Assessing the Uncertainties in Ozone and SOA Predictions due to Different Branching Ratios of the Cresol Pathway in the Toluene-OH Oxidation Mechanism

Oxidation of toluene by OH radicals plays a significant role in forming ozone (O₃) and secondary organic aerosol (SOA) in polluted urban atmospheres. However, the branching ratio of the cresol formation pathway after OH addition to the aromatic ring remains uncertain, affecting model predictions of O₃ and SOA. In this study, SOA formation under low (18%) and high (48%) cresol branching ratio conditions are determined by modeling chamber experiments on toluene SOA formation, using a photochemical box model with the semiexplicit master chemical mechanism (MCM) v3.2 and an SOA module for the equilibrium gas-to-particle partitioning of semivolatile products. The modeled SOA concentrations are fitted to determine the SOA yields and saturation concentrations using the classical two-product representation. These parameters are then applied in the community multiscale air quality (CMAQ) model to assess the impact of the cresol branching ratio on SOA formation. The reaction products of ARO1 (the lump species that includes mostly toluene) with OH are also modified to reflect the higher cresol branching ratio. Two sets of CMAQ simulations for China (C0, with low SOA yields and unmodified ARO1 + OH reaction, and C1, with high SOA yields and modified ARO1 + OH reaction) are conducted for January and July 2013. Predicted monoaromatic compound concentrations in major urban areas are ∼4–7 ppb in January and ∼1.5–3 ppb in July, which generally agree with measurements. The higher cresol branching ratio simulations lead to slightly lower OH radicals and O₃ predictions. Less than 1 ppb decrease of monthly average daily maximum 8 h and peak hour O₃ is found in the urban areas in July and with broader spatial coverage in January. The increase in January ARO1 SOA is approximately 1.2 μg m^–3, corresponding to a relative increase of 40–70% to ARO1 SOA or an ∼10% increase of total SOA. This change reflects the combined effects of increasing ARO1 SOA due to higher yields and reduced formation of semivolatile organic products and glyoxal and methylglyoxal due to lower OH radicals.