Size distribution of carbonaceous aerosols at a high-altitude site on the central Tibetan Plateau (Nam Co Station, 4730 m a.s.l.)

Abstract

The chemical composition and size distribution characteristics of atmospheric aerosols have important effects on the environment, human health and climate change. In this paper, we study the size distribution of carbonaceous aerosols at the remote and pristine site, Nam Co Monitoring and Research Station for Multisphere Interactions, in the inland Tibetan Plateau (TP) based on collected size-segregated aerosols during 2012. The samples were quantified using the thermal/optical (TOR) method. The overall average concentrations of OC and EC in TSP, PM9.0, PM2.1, and PM1.0 were 4.61μgm−3 and 0.19μgm−3, 4.52μgm−3 and 0.18μgm−3, 2.72μgm−3 and 0.11μgm−3, and 2.11μgm−3 and 0.09μgm−3, respectively. Generally, the highest concentration of OC and EC in different aerosol size occurred during winter. The low level of EC indicated that direct anthropogenic disturbances in the interior of the TP still remain insignificant.The size distributions of OC and EC concentrations presented bimodal variations. In winter, pre-monsoon, monsoon, and post-monsoon seasons, the peaks for OC were in droplet mode (0.43–0.65μm) and coarse mode (4.7–5.8μm); while in the monsoon period, the coarse mode shifted to a smaller size bin (3.3–4.7μm). The coarse mode may be due to dust particles while the droplet mode may be due to the growth process of particles. For EC, the peaks variations in coarse mode were as same as OC, while the other peaks were complicated: the peaks during winter, pre-monsoon, and monsoon seasons exhibited in droplet mode (1.1–2.1μm, 0.65–1.1μm, and 0.43–0.65μm, respectively), and in post-monsoon period, the peak located in condensation mode. The highest peak concentrations for OC and EC occurred in winter and the pre-monsoon period, while the lowest peak values in the monsoon and post-monsoon periods, respectively. The size distribution variations may be caused by deposition, gas/particles exchange, hygroscopic growth, external mixing, and secondary organic carbon formation. OC/EC ratios in aerosols over the TP mostly exhibit high values, which emphasizes the importance of OC over this region.