2.5 by the AIM-IC during the two PCAP events,accounting for roughly 5 % of total mass loading. The presence of asignificant particle mass and surface area in the coarse mode during thefirst PCAP event was indicated by size-resolved particle measurements froman aerodynamic particle sizer. Taken together, these observations imply thatatmospheric measurements of the gas-phase and fine-mode particle nitrate maynot represent the total burden of nitrate in the atmosphere, implying apotentially significant role for uptake by coarse-mode dust. Using theNO3- : NH4+ ratio observed in the snowpack to estimate theproportion of atmospheric nitrate present in the coarse mode, we estimatethat the amount of secondary NH4NO3 could double in the absence ofthe coarse-mode sink. The underestimation of total nitrate indicates anincomplete account of the total oxidant production during PCAP events. Theability of coarse particles to permanently remove HNO3 and influencePM2.5 formation is discussed using information about particlecomposition and size distribution.

The role of coarse aerosol particles as a sink of HNO&lt;sub&gt;3&lt;/sub&gt; in wintertime pollution events in the Salt Lake Valley

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