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      An MCM modeling study of nitryl chloride (ClNO<sub>2</sub>) impacts on oxidation, ozone production and nitrogen oxide partitioning in polluted continental outflow

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          Abstract

          Nitryl chloride (ClNO<sub>2</sub>) is produced at night by reactions of dinitrogen pentoxide (N<sub>2</sub>O<sub>5</sub>) on chloride containing surfaces. ClNO<sub>2</sub> 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<sub>2</sub>O<sub>5</sub> 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<sub>2</sub> as a daytime Cl· source and a nighttime NO<sub>x</sub> reservoir. However, the fate of the Cl· and the overall impact of ClNO<sub>2</sub> on regional photochemistry remain poorly constrained by measurements and models. To this end, we have incorporated ClNO<sub>2</sub> 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<sub>2</sub>O<sub>5</sub>, ClNO<sub>2</sub>, HOCl, ClONO<sub>2</sub>, and Cl<sub>2</sub>, none of which are currently part of the MCM, have been added to the mechanism. The presence of ClNO<sub>2</sub> produces significant changes to oxidants, ozone, and nitrogen oxide partitioning, relative to model runs excluding ClNO<sub>2</sub> formation. From a nighttime maximum of 1.5 ppbv ClNO<sub>2</sub>, the daytime maximum Cl· concentration reaches 1 × 10<sup>5</sup> atoms cm<sup>−3</sup> 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<sub>2</sub>. In the presence of several ppbv of nitrogen oxide radicals (NO<sub>x</sub> = NO + NO<sub>2</sub>), these perturbations lead to similar enhancements in hydrogen oxide radicals (HO<sub>x</sub> = OH + HO<sub>2</sub>). Neglecting contributions from HONO, the total integrated daytime radical source is 17% larger when including ClNO<sub>2</sub>, 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.

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          Author and article information

          Journal
          Atmospheric Chemistry and Physics
          Atmos. Chem. Phys.
          Copernicus GmbH
          1680-7324
          2014
          April 2014
          : 14
          : 8
          : 3789-3800
          Article
          10.5194/acp-14-3789-2014
          7010ab22-6522-4c94-810d-126fc030ea48
          © 2014

          http://creativecommons.org/licenses/by/3.0/

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