Contribution of Regional Transport to Surface Ozone at an Island Site of Eastern China

The surface ozone (O-3) in non-urban areas can be strongly influenced by transported regional air pollutants from distant urban agglomerations. Comprehensively using the polar plot graphical technique, and backward trajectory and potential source contribution analyses, the variations of surface O-3 and its potential source regions at an island site of Zhejiang, China, were analyzed based on data from June 2013 to October 2016. Relatively high hourly O-3 levels (max.: 154.0 ppb) were often observed, with the total number of hourly O-3 exceeding the national standard (75 ppb) being 583, indicating high health risks from O-3 exposure. An obvious time-lag for the diurnal O-3 peak was observed, with the daily maxima often occurring at 16:00. A decreased O-3 level was observed in summer, which was probably due to the comprehensive influence of intense rainfall, high relative humidity and clean marine airmasses during this season. The transport of external air pollutants played a dominant role in affecting local O-3, with relatively high O-3 concentrations (> 50 ppb) being observed under high wind speeds (> 5 m s(-1)). Based on all the source identification analyses, the major regions contributing to the surface O-3 lay to the north and northwest of the study area. Long-range transported air pollutants from the coastal provinces of the North China Plain may have significantly enhanced surface O-3 at the study site, while contributions from local areas and areas to the south of the study site were rather small. A high incidence (64.3%) of O-3 pollution was accounted for by the inflowing airmasses from outside of Zhejiang, which quantitatively confirmed the significant background transport of O-3 to this province. Coordinated inter-regional control on pollutant emissions should be carried out to reduce O-3 in Zhejiang Province.