Abstract: Using meteorological and environmental monitoring data,the backward trajectory analysis,and satellite fire-monitoring data,the characteristics of a persistent heavy air pollution event in Harbin from October 18 to 20,2017 were analyzed,including atmospheric circulation,meteorological elements,pollutant sources,and transport.The results indicate that the heavy and severely heavy air pollution periods last for 48 h,with the mean and maximum PM_2.5 (primary air pollutant) concentration reaching 438 μg·m^-3 and 1487 μg·m^-3,respectively.This heavy pollution event can be divided into two stages where double peaks of pollutant concentrations are observed.During this heavy pollution event,atmospheric circulation at upper levels are smooth and dominated by warm advection ahead of a shallow trough,and meanwhile,the surface is controlled by a weak low pressure.The surface wind speed is low,about 1.5 m·s^-1 on average,and the occurrence frequency of wind speed ≤ 1.5 m·s^-1 is 71%.The convergence of wind field favored the accumulation of air pollutants.During the development of heavy pollution,the increase of relative humidity (RH) contribute to the hygroscopic growth of particulate matters and thus enhance the pollution process.The decrease of PM_2.5 concentration to the minimum lags 4-5 h behind that of RH to its minimum value.A strong surface temperature inversion layer is observed in the planetary boundary layer,with the top up to about 200 m and the intensity larger than 2.0℃·(100 m)^-1.The stable stratification makes poor vertical diffusion.These air pollutants dominantly come from straw burning,followed by the coal combustion for building heating.The heavy pollution event in Harbin is due to the accumulation of local pollutants under stable meteorological conditions and the long-distance transport of massive pollutants caused by straw burning.

Key words: Heavy pollution, Straw burning, Meteorological condition