Prof Jacqui Hamilton (Rickard) (@jacquirickard.bsky.social)

Comparison of Isoprene-Derived Secondary Organic Aerosol Formation Pathways at an Urban and a Forest Site In isoprene oxidation, epoxides are crucial for forming isoprene-derived secondary organic aerosol (iSOA), significantly impacting the global atmospheric aerosol burden and composition. Isoprene dihydroxyepoxydiols (IEPOX) and hydroxymethyl–methyl-α-lactone (HMML) are key reactive gas-phase intermediates under low- and high-NO conditions, respectively. IEPOX and/or HMML can undergo nucleophilic reactions with both sulfate and water, producing isoprene-derived organosulfates (iOSs) and oxygen-containing tracers (iOTs). In this study, high temporal observations of iSOA species indicate that although OS formation in the urban and forest areas exhibited similar diurnal variations, the average concentrations of biogenic OSs decreased significantly from the urban site to the forest site. Additionally, at the urban site, the nucleophilic addition reaction is more likely to occur with sulfate (iOSs/iOTs = 1.51), while at the forest site, water serves as the dominant reactant (iOSs/iOTs = 0.81). We have also estimated the branching ratios of nucleophilic reaction pathways with sulfate and water at the different sites. At the Guangzhou urban site, the branching ratio of IEPOX with sulfate:water has a mean of 42.4%:57.6%, whereas for HMML, it is 64.3%:35.7%. At the forest site, the branching ratio of IEPOX with sulfate:water is roughly equal at 48.3%:51.7%, while for HMML, the preference is for reaction with water with a ratio of 33.7%:66.3%. Furthermore, the concentrations of iOSs were significantly correlated with the product of [O3]·[SO42–]. This correlation was stronger than the individual correlations of major iOSs with [O3] and [SO42–]. Thus, higher levels of OS were attributed to enhanced photochemical processing and increased levels of sulfate. This research offers valuable insights into the impacts of biogenic–anthropogenic interactions and the factors controlling the formation of iSOA in polluted areas via nucleophilic addition reactions. https://pubs.acs.org/doi/full/10.1021/acsearthspacechem.4c00398