Environ. Sci. Technol. 2005, 39, 9229-9236
Products and Mechanism of Secondary Organic Aerosol Formation from Reactions of n-Alkanes with OH Radicals in the Presence of NOx Y O N G B I N L I M † A N D P A U L J . Z I E M A N N * ,‡ Air Pollution Research Center, University of California, Riverside, California 92521
Secondary organic aerosol (SOA) formation from reactions of n-alkanes with OH radicals in the presence of NOx was investigated in an environmental chamber using a thermal desorption particle beam mass spectrometer for particle analysis. SOA consisted of both first- and highergeneration products, all of which were nitrates. Major firstgeneration products were δ-hydroxynitrates, while highergeneration products consisted of dinitrates, hydroxydinitrates, and substituted tetrahydrofurans containing nitrooxy, hydroxyl, and carbonyl groups. The substituted tetrahydrofurans are formed by a series of reactions in which δ-hydroxycarbonyls isomerize to cyclic hemiacetals, which then dehydrate to form substituted dihydrofurans (unsaturated compounds) that quickly react with OH radicals to form lower volatility products. SOA yields ranged from ∼0.5% for C8 to ∼53% for C15, with a sharp increase from ∼8% for C11 to ∼50% for C13. This was probably due to an increase in the contribution of firstgeneration products, as well as other factors. For example, SOA formed from the C10 reaction contained no firstgeneration products, while for the C15 reaction SOA was ∼40% first-generation and ∼60% higher-generation products, respectively. First-generation δ-hydroxycarbonyls are especially important in SOA formation, since their subsequent reactions can rapidly form low volatility compounds. In the atmosphere, substituted dihydrofurans created from δ-hydroxycarbonyls will primarily react with O3 or NO3 radicals, thereby opening reaction pathways not normally accessible to saturated compounds.
Introduction Secondary organic aerosol (SOA) is formed from the atmospheric oxidation of volatile organic compounds (VOCs), including alkanes, alkenes, and aromatic hydrocarbons, by OH and NO3 radicals and O3 (1, 2). In urban areas, VOCs are typically ∼40-50% alkanes, ∼20-30% aromatic hydrocarbons, and ∼5-10% alkenes, with the remainder being carbonyl compounds and unidentified species (3, 4). Alkanes and aromatic hydrocarbons in urban and regional areas arise largely from anthropogenic emissions, including vehicle * Corresponding author phone: (951) 827-5127; fax: (951) 8275004; e-mail:
[email protected]. † Also in the Department of Chemistry. ‡ Also in the Department of Environmental Sciences, Department of Chemistry, and Environmental Toxicology Graduate Program. 10.1021/es051447g CCC: $30.25 Published on Web 11/02/2005
2005 American Chemical Society
emissions (5-7), while alkenes are emitted from anthropogenic and biogenic sources (8, 9). In recent years, many laboratory studies have measured SOA products (10-15) and yields (16-18), and it is now recognized that atmospheric reactions of alkenes and aromatic hydrocarbons lead to significant amounts of SOA containing multifunctional compounds, including oligomeric species. The VOCs chosen for this study are alkanes, a class of compounds whose atmospheric chemistry has been largely neglected, both with regards to gas-phase photooxidation and SOA formation. This is although alkanes comprise the largest class of VOC emissions from anthropogenic sources, and modeling studies have indicated that alkane photooxidation may be a significant contributor to SOA formation in urban areas (19-21). In the troposphere, alkanes react predominantly with OH radicals (22). Because alkanes are less reactive than alkenes and aromatics, their conversion to SOA should occur more slowly, with the result being that SOA forms mainly downwind of urban centers and therefore impacts regional and global aerosol loading. In addition to being an atmospherically important class of compounds, alkanes are an ideal system for exploring basic gas-phase reaction mechanisms (i.e., hydrogen abstraction, peroxy radical reactions, and decomposition and isomerization of alkoxy radicals) involved in the formation of condensable compounds from atmospheric oxidation of hydrocarbons.
Experimental Section Environmental Chamber Technique. In these experiments, C8-C15 n-alkanes were reacted with OH radicals in the presence of NOx in an ∼6600 L PTFE environmental chamber at ∼25 °C and ∼97 kPa. The chamber was filled with clean, dry air (
