Abstract

Abstract. Nitryl chloride (ClNO2), an important precursor of Cl atoms, significantly affects atmospheric oxidation capacity and O3 formation. However, sources of ClNO2 in inland China have not been fully elucidated. In this work, laboratory experiments were conducted to investigate heterogeneous reactions of N2O5 with eight saline mineral dust samples collected from different regions in China, and substantial formation of ClNO2 was observed in these reactions. ClNO2 yields, φ(ClNO2), showed large variations (ranging from <0.05 to ∼0.77) for different saline mineral dust samples, depending on mass fractions of particulate chloride. In addition, φ(ClNO2) could increase, decrease or show insignificant change for different saline mineral dust samples when relative humidity (RH) increased from 18 % to 75 %. We further found that current parameterizations significantly overestimated φ(ClNO2) for heterogeneous uptake of N2O5 onto saline mineral dust. In addition, assuming a uniform φ(ClNO2) value of 0.10 for N2O5 uptake onto mineral dust, we used a 3-D chemical transport model to assess the impact of this reaction on tropospheric ClNO2 in China and found that weekly mean nighttime maximum ClNO2 mixing ratios could have been increased by up to 85 pptv during a severe dust event in May 2017. Overall, our work showed that heterogeneous reaction of N2O5 with saline mineral dust could be an important source of tropospheric ClNO2 in inland China.

Highlights

  • The formation of O3 and secondary aerosols, two major air pollutants, is closely related to atmospheric oxidation processes (Lu et al, 2019)

  • Rate constants for reactions of certain volatile organic compounds (VOCs) with Cl atoms can be a few orders of magnitude larger than those reacting with OH radicals (Atkinson and Arey, 2003; Atkinson et al, 2006); despite its lower concentrations in the troposphere, Cl can contribute significantly to atmospheric oxidation capacity in some regions (Saiz-Lopez and von Glasow, 2012; Simpson et al, 2015; Wang et al, 2019)

  • One major source of tropospheric Cl atoms is daytime photolysis of nitryl chloride (ClNO2) (Thornton et al, 2010; Simpson et al, 2015), which is formed in heterogeneous reaction of N2O5 with chlorine-containing particles (R1) at nighttime (Osthoff et al, 2008; Thornton et al, 2010): N2O5(g)+Cl−(aq) → φClNO2(g)+(2−φ)NO3−(aq). (R1)

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Summary

Introduction

The formation of O3 and secondary aerosols, two major air pollutants, is closely related to atmospheric oxidation processes (Lu et al, 2019). Major tropospheric oxidants include OH radicals, NO3 radicals and O3, and in the last 2 decades Cl atoms have been proposed as an important oxidant (Saiz-Lopez and von Glasow, 2012; Simpson et al, 2015; Wang et al, 2019). Rate constants for reactions of certain volatile organic compounds (VOCs) with Cl atoms can be a few orders of magnitude larger than those reacting with OH radicals (Atkinson and Arey, 2003; Atkinson et al, 2006); despite its lower concentrations in the troposphere, Cl can contribute significantly to atmospheric oxidation capacity in some regions (Saiz-Lopez and von Glasow, 2012; Simpson et al, 2015; Wang et al, 2019). One major source of tropospheric Cl atoms is daytime photolysis of nitryl chloride (ClNO2) (Thornton et al, 2010; Simpson et al, 2015), which is formed in heterogeneous reaction of N2O5 with chlorine-containing particles (R1) at nighttime (Osthoff et al, 2008; Thornton et al, 2010): N2O5(g)+Cl−(aq) → φClNO2(g)+(2−φ)NO3−(aq). (R1)

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