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^•) 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 ∼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.