Heterogeneous oxidation of SO2 is an effective production pathway of sulfate in the atmosphere. We recently reported a novel pathway for the heterogeneous oxidation of SO2 by in-particle oxidants (OH, NO2, and NO2 -/HNO2) produced from particulate nitrate photolysis (Environ. Sci. Technol. 2019, 53, 8757-8766). Particulate nitrate is often found to coexist with chloride and other halide ions, especially in aged sea-salt aerosols and combustion aerosols. Reactive uptake experiments of SO2 with UV-irradiated nitrate particles showed that sulfate production rates were enhanced by a factor of 1.4, 1.3, and 2.0 in the presence of Cl-, Br-, and I-, respectively, compared to those in the absence of halide ions. The larger sulfate production was attributed to enhanced nitrate photolysis promoted by the increased incomplete solvation of nitrate at the air-particle interface due to the presence of surface-active halide ions. Modeling results based on the experimental data showed that the nitrate photolysis rate constants increased by a factor of 2.0, 1.7, and 3.7 in the presence of Cl-, Br-, and I-, respectively. A linear relation was found between the nitrate photolysis rate constant, jNO3-, and the initial molar ratio of Cl- to NO3 -, [Cl-]0/[NO3 -]0, as jNO3- = 9.7 × 10-5[Cl-]0/[NO3 -]0 + 1.9 × 10-5 at [Cl-]0/[NO3 -]0 below 0.2. The present study demonstrates that the presence of halide ions enhances sulfate production produced during particulate nitrate photolysis and provides insights into the enhanced formation of in-particle oxidants that may increase atmospheric oxidative capacity.
ASJC Scopus subject areas
- Environmental Chemistry