Observation of N₂O₅Deposition and ClNO₂Production on the Saline Snowpack

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.