Road-Salt Aerosol Reacts More Efficiently with N2O5 than Sea Spray Aerosol to Produce ClNO2

Abstract

Atmospheric chemical models underpredict the inland production of nitryl chloride (ClNO₂), a precursor to atomic chlorine (Cl·), due to a lack of both a complete inventory of aerosol-chloride sources and their reactive uptake kinetics. One source not represented in atmospheric models is road salt, potentially driving underpredictions of Cl· in regions that apply them. We measured the soluble ion content, dinitrogen pentoxide reactive uptake coefficient (γN₂O₅), and ClNO₂ yield (φClNO₂) of several road-salt samples. To account for diverse deicing practices, we investigated several road-salt additives, including samples with sand abrasives and brines. γN₂O₅ for all samples ranged from 0.003 to 0.067, while φClNO₂ ranged from nondetectable to unity, with both parameters depending on soluble ion composition and relative humidity (RH). Under low RH conditions (<40%), road salts without additives had low values of γN₂O₅ and φClNO₂, due to sodium chloride (NaCl) efflorescence, while samples with sand and/or brine additives, including beet juice extract, showed higher γN₂O₅ and variable φClNO₂. When RH was above 40%, all but one sample showed values of γN₂O₅ and φClNO₂ similar to NaCl, suggesting soluble chloride is driving N₂O₅-road-salt reactions at elevated RHs and can be predicted using current parametrizations. A comparison of N₂O₅-road-salt chemistry demonstrated that road salt reacts more efficiently than several known aerosol-chloride sources, including sea spray aerosol. We estimate that road salt can produce ClNO₂ approximately 7.5 times more efficiently than sea spray aerosol, highlighting the importance of including road salt as an efficient halogen source in atmospheric models.