Temperature inversions in severe polluted days derived from radiosonde data in North China from 2011 to 2016

Temperature inversion tends to inhibit the transfer of momentum, heat and moisture in the atmospheric boundary layer, which is often accompanied by severe air pollution. Recently, severe haze pollution has frequently occurred in North China. In this study, the characteristics of temperature inversion on severe polluted days (SPDs) in Beijing were investigated by using radiosonde data with standard pressure levels from 2011 to 2016. Both surface-based inversion (SI) and elevated inversion (EI) were analyzed. 93% of the SPDs were accompanied by temperature inversion, most of which occurred in wintertime. Annual frequency of SI (FSI) and EI (FEI) showed slight fluctuations with mean value of 0.18 and 0.67, respectively. Overall, the annual SI was stronger and deeper than annual EI. Seasonally, the SI was most frequent (0.39) in autumn, in contrast to EI that occurred most frequently (0.95) in summer. Both SI and EI were weakest in summer and strongest in winter. Average monthly SI strength was about 0.38 °C in summer and 2.40 °C in winter, average monthly EI strength was about 0.64 °C in summer and 2.20 °C in winter. The average monthly SI and EI were deepest in winter and shallowest in summer. SI depth were 778 m and 221 m in winter and summer, EI were 630 m and 336 m in winter and summer. The substantially strong liner relationship was found between seasonal inversion strength and PM2.5 concentration, and the inversion strength was found to be better compared with the inversion depth at predicting the PM2.5 concentration during SPDs. Obvious lower air outflow and turbulent kinetic energy were found in SPDs compared to non-SPDs, which indicated weaker turbulence in SPDs. Future efforts should focus on accurate model simulations of temperature inversions in SPDs.