Multiphase Photochemistry of Iron-Chloride Containing Particles as a Source of Aqueous Chlorine Radicals and Its Effect on Sulfate Production

Photolysis of iron chlorides is a well-known photolytic source of Cl^• in environmental waters. However, the role of particulate chlorine radicals (Cl^• and Cl₂^•-) in their multiphase oxidative potential has been much less explored. Herein, we examine the effect of Cl^•/Cl₂^•- produced from photolysis of particulate iron chlorides on atmospheric multiphase oxidation. As a model system, experiments on multiphase oxidation of SO₂ by Cl^•/Cl₂^•- were performed. Fast sulfate production from SO₂ oxidation was observed with reactive uptake coefficients of ∼10^-5, comparable to the values necessary for explaining the observations in the haze events in China. The experimental and modeling results found a good positive correlation between the uptake coefficient, Υ_SO2, and the Cl^• production rate, d[Cl^•]/dt, as Υ_SO2 = 5.3 × 10^-6 × log(d[Cl^•]/dt) + 4.9 × 10^-5. When commonly found particulate dicarboxylic acids (oxalic acid or malonic acid) were added, sulfate production was delayed due to the competition of Fe³⁺ between chloride and the dicarboxylic acid for its complexation at the initial stage. After the delay, comparable sulfate production was observed. The present study highlights the importance of photochemistry of particulate iron chlorides in multiphase oxidation processes in the atmosphere.