Mechanistic investigations of the formation of highly oxidized products from the multi-generation OH oxidation of styrene

Abstract. Styrene is the second most efficient aromatic compound in the formation of secondary organic aerosol (SOA) after toluene. Recent experiments have identified C₇ and C₈ series compounds as the main components of SOA in the photooxidation of styrene. However, their molecular structures and formation pathways remain largely uncharacterized. Herein, the formation mechanisms of highly oxidized products from the multi-generation ·OH oxidation of styrene are studied using the quantum chemistry methods. The calculations show that the multi-generation ·OH oxidation mechanisms of styrene are modulated by RO₂ and RO radicals. For the first generation ·OH oxidation, the addition of OH radicals to the C_β-site of vinyl group is the dominant pathway, and the main C₇- and C₈-products are benzaldehyde, 1^st-ROOH (C₈H₁₀O₃) and 1^st-RONO₂ (C₈H₉NO₃). For the second generation ·OH oxidation, OH-addition reaction occurring at the ortho-position of 1^st-ROOH and 1^st-RONO₂ has a significant dominance. The peroxide bicyclic peroxy radicals (BPR) can react with HO₂·and NO to form the C₈-products 2^nd-ROOH (C₈H₁₂O₈) and 2nd-RONO2 (C₈H₁₀N₂O₁₀). The formed peroxide bicyclic alkoxy radical (BAR) can proceed either the ring-opening reactions to form the multifunctionalized dicarbonyls and ketene-enols, or the cyclization reactions to generate the highly oxidized C₆-epoxides with the branching ratios of ~30 %. For the third generation ·OH oxidation, syn-OH-addition occurring at the C=C double bond of 2^nd-ROOH and 2^nd-RONO₂ has the smallest barrier. The major C₈-products are the multifunctionalized hydroperoxides and organic nitrates. The volatility of the multi-generation ·OH oxidation products significantly decreases with increasing the number of ·OH oxidation steps.