Simultaneous formation of sulfate and nitrate via co-uptake of SO ₂ and NO ₂ by aqueous NaCl droplets: combined effect of nitrate photolysis and chlorine chemistry

SOinline-formula₂ and NOinline-formula₂ 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 NOinline-formula₂ and SOinline-formula₂ into NaCl droplets at different RHs under irradiation and dark conditions. A significant formation of nitrate attributable to NOinline-formula₂ hydrolysis was observed during the NOinline-formula₂ uptake under all conditions, and its formation rate increases with decreasing RH. The averaged NOinline-formula₂ uptake coefficient, inline-formula $M12inlinescrollmathml{\mathrm{italic\; \gamma }}_{chem{\mathrm{normal\; NO}}_{\mathrm{normal\; 2}}}$ 24pt12ptsvg-formulamathimg9994a0145bf18567b5d51cd11b781e64 acp-23-6113-2023-ie00001.svg24pt12ptacp-23-6113-2023-ie00001.png , from the unary uptake of NOinline-formula₂ into NaCl droplets under dark conditions is 1.6 inline-formula× 10inline-formula⁻⁵, 1.9 inline-formula× 10inline-formula⁻⁵, and 3.0 inline-formula× 10inline-formula⁻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, NOinline-formula₂, and N(III)) that directly react with S(IV) to produce sulfate. The OH radicals generated 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 SOinline-formula₂ uptake coefficient, inline-formula $M22inlinescrollmathml{\mathrm{italic\; \gamma }}_{chem{\mathrm{normal\; SO}}_{\mathrm{normal\; 2}}}$ 23pt12ptsvg-formulamathimg7b9410df74d6fda480a64fc8606aaf92 acp-23-6113-2023-ie00002.svg23pt12ptacp-23-6113-2023-ie00002.png , as a function of the nitrate photolysis rate, inline-formula $M23inlinescrollmathml{P}_{chem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}}$ 25pt16ptsvg-formulamathimgda19ac2768e7dcd8b97125e02ffb8017 acp-23-6113-2023-ie00003.svg25pt16ptacp-23-6113-2023-ie00003.png (inline-formula $M24inlinescrollmathml{j}_{chem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}}$ 23pt16ptsvg-formulamathimga1ec66b903008f20411aa0f52124fb6e acp-23-6113-2023-ie00004.svg23pt16ptacp-23-6113-2023-ie00004.png  inline-formula× [NOinline-formula $M26inlinescrollmathml{}_{\mathrm{normal\; 3}}^{-}$ 9pt16ptsvg-formulamathimgd96e0e0e6a6172a7d34ac185b1d0a8a7 acp-23-6113-2023-ie00005.svg9pt16ptacp-23-6113-2023-ie00005.png ]), and chloride photolysis rate, inline-formula $M27inlinescrollmathml{P}_{chem{\mathrm{normal\; Cl}}^{-}}$ 21pt12ptsvg-formulamathimgc20a22d03150bc6a02a165a42d80bf4f acp-23-6113-2023-ie00006.svg21pt12ptacp-23-6113-2023-ie00006.png (inline-formula $M28inlinescrollmathml{j}_{chem{\mathrm{normal\; Cl}}^{-}}$ 19pt12ptsvg-formulamathimg5a5e5821e30a4aa083481a712d7b2f61 acp-23-6113-2023-ie00007.svg19pt12ptacp-23-6113-2023-ie00007.png  inline-formula× [Clinline-formula⁻]), was derived as inline-formula $M31inlinescrollmathml{\mathrm{italic\; \gamma }}_{chem{\mathrm{normal\; SO}}_{\mathrm{normal\; 2}}}$ 23pt12ptsvg-formulamathimg80233142351d2e72f6461524fc3ad109 acp-23-6113-2023-ie00008.svg23pt12ptacp-23-6113-2023-ie00008.png  inline-formula= 0.41 inline-formula× inline-formula $M34inlinescrollmathml{P}_{chem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}}$ 25pt16ptsvg-formulamathimgb0783186d815e9fba508cdfddc2f7861 acp-23-6113-2023-ie00009.svg25pt16ptacp-23-6113-2023-ie00009.png  inline-formula+ 0.34 inline-formula× inline-formula $M37inlinescrollmathml{P}_{chem{\mathrm{normal\; Cl}}^{-}}$ 21pt12ptsvg-formulamathimg6c8feedebb5852880e99969496449ecd acp-23-6113-2023-ie00010.svg21pt12ptacp-23-6113-2023-ie00010.png . Our findings open up new perspectives on the formation of secondary aerosol from the combined effect of nitrate photolysis and chlorine chemistry.