| The Tibetan Plateau,known as the"third pole of the earth",has significant dynamical and thermal effects on the circulation and monsoon of the surrounding areas and even the global atmosphere.Aerosols affect the thermal driving process of the Tibetan Plateau by changing the balance of ground and air radiation on the plateau,which in turn affects the circulation situation in downstream areas and even the global climate change and ecological environment.An in-depth study of the spatial and temporal distribution characteristics and trends of aerosols on the Tibetan Plateau is of positive significance to further study the feedback effects of aerosols on the climate and environment of the Tibetan Plateau.In this paper,we analyze Nam co optical properties of atmospheric aerosols on the central Tibetan Plateau using the data from the Second Integrated Scientific Research Project on the Tibetan Plateau2020 Nam co observation;we discuss the applicability of MERRA-2 AOD on the Tibetan Plateau using long-term MERRA-2 aerosol optical depth(AOD)data in combination with AERONET 500nm AOD data from 2011 to 2020,and analyze the the spatial and temporal distribution characteristics and trends of the five aerosol optical depthes on the Tibetan Plateau were analyzed.The main conclusions are as follows:(1)In monsoon period,average AOD in Nam co was 0.025±0.01,and the frequency distribution was concentrated in 0.02±0.03.The depolarization ratio was 0.033±0.015,and the maximum frequency distribution was between 0.02 and0.03.The mean aerosol concentration was 129.73 N/cm~3.The average concentration of BC was 37.36 ng/m~3.The vertical distribution of aerosol depolarization ratio was about 0.03?0.05,and the vertical distribution of extinction coefficient was between0.001?0.025.The extinction coefficient showed a stable peak value at 0.7 km,ranging from 0.008 to 0.023 km-1,and then decreases rapidly to a lower level at 1.2km.Aerosol extinction coefficient decreases slowly with the height of 1.2?3 km,and suddenly decreases to 0 at 3 km.The weak peak value of aerosol extinction coefficient appeared at 2?3 km,and there might be an external aerosol layer.(2)In the case of high aerosol number concentration,the aerosols emerged at a height of 2.5 km and the height of the aerosol layer decreased gradually over time.In the case of Black carbon(BC)events,BC mainly came from the Taklimakan Desert in the northwest of the Tibetan Plateau,and mainly came from South Asia.The results of the backward trajectory from 2011 to 2020 show that,the potential source area of aerosol in Nam co is mainly the southern foothills of the Himalayas,and a small amount of aerosol falls in the desert area of the Tibetan Plateau in spring.In summer,the backward track fell between 88°E and 92°E to the south of 30°N,and a small amount of air mass came from the northwest of Nam co.In autumn and winter,the track from the northern Tibetan Plateau gradually disappeared.(3)MERRA-2 AOD is applicable on the Tibetan Plateau.The correlation coefficient between MERRA-2 and AERONET AOD is 0.66.However,Merra-2AOD was larger than AERONET AOD,especially in the range of AOD<0.2 and from March to April.The highest correlation was found in the central part of the Tibetan Plateau(Nam co),with a correlation coefficient of 0.73;the worst correlation was found in Qoms station,with a correlation coefficient of 0.5 only.(4)From 2011 to 2021,dust AOD,sulfate AOD,OC AOD,BC AOD and sea salt AOD accounted for 42.88%,35.06%,15.93%,5.03%and 1.10%of the total AOD,respectively.Dust aerosols contributed the most to total AOD in spring and summer,and sulfate contributed the most to total AOD in autumn and winter.Both seasonal sequence of dust AOD and dust AOD/total AOD were spring>summer>autumn>winter;the seasonal variation of sulfate AOD was small,but sulfate AOD/total AOD increased significantly in the order of spring-summer-autumn-winter.(5)The annual variation of five types aerosols are unimodal.The annual variation of dust aerosol optical thickness was large,with dust AOD greater than0.08 from April to May,and the dust AOD was the lowest in December;the annual variation of sulfate aerosol is relatively small,with the peak(0.044)occurring in August;the OC AOD peak(>0.02)occurs in March?April,and OC AOD varies between 0.010 and 0.013 from June to December;the highest value of BC AOD(0.0061)occurs in July and the lowest value(0.004)occurs in November;only in March-July the sea salt AOD of the Tibetan Plateau is>0.001.(6)The average AOD of the Tibetan Plateau was 0.098,with higher AOD in the southeastern,eastern and northern parts of the Tibetan Plateau and within 0.1 in all other areas.The dust AOD decreased gradually from north to south,and in spring the dust AOD was above 0.04 in the whole Tibetan Plateau,and in summer the dust AOD did not change significantly in the north and decreased significantly in the south.Sulfate AOD decreased gradually from northeast to southwest.The OC AOD decreases from southeast to northwest,and the OC AOD is the highest in spring(0.025),and only a small amount of OC exists in the southeastern part of the Tibetan Plateau in other seasons.BC AOD is the highest in summer,and more in the southern part of the Tibetan Plateau and decreases towards the north.Sea salt aerosol is mainly concentrated in the southeast,and a small amount of sea salt aerosol(0.02?0.08)exists in the northwest in spring and summer.(7)The rate of change of AOD on the Tibetan Plateau from 2011 to 2020 was-0.2×10-2±0.4×10-3yr-1.AOD on the Tibetan Plateau is decreasing in more than90%of the areas,only in the northern dust belt AOD is increasing,which is mainly caused by the increase of dust AOD.The southeastern part of the Tibetan Plateau was showing a faster decreasing trend.2011 to 2020 the rate of change of dust,sulfate,organic carbon,black carbon and sea salt AOD change rates are-0.6×10-3±0.4×10-3yr-1,-0.9×10-3±0.1×10-3yr-1,-0.4×10-3±0.1×10-3yr-1,-0.5×10-4±0.2×10-4yr-1,-0.6×10-4±0.2×10-4yr-1,respectively.The continuous reduction of dust aerosol and sulfate aerosol is the main reason for the overall decreasing trend of AOD,based on which the occurrence of dust events makes AOD with certain fluctuations. |
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