Land-Sea Interactions Lead to Contrasting Fates of Aliphatic and Aromatic Intermediate Volatile Organic Compounds

Intermediate volatile organic compounds (IVOCs) contribute significantly to secondary organic aerosol (SOA) in the coastal atmosphere, yet their transformation processes under land-sea interactions remain elusive. Here, the aqueous transformations of three representative IVOCs, namely, aliphatic cis-pinonic acid (CPA), aromatic eugenol, and dihydroeugenol, were systematically investigated. Terrestrial anthropogenic nitrates and marine natural halide ions were incorporated with varying halide-to-nitrate ratios and pH to simulate land-sea interactions. Results showed that at pH 5.5, elevated halide-to-nitrate ratios suppressed the degradation of all IVOCs by scavenging hydroxyl radicals formed from nitrate photolysis. However, at lower pH (2.6), the suppressive effect was mitigated and even overridden for aromatic IVOCs due to the enhanced conversion of nonselective hydroxyl radicals to reactive halogen species (RHS). Selective RHS preferentially oxidized electron-rich aromatic IVOCs via single-electron transfer reactions, forming phenoxy radicals that easily coupled to less oxidized oligomers. In contrast, RHS reacted slowly with electron-deficient CPA while inducing some secondary reactions. Enhanced oligomer formation reduced product volatility, and combined with the suppression of strongly absorbing nitrogen-containing products attenuated nitrate-induced light absorption increase of aromatic IVOCs. These results reveal contrasting fates of aliphatic and aromatic IVOCs under land-sea interactions associated with varying halide-to-nitrate ratios. Such processes should be considered when evaluating SOA formation and impacts in coastal regions.