Synoptic circulation pattern and boundary layer structure associated with PM2.5 during wintertime haze pollution episodes in Shanghai

Rapid industrialization and urbanization have caused severe air pollution in Shanghai, China, which occurs frequently in recent winters. Here, the relationships between boundary layer structure and PM2.5 under different synoptic patterns were analysed. Firstly, the wintertime synoptic circulation patterns in Shanghai have been classified into seven types using the obliquely rotated Principal Components in T-mode (PCT) method based on the 925 hPa geopotential data. Then, the fine-resolution sounding-derived structures of planetary boundary layer (PBL) in these patterns, including PBL height (PBLH), inversion base height, and inversion intensity, combined with the relative humidity (RH) and temperature were analysed with respect to PM2.5. The results showed that three typical patterns, including high pressure located to the west, and those to the southwest and south of Shanghai, accounted for 62.17% of the total number of cases characterized by high PM2.5 concentrations. The north-westerly winds were found to dominate these specific synoptic patterns, causing severe pollution when polluted air masses from the northwest region of China were brought into Shanghai. The local meteorological factors within the PBL also influenced PM2.5 to varying extent in Shanghai under different synoptic conditions. Particularly, the PM2.5 concentrations were anti-correlated with PBLH (R = -0.2 at 0800 Beijing Local Time (BJT); R = -0.3 at 2000 BJT). Also, temperature inversion played an important role in aerosol pollution. The intensity of temperature inversion was slightly correlated with PM2.5 (R = 0.13). The base height of the temperature inversion was more closely anti-correlated with PM2.5 (R = -0.25). The frequent near-surface temperature inversions in the latter two synoptic patterns were unfavourable for the spread and dispersion of aerosol pollutants in the PBL, thus leading to severe aerosol pollution. This study has important implications for better understanding of the association of atmospheric circulation patterns with air pollution episodes in Shanghai.