Abstract
Abstract. 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 NOx increased ozone and SOA production. The synergetic effect of accelerated NO2 conversion and NO reaction with AHs boosted ozone and volatile intermediate formation. Promoting AH concentration in the VOC / NOx 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.
Highlights
As an abundant group of volatile organic compounds (VOCs), aromatic hydrocarbons (AHs) are important precursors of ozone (O3) and secondary organic aerosols (SOAs) in the atmospheric environment (Peng et al, 2017; Tong et al, 2020), directly or indirectly threatening air quality and public health (Henze et al, 2008; Lane et al, 2008; Yang et al, 2016)
Previous studies indicated that the rate constants of AHs with qOH were found to follow the order of toluene (5.61 ± 0.08) × 10−12 cm3 molecule−1 s−1 < xylene
All these results demonstrated that the increase of substituent number on the phenyl ring inhibited the generation of lowvolatility products, reducing the generation of SOA particles and leading to the decrease of SOA yield
Summary
As an abundant group of volatile organic compounds (VOCs), aromatic hydrocarbons (AHs) are important precursors of ozone (O3) and secondary organic aerosols (SOAs) in the atmospheric environment (Peng et al, 2017; Tong et al, 2020), directly or indirectly threatening air quality and public health (Henze et al, 2008; Lane et al, 2008; Yang et al, 2016). The important role of substituent position has been observed in the OH-initiated alkane and alkene oxidation (Atkinson, 2007; Ziemann, 2011) All these previous studies inspire us that the influence of substituents, including the number and position, on the photochemical transformation of AHs to O3 and SOA cannot be ignored. Both O3 and SOA generated from different AHs have been studied in laboratory smog chamber simulations, the role of AH substituents in the formation kinetics and mechanisms of O3 and SOA, as well as their relationship with the oxidation intermediates, has not been systematically investigated and established. The results of this work will further elucidate the photochemical behavior of AHs in the atmosphere and provide reliable experimental data for modeling and prediction in the future
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