Exploring Surface-Enhanced Heterogeneous Oxidation of Isoprene: Evidence for Atmospheric Haze Chemistry

Abstract

Solid atmospheric particulates can act as heterogeneous drivers for gas loss and particle aging during haze episodes. Observational and experimental evidence reveals an unidentified competitive mechanism involving transition metal ions (TMIs) that catalyze the heterogeneous oxidation of isoprene. Hydroxyl radicals (OH) were generated through the reaction of singlet oxygen (O(¹D)) with molecular water at the surface of earth-abundant manganese (Mn) nanoparticles. The energy threshold for OH production was minimized to 213 kJ mol⁻¹ in the presence of alkali K⁺ ions, significantly lower than the 392 kJ mol⁻¹ required for ozone photolysis. The rapid loss of isoprene (1.60 × 10⁻² s⁻¹) for the particulate mixtures resulted in the formation of approximately 70% C₁–C₄ carbonyl oligomers via interfacial binding modes, which promoted particle growth. This contrasts with the higher yields of C₅ products typically observed in gas-phase reactions of isoprene with OH radicals. The findings could enhance the understanding of severe haze formation, particularly under complex air pollution conditions.