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

Abstract

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, γSO2, and the Cl• production rate, d­[Cl•]/dt, as γSO2 = 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.

Multiphase Oxidation of Sulfur Dioxide in Aerosol Particles: Implications for Sulfate Formation in Polluted Environments

“…The large uncertainty arises from the uncertainties in the reaction kinetics. In addition to nitrate photolysis, it was found that photolysis of iron-chloride containing aerosol particles produces Cl•/Cl 2 • – that also lead to rapid multiphase oxidation of SO 2, with a γ SO2 of ∼10 –5 …”

“…In addition to nitrate photolysis, it was found that photolysis of iron-chloride containing aerosol particles produces Cl•/Cl 2 • − that also lead to rapid multiphase oxidation of SO 2, with a γ SO2 of ∼10 −5 . 109 Photosensitization chemistry is an emerging field in aerosol chemistry 110 that is founded in concepts explored originally in the natural waters photochemistry community. Solar radiation excites a photosensitizing molecule to a triplet state that can react with another molecule through energy transfer, electron transfer, or hydrogen atom abstraction mechanisms.…”

Aging of atmospheric aerosols and the role of iron in catalyzing brown carbon formation

“…167,168 These diacids, along with inorganic salts, influence the pH, ionic strength, water activity, and viscosity of the aerosol liquid water, and also compete for binding to iron, specifically the anionic species. 169 Using aerosol liquid water content of 6×10 -8 L m -3 of air, Gen et al 147 estimated the molar concentration of C2-C4 diacids to range from 3-95 mM depending on the diacid over a number of locations in 159,161,179 .…”

Single-particle Raman spectroscopy for studying physical and chemical processes of atmospheric particles

“…Hence, hydrophobic substrates are often used to study supersaturated aqueous droplets by reducing the particle and substrate contact area. Investigations on photochemistry can be easily performed by coupling a transparent substrate and an additional light source beneath the quartz window at a flow cell's bottom (Gen et al, 2019c(Gen et al, , 2020. A single particle can be retained for an infinite time deposited, facilitating studies of slow processes in the atmosphere.…”

“…4). Later, they also reported sulfate formation by multiphase oxidation of SO 2 during FeCl 3 photolysis (Gen et al, 2020). Recently, sulfate formation was used to indicate the synergistic effect of the coexistence of iron-organic complexes and nitrate under irradiation as a source of N(III), based on the reaction between SO 2 and N(III) (Gen et al, 2021).…”