Ozone pollution around a coastal region of South China Sea: interaction between marine and continental air

Abstract. Marine atmosphere is usually considered to be a clean environment, but this study indicates that the near-coast waters of the South China Sea (SCS) suffer from even worse air quality than coastal cities. The analyses were based on concurrent field measurements of target air pollutants and meteorological parameters conducted at a suburban site (Tung Chung, TC) and a nearby marine site (Wan Shan, WS) from August to November 2013. The observations showed that the levels of primary air pollutants were significantly lower at WS than those at TC, while the ozone (O₃) value was greater at WS. Higher O₃ levels at WS were attributed to the weaker NO titration and higher O₃ production rate because of stronger oxidative capacity of the atmosphere. However, O₃ episodes were concurrently observed at both sites under certain meteorological conditions, such as tropical cyclones, continental anticyclones and sea–land breezes (SLBs). Driven by these synoptic systems and mesoscale recirculations, the interaction between continental and marine air masses profoundly changed the atmospheric composition and subsequently influenced the formation and redistribution of O₃ in the coastal areas. When continental air intruded into marine atmosphere, the O₃ pollution was magnified over the SCS, and the elevated O₃ ( >  100 ppbv) could overspread the sea boundary layer  ∼  8 times the area of Hong Kong. In some cases, the exaggerated O₃ pollution over the SCS was recirculated to the coastal inshore by sea breeze, leading to aggravated O₃ pollution in coastal cities. The findings are applicable to similar mesoscale environments around the world where the maritime atmosphere is potentially influenced by severe continental air pollution.