Current status of model predictions on volatile organic compounds and impacts on surface ozone predictions during summer in China

Abstract. Volatile organic compounds (VOCs) play a crucial role in the formation of tropospheric ozone (O₃) and secondary organic aerosols. VOC emissions are generally considered to have larger uncertainties compared to other pollutants, such as sulphur dioxide and fine particulate matter (PM_2.5). Although predictions of O₃ and PM_2.5 have been extensively evaluated in air quality modelling studies, there has been limited reporting on the evaluation of VOCs, mainly due to a lack of routine VOCs measurements at multiple sites. In this study, we utilized VOCs measurements from the ATMSYC project at 28 sites across China and assessed the predicted VOCs concentrations using the Community Multiscale Air Quality (CMAQ) model with the widely used Multi-resolution Emission Inventory for China (MEIC). The ratio of predicted to observed total VOCs was found to be 0.74 ± 0.40, with underpredictions ranging from 2.05 to 50.61 ppbv (5.77 % to 85.40 %) at 24 sites. A greater bias in VOCs predictions was observed in industrial cities in the north and southwest, such as Jinan, Shijiazhuang, Lanzhou, Chengdu, and Guiyang. In terms of different VOC components, alkanes, alkenes, non-naphthalene aromatics (ARO2MN), and alkynes were consistently underpredicted, with prediction to observation ratios of 0.53 ± 0.38, 0.51 ± 0.48, 0.31 ± 0.38, and 0.41 ± 0.47, respectively. Sensitivity experiments were conducted to assess the impact of the VOCs prediction bias on O₃ predictions. While emission adjustments improved the model performance for VOCs, resulting in a ratio of total VOCs to 0.86 ± 0.47, they also exacerbated O₃ overprediction relative to the base case by 0.62 % to 6.27 % across the sites. This study demonstrates that current modelling setups and emission inventories are likely to underpredict VOCs concentrations, and this underprediction of VOCs contributes to lower O₃ predictions in China.