The vertical distribution of PM2.5 and boundary-layer structure during summer haze in Beijing

Between August 1, 2009, and August 16, 2009, physical observations of the atmospheric boundary layer and synchronous vertical observations of atmospheric particles were conducted from the Beijing 325 m meteorological tower, where the particulate matter with a diameter of 2.5 μm or less (PM2.5) analysis meters were stationed at a three-floor platform with altitudes of 8 m, 120 m and 280 m, respectively. Meanwhile, the atmospheric temperatures, relative humidity, wind speeds and wind directions between 8 m and 320 m were observed online at 15 different altitude intervals. The backscattering coefficient of aerosols in the boundary-layer atmosphere within 2.5 km height was also observed using a backscattered laser ceilometer. The observations showed that the PM2.5 pollution in the atmosphere from the ground up to 280 m in Beijing was quite high on August 2009, with a maximum of 200 μg m−3. Within 280 m, the vertical distribution of PM2.5 was inhomogeneous, with a maximum difference of up to 116 μg m−3 between levels in the night residual layer and at the ground. The high concentration of particles in the residual layer reached the ground by the next morning through convection, thus becoming severe pollutants. The PM2.5 in the near-surface layer was directly related to the reduction of ultra-violet radiation (UV), with a correlation coefficient of −0.57. Under steady weather conditions, the topographic mountain-valley breezes in Beijing superposed the land-sea breezes, resulting in a specific breath-like diurnal variation in wind direction. As a result, the PM2.5 mixed and increased in the regional area, leading to serious dust-haze pollution. Within 320 m of the boundary layer, the vertical distributions of temperature, humidity, wind speed and wind direction were inhomogeneous, and these patterns were the major factors influencing the distribution and variation of PM2.5 concentration. Under steady weather conditions, the reverse distribution of relative humidity became more significant, while the low temperatures at higher altitudes facilitated the formation of organic aerosols. Sometimes, the PM2.5 levels increased due to long-range transmission of smoke plume into the residual layer. By the synchronous effect of these factors, the moisture absorption of PM2.5 in the upper layer increased, at times resulting in a “higher-top and lower-bottom” pattern of the PM2.5 distribution.