Abstract
Abstract. The atmospheric multiphase reaction of dinitrogen pentoxide (N2O5) with chloride-containing aerosol particles produces nitryl chloride (ClNO2), which has been observed across the globe. The photolysis of ClNO2 produces chlorine radicals and nitrogen dioxide (NO2), which alter pollutant fates and air quality. However, the effects of local meteorology on near-surface ClNO2 production are not yet well understood, as most observational and modeling studies focus on periods of clear conditions. During a field campaign in Kalamazoo, Michigan, from January–February 2018, N2O5 and ClNO2 were measured using chemical ionization mass spectrometry, with simultaneous measurements of atmospheric particulate matter and meteorological parameters. We examine the impacts of atmospheric turbulence, precipitation (snow, rain) and fog, and ground cover (snow-covered and bare ground) on the abundances of ClNO2 and N2O5. N2O5 mole ratios were lowest during periods of lower turbulence and were not statistically significantly different between snow-covered and bare ground. In contrast, ClNO2 mole ratios were highest, on average, over snow-covered ground, due to saline snowpack ClNO2 production. Both N2O5 and ClNO2 mole ratios were lowest, on average, during rainfall and fog because of scavenging, with N2O5 scavenging by fog droplets likely contributing to observed increased particulate nitrate concentrations. These observations, specifically those during active precipitation and with snow-covered ground, highlight important processes, including N2O5 and ClNO2 wet scavenging, fog nitrate production, and snowpack ClNO2 production, that govern the variability in observed atmospheric chlorine and nitrogen chemistry and are missed when considering only clear conditions.
Highlights
Atmospheric halogen radicals are highly oxidizing agents of tropospheric pollutants (Simpson et al, 2015)
Through vertical gradient measurements on select nights of the SNow and Atmospheric Chemistry in Kalamazoo (SNACK) field campaign, we showed that N2O5 deposits at the same rates over bare and snow-covered ground; whereas, while ClNO2 deposits on bare ground, it can be emitted from the saline snow-covered ground, with snow chamber experiments confirming saline snow ClNO2 production 95 (McNamara et al, 2021)
It is likely that both N2O5 and HNO3 uptake, followed by aqueous-phase nitrate formation, led to the increased PM2.5 NO3observed during fog.To conclude our examination of N2O5 and ClNO2 abundances during different weather conditions, we examine trends in temperature and relative humidity (RH) during clear conditions, snowfall, rainfall, 485 and fog
Summary
Atmospheric halogen radicals are highly oxidizing agents of tropospheric pollutants (Simpson et al, 2015). Following nighttime formation, the photolysis of nitryl chloride (ClNO2) upon sunrise is a source of chlorine radicals (R1) at a time when other oxidants, including the hydroxyl radical (OH), are less abundant (Young et al, 2014), leading to enhanced oxidation of volatile organic compounds (Osthoff et al, 2008). ClNO2 photolysis releases NO2 (R1), recycling nitrogen oxides (NOx) that drive. R1 CCCCCCOO22 + hhhh → CCCC + NNOO22 ClNO2 is formed by the multiphase reaction of dinitrogen pentoxide (N2O5) on a chloridecontaining surface (R2), sea spray aerosol (Finlayson-Pitts and Pitts, 1989; Osthoff et al., 2008). NN22OO55 (gg) + CCCC(−aaaa) → CCCCCCOO22 (gg) + NNNN3−3 (aaaa) R2. ClNO2 production has been previously studied in the laboratory following the reaction of N2O5 with aqueous aerosols
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