Characteristics and trends of Ambient Ozone and Nitrogen Oxides at urban, suburban, and rural sites from 2011 to 2017 in Shenzhen, China

Abstract

The emissions of nitrogen oxides (NOx) decreased under China's air quality control policies. However, concern remains regarding the response of ozone (O3) in the metropolitan areas. The characteristics and trends of ambient O3 and NOx in Shenzhen were investigated during the 2011-2017 period. Both the human population and vegetation are exposed to higher O3 at suburban and rural sites than at the urban site. The O3 weekend effect is significant (p = 0.062) at the urban site, with O3 levels 1.19 ppb higher on Sunday than on weekdays. Solar radiation, precipitation, and relative humidity are the most relevant meteorological factors that affect O3 daily variations. Wind speed is the least relevant factor, but wind direction is related to the presence of high O3 air concentrations. Both 1-h and 8-h O3 exhibit an increase, opposite to the trend of NOx. A slight decline in O3 occurs in autumn at less urbanized sites. The increase in O3 is more prevalent and rapid in the winter at more urbanized sites. This can be due to the transport of increased O3 from northern China, as well as a lowered O3 titration effect with NOx reduction. O3 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 O3 and 2.5% (95% CIs: -0.46, 6.12) per year (p > 0.1) for 1-h O3, posing greater human health risks to areas with high population density.