Impact of Molecular Structure on Secondary Organic Aerosol Formation from Aliphatic Carbonyls

Aliphatic carbonyls widely exist in source emissions and the atmosphere, serving as secondary organic aerosol (SOA) precursors. Molecular structure including carbon number and functional groups influences SOA formation. However, limited research was conducted on the vastly different SOA yields among aliphatic carbonyls, and the impact of the molecular structure remains unclear and controversial. In this study, 11 aliphatic carbonyls were photooxidized to study the SOA formation and gas-phase products in an oxidation flow reactor (OFR). The maximum SOA yields of C₁₅ and C₁₃ straight-chain carbonyls are 0.41-0.52 and 0.18-0.23, lower but within a comparable range with long-chain alkanes, whereas those for C₈-C₁₀ carbonyls are less than 0.04. Aliphatic carbonyls with more carbon have significantly higher SOA yields, accompanied by additional multigenerational products. The SOA yield of <i>n</i>-aldehyde is moderately higher than that of <i>n</i>-alkan-2-one with the same carbon number, due to the enhanced formation of long-chain acids with lower volatility than dicarbonyls. The saturated cyclic structure increases the SOA yield by forming more multifunctional products with additional aldehyde groups upon ring-opening, whereas the branched structure easily fragmented and decreased the SOA yield. This study highlights that the aliphatic carbonyls with more carbon numbers and a cyclic structure with less branching may have higher SOA yields.