Simulation of sea-land breezes and a discussion of their implications on the transport of air pollution during a multi-day ozone episode in the Pearl River Delta of China

Sea-land breezes (SLBs) play an important role in transporting air pollution from urban areas on the coast. In this study, a meso-scale model MM5 was used to simulate SLBs and to understand the transport of pollution during a prolonged ozone episode observed in Hong Kong in September 2001. With the designed settings, the model performed very well in the simulation of SLBs. The simulated surface winds and the planetary boundary layer (PBL) heights are presented to contrast the characteristics of dispersion and transport on pre-episode and episode days. The diurnal variations in horizontal and vertical winds on an episode day are then presented to illustrate the evolution of SLBs. The results indicate that on episode days the onset of sea breezes (SBs) was delayed to noontime due to offshore synoptic winds, while on pre-episode days the SBs had already penetrated deep inland by early afternoon. The simulation shows that SBs propagated in both onshore and offshore directions in the afternoon, leading to the formation of nocturnal regional-scale SBs. The maximum distance for the inland penetration of an SB front (SBF) was about 60–80 km, in contrast to 120–150 km for offshore propagation. With the aid of high-resolution trajectories, the main meteorological factors contributing to the occurrence of the observed ozone episode are discussed. It is believed that the offshore synoptic wind, the delayed SBs, as well as the low mixing height contributed to the daytime transport of pollution and high ozone on the coast. The trajectory analysis also indicates important contributions from regional sources of emission.