[Microscopic characteristics and size distribution of summer PM10 in the air of a northwestern urban site and a clean air site in Beijing].

Microscopic characteristics and size distribution of individual particles in the summer (2001) PM10 collected at a northwestern urban site and a clean air site in Beijing have been investigated using high resolution Field Emission Scanning Electron Microscopy (FESEM) and image analysis technologies. The soot aggregates were observed at both sampling sites, showing a characteristic regional pollution. The irregular mineral particles existed commonly at the urban site but were rarely seen at the clean air site. The elongated gypsums and biological particles were observed only at the clean air site. PM10 displayed a biomodal number-size distribution, with a higher peak in 0.2-0.5 micron range and a lower peak in 1-2.5 microns range. In contrast, volume-size distribution of PM10 exhibited a unimodal pattern with the peak in 1-2.5 microns range. The coarse particles, mainly minerals, had a considerable contribution to the PM10 mass, although they were insignificant in number. In PM2.5, however, the soot aggregates were dominant both in number and mass.