The onset of the South China Sea summer monsoon (SCSSM) has profound impacts on meteorological conditions over East Asia. However, whether the interannual variability in monsoon onset date impacts ozone (O3) pollution remains unclear. Here, we investigate the relationship between early onset of SCSSM and late spring O3 in southern China. Our results show notable differences in surface O3 concentrations before and after SCSSM onset during early onset events in southern China. The enhanced O3 of 11.1 mu g m-3 is supported by increased air temperature and solar radiation of 1.1 K and 30.9 W m-2 and reduced relative humidity of -5.7%. Both observation and model simulations confirm that O3-favorable meteorological conditions modulated by early SCSSM onset can be found in May. It increases the boundary layer height and biogenic emissions of volatile organic compounds, enhancing O3 by 10 mu g m-3 over southern China. Chemical processes dominate such increases in O3 with enhanced chemical production of 0.27 Tg month-1. Descending motion in southern China vertically transports O3 to surface by 0.10 Tg month-1, whereas horizontal advection reduces O3 concentration by 0.12 Tg month-1. The meteorological responses to colder sea surface temperature (SST) in the central equatorial Pacific are pronounced, leading to higher O3 concentrations over the Yangtze River Delta, while warmer SST in the Philippine Sea contributes O3 over the Pearl River Delta and eastern China. This study highlights the importance of SCSSM onset with respect to O3 in southern China, with promising applications in management of air pollution and agriculture. The onset of the South China Sea summer monsoon (SCSSM), a spectacular feature of the Asian summer monsoon, has significant impacts on meteorological conditions over East Asia. It remains unclear whether the interannual variability in monsoon onset date affects ozone (O3) pollution. In this study, we demonstrated that the early onset of SCSSM can create warmer and drier conditions with enhanced solar radiation in May, which increases boundary layer height, boosts biogenic emissions of volatile organic compounds and worsens O3 pollution in southern China. By integrating reconstructed O3 data set and meteorological reanalysis together with model simulations, we found that increased O3 concentrations of approximately 10 mu g m-3 over southern China were dominated by enhanced chemical production. Both sea surface temperature anomalies in the central equatorial Pacific and Philippine Sea contributed to the O3-favorable meteorological conditions modulated by early SCSSM onset. Our study emphasizes the importance of considering the SCSSM onset in understanding O3 pollution in southern China, with promising applications in air pollution management and agriculture. Early onset of the South China Sea summer monsoon can create warmer and drier conditions with enhanced solar radiation in May Enhanced chemical production dominates the worsening late spring ozone pollution in southern China during early onset events Both sea surface temperature anomalies in the central equatorial Pacific and Philippine Sea contribute to higher ozone in southern China
