Wintertime Atmospheric Chlorine Chemistry on Aerosol Particles and Snow

Atmospheric chlorine chemistry is prevalent in wintertime mid-latitude environments from the dark reaction of N2O5 with chloride-containing surfaces to produce ClNO2. Nitryl chloride (ClNO2) photolyzes to produce highly reactive Cl atoms and NO2, which alter atmospheric oxidation pathways and air quality. We conducted measurements of atmospheric trace gases, particles, and snow in two polluted urban locations in Michigan during wintertime to investigate the dominant chloride-containing surfaces resulting in this chemistry. We observed road salt aerosol to be the primary chloride source resulting in ClNO2 production. Our single-particle measurement methods showed that only a fraction of the particulate surface area contained chloride. We developed a new approach to parametrize N2O5 uptake and ClNO2 yield that considers the heterogeneity of the aerosol population. This new approach more accurately simulated ClNO2, compared to traditional mass-based methods, which assume homogeneous distribution of chemical components across all particles and which frequently overestimate ClNO2 production. In addition, we conducted vertical gradient and snow chamber experiments showing that ClNO2 can be produced from the reaction of N2O5 on the saline snowpack, serving as an addition chloride source in the wintertime environment. This work improves our understanding of wintertime chlorine chemistry, and our new approach to simulating N2O5 uptake and ClNO2 production is expected to be applicable to other heterogeneous reactions.