Urban inland wintertime N<sub>2</sub>O<sub>5</sub> and ClNO<sub>2</sub> influenced by snow-covered ground, air turbulence, and precipitation

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.

Quantifying the Contributions of Aerosol- and Snow-Produced ClNO2 through Observations and 1D Modeling

“…The large measurement uncertainties in the snowpack ClNO 2 emission fluxes resulted in a significant range of modeled ClNO 2 (Figure b), indicating that the ClNO 2 simulations were highly sensitive to the snowpack emissions. The important role of snowpack ClNO 2 production in the simulations is consistent with the whole-campaign observations by Kulju et al, who found higher ClNO 2 mole ratios when snow-covered ground was present, which could not be explained by air turbulence, N 2 O 5 , or several other variables, and attributed this finding to the snowpack ClNO 2 flux. The NO 2 level did not have a significant impact on the simulated ground-level ClNO 2 , as shown in Figure S12, since the corresponding model layer was constrained with the measured N 2 O 5 at the field site.…”

“…N 2 O 5 and ClNO 2 observations for the full SNACK campaign were previously reported by Kulju et al Over the full campaign, N 2 O 5 mole ratios were not statistically significantly different between the snow-covered and bare ground periods . In contrast, on average over the full campaign, ClNO 2 mole ratios were higher over snow-covered ground compared to bare ground due to snowpack ClNO 2 production . Here, we focus this modeling study on two case studies, the nights of Jan 31 and Jan 30, which were chosen to represent the bare ground and snow cover periods, respectively (Figure ).…”

“…Field observations show that ClNO 2 is ubiquitous in the atmospheric boundary layer in both coastal − and inland regions. − Enhanced levels of ClNO 2 are attributed to air masses affected by sea salt aerosols, ,, biomass burning, and coal burning activities, ,, with playa dust also having been found to be a ClNO 2 source. , While understudied, road salt can also be a significant source of chloride in the aerosol phase, as well as urban snowpack. ,− Large amounts of road salts are used globally in wintertime environments for deicing purposes. In 2019, ∼18 million tons of road salt was used in the U.S .…”

“…In Kalamazoo, Michigan, U.S., a maximum of ∼90 ppt of ClNO 2 was observed at 1.5 m over snow-covered ground. 28 Snowpack consists of interstitial air and snow grains with brine patches containing solutes excluded from the ice. 48 Therefore, snowpack is a highly porous medium that serves as a unique matrix for multiphase reactions.…”

Distinctive Heterogeneous Reaction Mechanism of ClNO2 on the Air–Water Surface Containing Cl

“…Therefore, at low temperatures, R moves right, namely, N 2 O 5 formation. As a result, N 2 O 5 dissociation (left direction of R ) slows; thus, net N 2 O 5 formation is favorable at low temperatures (right direction of R ), , forming ClNO 2 (R ) that competes with the N 2 O 5 hydrolysis process (R ). This low-temperature process illustrates ClNO 2 formation on Cl-containing spray aerosols along the Arctic coast or in coastal regions in winter.…”