Photooxidation of volatile organic compounds (VOC) produces secondary organic aerosol (SOA) and light–absorbing brown carbon (BrC). SOA formation proceeds via multiple pathways, reflecting significant chemical complexity relevant to gaseous oxidation and subsequent gas-to-particle conversion. Toluene is an important VOC under urban conditions, but its fundamental chemical mechanisms leading to SOA and BrC formation remain uncertain. Here we elucidate multi-generation SOA production from toluene by simultaneously tracking the evolutions of gas-phase oxidation and aerosol formation in a reaction chamber. Significant size increase and browning of monodisperse sub-micrometer particles occur shortly after initiating photooxidation at 10-90% relative humidity (RH). The temporally resolved evolution of gaseous products and aerosol properties (i.e., size, density, and optical properties) as well as chemical speciation of the particle-phase products indicate that aerosol growth and browning result from earlier generation products consisting of the di-carbonyl and carboxylic functional groups. Moreover, while volatile dicarbonyls engage in aqueous reactions to yield non-volatile oligomers and light-absorbing nitrogen-heterocycles/heterochains in the presence of NH<sub>3</sub> at high RH (i.e., above the efflorescent point), low-volatility organic acids contribute to aerosol carboxylates via partitioning or acid-base reaction in a wide RH range. We conclude that toluene contributes importantly to SOA and BrC formation via primarily dicarbonyls and organic acids resulting from their prompt and high yields from oxidation as well as their unique functionalities for participation in particle-phase reactions. Our findings explain atmospheric measurements of large SOA growth under polluted urban conditions and highlight the importance for understanding multi-generation production of condensable oxidized organics and their functionality-reactivity relationship.

Multi-generation production of secondary organic aerosol and brown carbon from toluene photooxidation

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.