Ozone (O3) pollution is increasing in China and the underlying reason for this is unknown, making effective control unrealistic. Using an innovative approach, we quantitatively identified the impact of meteorology and precursor emission changes, both local and nonlocal, on the long-term O3 trend in the PRD. Meteorology can contribute to up to 15 % of long-term O3 variations. The undesirable NOx/VOC control ratio over the past few years is most likely responsible for the O3 increase in the PRD.