Abstract. China is experiencing increasingly serious ambient ozone pollution, including the economically developed Pearl River Delta (PRD) region. However, the underlying reasons for ozone increase remain largely unclear, leading to perplexity in formulating effective ozone control strategies. In this study, by developing a statistical analysis framework combining meteorological adjustment and source apportionment, we examine quantitatively the impacts of meteorology and precursor emissions from within and outside the PRD on the evolution of ozone during the past decade. We found that meteorological condition has mitigated ozone increase, and its variation can account for at most 15 % of annual ozone concentration in the PRD. Precursor emission from outside the PRD (non-local) makes the largest contribution to ambient ozone in the PRD and shows a consistently increasing trend, while that from within the PRD (local) shows a significant spatial heterogeneity and plays a more important role during ozone episodes over southwestern. Under general conditions, the impact on northeastern is positive but decreasing, and on southwestern is negative but increasing. During ozone episodes, the impact on northeastern is negative and decreasing, while on southwestern is positive but decreasing. Central and western PRD is the only area with increasing local ozone contribution. The spatial heterogeneity in both local ozone contribution and its trend under general conditions and ozone episodes are well interpreted by a conceptual model collectively taking into account ozone precursor emissions and their changing trends, ozone formation regimes, and the monsoonal and micro-scale synoptic conditions over different sub-regions of the PRD. In particular, we conclude that the inappropriate NOx / VOC control ratio within the PRD over the past years is most likely responsible for the ozone increase over southwestern, both under general conditions and during ozone episodes. By investigating the ozone evolution influenced by emission changes within and outside PRD during the past decade, this study highlights the importance of establishing a dichotomous ozone control strategy to tackle with general conditions and pollution events separately. NOx emission control should be further strengthened to alleviate peak ozone level during episodes. Detailed investigation is needed to retrieve appropriate NOx / VOC ratios for different emission and meteorological conditions, so as to maximize the ozone reduction efficiency in the PRD.
Quantitative impacts of meteorology and precursor emission changes on the long-term trend of ambient ozone over the Pearl River Delta, China and implications for ozone control strategy
