Abstract: The variations of meteorological conditions,vertical wind field,features of planetary boundary layer and pollutant sources during a heavy PM_2.5 pollution event in Shenyang in November of 2015 were comprehensively analyzed using an observational dataset.The results show that the period of PM_2.5 concentrations higher than 250 μg·m^-3 lasts for 81 hours during this event,with a maximum PM_2.5 concentration reaching 1287 μg·m^-3 and a maximum PM_2.5/PM₁₀ ratio reaching 90%.Affected by an inverted trough at the surface and the Huanghuai cyclone,the temperature inversion layer,high humidity conditions and weak northerly winds remain in the near-surface layer,which are favorable for the hygroscopic growth and long-term accumulation of particles.The observational data from a wind profile radar showed that during the heavy pollution period,low level winds,fickle wind directions and weak subsidence of airflows near the surface remained.Based on the radar data,the ventilation index (VI) and recirculation index (R) are calculated.The values of PM_2.5 concentrations have a negative correlation with the values of VI,and the mean VI on clear days is twice of that on heavy pollution days.The values of R are larger than 0.9 before the heavy pollution period and became smaller than 0.8 during this pollution period.Through analyzing the backward trajectories and fire point monitoring data,the air masses at 300 m altitude over Shenyang originate from areas with the dense biomass burning.Moreover,the variation of NO₂ and CO concentrations in Shenyang are consistent with that of PM_2.5.This heavy pollution event may be also associated with the biomass burning.

Key words: PM_2.5, Heavy pollution, Meteorological condition, Wind profile radar, Air mass trajectory, Biomass burning