Products and Mechanism of Secondary Organic Aerosol Formation from Reactions of Linear Alkenes with NO3Radicals

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Abstract
Secondary organic aerosol (SOA) formation from reactions of linear alkenes with NO3 radicals was investigated in an environmental chamber using a thermal desorption particle beam mass spectrometer for particle analysis. A general chemical mechanism was developed to explain the formation of the observed SOA products. The major first-generation SOA products were hydroxynitrates, carbonylnitrates, nitrooxy peroxynitrates, dihydroxynitrates, and dihydroxy peroxynitrates. The major second-generation SOA products were hydroxy and oxo dinitrooxytetrahydrofurans, which have not been observed previously. The latter compounds were formed by a series of reactions in which δ-hydroxycarbonyls isomerize to cyclic hemiacetals, which then dehydrate to form substituted dihydrofurans (unsaturated compounds) that rapidly react with NO3 radicals to form very low volatility products. For the ∼1 ppmv alkene concentrations used here, aerosol formed only for alkenes C7 or larger. SOA formed from C7−C9 alkenes consisted only of second-generation products, whereas for larger alkenes first-generation products were also present and contributions increased with increasing carbon number apparently due to the formation of lower volatility products. The estimated mass fractions of first- and second-generation products were approximately 50:50, 30:70, 10:90, and 0:100, for 1-tetradecene, 1-dodecene, 1-decene, and 1-octene SOA, respectively. This study shows that δ-hydroxycarbonyls play a key role in the formation of SOA in alkene−NO3 reactions and are likely to be important in other systems because δ-hydroxycarbonyls can also be formed from reactions of OH radicals and O3 with hydrocarbons.