Physical And Chemical Properties And Sources Of Aerosols Over The Central And Western Tibetan Plateau

The Qinghai-Tibet Plateau（TP）is extremely sensitive to climate change,and its unique topographic,thermal and dynamic effects have a profound impact on global weather and climate.The radiation effects of atmospheric aerosols and the aerosol-cloud interaction affect the climate,hydrological cycle and ecosystem in the TP,and thus have an important impact on the global climate and environment.The central part of TP has a complex landscape and fragile ecology;the western part of the plateau has a large number of desert areas,controlled by the westerly wind belt and the Indian monsoon.With less industrial pollution and pristine landscapes in the central and western parts of TP,research on the sources and components of aerosols can be carried out,which can help reveal the differences in physicochemical properties and sources of aerosols in the central and western parts of the plateau and provide further insight into the special characteristics and complexity of TP atmospheric environment.Previous studies have carried out some observations related to aerosol components in TP,but the understanding and quantification of the physicochemical properties of aerosols are still inadequate,which limits further insight into TP aerosols and makes the sources of some aerosols unclear,causing large uncertainties in the assessment of aerosol radiative effects,especially in the central and western parts of TP where observational studies are scarce.In view of the above issues,this paper selected Nam Co station in central TP and Shiquanhe station in western TP as representative stations,conducted comprehensive field observations,and combined with reanalysis data to study the physical and chemical characteristics and sources of aerosols in central and western TP,and compared the differences between them.The main results and conclusions are as follows:（1）The spatial and temporal distribution of different types of aerosols on the TP was revealed using MERRA-2 reanalysis data.The dust aerosol came from natural sources,and the distribution was high in the north and low in the south.The carbon aerosol showed a high value in the southern margin of the TP.In spring and summer,frequent dust events in the Taklimakan Desert lead to more dust in the northern part of the TP,and the spatial distribution of organic carbon is similar in different seasons.The concentration of carbon aerosol in the central TP was higher than that in the western TP,while the concentration of dust aerosol was lower than that in the western TP.The Interannual variation of dust is similar in central and western TP,while the interannual variation of black carbon and sulfate is consistent.The proportion of dust in central TP decreases significantly in autumn and winter,and the dust concentration is higher in western TP in summer,which is a combination of dust transport from northern India,dust outbreaks from the Taklamakan Desert,and local elevation of dust.The carbon aerosols in the central and western regions were higher in spring and summer and lower in autumn and winter,which was related to the long-distance transport of pollutants in South Asia and northern India.（2）The comprehensive observation of aerosol physical-chemical properties was carried out from July 8 to August 1,2019 at Shiquanhe Station and from August 6 to September 11,2020 at Nam Co Station.The mean values of single scattering albedo（SSA）and scattering?ngstr?m exponent（SAE）at Shiquanhe Station were 0.73±0.18and 1.04±0.42,respectively.The higher SAE may be due to the influence of dust aerosol,while the lower SSA may be due to the strong absorption of fine mode aerosol.The mean values of SSA and SAE at Nam Co Station were 0.84±0.09 and 0.59±0.14,respectively.The higher SSA may be caused by the mixture of fresh biomass combustion and coarse dust aerosol.（3）The concentrations of total suspended particulates（TSP）at Nam Co Station and Shiquanhe Station were 10.11±5.36μg·m^-3 and 15.71±5.92μg·m^-3,respectively,and the average ratio of organic carbon（OC）to elemental carbon（EC）was 15.3 and17.4,respectively,indicating that the two stations were influenced by biomass aerosols.The average mass concentration of Calcium（Ca）at the two stations was 0.27±0.17and 0.93±0.44μg·m^-3.The highest proportion of Ca indicated that crustal mass was the main contributor to TSP in the central and western of the TP.The higher contribution of Ca²⁺to SO₄^2-and NO₃^-from the two stations reflects the influence of mineral dust（MD）in the central and western parts of the TP,which is greater than that from anthropogenic fossil fuel consumption.The mass concentrations of organic matter（OM）,EC,MD and water-soluble ion（nd-WSII）at Nam Co Station were 4.20±1.90,0.12±0.13,1.83±1.24 and 0.42±0.15μg·m^-3,respectively.The mass concentrations of OM,EC,MD and nd-WSII at Shiquanhe Station were 3.55±0.96,0.09±0.11,4.38±2.00 and 1.20±0.41μg·m^-3,respectively.This indicates that OM and MD are the main components of TSP in the central and western TP in summer.（4）The results of enrichment factor（EF）method showed that Al,Ti and K of the two stations mainly came from natural sources.The high EF values of heavy metallic elements indicate that heavy metals from anthropogenic pollutants in South Asia can be transported to the central and western TP during the summer monsoon period.The correlation of aerosol composition was used to determine potential sources.The correlation of K⁺with OC and EC was weak at both stations,and the contribution of plant burning was small.The lower correlation between wind speed and metal elements at Nam Co station indicates that the contribution of local sand initiation is small.The high correlation between wind speed and metal elements（r>0.5）at Shiquanhe Station indicates that local wind-driven sand has an important contribution to dust aerosol.There was a high correlation between OC and Ca²⁺（r=0.84）,indicating the same source of mineral dust and organic matter at Nam Co station.The results of electron microscopy analysis of single particles indicated at Nam Co station showed that 53.7%of the organic matter and 14.3%of the mineral dust were at submicron size and that there was irregular mixing of mineral dust with biomass aerosols.（5）The aerosol sources from the central and western parts of TP were analyzed by combining backward trajectories and potential source contribution function（PSCF）methods.The trajectories of air masses from the interior of TP are more frequent at Nam Co and Shiquanhe stations.The PSCF results show that OC and Ca have similar source area distributions at Nam Co and Shiquanhe stations,and the important source areas are all located in the interior of the TP,further indicating that OC and Ca have the same source areas.The two main potential source areas of OC at Shiquanhe station are located in the interior of the TP,and the PSCF large value areas of Ca are all in the interior of the TP,indicating that the aerosol transport in the interior of the plateau is not negligible.