Secondary Organic Aerosol Formation From Radical-Initiated Reactions of Alkenes: Development of Mechanisms
The products and mechanism of secondary organic aerosol (SOA) formations from 1-alkenes, internal alkenes, and 2-methyl-1-alkenes (OH radical reaction only) with OH and NO3 radicals (in the presence of NOx for OH radical reactions) were investigated in an environmental chamber, and quantitative chemical mechanisms with gas-partiticle partitioning corrections were developed for some systems to estimate SOA yields. Aerosol products chemical compositions were analyzed using a thermal desorption particle beam mass spectrometer, and volatile compounds were quantified using a gas chromatograph with flame-ionization detector. Nitrate products were analyzed using a high-performance liuid chromatograph with UV-vis detector for yield measurements and 1H NMR for identification. The major products observed in OH radical-initiated reactions with linear alkenes were β-hydroxynitrates, dihydroxynitrates, cyclic hemiacetals, dihydrofurans, and dimers formed from dihydroxycarbonyls. Trihydroxynitrates and trihydroxycarbonyls were observed for 2-methyl-1-alkene reactions in addition to the products listd above, but dimers were not observed due to reduction of driving force of the formation. The yields of β-hydroxynitrates, dihydroxynitrates, and trihydroxynitrates were measured to calculate the relative ratios for forming primary, secondary, and tertiary β-hydroxyalkyl radicals by OH radical addition to the double bond, calculated as 1.0:1.9:4.3, and the branching ratios for forming β-hydroxynitrates from reactions of primary, secondary, and tertiary β-hydroxyperoxy radicals with NO, calculated as 0.12, 0.15, and 0.25. The branching ratios of β-hydroxynitrate formations were lower than that of alkylnitrate formations due to hydrogen bonding between hydroxy and peroxy groups in hydroxyperoxy radical-NO complexes. The effect is enhanced with the addition of NH3 for 1-alkene reactions. SOA yields estimated with a developed model agreed well with the measured SOA yields for 2-methyl-1-alkene reactions, but the agreement was limited for 1-alkenes to the region where gas-particle partitioning does not affect due to difficulty in vapor pressure estimation of the products. The major products observed in NO3 radical-initiated reactions with linear alkenes include β-hydroxynitrates, carbonylnitrates, dihydroxynitrates, and hydroxyl and oxo dinitrooxytetrahydrofurans which have not been observed previously. The products through the pathway of isomerizing δ-hydroxycarbonyls to cyclic hemiacetals were observed in all linear alkene and 2-methyl-1-alkene reactions, and second-generation products were observed in NO3 radical-initiated reactions.