Abstract. To understand the characteristics and changes of baseline ozone (O3) in
oceanic air in eastern China, a 6-year measurement of O3 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 O3 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 O3 increases observed at urban and
rural sites in Shanghai, observed O3 concentrations at SSI exhibited
statistically insignificant increasing changes (1.12 ppbv yr−1, α>0.10) during the observation period, suggesting less impacts
of anthropogenic emissions on O3 levels in oceanic air. In addition, an
insignificant decreasing change (−0.72 ppbv yr−1, α>0.10) was detected in O3 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 O3 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, O3 could be continuously produced during daytime in
aged city plumes, resulting in elevated O3 concentrations transported
to SSI. The impacts of the offshore O3 on O3 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 O3 in
the oceanic air inflows can lead to 20 %–30 % increases in urban O3
concentrations, which should be crucially considered in dealing with urban
O3 pollution in large coastal cities like Shanghai.
How does urbanization affect vegetation productivity in the coastal cities of eastern China?
