Climatic features of wind field and their impact on local air quality in Taiyuan Basin.

Taiyuan Basin has long been considered as one of the most air-polluted regions in China, closely related to its enclosed terrain and local climatic conditions. In this study, the wintertime climatic characteristics of the wind field and the corresponding boundary layer structure were investigated for their potential impact on local air quality in Taiyuan Basin, utilizing long-term Weather Research and Forecasting model simulations from 2015 to 2020. Prevailing southwesterly wind in this basin was revealed. It was demonstrated that the blocking effect of the mountains to the dominant airflow of winter monsoon, and the guidance of the basin orientation to the diverted wind, lead to this fundamental feature. Eight synoptic patterns were identified based on the obliquely rotated T-mode Principal Component Analysis (T-PCA) and further grouped into pollution-prone and relatively clean pattern types. Weak winds, low boundary layer height, higher temperature and higher relative humidity were associated with pollution-prone synoptic patterns, while the opposite conditions were related to the clean patterns. Lower and higher ventilation indexes (<3000 m s and > 4000 m s) were also distinct the former from the latter patterns. Two case studies revealed the detailed vertical structure of boundary layer wind and temperature, as well as their temporal variation during the pollution episode. The influence of upper layer southwest warm advection was emphasized. In contrast, for clean events in this basin, the removal of air pollutants heavily depended on the air-flushing process brought by the stronger weather system, which occurred in frequency approximately 36 % in the winter season.