Chemistry of Secondary Organic Aerosol Formation from OH Radical-Initiated Reactions of Linear, Branched, and Cyclic Alkanes in the Presence of NOx

The effects of molecular structure on the products and mechanisms of SOA formation from OH radical-initiated reactions of linear, branched, and cyclic alkanes in the presence of NOx were investigated in a series of environmental chamber experiments. SOA mass spectra were obtained in real time and off line using a thermal desorption particle beam mass spectrometer and used to identify reaction products. Real-time mass spectra were used to classify products according to their temporal behavior, and off-line temperature-programmed thermal desorption analysis of collected SOA was used to separate products by volatility prior to mass spectral analysis and to gain information on compound vapor pressures. A reaction mechanism that includes gas- and particle-phase reactions was developed that explains the formation of SOA products and is consistent with the various lines of mass spectral information. Results indicate that the SOA products formed from the reactions of linear, branched, and cyclic alkanes are similar, but differ in a few important ways. Proposed first-generation SOA products include alkyl nitrates, 1,4-hydroxynitrates, 1,4-hydroxycarbonyls, and dihydroxycarbonyls. The 1,4-hydroxycarbonyls and dihydroxycarbonyls rapidly isomerize in the particle phase to cyclic hemiacetals that then dehydrate to volatile dihydrofurans. This conversion process is catalyzed by HNO3 formed in the chamber and is slowed by the presence of NH3. Volatile products can react further with OH radicals, forming multi-generation products containing various combinations of the same functional groups present in first-generation products. For linear and branched alkanes, the products are acyclic or monocyclic, whereas for cyclic alkanes they are acyclic, monocyclic, or bicyclic. Some of the products, especially those formed from ring-opening reactions of cyclic alkanes appear to be low volatility oligomers. The implications of the results for the formation of atmospheric SOA are discussed.