Elevated Soot Layer in Polluted Urban Atmosphere: A Case Study in Beijing

A layered structure of aerosol particles from surface to 1080 m was observed in Beijing on 8 December 2007. Below 700 m, the particles were well mixed vertically. From 700 to 1000 m was an elevated aerosol layer (EAL), in which the aerosol concentration was remarkably higher than in the lower and upper layers. Electron microscopic analyses of particles in the size range of 0.2-1.3 mu m at different altitudes revealed that soot particles were the majority in all layers. There were fresh, young and aged soot particles in the lower layer. In contrast, soot particles in the EAL were well aged, showing the structure of shrunken soot inclusions coated with weakly absorbing materials. The geometric mean equivalent diameter of the soot particles in the EAL was 0.4-0.6 mu m while that of their inclusions was about 0.1 mu m. The EAL coincided with the remaining nocturnal layer aloft, which was the residual left by the daytime upward convective mixing. These results suggest that the fate of soot particles was closely dependent on the evolution of the boundary layer. While particles emitted from the surface were efficiently mixed upward in daytime, the residual nocturnal layer acted as a cap and produced an EAL abundant in well-aged soot particles. In addition, the lapse rate in the EAL had an obvious decrease. However, estimation of aerosol absorption showed a limited warming effect and the inversion intensification could not be explained by the absorption. Meteorological records indicated that the intensification was more likely the consequence of air subsidence.