An MCM modeling study of nitryl chloride (ClNO 2) impacts on oxidation, ozone production and nitrogen oxide partitioning in polluted continental outflow
Nitryl chloride (ClNO 2) is produced at night by reactions of dinitrogen pentoxide (N 2O 5) on chloride containing surfaces. ClNO 2 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 N 2O 5 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 ClNO 2 as a daytime Cl· source and a nighttime NO x reservoir. However, the fate of the Cl· and the overall impact of ClNO 2 on regional photochemistry remain poorly constrained by measurements and models. To this end, we have incorporated ClNO 2 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 N 2O 5, ClNO 2, HOCl, ClONO 2, and Cl 2, none of which are currently part of the MCM, have been added to the mechanism. The presence of ClNO 2 produces significant changes to oxidants, ozone, and nitrogen oxide partitioning, relative to model runs excluding ClNO 2 formation. From a nighttime maximum of 1.5 ppbv ClNO 2, the daytime maximum Cl· concentration reaches 1 × 10 5 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 ClNO 2. In the presence of several ppbv of nitrogen oxide radicals (NO x = NO + NO 2), these perturbations lead to similar enhancements in hydrogen oxide radicals (HO x = OH + HO 2). Neglecting contributions from HONO, the total integrated daytime radical source is 17% larger when including ClNO 2, 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.