Enhanced late spring ozone in southern China by early onset of the South China Sea summer monsoon

The onset of the South China Sea summer monsoon (SCSSM) profoundly impacts 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 the 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 µg 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 µg 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 the 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.