Enhanced Sulfate Formation through Synergistic Effects of Chlorine Chemistry and Photosensitization in Atmospheric Particles

Numerous studies have demonstrated that organic photosensitizers from biomass burning can generate oxidants to effectively convert inorganic and organic precursors into secondary aerosols. Particulate chloride ions can be internally mixed with organic photosensitizers in biomass burning particles. In this study, we investigate the impact of the interaction of chlorine chemistry and photosensitization on the oxidative potential of aerosols by utilizing SO₂ oxidation to form sulfate as an indicator. Mixed particles of chloride with glyoxal and its reaction products of ammonia of imidazole-2-carboxaldehyde (IC) were studied. Premixed NH₄Cl + glyoxal particles have a 4–5 times higher sulfate formation rate than premixed NaCl + glyoxal, particularly at low relative humidity, suggesting the role of photosensitization. Furthermore, the addition of IC resulted in an ∼73-fold increase in sulfate production rate compared to NH₄Cl alone. No noticeable sulfate formation was observed in the presence of IC alone, likely due to the high particle acidity in this study (i.e., pH = 2). The kinetic analysis of these particles results yields a reaction rate constant of chloride ions with the triplet state of IC, ³IC*, ∼3 orders of magnitude higher than previously reported values in bulk solution. These findings underscore the significance of the synergetic effect of chlorine chemistry and photosensitization in enhancing atmospheric oxidative capacity. © 2024 American Chemical Society