A Measurement And Model Study On Ozone Characteristics In Marine Air At A Remote Island Station And Its Interaction With Urban Ozone Air Quality In Shanghai, China

To understand the characteristics and changes of baseline ozone (O-3) in oceanic air in eastern China, a 6-year measurement of O-3 concentration was conducted from 1 January 2012 to 15 September 2017 at a remote offshore station located on Sheshan Island (SSI) near the megacity of Shanghai. The observed monthly mean O-3 concentrations at SSI ranged from 33.4 to 61.4 ppbv during the study period, which were about 80 % and 12 % higher, respectively, than those measured at downtown and rural sites in Shanghai. Compared to the remarkable O-3 increases observed at urban and rural sites in Shanghai, observed O-3 concentrations at SSI exhibited statistically insignificant increasing changes (1.12 ppbv yr(-1), alpha >0.10) during the observation period, suggesting less impacts of anthropogenic emissions on O-3 levels in oceanic air. In addition, an insignificant decreasing change (-0.72 ppbv yr(-1), alpha>0.10) was detected in O-3 concentrations at SSI in September and October when the influence of regional transport was minimum throughout the year, providing a good proxy to study the baseline oxidation capacity of the oceanic atmosphere. City plumes from Shanghai usually carried higher levels of NOx, resulting in decreased O-3 concentrations at SSI during southwesterly and westerly winds. However, In MAM (March-May) and JJA (June-August), due to the enhanced production of oxygenated volatile organic compounds, O-3 could be continuously produced during daytime in aged city plumes, resulting in elevated O-3 concentrations transported to SSI. The impacts of the offshore O-3 on O-3 levels in Shanghai are quantified during an easterly wind dominant episode (1-30 September 2014) using the WRF-Chem model (Weather Research and Forecasting model coupled with Chemistry). Sensitivity results suggest that O-3 in the oceanic air inflows can lead to 20 %-30 % increases in urban O-3 concentrations, which should be crucially considered in dealing with urban O-3 pollution in large coastal cities like Shanghai.