Products and Mechanism of Secondary Organic Aerosol Formation from Reactions ofn-Alkanes with OH Radicals in the Presence of NOx

Abstract

Secondary organic aerosol (SOA) formation from reactions of n-alkanes with OH radicals in the presence of NO_x was investigated in an environmental chamber using a thermal desorption particle beam mass spectrometer for particle analysis. SOA consisted of both first- and higher-generation products, all of which were nitrates. Major first-generation products were δ-hydroxynitrates, while higher-generation 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 first-generation products, as well as other factors. For example, SOA formed from the C₁₀ reaction contained no first-generation 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.