Characterizations of aerosols over the Beijing region: A case study of aircraft measurements

Abstract

Using optical spectrometer probes mounted on a light aircraft, fine-mode and coarse-mode aerosol size distributions were recorded within and above the planetary boundary layer in the vicinity of Beijing on 29 April 2005. A regional transport and dynamical model (Weather Research and Forecast coupled with a tracer transport model—WRF-Tracer) and satellite (moderate-resolution imaging spectroradiometer—MODIS) data were applied to analyze and interpret the aircraft measurements. The results show that the aerosol pollution in the Beijing region was very heavy on 29 April 2005. The satellite measured aerosol optical depth (AOD) over this region was exceeded to 2.0. The aerosol mass concentrations reached up to 160 μg m −3 in some cases. The analysis suggests that three important source regions influenced this heavy aerosol pollution in the Beijing region, including: (1) the long-range transport from the Gobi Desert with diameters ranging from 0.5 to 4 μm, (2) the transport from the south of Beijing area, and (3) the local emissions over Beijing. A strong vertical gradient of aerosol concentrations in Beijing at Shahe Airport was observed. The fine aerosol particles are dominated below the planetary boundary layer (PBL) with number concentrations of 5000 cm −3, diameters of 0.3 μm, and mass concentrations of 144 μg cm −3. However, the coarse aerosol particles (dust) are dominated above the PBL with number concentrations of 2.5 cm −3, diameters of 2.5 μm, and mass concentrations of 120 μg m −3. There is an inversion layer at the top of the PBL, and this inversion layer acts to suppress mixing between the PBL and overriding troposphere and stabilizes desert dust layer that is being transported overhead. This study demonstrates that the measurement from the aircraft together with the WRF-tracer and MODIS data provides good information to understand the origins and spatial distribution of aerosol particles in Beijing. Note that this analysis is based on the measurement on 29 April 2005. Thus it is considered as an example or a case study.