Characteristics and Trends of Ambient Ozone and Nitrogen Oxides at Urban, Suburban, and Rural Sites from 2011 to 2017 in Shenzhen, China

The emissions of nitrogen oxides (NOx) decreased under China's air quality control policies. However, concern remains regarding the response of ozone (O-3) in the metropolitan areas. The characteristics and trends of ambient O-3 and NOx in Shenzhen were investigated during the 2011-2017 period. Both the human population and vegetation are exposed to higher O-3 at suburban and rural sites than at the urban site. The O-3 weekend effect is significant (p = 0.062) at the urban site, with O-3 levels 1.19 ppb higher on Sunday than on weekdays. Solar radiation, precipitation, and relative humidity are the most relevant meteorological factors that affect O-3 daily variations. Wind speed is the least relevant factor, but wind direction is related to the presence of high O-3 air concentrations. Both 1-h and 8-h O-3 exhibit an increase, opposite to the trend of NOx. A slight decline in O-3 occurs in autumn at less urbanized sites. The increase in O-3 is more prevalent and rapid in the winter at more urbanized sites. This can be due to the transport of increased O-3 from northern China, as well as a lowered O-3 titration effect with NOx reduction. O-3 increases fastest at the urban site, with an estimated rate of 4.3% (95% confidence intervals (CIs): 0.96, 8.25) per year (p < 0.05) for 8-h O-3 and 2.5% (95% CIs: -0.46, 6.12) per year (p > 0.1) for 1-h O-3, posing greater human health risks to areas with high population density.