Enhanced Sulfate Production by Nitrate Photolysis in the Presence of Halide Ions in Atmospheric Particles

摘要

Heterogeneous oxidation of SO_2 is an effective production pathway of sulfate in the atmosphere. We recently reported a novel pathway for the heterogeneous oxidation of SO_2 by in-particle oxidants (OH, NO_2, and NO_2~-/HNO_2) 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 SO_2 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, j_(NO3-), and the initial molar ratio of Cl~- to NO_3~-, [Cl~-]_0/[NO_3~-]_0, as j_(NO3-) = 9.7 × 10~(-5)[Cl~-]_0/[NO_3~-]_0 + 1.9 × 10~(-5) at [Cl~-]_0/ [NO_3~-]_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.