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.

In a recent study published in Atmospheric Chemistry and Physics, researchers from the Institute of Earth Environment of the Chinese Academy of Sciences investigated the formation mechanisms of multifunctional productsfrom the multi-generation hydroxyl radical (·OH) oxidation of styrene under different NO_x conditions by using quantum chemical methods.

The researchers found that during the first generation of ·OH oxidation, the addition of ·OH radicals to terminal carbon (C_β-site) of a vinyl group is the dominant pathway. This process primarily produces the closed-shell compounds 1^st-ROOH (C₈H₁₀O₃), benzaldehyde (C₇H₆O), and 1^st-RONO₂ (C₈H₉NO₃).

As oxidation proceeds, the reaction pathways shift. In the second generation of ·OH oxidation, ·OH-addition reaction occurring at the ortho-site of 1^st-ROOH and 1^st-RONO₂ has a significant dominance. During the third generation, the addition of ·OH radicals to the carbon–carbon double bond in 2^nd-ROOH and 2^nd-RONO₂ dominates the reaction process. 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 researchers also found that the volatility of the oxidation products significantly decreases with increasing the number of ·OH oxidation steps in the multi-generation ·OH oxidation ofstyrene. Ultimately, these products 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 was supported by the National Natural Science Foundation of China.