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 Cl) in their multiphase oxidative potential has been much less explored. Herein, we examine the effect of Cl/Cl produced from photolysis of particulate iron chlorides on atmospheric multiphase oxidation. As a model system, experiments on multiphase oxidation of SO by Cl/Cl were performed. Fast sulfate production from SO oxidation was observed with reactive uptake coefficients of ∼10, 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, γ, and the Cl production rate, d[Cl]/dt, as γ = 5.3 × 10 × log(d[Cl]/dt) + 4.9 × 10. When commonly found particulate dicarboxylic acids (oxalic acid or malonic acid) were added, sulfate production was delayed due to the competition of Fe 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.