Mechanistic investigations of the formation of multifunctional products from the multi-generation ●OH oxidation of styrene

Styrene is a highly reactive aromatic hydrocarbon that has been identified as a key secondary organic aerosol (SOA) precursor. Recent laboratory chamber experiments have identified C 7 and C 8 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 multifunctional products from the multi-generation ⚫OH oxidation of styrene are studied using the quantum chemistry methods. The calculations show that the first generation RO 2 radicals can either proceed unimolecular decomposition to yield benzaldehyde (C 7 H 6 O), or undergo bimolecular reactions with HO 2 ⚫/NO to form the first generation closed-shell C 7 - and C 8 -products, hydroperoxide 1st-ROOH (C 8 H 10 O 3 ), benzaldehyde, and organic nitrate 1st-RONO 2 (C 8 H 9 NO 3 ). For the second generation ⚫OH oxidation, OH-addition reaction occurring at the ortho -site of 1st-ROOH and 1st-RONO 2 has a significant dominance. The ortho -OH-addition products can proceed through two O 2 -addition steps and a cyclization process to produce the peroxide bicyclic peroxy radicals (BPR). BPR can further react with HO 2 ⚫/NO to form the second generation closed-shell C 8 -products, hydroperoxide 2nd-ROOH (C 8 H 12 O 8 ), organic nitrate 2nd-RONO 2 (C 8 H 10 N 2 O 10 ), and other multifunctional products, in which the first two products have fractional yields of 41.4 % and 4.8 %, respectively. For the third generation ⚫OH oxidation, OH-addition occurring at the C=C double bond of 2nd-ROOH and 2nd-RONO 2 has the lowest barrier. The major third generation closed-shell C 8 -products are the multifunctional hydroperoxides and organic nitrates. These findings carry important implications for advancing our understanding of the chemical composition and formation mechanisms of aromatic SOA.