Size-resolved carbonaceous aerosols at near surface level and the hilltop in a typical valley city, China

Özet

The lack of light–absorbing aerosols vertical distributions data largely limited to revealing the formation mechanismof severe haze pollution in Chinese cities. Based on the synchronous measurements of size–resolvedcarbonaceous aerosols and meteorological data at near surface level and hilltop (about 620m above the valley)in Lanzhou of northwest China, this study compared organic and elemental carbon (OC, EC) size distributions atthe two altitudes and revealed the key influencing factors in a typical urban valley, China. The winter OC sizedistributions were typically bimodal with two comparable peaks in the accumulation and coarse modes, whilethose in summer were unimodal with the highest value in the size bin of 4.7–5.8 μm. The size-resolved OC andEC at near the surface were significantly higher than those at the hilltop. The difference (concentrations and sizedistributions) of OC and EC between the surface and hilltop in summer was much smaller than that in winter dueto stronger vertical mixing and larger summer SOC contributions at the hilltop. The winds paralleling withrunning urban valley were conducive to dispersing the air pollutants from near the surface to the upper air. Theroles of horizontal and vertical dispersions to carbonaceous aerosols were comparable at near the surface, whilehorizontal dispersion was more important at the hilltop. Furthermore, the vertical dispersion was a main factorcontrolling size–resolved carbonaceous aerosols under highly polluted conditions in a typical urban valley. Thisstudy will provide the basis for regulation of severe haze pollution over complex terrain.