Using observational and modeling studies, the importance of the mechanism driving regional O₃ transport in the residual layer (RL) with respect to summer smog over the Yangtze River Delta region in eastern China was revealed. This mechanism was also examined in association with diurnal change in the atmospheric boundary layer. Regional O₃ transport through the nocturnal RL is believed to have great implications for understanding urban and regional O₃ pollution in this area.