Products and mechanism of secondary organic aerosol formation from reactions of n-alkanes with OH radicals in the presence of NOx

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 higher-generation products,all of which were nitrates, Major first-generation products were 6-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 delta-hydroxycarbonyis 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 similar to 0.5% for C-8 to similar to 53% for C-15, with a sharp increase from similar to 8% for C-11 to similar to 50% for C-13. 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 C-10 reaction contained no first-generation products, while for the C15 reaction SOA was similar to 40% first-generation and similar to 60% higher-generation products, respectively. First-generation delta-hydroxycarbonyls are especially important in SOA formation, since their subsequent reactions can rapidly form low volatility compounds. In the atmosphere, substituted dihydrofurans created from 6-hydroxycarbonyls will primarily react with O-3 or NO3 radicals, thereby opening reaction pathways not normally accessible to saturated compounds.