Aqueous‐Phase Process Accelerates OH Oxidation of Biomass‐Burning PAHs and Generates New Multifunctional Products

Abstract

Biomass burning (BB) emits large amounts of polycyclic aromatic hydrocarbons (PAHs) and is an important source of secondary organic aerosol (SOA). Recent studies suggest that, compared with gas‐phase oxidation, the oxidation of BB‐emitted organics in atmospheric aqueous phases can substantially enhance the formation of aqueous SOA (aqSOA). However, the aqueous‐phase transformation and environmental fate of PAHs from BB remain poorly understood. This study reports the OH‐initiated aqueous‐phase oxidation kinetics and reaction mechanisms of three representative biomass‐burning PAHs: naphthalene (NAP), acenaphthene (ACE), and acenaphthylene (ACY). The second‐order aqueous‐phase rate constants for reactions with OH were measured to be (1.63 ± 0.55) × 10 ¹⁰ , (3.37 ± 1.15) × 10 ¹⁰ , and (2.07 ± 0.70) × 10 ¹⁰  M ⁻¹  s ⁻¹ for NAP, ACE, and ACY, respectively. The estimated aqueous‐phase lifetimes were generally shorter than the corresponding gas‐phase lifetimes under the representative OH conditions considered here, highlighting the potential importance of atmospheric aqueous environments in promoting the aging and transformation of these PAHs. Product analysis and pathway inference showed that NAP and ACE preferentially formed lower‐carbon, more highly oxidized products, whereas ACY more readily produced oxygenated products that retained larger carbon skeletons. Furthermore, this study identified six previously unreported aqueous‐phase oxidation products, reflecting ongoing oxidation and structure‐dependent patterns of product evolution. Field observations showed that five newly identified products were also detected in ambient PM _2.5 samples, supporting the atmospheric relevance of the laboratory‐identified products. These results improve our understanding of the aqueous‐phase transformation of PAHs and their potential contribution to aqSOA formation.