Fig. 1. Time distribution of LLJ

Fig. 2. Line chart of the frequency of LLJ in different months

Fig. 3. LLJ characteristics. (a)LLJ wind speed; (b) LLJ height; (c) LLJ wind direction; (d) diurnal variation of LLJ frequency

Fig. 4. Diurnal variation line chart of LLJ characteristics under different jet wind directions. (a) (d) LLJ wind speed; (b) (e) LLJ height; (c) (f) LLJ frequency

Fig. 5. Wind direction distribution histogram. (a) Winter and (b) spring of LLJ wind profiles; (c) winter and(d) spring of all wind profiles

Fig. 6. Monthly average results of the ERA5 mean sea level pressure (November 2020 to April 2021)

Fig. 7. Statistics histograms of daily variation of 1000 hPa land-sea temperature difference in winter. (a) December 2020; (b) January 2021; (c) February 2021

Fig. 8. Observation results of LLJ process (December 11st, 2020). (a) Horizontal wind speed; (b) horizontal wind direction; (c) vertical wind speed standard deviation; (d) PM_2.5 mass concentration time sequence diagram

Fig. 9. Scatter chart of PM_2.5 mass concentration relative change during LLJ. (a) 00:00-06:00; (b) 18:00-23:00