Seasonal and diurnal variations in aerosol concentrations at a high-altitude site on the northern boundary of Qinghai-Xizang Plateau

The concentration and size distribution of aerosol particles in the range of 0.25–32 μm was monitored with an optical particle counter from 2009 to 2011 at the Qilian Shan Station of Glaciology and Ecologic Environment (QSS) (39.50N, 96.51E; 4180 m a.s.l.), in the northern Qinghai-Xizang (Tibet) Plateau. The average total number concentration N was 27 cm− 3 (1σ = 31), with submicrometer particles (N0.25 < d < 1 μm) accounting for 97%. A clear seasonal variability of N was observed with high values in spring and summer, peaking in April and August, respectively. The peak in April corresponded to the highest frequency of dust storms in northern China, while the peak in August was likely because of polluted air mass transport, as seen from the increased fraction of N0.25 < d < 1 μm and the increased concentration of black carbon. The N also exhibited an evidently diurnal pattern depending on seasons. The cold seasons (winter and spring) presented a bimodal structure with one peak in the morning and another in early evening, while only one early evening peak was seen in the warm seasons (summer and fall). The diurnal pattern in the warm seasons was mainly related to the development of warm thermal dynamic processes in the planetary boundary layer, while it was usually associated with synoptic-scale weather patterns during the cold seasons. The polluted air masses were mainly transported upslope from the cities that located at lower elevations.