Effects of photochemical aging on the molecular composition of organic aerosols derived from agricultural biomass burning in whole combustion process

Biomass burning organic aerosols (BBOA) are key components of atmospheric particulate matter, yet the effects of aging process on their chemical composition and related properties remain poorly understood. In this study, fresh smoke emissions from the combustion of three types of agricultural biomass residues (rice, maize, and wheat straws) were photochemically aged in an oxidation flow reactor. The changes in BBOA composition were characterized by offline analysis using ultrahigh performance liquid chromatography coupled with Orbitrap mass spectrometry. The BBOA molecular composition varied dramatically with biomass type and aging process. Fresh and aged BBOA were predominated by CHO and nitrogen-containing CHON, CHN, and CHONS species, while with very few CHOS and other non‑oxygen species. The signal peak area variations revealed that individual molecular species underwent dynamic changes, with 77–81 % of fresh species decreased or even disappeared and 33–46 % of aged species being newly formed. A notable increase was observed in the number and peak area of C_xH_yO_≥6 compounds in aged BBOA, suggesting that photochemical process served as an important source of highly oxygenated species. Heterocyclic C_xH_yN₂ compounds mostly dominated in fresh CHN species, whereas C_xH_yN₁ were more abundant in aged ones. Fragmentation and homologs oxidation by addition of oxygen-containing functional groups were important pathways for the BBOA aging. The changes in BBOA composition with aging would have large impacts on particle optical properties and toxicity. This study highlights the significance of photochemical aging process in altering chemical composition and related properties of BBOA.