Modeling SOA formation from OH reactions with C8–C17n-alkanes

Recent laboratory studies show that δ-hydroxycarbonyls formed via OH-initiated reactions with alkanes cyclize and then dehydrate to form substituted dihydrofurans. These dihydrofurans are highly reactive, with estimated lifetimes in the atmosphere of 1.3 h (OH), 24 s (NO3), and 7 min (O3). These studies also show that secondary organic aerosol (SOA) yields from alkanes increase with carbon number from 4% for C8 to 44% for C13 to almost 90% for C17. The reaction mechanism proposed for these observations has been incorporated explicitly into the Caltech Atmospheric Chemistry Mechanism (CACM) to investigate the factors controlling the yield curve over the homologous series of C8–C17n-alkanes. It was found that the hypothesized chemical reaction sequence was incomplete. Results from simulations indicate as yet unknown chemistry involving the carbonylester products may explain the discrepancies between observed and simulated SOA yields. Using the carbonylesters (which do not contribute directly to SOA) as proxies for their SOA-forming products, the SOA yield curve was reproduced. Prior versions of CACM did not include SOA formation from medium-chain alkanes. Laboratory data show SOA yields from these compounds range from 4% to 35% (C8–C12). The majority of SOA for these alkanes derives from second- and third-generation compounds (99–88% over the C8–C12 interval) that had not been represented before. The long-chain alkanes in CACM previously were allowed to form aerosol, but only the first-generation products were represented. Here, the second- and third-generation products were found to constitute 78–69% of the SOA mass over the C13–C17 interval, indicating the importance of including this additional chemistry in simulations of SOA formation from n-alkanes.