Elevated aerosol layer embedded with aged soot particles in a polluted urban atmosphere

Abstract. A layered structure of aerosol particles from surface to 1080 m was observed in Beijing during daytime on 8 December 2007. Under about 700 m, particles were well mixed vertically. From 700 to 1000 m was an elevated aerosol layer (EAL), in which aerosol concentration was remarkably larger than those in the lower and upper layers. Electron microscopic analysis of particles in the size range of 0.2~1.3 μm at different altitudes revealed that soot particles were predominant in all layers. There were fresh, young and aged soot particles in the lower layer. In contrast, soot particles in the EAL were all well aged, showing the structure of shrunk soot inclusions coated with weak absorbing species (the so-called core-shell structure). The geometric mean equivalent diameter of soot particles in the EAL was approximately 0.4~0.6 μm while that of their soot inclusions was about 0.1 μm. The EAL coincided with the remaining nocturnal layer aloft, which was the residual left by the daytime upward convective mixing in the boundary layer. The lapse rate in the lower part of the EAL had an obvious decrease in the morning, indicating the absorption effect of soot particles there. These results suggest that the fate of soot particles was closely dependent on the evolution of the boundary layer. While particles emitted from surface were efficiently mixed upward in daytime, residual nocturnal layer as a cap lid produced an EAL abundant in well-aged soot particles, in which the aerosols, as a feedback, enhanced the stability of the layer by absorbing solar radiation.