Preprints
https://doi.org/10.5194/egusphere-2023-223
https://doi.org/10.5194/egusphere-2023-223
14 Feb 2023
 | 14 Feb 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Simultaneous Formation of Sulfate and Nitrate via Co-uptake of SO2 and NO2 by Aqueous NaCl Droplets: Combined Effect of Nitrate Photolysis and Chlorine Chemistry

Ruifeng Zhang and Chak Keung Chan

Abstract. SO2 and NO2 are the critical precursors in forming sulfate and nitrate in ambient particles. We studied the mechanism of sulfate and nitrate formation during the co-uptake of NO2 and SO2 into NaCl droplets at different RHs under irradiation and dark conditions. A significant formation of nitrate attributable to NO2 hydrolysis was observed during the NO2 uptake under all conditions, and its formation rate increases with decreasing RH. The averaged NO2 uptake coefficient, γNO2, from the unary uptake of NO2 into NaCl droplets under dark are 1.6 × 10-5, 1.9 × 10-5, and 3.0 × 10-5 at 80 %, 70 %, and 60 % RH, respectively. Chloride photolysis and nitrate photolysis play a crucial role in sulfate formation during the co-uptake. Nitrate photolysis generates reactive species (e.g., OH radicals, NO2, and N(III)) that directly react with S(IV) to produce sulfate. The generated OH radicals from nitrate photolysis can also react with chloride ions to form reactive chlorine species and then sulfate. To parameterize the role of nitrate photolysis and chloride photolysis in forming sulfate, the SO2 uptake coefficient, γSO2, as a function of nitrate photolysis rate, PNO3- (= jNO3- × [NO3-]), and chloride photolysis rate, PCl- (= jCl- × [Cl-]), as γSO2 = 0.41 × PNO3- + 0.34 × PCl- was derived. Our findings open up new perspectives on the formation of secondary aerosol from the combined effect of nitrate photolysis and chlorine chemistry.

Ruifeng Zhang and Chak Keung Chan

Status: open (until 28 Mar 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-223', Anonymous Referee #1, 12 Mar 2023 reply
  • RC2: 'Comment on egusphere-2023-223', Anonymous Referee #2, 18 Mar 2023 reply

Ruifeng Zhang and Chak Keung Chan

Ruifeng Zhang and Chak Keung Chan

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Short summary
Research in sulfate and nitrate formation from co-uptake of NO2 and SO2, especially under irradiation, is rare. We studied the co-uptake of NO2 and SO2 by NaCl droplets under various conditions, including irradiation/dark and different RHs, using Raman spectroscopy/Flow cell and kinetic model simulation. A significant nitrate from NO2 hydrolysis can be photolyzed to generate OH radicals that can further react with chloride to produce reactive chlorine species and promote sulfate formation.