An MCM modeling study of nitryl chloride (ClNO₂) impacts on oxidation, ozone production and nitrogen oxide partitioning in polluted continental outflow

Abstract. Nitryl chloride (ClNO₂) is produced at night by reactions of dinitrogen pentoxide (N₂O₅) on chloride containing surfaces. ClNO₂ is photolyzed during the morning hours after sunrise to liberate highly reactive chlorine atoms (Cl·). This chemistry takes place primarily in polluted environments where the concentrations of N₂O₅ precursors (nitrogen oxide radicals and ozone) are high, though it likely occurs in remote regions at lower intensities. Recent field measurements have illustrated the potential importance of ClNO₂ as a daytime Cl· source and a nighttime NO_x reservoir. However, the fate of the Cl· and the overall impact of ClNO₂ on regional photochemistry remain poorly constrained by measurements and models. To this end, we have incorporated ClNO₂ production, photolysis, and subsequent Cl· reactions into an existing master chemical mechanism (MCM version 3.2) box model framework using observational constraints from the CalNex 2010 field study. Cl· reactions with a set of alkenes and alcohols, and the simplified multiphase chemistry of N₂O₅, ClNO₂, HOCl, ClONO₂, and Cl₂, none of which are currently part of the MCM, have been added to the mechanism. The presence of ClNO₂ produces significant changes to oxidants, ozone, and nitrogen oxide partitioning, relative to model runs excluding ClNO₂ formation. From a nighttime maximum of 1.5 ppbv ClNO₂, the daytime maximum Cl· concentration reaches 1 × 10⁵ atoms cm⁻³ at 07:00 model time, reacting mostly with a large suite of volatile organic compounds (VOC) to produce 2.2 times more organic peroxy radicals in the morning than in the absence of ClNO₂. In the presence of several ppbv of nitrogen oxide radicals (NO_x = NO + NO₂), these perturbations lead to similar enhancements in hydrogen oxide radicals (HO_x = OH + HO₂). Neglecting contributions from HONO, the total integrated daytime radical source is 17% larger when including ClNO₂, which leads to a similar enhancement in integrated ozone production of 15%. Detectable levels (tens of pptv) of chlorine containing organic compounds are predicted to form as a result of Cl· addition to alkenes, which may be useful in identifying times of active Cl· chemistry.