Characteristics of the air pollution and the causes of heavy air pollution in winter in the northern slope of Tianshan Mountains: case study of Shihezi City

Taking Shihezi City, a typical city on the northern slope of Tianshan Mountains, as the case, based on the ground conventional pollutant monitoring, meteorological observation, the LiDAR observation, and the mesoscale Weather Research and Forecast (WRF) model simulation results, the impact of meteorological parameters and boundary layer structure changes on the air quality in Shihezi City were comprehensively analyzed. The results showed that the seasonal differences of air quality in Shihezi City were quite significant, with the highest difference of 11.4 times of PM_2.5 concentrations between winter and summer, and the occurrence rate of air pollution episodes in December to February in winter is as high as 81.2%, with 59.1% of heavy and above polluted days. In the winter of 2020-2021, four heavy pollution processes occurred, and each heavy pollution process lasted for 7-27 d, with the interval period of only 1-3 d. Each process was dominated by fine particle pollution. In total, the period of December-February could be considered as a continuous "pollution season", with the peak value of 373-425 μg/m³ of PM_2.5, and the average value of 0.82 for PM_2.5/PM₁₀. After entering autumn and winter, the continuous low temperature and high humidity meteorological conditions on the ground had a significant negative effect on PM_2.5 concentration, and the main influence conditions were T<−3 ℃ and 65%<RH<92%. Under these conditions, the significant declining of boundary layer height and the change of near-ground diffusion caused by continuous strong inversion were the fundamental reasons for the formation of winter "pollution season". In the heavy pollution process on January 16-22, 2021, the continuous low temperature, high humidity and breezy/still wind conditions were the dominant ground meteorological conditions, and the generation and dissipation of heavy pollution only changed with the boundary layer and inversion conditions. The boundary layer height in the pollution accumulation period decreased by nearly 5 times compared with that in the clean days, and the intensity of the inversion temperature exceeded 1.5 ℃/(100 m). Immediately after the heavy pollution episode, 3 clean days appeared following with the receding of inversion temperature and the rising of boundary layer height.