Enhanced Sulfate Formation in Mixed Biomass Burning and Sea-salt Particles Mediated by Photosensitization: Effects of Chloride and Nitrogen-containing Compounds

Abstract. Recent research has suggested that photosensitized oxidation can be an effective pathway for the oxidation of SO₂ based on a limited number of model photosensitizers. However, there is a notable dearth of research conducted on complex chemical systems, impeding a comprehensive understanding of sulfate formation in photosensitization. This work studied sulfate formation by mixing real biomass burning (BB) extracts and NaCl, mimicking internal mixtures of BB and sea-salt particles. Significant enhancement of sulfate formation was observed for BB-NaCl particles compared to incense burning (IS)-NaCl particles. For fresh particles, the sulfate formation rate followed the trend of corn straw (CS)-NaCl>rice straw (RS)-NaCl>wheat straw (WS)-NaCl>IS-NaCl. Aged particles were produced by irradiating the filters directly with UV lights. Aged particles showed changes in sulfate formation rates, with the highest enhancement by RS-NaCl due to interactions between RS and NaCl. Model experiments spiked with nitrogen-containing organic compounds (NOCs), such as pyrazine (CHN) and 4-nitrocatechol (CHON), revealed positive effects of chloride in the PS-CHON system and negative effects in the PS-CHN system. Our work suggests that BB reaching or near coastal areas could affect sulfate formation via photosensitizer-mediated reactions, potentially exacerbating air quality concerns.