Observationally Constrained 1-Dimensional Modeling of Wintertime Inland ClNO2 Over Urban Snowpack

Chlorine radicals (Cl) can significantly impact regional air quality by reacting with volatile organic compounds leading to production of secondary air pollutants such as tropospheric ozone and secondary organic aerosols. Nitryl chloride (ClNO2) is a nighttime radical reservoir generated from the heterogeneous reactions of gas-phase dinitrogen pentoxide (N2O5) and chloride containing surfaces. During the day, ClNO2 rapidly photolyzes to produce Cl. Previously, most studies have focused on ClNO2 production driven by atmospheric particulate chloride. However, recent (but limited) studies have reported the road salt contaminated snowpack as a source of wintertime ClNO2 in urban regions. In this study, ClNO2 was simulated using an atmosphere-snow coupled 1-dimensional model, during two case days (bare ground vs snow cover) in Kalamazoo, MI, in the winter. The model was constrained with measured trace gases, aerosol composition, meteorological conditions, and snow parameters. The two main questions of the modeling study are: 1) how high can the snow emitted ClNO2 transport vertically in the nocturnal boundary layer? and 2) what is the vertically-resolved relative importance of aerosols versus the snowpack as a source of ClNO2? The results show that ClNO2 is significantly over-predicted, compared to measurements, when simulated by the snow module in the model framework. For both case days, model simulations showed improved agreement with observations when constrained with observationally driven ClNO2 fluxes from snowpack and when using the newly-developed single-particle parameterization method for N2O5 uptake and ClNO2 yield on particles.