Characteristics of surface ozone and nitrogen oxides at urban, suburban and rural sites in Ningbo, China

Surface ozone (O-3) is a harmful air pollutant that has attracted growing concern in China. In this study, the mixing ratios of O-3 and nitrogen oxides (NOx) at three different sites (urban, suburban and rural) of Ningbo were continuously measured to investigate the spatiotemporal characteristics of O-3 and its relationships with environmental variables. The diurnal O-3 variations were characterized by afternoon maxima (38.7-53.1 ppb on annual average) and early morning minima (11.7-26.2 ppb) at all the three sites. Two seasonal peaks of O-3 were observed in spring (April or May) and autumn (October) with minima being observed in winter (December). NO levels showed generally opposite variations to that of O-3 with diurnal and seasonal maxima occurring in morning/evening rush-hours and in winter, respectively. As to the inter-annual variations of air pollutants, generally decreasing and increasing trends were observed in NO and O-3 levels, respectively, from 2012 to 2015 at both urban and suburban sites. O-3 levels were positively correlated with temperature but negatively correlated with relative humidity and NO levels. Significant differences in O-3 levels were observed for different wind speeds and wind directions (p < 0.001). O-3 levels varied non-linearly with wind speed ranges with an increasing trend within 4 m/s. Higher mixing ratio of O-3 was observed for wind blowing from the sea, which indicates that the coastal air mass might carry more O-3. A decreasing trend in O-3 levels was observed from weekdays to weekends at all the three sites. As to the spatial variation, higher levels of O-3 were observed at the suburban and rural sites where less O-3 was depleted by NO titration. In contrast, the urban site exhibited lower O-3 but higher NOx levels due to the influence of traffic emissions. Larger amplitudes of diurnal and monthly O-3 variations were observed at the suburban site than those at the urban and rural sites. In general, the O-3 levels at the non-urban sites were more affected by the background transport, while both the local and regional contributions played roles in urban O-3 variations. The annual average O-3 mixing ratios (22.7-37.7 ppb) in Ningbo were generally similar to those of other regions around the world. However, the recommended air quality standards for O-3 were often exceeded during warm seasons, which could be a potential threat to both local population and plant growth. (C) 2016 Elsevier B.V. All rights reserved.