Based on measurements of the Beijing 325-m Meteorological Tower,this study reports on the analysis of atmospheric stability conditions and turbulent heat exchange during consecutive episodes of particle air pollution in Beijing,as induced mainly by haze and dust events (April 15–30,2012); of particular interest are their vertical variations within the lower urban boundary layer.First,the haze and dust events are characterized by different atmospheric conditions,as notably lower wind speed and higher humidity are observed during the haze events.Moreover,for description of stability conditions,the bulk Richardson number (RiB) is inter-compared among three different height intervals,namely 8–47,47–140,and 140–280 m,presumably representing the urban canopy layer,roughness sub-layer,and surface layer,respectively.Values of RiB demonstrate an apparent increase in the occurrence frequency of stably-stratified air layers higher above — for the 140–280-m height interval,positive values (RiB > 0 for stable stratification) are determined for about 85% of the time.As confirmative evidence,downward transport of sensible heat is observed at 280 m for the full diurnal cycle,which,by contrast,is rarely seen at 140 m during daytime; meanwhile,the vertical transport efficiency of sensible heat is significantly lower at the height of 280 m.It is thus important,for improved mechanistic understanding of the haze/dust-induced urban particle air pollution,both to implement high-resolution vertical profile observations of the complex urban boundary layer and to delve into issues related to the stably-stratified flows.