Observation of N₂O₅ Deposition and ClNO₂ Production on the Saline Snowpack

Abstract

Nitryl chloride (ClNO2), a precursor to highly reactive chlorine radicals and a reservoir for nitrogen dioxide (NO2), is formed from the reaction of chloride with N2O5, which has a longer atmospheric lifetime during the winter. Previous field observations, modeling, and laboratory ice flow tube results led to the hypothesis that saline snow is a source of ClNO2 following the deposition of dinitrogen pentoxide (N2O5). Due to the widespread use of road salt (primarily halite) and its deposition to the snowpack, the saline snowpack in Kalamazoo, Michigan, was investigated for the potential for direct ClNO2 production following N2O5 deposition. Vertical gas profile and snow chamber experiments were conducted during January–February 2018 with chemical ionization mass spectrometry measurements of ClNO2 and N2O5. The vertical gas profile measurements showed N2O5 and ClNO2 deposition over both bare and snow-covered ground. However, positive (upward) ClNO2 fluxes were only observed over the snow-covered ground, showing that the saline snowpack can serve as a source of ClNO2. A fraction of the ClNO2 profiles over the snow-covered ground did not exhibit gradients, indicative of a balance between ClNO2 production and loss, including through hydrolysis. Exposure of local snow to synthesized N2O5 during chamber experiments resulted in ClNO2 production that depended on the snowpack physical structure. Together, these results demonstrate a saline snowpack source of ClNO2, with expected relevance to both wintertime inland and coastal regions with snow.

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

“…ClNO2 flux over snow-covered ground, and snow chamber experiments showed that synthesized N2O5 reacted with the local saline snow to produce ClNO2 (McNamara et al, 2021). This finding is also consistent with the laboratory study by Lopez-Hilfiker et al (2012), which showed that N2O5 can react on halide-doped ice surfaces to produce ClNO2.…”

“…5a). This is consistent with measurements of similar net negative (deposition) fluxes of N2O5 over both snow-covered and bare ground (McNamara et al, 2021). In contrast, Figure 5b shows significantly higher (p<0.05, t-test) average ClNO2 mole ratios observed over snow-covered ground at 19:30-22:00, 23:00-00:00, 01:00-01:30, 03:00, and 07:30.…”

“…In our previous publication from this study, we showed photochemical snowpack HONO production due to snow nitrate photolysis (Chen et al, 2019). Through vertical gradient measurements on select nights of the SNACK field campaign, we showed that N 2O5 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 ClNO 2 production (McNamara et al, 2021). Here, we focus on the observational time series of near-surface ClNO2 and its precursor N2O5 and examine the influences of precipitation (rain, snow) and fog, atmospheric turbulence, (Christiansen et al, 2020).…”

“…Recently, a study in Ann Arbor, Michigan identified road salt aerosol as the dominant aerosol chloride source for ClNO2 production during winter (McNamara et al, 2020). Measurements in Kalamazoo, Michigan also identified the road salt-contaminated snowpack as a ClNO2 source (McNamara et al, 2021). A study in coastal British Columbia, Canada suggested scavenging of ClNO2 by rain and/or fog droplets as a potential loss process (Osthoff et al, 2018).…”

“…It is important to note that we do not expect the soil to be a significant NOx source; a wintertime study by Seok et al (2015) in northern Michigan showed no measureable soil NO flux, with NO2 only released from the snow during daytime from nitrate photolysis. Vehicle NOx emissions from the nearby roadway location ~80 m away (McNamara et al, 2021) are suggested to control the magnitude of the nighttime titration effect at the field site, as few time periods overnight were statistically different in O3 mole ratios between the lower and higher turbulence conditions, on average (Fig. S8).…”