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. To investigate the spatiotemporal variability of the mixing layerheight (MLH) on the North China Plain (NCP), multi-site and long-termobservations of the MLH with ceilometers at three inland stations (Beijing, BJ; Shijiazhuang, SJZ; Tianjin, TJ) andone coastal site (Qinhuangdao) were conducted from 16 October 2013 to 15 July 2015. The MLH of the inlandstations in the NCP were highest in summer and lowest in winter, while theMLH on the coastal area of Bohai was lowest in summer and highest in spring.As a typical site in southern Hebei, the annual mean of the MLH at SJZ was464  ±  183 m, which was 15.0 and 21.9 % lower than that at the BJ(594  ±  183 m) and TJ (546  ±  197 m) stations, respectively.Investigation of the shear term and buoyancy term in the NCP revealed thatthese two parameters in southern Hebei were 2.8 times lower and 1.5 timeshigher than that in northern NCP within 0–1200 m in winter, respectively,leading to a 1.9-fold higher frequency of the gradient Richardson number > 1 in southern Hebei compared to the northern NCP.Furthermore, combined with aerosol optical depth and PM 2.5 observations,we found that the pollutant column concentration contrast (1.2 times) betweenthese two areas was far less than the near-ground PM 2.5 concentrationcontrast (1.5 times). Through analysis of the ventilation coefficient in theNCP, the near-ground heavy pollution in southern Hebei mainly resulted fromthe lower MLH and wind speed. Therefore, due to the importance of unfavorableweather conditions, heavily polluting enterprises should be relocated andstrong emission reduction measures should be introduced to improve the air quality insouthern Hebei.

Mixing layer height on the North China Plain and meteorological evidence of serious air pollution in southern Hebei