Dominant mechanism underlying the explosive growth of summer surface O3 concentrations in the Beijing-Tianjin-Hebei Region, China

Despite the implementation of stringent emission reduction measures in China since 2013, ozone (O3) 3 ) pollution remains a serious concern. Many O3 3 pollution processes are accompanied by the explosive growth of O3 3 concentration, meanwhile this explosive growth will trigger O3 3 pollution with a high probability. This study investigates the chemical and physical mechanisms behind the explosive growth of O3 3 in the summer within the Beijing-Tianjin-Hebei (BTH) region based on the atmospheric circulation clustering and WRF-CMAQ simulation. Ninety-four instances of O3 3 explosions from the summer of 2014-2021 were selected based on their day-to-day variation in O3 3 concentration, and the corresponding meteorological conditions were classified into four clusters using K-means analysis. Cluster 1 revealed that large-scale regional transmission of O3 3 and its precursors from the southern BTH region led to a renewed burst of O3 3 growth based on mild pollution. Nighttime reduction in NO2 2 concentration decreases titration consumption, causing a nocturnal rise in O3 3 concentration, detectable through observed data and progress analysis. In Cluster 2 and Cluster 3, the BTH region is located in the northeast wind area ahead of the ridge and northly wind area of post-trough situation, respectively. The vertical downward motion with the markable increase in surface solar radiation (SSR), 2-m temperature (T2M), and decrease in relative humidity (RH) contribute to photochemical O3 3 generation. Additionally, enhancements in boundary layer stability and reductions in dry deposition and vertical diffusion are favorable for the O3 3 accumulation. Cluster 4 showed the explosive growth after rainfall event, with the largest increment. The post-rain increase in SSR and T2M, coupled with decreased RH, promotes photochemical O3 3 formation. Moreover, progress analysis reveals the reduced cloud cover slows down O3 3 removal, contributing to O3 3 accumulation. The dominant mechanism behind the explosive growth of O3 3 will provide certain guidance for the prediction of O3.