Rural vehicle emission as an important driver for the variations of summertime tropospheric ozone in the Beijing-Tianjin-Hebei region during 2014-2019.

Tropospheric ozone (O) pollution is increasing in the Beijing-Tianjin-Hebei (BTH) region despite a significant decline in atmospheric fine aerosol particles (PM) in recent years. However, the intrinsic reason for the elevation of the regional O is still unclear. In this study, we analyzed the spatio-temporal variations of tropospheric O and relevant pollutants (PM, NO, and CO) in the BTH region based on monitoring data from the China Ministry of Ecology and Environment during the period of 2014-2019. The results showed that summertime O concentrations were constant in Beijing (BJ, 0.06 µg/(m•year)) but increased significantly in Tianjin (TJ, 9.09 µg/(m•year)) and Hebei (HB, 6.06 µg/(m•year)). Distinct O trends between Beijing and other cities in BTH could not be attributed to the significant decrease in PM (from -5.08 to -6.32 µg/(m•year)) and CO (from -0.053 to -0.090 mg/(m•year)) because their decreasing rates were approximately the same in all the cities. The relatively stable O concentrations during the investigating period in BJ may be attributed to a faster decreasing rate of NO (BJ: -2.55 µg/(m•year); TJ: -1.16 µg/(m•year); HB: -1.34 µg/(m•year)), indicating that the continued reduction of NO will be an effective mitigation strategy for reducing regional O pollution. Significant positive correlations were found between daily maximum 8 hr average (MDA8) O concentrations and vehicle population and highway freight transportation in HB. Therefore, we speculate that the increase in rural NO emissions due to the increase in vehicle emissions in the vast rural areas around HB greatly accelerates regional O formation, accounting for the significant increasing trends of O in HB.