Abstract
Abstract. Wintertime ammonium nitrate (NH4NO3) pollution events burden urban mountain basins around the globe. In the Salt Lake Valley of Utah in the United States, such pollution events are often driven by the formation of persistent cold-air pools (PCAPs) that trap emissions near the surface for several consecutive days. As a result, secondary pollutants including fine particulate matter less than 2.5 µm in diameter (PM2.5), largely in the form of NH4NO3, build up during these events and lead to severe haze. As part of an extensive measurement campaign to understand the chemical processes underlying PM2.5 formation, the 2017 Utah Winter Fine Particulate Study, water-soluble trace gases and PM2.5 constituents were continuously monitored using the ambient ion monitoring ion chromatograph (AIM-IC) system at the University of Utah campus. Gas-phase NH3, HNO3, HCl, and SO2 along with particulate NH4+, Na+, K+, Mg2+, Ca2+, NO3-, Cl−, and SO42- were measured from 21 January to 21 February 2017. During the two PCAP events captured, the fine particulate matter was dominated by secondary NH4NO3. The comparison of total nitrate (HNO3 + PM2.5 NO3-) and total NHx (NH3 + PM2.5 NH4+) showed NHx was in excess during both pollution events. However, chemical composition analysis of the snowpack during the first PCAP event revealed that the total concentration of deposited NO3- was nearly 3 times greater than that of deposited NH4+. Daily snow composition measurements showed a strong correlation between NO3- and Ca2+ in the snowpack. The presence of non-volatile salts (Na+, Ca2+, and Mg2+), which are frequently associated with coarse-mode dust, was also detected in PM2.5 by the AIM-IC during the two PCAP events, accounting for roughly 5 % of total mass loading. The presence of a significant particle mass and surface area in the coarse mode during the first PCAP event was indicated by size-resolved particle measurements from an aerodynamic particle sizer. Taken together, these observations imply that atmospheric measurements of the gas-phase and fine-mode particle nitrate may not represent the total burden of nitrate in the atmosphere, implying a potentially significant role for uptake by coarse-mode dust. Using the NO3- : NH4+ ratio observed in the snowpack to estimate the proportion of atmospheric nitrate present in the coarse mode, we estimate that the amount of secondary NH4NO3 could double in the absence of the coarse-mode sink. The underestimation of total nitrate indicates an incomplete account of the total oxidant production during PCAP events. The ability of coarse particles to permanently remove HNO3 and influence PM2.5 formation is discussed using information about particle composition and size distribution.
Highlights
Several studies conducted in northern Utah have shown that PM2.5 during these persistent cold-air pools (PCAPs) episodes is predominantly composed of ammonium nitrate (NH4NO3), accounting for roughly 60 %–80 % of the total dry particle mass (Baasandorj et al, 2017; Franchin et al, 2018; Hansen et al, 2013; Kelly et al, 2013; Kuprov et al, 2014; Long et al, 2002, 2003; Mangelson et al, 1997)
The ambient ion monitoring ion chromatograph (AIM-ion chromatographs (ICs)) measurements confirmed the dominance of NH4NO3 in wintertime PM2.5 pollution events in the Salt Lake Valley (SLV)
The detection of non-volatile cations in PM2.5 by the Aerosol Instrument Manager (AIM)-IC suggests the presence of mineral dust and/or salt during PCAP pollution events, which can potentially impact the availability of HNO3
Summary
Episodes with high particulate matter (PM) pollution occur frequently in urban air basins across the globe during winter months when a stable boundary layer persists for multiple days, including in the mountain valleys in the western USA (Baasandorj et al, 2017; Bares et al, 2018; Green et al, 2015; Silcox et al, 2012; Whiteman et al, 2014), the Po Valley in Italy (Bernardoni et al, 2017; Vecchi et al, 2018), and the Sichuan (Tian et al, 2019) and Twin-Hu (Gao et al, 2019) basins in China These mountain basins, including the Salt Lake Valley (SLV) in northern Utah, experience strong temperature inversions that develop into persistent cold-air pools (PCAPs), which suppress vertical mixing and trap emissions within a shallow boundary layer (Lareau et al, 2013; Whiteman et al, 2014). NH4NO3 formation is thermodynamically favorable under conditions with low temperatures and high relative humidity based on equilibrium partitioning with gas-phase ammonia (NH3) and nitric acid (HNO3) (Mozurkewich, 1993; Nowak et al, 2010; Seinfeld and Pandis, 2006), illustrated in the right-hand portion of Fig. 1
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