Impact of Reactive Chlorine on Atmospheric Oxidative Capacity in a Snowy Polluted Environment

Reactive chlorine species can significantly influence the formation of secondary air pollutants. Due to limited observational data, their contribution to haze formation in cold environments remains poorly constrained. In this study, we conducted field measurements of reactive chlorine species in snowy Northeast China, a region frequently affected by wintertime haze events. The average nitryl chloride (ClNO₂) and molecular chlorine (Cl₂) were 110 ± 193 and 13 ± 13 ppt during the day, and 186 ± 216 and 12 ± 17 ppt at night, respectively. In addition, we performed vertical profile experiments to determine the air-snow surface exchange fluxes of ClNO₂ and Cl₂. They generally exhibited net deposition to the snowpack at night but frequently showed net emissions from the snowpack during the day, suggesting photochemical production on the snowpack. On average, photolysis of Cl₂ produced three times more chlorine radicals than ClNO₂ on a daily basis. The combined chlorine radical production rate from Cl₂ and ClNO₂ reached approximately one-third of the hydroxyl radical production rate from nitrous acid, highlighting their substantial role in wintertime atmospheric oxidation. Our results contribute to a better understanding of atmospheric oxidation in the snowy, polluted regions worldwide.