Worsening ozone air pollution with reduced NO and VOCs in the Pearl River Delta region in autumn 2019: Implications for national control policy in China.

Ozone (O) pollution has emerged as a major air quality issue in China. Here we emphasize the great challenges in controlling O pollution by analyzing the recent experience of the Pearl River Delta (PRD) in southern China in reducing the autumn O peaks. Despite significant reductions in the concentration of O precursors, i.e., nitrogen oxides (NO) and volatile organic compounds (VOCs), regional O pollution in the PRD was largely worse in autumn 2019 than in autumn 2018. We found that the supra-regional and regional background concentrations of O increased significantly in the PRD in autumn 2019 due to increased concentrations of O in the vast surrounding areas. We also observed slight increases in the concentrations of PRD-regionally and Guangzhou-locally produced O. A chemical box-model analysis confirmed a slight increase in the in-situ production of O and revealed that increased biogenic VOCs (BVOCs) and decreased NO levels negated the effect of significant decrease in the anthropogenic VOCs. Taken together, these aspects exacerbated O pollution in the PRD region in autumn 2019 relative to autumn 2018. The findings from this study highlight the strong interactions of O pollution over multiple regions and the need for collaborative inter-regional efforts to control O pollution. The experience of PRD also underlines the key role of BVOCs and the importance of science-based strategies to decrease VOCs and NO.