Oxidation Products of Semi-volatile Alkanes by Hydroxyl Radicals

Alkanes are ubiquitous in the atmosphere and are important components that influence atmospheric chemistry. Semi-volatile alkanes are partitioned between the gas- and the particle-phases and can be readily oxidized in both phases. Previous studies have demonstrated that reaction rates and the products of OH oxidation are very different for organic compounds in the gas- and particle phases. In the present study, n-octadecane (C18H38), n-eicosane (C20H42), n-docosane (C22H46), n-tricosane (C24H50), and n-pentadecylcyclohexane (C21H42) were chosen as model compounds for semi-volatile alkanes to examine their OH-initiated oxidation reactions in a flow tube reactor. OH exposure was varied in the experiments, equivalent to oxidation of up to one week in the atmosphere. Oxidation products were collected on filters and analyzed using two-dimensional gas chromatography coupled to a high-resolution time-of-flight electron impact ionization and vacuum ultraviolet photoionization mass spectrometer. Most of the oxygenated higher molecular weight isomers were separated and quantified. Our results suggest that aerosol samples formed in the n-octadecane experiment were more oxidized than the other model compounds (i.e., functionalization products with three oxygen atoms per molecule compared to two oxygen atoms per molecule) at similar OH exposures and aerosol mass loadings. This is likely due to the concentration of n-octadecane in the gas phase where oxidation is more rapid. We find that the first-generation gas-phase oxidation products quickly partition to the particle phase after which higher-generation oxidation likely occurs in the particle phase. Interestingly, functionalized carbonyl isomers for the normal alkanes were only observed on the 4 carbon positions closest to the molecule end in all cases, which is in contrast to structure-reactivity relationship (SRR) predictions for gas-phase reactions. For n-octadecane, the concentrations of first-generation functionalization products decreased on carbon positions closer to the molecule end. This trend is reversed for the other three normal alkanes, which show similar results to a recent study on heterogeneous oxidation of non-volatile alkane aerosols. Oxidation of n-pentadecylcyclohexane produces carbonyl and alcohol products on both the hexane ring and the side chain. Ring-opening products were not observed, likely due to the low ring strain energy of the hexane ring. This work provides insights into the oxidation mechanisms of semi-volatile organics leading to secondary aerosol formation and aging.