Characterization And Source Analysis Of Atmopheric Aerosol Over Qinghai-Tibet Plateau

The Qinghai-Tibet Plateau is a sensitive and vulnerable area for climate change. In recent decades, the rapid increase in the aerosol pollution in Qinghai-Tibet Plateau has seriously affected the climate and ecological environment. Bulk aerosol samples were collected at two different sites in Tibetan Plateau(TP)—the Qinghai Lake Obsevatory on the northeast and the Lulang site on the southeast of TP. We report aerosol mass levels and the concentrations of selected elements, carbonaceous species, and water-soluble inorganic ions, objectives were to determine chemical characteristics of the aerosol and identify its major sources. The cause, source and influnce of different types of pollution events in TP were explored in this study. Finally, source apportionment methods were used to identify presumptive sources and potential source regions to the sampling sites.The PM2.5 ranged from 5.7 to 149.7 μg m-3 in Qinghai Lake, and it was predominately crustal material(~40% on average). The combined mass of eight water-soluble inorganic ions ranged from 1.0 to 41.5 μg m-3, with the largest contributions from SO42-, NO3-, and Ca2+. Low abundances of organic carbon(OC, range: 1.0 to 8.2 μg m-3) and elemental carbon(EC, 0.2 to 2.3 μg m-3) were found in QHL. Weak seasonality in the OC/EC ratio(4.5 ± 2.0) indicated simple and stable sources for carbonaceous particles. The water-soluble ions, OC and EC accounted for ~30%, 10% and 2% of the PM2.5, respectively. Water-soluble organic carbon(WSOC, range: 0.5 to 4.3 μg m-3) accounted for 47.8% of the OC. Both OC and WSOC were positively correlated with water-soluble K+(r = 0.70 and 0.73 respectively), an indicator of biomass burning. Higher WSOC and stronger correlations between WSOC and EC in spring and winter compared with summer and autumn are evidence for primary biomass burning aerosols. The concentrations of mass and major compositions were 2~10 times higher than those for some TP or continental background sites but much lower than urban areas. Compared with particles produced from burning yak dung(a presumptive source material), PM2.5 had higher SO42-/OC ratios. The higher ratios were presumed as a result of fossil fuel combustion. After excluding data for dust storms events(Normal days), the relative percentages of OM, EC, K+, NH4+, NO3- and mineral dust showed little difference among seasons despite different monsoons dominated in four seasons; implying that the PM2.5 sources were relatively stable. The results from Qinghai lake evidently reflect regional characteristics of the aerosol. Potential source contribution function(PSCF) model was used to evaluate the potential source region of Qinghai Lake PM2.5 mass concentration. There were six potential source regions that had effect on PM2.5concentration in Qinghai Lake, including Tarim Basin(Taklimakan desert), Turpan Basin in Xinjiang, the junction areas of Qinhai, Xinjiang and Gansu, Qaidam Basin in Qinghai, Hexi Corridor in Gansu Province, and the urban zones in the eastern of Qinghai Lake.Four dust storm events(DS) and three open biomass burning events(OB) had significant impact on aerosol population and concentrations in Qinghai Lake. The averaged mass concentrations of dust storms reach 99.73μg m-3. Combined with backward trajectory analysis and data comparison with source region, we assumed that “air masses replacement” make the fourth DS event seems like a “fresh/pure” one. The dust strom carried significant quantities of pollutants(such as sulfate/nitrate) may be directly from the source region, not always caused by the reactions on the surface of dust particles with pollutants. In OB events, the averaged mass level reach 75.33μg m-3. On the relative contributions of major chemical species to the aerosol mass,.SNA(SO42-/NO3-/NH4+) was the most abundant species, accounting for 44% of the mass, while K+ and EC concentrations had no significant changes during the episode. After excluding the pollution events(Normal days), the percentages of OM, EC and mineraldust in four seasons showed no significant difference, which implied the pollution sources in Qinghai Lake were relatively stable. Dramatic increase or change of pollution sources may not occur in Qinghai Lake. Natural sources have significant impact on the sampling site relative to anthropogenic sources.Significant buildup of aerosol mass and chemical species(organic carbon, element carbon, nitrate, and sulfate) occurred during the premonsoon, while lower concentrations were observed during the monsoon in Lulang. Seasonal variations in aerosol and chemical species were driven by precipitation scavenging and atmospheric circulation. Two kinds of high-aerosol episodes were observed: one was enriched with dust indicators(Fe and Ca2+), and the other was enhanced with OC, EC, SO42_, NO3-, and Fe. The TSP loadings during the latter were 3 to 6 times those on normal days. The greatest aerosol optical depths(National Centers for Environmental Protection/National Center for Atmospheric Research reanalysis) occurred upwind, in eastern India and Bangladesh, and trajectory analysis indicates that air pollutants were transported from the southwest. Northwesterly winds brought high levels of natural emissions(Fe, Ca2+) and low levels of pollutants(SO42-, NO3-, K+, and EC); this was consistent with high aerosol optical depths over the western deserts and Gobi.Our work provides evidence that both geological and pollution aerosols from surrounding regions impact the aerosol population of the TP.