Formation kinetics and mechanisms of ozone and secondary organic aerosols from photochemical oxidation of different aromatic hydrocarbons: dependence on NO _x and organic substituents

Aromatic hydrocarbons (AHs) contribute significantly to ozone and secondary organic aerosol (SOA) formation in the atmosphere, but their formation mechanisms are still unclear. Herein, the photochemical oxidation of nine AHs was investigated in a chamber. Only a small amount of ozone was produced from the direct photochemical oxidation of AHs, while a lower number of AH substituents resulted in higher concentrated ozone. Addition of NOinline-formula_x increased ozone and SOA production. The synergetic effect of accelerated NOinline-formula₂ conversion and NO reaction with AHs boosted ozone and volatile intermediate formation. Promoting AH concentration in the VOC inline-formula $M4inlinescrollmathml/$ 8pt14ptsvg-formulamathimg57ee8123d9c9aefcf23d9c7f6463c158 acp-21-7567-2021-ie00001.svg8pt14ptacp-21-7567-2021-ie00001.png  NOinline-formula_x ratio further increased formation rates and concentrations of both ozone and SOA. Additionally, ozone formation was enhanced with increasing AH substituent number but negligibly affected by their substituent position. Differently, SOA yield decreased with an increased substituent number of AHs but increased with ortho-methyl-group-substituted AHs. Model fitting and intermediates consistently confirmed that increasing the substituent number on the phenyl ring inhibited generation of dicarbonyl intermediates, which however were preferentially produced from oxidation of ortho-methyl-group-substituted AHs, resulting in different changing trends of the SOA yield. The restrained oligomerization by increased substituent number was another main cause for decreased SOA yield. These results are helpful to understand the photochemical transformation of AHs to secondary pollutants in the real atmosphere.