The Spatial And Temporal Distribution And Radiative Forcing Of Absorbing Aerosols Over East Asia

Absorbing aerosols(i.e.black carbon,organic carbon and dust)in the atmosphere can strongly absorb solar radiation and have a profound impact on the various aspects of the climate system,including radiation budget,water cycle,monsoon,etc.East Asia is not only one of the most important dust source regions,but also the area with largest emission of black carbon in the world.Therefore,the study of the abosoring aerosols over East Asia is of great significance.This study systematicly and thoroughly discusses the spatial-temporal distribution of the absorbing aerosols over the East Asia and the difference of radiative forcing(RF)over different climatic regions based on WRF-Chem model in combination with multi-type satellite and ground-based observation data.The main results are summarized as follows:(1)The spatial-temporal characteristics of the absorbing aerosol over the East Asia are analyzed using OMAERUV product from Ozone Monitoring Instrument(OMI)during 2006-2016.The absorbing aerosol optical depth(AAOD)reaches its maximum in the spring and minimum in the summer,and its high-value regions mainly distribute on the southern side of the dividing line between the humid area and arid area(i.e.aridity index is equal to 0.65)in the winter.The low-value regions of single scattering albedo(SSA)can be found at dust source regions and Southeast Asia.In the summer,SSA is comparatively high on the southern side of the dividing line between the humid area and arid area,which means the scattering of the aerosols are dominant.In general hyper-arid and arid regions have highest aerosol optical depth(AOD)and AAOD among the five climatic regions,with springtime values up to 0.715 and 0.038,respectively.Humid and sub-humid regions have relatively high AOD and AAOD during the spring and winter and highest SSA during the summer.In the period of 2005-2016,AAOD over the East Asia shows significant upward trends,which is likely due to the increase of black carbon and organic carbon emissions.Total Consumption of coal and diesel increased by 91.51 and 7.42 million tons each year,respectively.SSA over the East Asia shows downward trends,indicating the enhancement of the aerosol absorption.In contrast to AAOD,AOD over the East Asia shows downward trends.The decrease of AOD in the southern China is probably related to the reduction of sulfate aerosols.SO2,as the precursor pollutant of the sulfate aerosol,has decreased by 0.68 million tons since 2006.(2)The spatial and temporal distribution of dust events over Tibetan Plateau(TP)are analyzed using dust events records of surface meteorological stations during 1961-2010.The Tibetan Plateau Dust Index(TPDI)for the spring and winter are also defined.It is found that the dust events over TP are characterized with a dominant declining trend during 1961-2010,accompany with annual and decadal fluctuations.1970 s witnessed active dust activity,and the occurrence of dust events has significantly decreased since then.Mann-Kendall test suggested that the decreasing trend was possibly an abrupt change in the 1990 s.Analysis shows that there are three potential causes for the change of dust events over Tibetan Plateau(TP).Firstly,surface wind speeds are positively correlated to the TPDI.The linear trends of surface wind speeds are-0.032 ms-1yr-1and-0.008 ms-1yr-1 for spring and winter.The averaged number days with strong winds also have decreased by 0.98 and 1.36 day each decade for spring and winter.Secondly,the Normalized Difference Vegetation Index(NDVI)is negatively correlated to the TPDI and its linear trend is 0.001 yr-1 for spring and winter,indicating the increase of vegetation cover over TP.Additionally,the ridge in the north of TP became stronger,while the westerly jet became weaker.(3)WRF-Chem model is applied to investigate the spatial characteristic of dust and black carbon(BC),which are two main types of absorbing aerosol,and to estimate the RF of absorbing aerosol in the different climatic regions over East Asia in 2007.The comparison between simulation results and satellite and ground-based observation indicates that WRF-Chem could reproduce the spatial characteristic of the aerosol during the simulation period.The average RF of dust over the simulation domain is-0.84 Wm-2 at the top of atmosphere(TOA),0.39 Wm-2 in the atmosphere and-1.23 Wm-2 at the surface,the average RF of BC is 1.06 Wm-2 at TOA,2.61 Wm-2 in the atmosphere and-1.55 Wm-2 at the surface.As a result of the distribution of the emission source and influence of the transport,the spatial distributions of RF due to dust and BC show evident geographical feature,with high-value area of dust RF mainly distribute on the northern side of the dividing line between the humid area and arid area while high-value area of BC RF distribute on the sourthern side.In general,dust RF is dominant over hyper-arid and arid region,while BC RF is dominant over humid and subhumid region.In the semi-arid region,the RF of dust and BC are roughly equivalent.In summary,this study deepens the overall understanding of the absorbing aerosols over East Asia and also establishes the foundation for further investigation of the role of absorbing aerosols in the regional difference of the warming rate.In addition,analysis of the trends of dust events and its potential causes over the Tibetan Plateau has a positive effect on the research of climate change in Tibetan Plateau.