Photochemical Cloud Processing of Water-Soluble Organic Gases Produced by Aqueous-Phase Photooxidation of α-Pinene, d-Limonene, and Toluene

Ambient water-soluble organic gases (WSOGs) are photochemically processed in atmospheric waters, which contributes to aqueous secondary organic aerosol (aqSOA). Chemical characterization of this atmospheric pathway has not been undertaken for monoterpene or aromatic oxidation products. In this work, gas-phase α-pinene, d-limonene, and toluene were oxidized via O3- and •OH-initiated reactions. Resulting water-soluble oxidation products were collected using a mist chamber, in which WSOGs were scrubbed by aqueous droplets. WSOG samples were then subjected to aqueous-phase photooxidation to quantify aqSOA mass yields and characterize aqSOA chemical composition. AqSOA particle mass concentrations and O/C ratios increased with aqueous-phase photooxidation time for monoterpene WSOGs, while only marginal increases were observed for toluene WSOGs. Mass spectra for α-pinene WSOGs showed an abundance of small carboxylic acids (C1–C4) and monomeric (C7– C10) carbonyls formed during the aqueous-phase photooxidation. The α-pinene WSOGs exhibited a distinct molecular composition and underwent different photochemical processes when contrasted with water-soluble organic particles (WSOPs) from the gas-phase oxidation of α-pinene. These results underscore the need for chemical-level characterization of monoterpene-derived WSOGs to quantitatively evaluate their potential contribution to the global atmospheric aqSOA burden.