Numerical Simulation Of Seasonal Differences In The Transport Of Black Carbon From South Asia To The Qinghai-Tibet Plateau

The black carbon aerosol(BC)deposited on the glacier surface will reduce the albedo over snow and absorb more solar radiation,thereby accelerating the melting of snow,which will affect the regional water cycle,while the warming of the atmosphere over the plateau will change the Indian monsoon circulation and affect the atmospheric circulation in the northern hemisphere.Previous studies have shown that black carbon from South Asia is one of the main sources of the Qinghai-Tibet Plateau.The transportation of black carbon to the Qinghai-Tibet Plateau presents obvious seasonal differences.The WRF-Chem model is used to study the transmission mechanism of South Asian black carbon to the Tibetan Plateau in April(pre-monsoon),July(summer monsoon)and December(winter monsoon).MIX emission inventory and Peking University's global black carbon emission inventory(PKU-BC)were involved to analyze the seasonal distribution of black carbon concentration,dry and wet deposition in the Qinghai-Tibet Plateau and South Asia,and the distribution of BC concentration and wind field at different altitudes.Combined with the vertical distribution of BC concentration across the Himalayas,the transport mechanism of black carbon in South Asia to Qinghai-Tibet Plateau in different seasons is studied.In South Asia,the total annual black carbon emissions from two inventories are relatively consistent,but show different seasonal variation.The PKU has the highest in spring in South Asia and the lowest in summer,while the MIX has the highest in winter and the lowest in summer.In South Asia,the black carbon emissions of PKU in the Ganges Plain and surrounding areas are higher than the MIX emission inventory,and the gap is strongest in spring;while in the central region of South Asia,the black carbon emissions of the PKU emission inventory are smaller than the MIX emission inventory.The black carbon concentrations obtained from the two emission sources show the same seasonal distribution characteristics.The concentration of black carbon on the ground in South Asia is the highest during the winter monsoon,and the concentration of black carbon on the ground in South Asia is the lowest during the summer monsoon.The concentration of surface black carbon on the Tibetan Plateau during the pre-monsoon period and the winter monsoon period in the Qinghai-Tibet Plateau was significantly greater than that during the summer monsoon period.The black carbon concentration gap between the two emission inventories in the Ganges Plain can reach 2-3 ?g/m3,and the increase rate on the southern slope can reach 50-80%;during the summer monsoon,the gap in the Ganges Plain is only0.1-0.5 ?g /m3,even some areas are smaller than the simulated value of MIX.At this time,the improvement rate of the southern slope has dropped to 30%.During the pre-monsoon,the prevailing southwest wind in the Bay of Bengal can transport black carbon from South Asia to the southeastern corner of the plateau;the prevailing northwest wind near the western Himalayas can carry black carbon to the western Himalayas.During the transportation of black carbon in South Asia,more dry deposition can be generated on the western slope of the plateau.The increase in black carbon concentration near the Ganges Plain will cause the black carbon to be transported farther in South Asia and produce more dry deposition in the hinterland of the plateau.At this time,there were both black carbon that passed through the Bay of Bengal to the southwest corner of the plateau,and black carbon that reached the Sichuan Basin and was uplifted and transported to the Qinghai-Tibet Plateau under the action of the southeast wind.A large amount of wet deposition of black carbon was produced in both areas.During the summer monsoon,black carbon accumulates at the foot of the Himalayas due to the prevailing southwesterly winds in the Bay of Bengal,and is transported to high altitudes under the updraft and climbing airflow.However,due to the weakening of the northward movement of the high-altitude westerly belt,the transportation of BC is weakened.At this time,black carbon in South Asia mainly settled on the plateau surface through wet deposition.BC was mainly transported to the plateau through the uplift of the terrain,so there was more wet deposition of black carbon throughout the Himalayas.During the winter monsoon,the prevailing northeasterly winds in South Asia are not conducive to the transportation of black carbon to the Qinghai-Tibet Plateau,but the prevailing northwest winds near the surface of the Ganges Plain make black carbon accumulate in the western Himalayas and are lifted into the air by the upward air currents along the slopes.The westerly jet stream of the country can carry it into the Tibetan Plateau or blow it to the southern Himalayas,and then transport it across the Himalayas to the Tibetan Plateau.At this time,the deposition of black carbon is not obvious.The accumulation of black carbon aerosols in the foothills of the Himalayas carried by the northwest wind is an indispensable part of the black carbon transportation process.Combining the difference in the distribution of emissions in South Asia between the PKU emission inventory and the MIX emission inventory and the simulated plateau black carbon concentration,we believe that 25°N-30°N,75°E-80°E is the most important area for the transportation of black carbon from South Asia to the Qinghai-Tibet Plateau.