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

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 conditions is 1.6 x 10^-5, 1.9 x 10^-5, and 3.0 x 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 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 SO₂ uptake coefficient, γSO₂, as a function of the nitrate photolysis rate, PNO₃^- (jNO₃^- x [NO₃^-]), and chloride photolysis rate, P_Cl- (j_Cl- x [Cl^-]), was derived as γ(SO₂) = 0.41 x PNO₃^- + 0.34 x P_Cl^-. Our findings open up new perspectives on the formation of secondary aerosol from the combined effect of nitrate photolysis and chlorine chemistry. © Author(s) 2023.