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

Nitryl chloride (ClNO2) is a radical reservoir that forms and accumulates in the nocturnal atmospheric boundary layer influenced by combustion emissions and chloride (e.g., sea salt and road salt). Upon sunrise, ClNO2 rapidly photolyzes to generate highly reactive chlorine radicals (Cl-center dot) that affect the air quality by generating secondary air pollutants. Recent studies have shown road salt aerosols and saline snowpack to be sources of ClNO2 in the wintertime urban environment; however, the quantitative contributions of each chloride source are not known. In this study, we examine the vertically resolved contributions of aerosol particles and saline snowpack as sources of ClNO2 by using an observationally constrained snow-atmosphere coupled one-dimensional model applied to wintertime Kalamazoo, Michigan, U.S. Model simulations show that ClNO2 emitted from urban snowpack can be vertically transported throughout the entire atmospheric boundary layer and can be a significant source of ClNO2, contributing up to similar to 60% of the ClNO2 budget near the surface. Modeled snowpack ClNO2 emission rates were 6 (+/- 7) times higher than the observationally derived emission rates, suggesting that not all snow chloride is available for reaction. ClNO2 production from both aerosol particles and snow emissions are required to best simulate the observed surface-level ClNO2. Using the traditional bulk parameterization for ClNO2 produced from particles significantly overestimated ClNO2 due to the assumption of having equivalent dinitrogen pentoxide (N2O5) uptake and chloride availability for the entire particle population. In comparison, the chemically resolved surface area-based parameterization slightly underestimated the observations and had uncertainties deriving from ClNO2 production from residential wood burning particles.