Identification And Interdecadal Characterization Of South Asian Aerosol Pollutant Transport To The Tibetan Plateau

South Asian aerosol pollutants can be transported to the Tibetan Plateau(TP)and affect the local atmospheric environment;however,few studies have produced statistical results for the long term.In this paper,we use aerosol reanalysis data combined with statistical methods to explore the main spatial and temporal distribution modes of aerosol optical thickness(AOD)in the TP region from 1980 to 2020.On this basis,an objective method is established to screen 41 years of pollution events transported from South Asia to the TP in combination with backward trajectory models to reveal the main pollution transport mechanisms,and further analyze the characteristics of interdecadal differences in pollution transport events and the influencing factors.The main conclusions are as follows:(1)The Modern-Era Retrospective analysis for Research and Applications,Version 2(MERRA-2)reanalysis aerosol data has good applicability in the TP region,where there are three areas of large values of AOD variation influenced by pollution transport from South Asia in the southern TP.The overall agreement between MERRA-2 AOD and wavelength index(AE)and site observations is good,with correlation coefficients of 0.88 and 0.59,respectively.MERRA-2 is also in good agreement with satellite inversion observations,which can represent the spatial distribution of AOD on the TP,and can reflect the interannual and seasonal variability of the TP.There are three main spatial and temporal distribution modes of AOD on the southern TP,and the large signal area is close to South Asia,and it matches the temporal variation of pollution emission in South Asia.(2)Strong pollution days affected by pollution transport from South Asia reached1702 days in 41 years on the TP and were able to be transported to the TP through circulation at different altitudes.2058 strong pollution days existed in the southern TP in 41 years,of which 1702 days(82.7%)were affected by pollution transport from South Asia.Among these 1702 days,the central region has the highest number of days with 661(38.8%),followed by the western and eastern regions with 544(32%)and 497(29.2%)days,respectively.South Asian pollutants accumulated on the southern side of the Himalayan foothills can be transported across the Himalayas to the TP by valley winds,north-south valley channels and cyclonic uplift.(3)The interdecadal variation in pollution transport events around 1999 is characterized,probably influenced by the intensification of pollution emissions from South Asia and the reanalysis of assimilation data.The number of pollution transport events is highest in spring,and the number of spring transport events shows a significant abrupt change in mean value around 1999.Compared with the pre-1999 period,the characteristics of AOD changes in the southeastern TP affected by South Asian pollution transport increased after 1999,and the main transport area of transport events changed from the west to the east,and the dominant aerosol type changed from dust to anthropogenic type.The difference in the characteristics of these two periods may be influenced by two factors: first,the relatively small number of sample observations assimilated by the reanalysis data before 1999,making the overall transport events in this period low;second,the subsequent increase in the level of anthropogenic emissions with economic development in South Asia,making the transport events after 1999 significantly enhanced.The results of this paper deepen the understanding of the transport of aerosol pollutants from South Asia to the TP,which helps to clarify the influencing factors and reveal the mechanisms of atmospheric changes on the TP,and provides a basis for further understanding of climate change on the TP.