Styrene has been identified as the second most efficient aromatic compound in the formation of secondary organic aerosol (SOA) after toluene, which is primarily emitted from the anthropogenic activities such as solvent usage and vehicle exhaust.

Recently, a research group from the Institute of Earth Environment of the Chinese Academy of Sciences (IEECAS) investigated the formation mechanisms of multifunctional productsfrom the multi-generation ·OH oxidation of styrene under different NO_x conditions by using quantum chemical methods.

They found that for the first generation ·OH oxidation of styrene, the addition of ·OH radicals to terminal carbon (C_β-site) of a vinyl group is the dominant pathway. The formation of the first generation closed-shell products mainly include 1^st-ROOH (C₈H₁₀O₃), benzaldehyde (C₇H₆O), and 1^st-RONO₂ (C₈H₉NO₃) (Fig. 1). For the second generation ·OH oxidation, OH-addition reaction occurring at the ortho-site of 1^st-ROOH and 1^st-RONO₂ has a significant dominance. For the third generation ·OH oxidation, the addition of OH radicals to the C=C bond in 2^nd-ROOH and 2^nd-RONO₂ is the dominant pathway. The fractional yields of multifunctional products formed from the reactions 2^nd-ROOH + ·OH and 2^nd-RONO₂ + ·OH are 26.3% and 2.6%, respectively. The volatility of the oxidation products significantly decreases with increasing the number of ·OH oxidation steps in the multi-generation ·OH oxidation ofstyrene (Fig. 2). Ultimately, they are transformed into extremely low volatility organic compounds, participating in the formation and growth of new aerosol particles.

This study deepens the traditional understanding of the atmospheric chemical reaction mechanism of aromatic hydrocarbons and is of great significance for improving the accuracy of model predictions and accurately assessing the contribution of anthropogenic source emissions to SOA formation.

This work, published in Atmospheric Chemistry and Physics, was supported by the National Natural Science Foundation of China.

Fig. 1 The potential energy surface for the first-stage oxidation of styrene initiated by OH radicals and the isomerization reactions at the M06-2X/6-311++G(3df,3pd)//M06-2X/6-31+g(d,p) level (Image by CHEN, Long)

Fig. 2 Molecular structures and volatility class of multi-generation oxidation products of styrene (Image by CHEN, Long)