[Spatio-Temporal Evolution Characteristics and Source Apportionment of O₃ and NO₂ in Shijiazhuang]

Ground-level O₃, NO₂, and meteorological (temperature, humidity, wind speed, precipitation, and sunshine duration) data from 18 counties in Shijiazhuang City from 2014 to 2017, and volatile organic compounds (VOCs) data for Summer 2017, were analyzed to explore the spatial patterns, evolution, influencing factors, and source apportionment of O₃ and NO₂ in Shijiazhuang City. Network analysis and inverse distance weighted (IDW) spatial autocorrelation and backward trajectories analyses were performed. The results indicate that O₃ concentrations increased between 2014 and 2017, and monthly variations showed a unimodal trend. The typical period of peak O₃ pollution (O₃ ≥ 160 μg·m^-3) was from May to September, characterized by high temperatures, low humidity, weak winds, and strong solar radiation. The O₃ concentrations were negatively correlated with the NO₂. Furthermore, O₃ concentrations increased year-on-year since 2015 in main urban area, and the dominant pollutant type had changed from NO₂ (2014 to 2016) to VOCs (2016 and 2017). However, the O₃ concentration of county-areas limited by the VOCs. The main factors affecting O₃ concentrations were industry, agriculture, economy, and population, and centers of O₃ pollution associated with secondary industry appeared in the main urban areas of Shijiazhuang and Luancheng. Moreover, VOCs trajectories during the summer monitoring period were clustered in three source directions:(A) East-northeast, 26.67%; (B) Northwest-west, 43.33%; and (C) Southeast-south, 30%). Trajectories (A) and (C) were the dominant directions of VOC transmission (east-southeast).