China has made significant progress decreasing the ambient concentrations of most air pollutants, but ozone (O3) is an exception. O3 mixing ratios during pollution episodes are far higher than the national standard in the Pearl River Delta (PRD), thus greater evidence-based control efforts are needed for O3 attainment. By using a validated O3 modeling system and the latest regional emission inventory, this study illustrates that control strategies for short-term O3 despiking and long-term attainment in the PRD may be contradictory. VOC-focused controls are more efficient for O3 despiking in urban and industrial areas, but significant NOx emission reductions and a subsequent transition to a NOx-limited regime are required for O3 attainment. By tracking O3 changes along the entire path toward long-term attainment, this study recommends to put a greater focus on NOx emission controls region-wide. Parallel VOC reductions around the Nansha port are necessary in summertime and should be extended to the urban and industrial areas in fall with a flexibility to be strengthened on days forecasted to have elevated O3. Contingent VOC-focused controls on top of regular NOx-focused controls would lay the groundwork for striking a balance between short-term despiking and long-term attainment of O3 concentrations in the PRD.

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Abstract. Marine atmosphere is usually considered to be a clean environment, but this study indicates that the nearcoast 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 (O 3 ) value was greater at WS. Higher O 3 levels at WS were attributed to the weaker NO titration and higher O 3 production rate because of stronger oxidative capacity of the atmosphere. However, O 3 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 O 3 in the coastal areas. When continental air intruded into marine atmosphere, the O 3 pollution was magnified over the SCS, and the elevated O 3 (> 100 ppbv) could overspread the sea boundary layer ∼ 8 times the area of Hong Kong. In some cases, the exaggerated O 3 pollution over the SCS was recirculated to the coastal inshore by sea breeze, leading to aggravated O 3 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.

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