Observational And Modeling Studies On The Aerosol Optical Properties Over The Eastern Edge Of Tibetan Plateau And Its Immediately Adjoining Sichuan Basin

Tibetan Plateau has a profound impact on the weather and climate changes as well as atmospheric environment in China.The eastern edge of Tibetan Plateau and the immediately adjoining Sichuan Basin can be regarded as an ideal region to investigate the aerosol optical properties and its influence mechanism with the distinct spatial differences between low and high aerosol optical depth under a special topography distribution.The study on the aerosol optical properties over the eastern edge of Tibetan Plateau and the immediately adjoining Sichuan Basin area could be beneficial to understand the impact of Tibetan Plateau to the variation of atmospheric environment.Aerosol optical properties such as Aerosol Optical Depth（AOD）,?ngstr?m Exponent（AE）and size distribution were retrieved by ground-based sunphotometer over Litang station in the eastern edge of Tibetan Plateau and their characteristics of variation at different time scale were analyzed.The relationship between AOD and AE at Litang station were discussed in different seasons.The Moderate-resolution Imaging Spectroradiometer（MODIS）retrieved AOD products over the eastern edge of Tibetan Plateau were validated by using the observational data retrieved from ground-based sunphotometer.During a severe haze episode over Sichuan Basin from 1st to 7th January 2017,the aerosol optical and radiative properties were analyzed by using ground-based observational data.The impact of local anthropogenic emissions to aerosol optical and radiative properties over Sichuan Basin was discussed by using model simulation.The major conclusions are as follow:1、The variations of aerosol optical and radiative properties over the eastern edge of Tibetan Plateau:At the Litang monitoring station,annual averaged values of 0.08±0.03 and0.72±0.23 were observed for AOD_440nm40nm and AE_440-870nm,respectively.During the whole observation period,both AOD_440nm and AE_440-870nm showed high values in summer and the low values in winter.The variation of monthly averaged AOD_440nm showed a maximum value of0.11±0.04 in July and a minimum value of 0.05±0.02 in January.The AE_440-870nm showed a maximum value of 1.08±0.19 in July and a minimum value of 0.48±0.32 in February.The bimodal size distribution with a higher peak value of coarse-mode aerosol and a lower peak value of fine-mode aerosol.The atmospheric condition with high AOD_440nm and low AE_440-870nm40-870nm was more frequent occurred in spring（15.57%）,reflected that Litang station was easier affected by the coarse-mode aerosol such as dust aerosol in spring.In summer,the frequency of atmospheric condition with high AOD_440nm and high AE_440-870nm was 54.52%,indicated that Litang station was affected by the anthropogenic aerosol.The frequency of atmospheric condition with low AOD_440nm and low AE_440-870nm was 50%in autumn,represented that Litang station was dominated by the background aerosol.In winter,the coarse-mode aerosol was primary particle at Litang.However,the AOD_440nm showed an increasing tendency with the AE_440-870nm increased,indicated that the extinction effect of fine-mode aerosol play an important role in winter AOD at Litang station.2、Validation of the MODIS aerosol optical depth products over the eastern edge of Tibetan Plateau by using ground-based sunphotometer:The accuracy and applicability of Moderate-resolution Imaging Spectroradiometer（MODIS）AOD products over eastern edge of Tibetan Plateau were validated by using retrieved data from CE-318 sunphotometer.The correlation coefficient between the CE-318 sunphotometer data and the MODIS₃K,MODIS_L2_DT and MODIS_L2_DB AOD products were 0.58,0.32 and 0.29,respectively.The percentage of the paired validation data falling within expected error envelope were 61%,47%and 92%for MODIS₃K,MODIS_L2_DT and MODIS_L2_DB,respectively.Overall,the MODIS₃K and MODSI_L2_DT showed an overestimation for the AOD over the eastern edge of Tibetan Plateau.The MODIS_L2_DB showed an underestimation for the AOD over the eastern edge of Tibetan Plateau.Within the margin of error,MODIS AOD product with 3km resolution（MODSI₃K）can show the variation tendency of AOD over the eastern edge of Tibetan Plateau.3、Variation of the aerosol optical and radiative properties during a severe haze pollution episode in Sichuan Basin:The aerosol optical and radiative properties during a severe haze pollution episode in Sichuan Basin were analyzed by using CE-318 sunphotometer.With the aggravating of haze pollution in Chengdu area,the AOD_440nm was increased from 0.61±0.34to 1.04±0.11 while the AE_440-870nm was decreased from 1.35±0.03 to 1.08±0.02.The single scattering albedo（SSA）was increased from 0.84±0.08 to 0.94±0.04.The aerosol direct radiative forcing（ADRF）at the bottom and at the top of atmosphere were enhanced from-106.18 W/m² to-120.87 W/m² and from-13.44 W/m² to-43.25 W/m²;while the aerosol direct radiative forcing was receded from 92.72 W/m² to 77.63 W/m².The cooling effect of atmospheric aerosol was enhanced both for the top and bottom of atmosphere;while the warming effect of atmospheric aerosol to the atmosphere was receded.The averaged peak value of fine-mode aerosols was raise from 0.055μm³/μm² to 0.096μm³/μm² with the effective radius of fine-mode aerosols increased;while the peak value and the effective radius of coarse-mode aerosols were almost unchanged.The variations of aerosol optical and radiative properties were attributed to the variations of fine-mode aerosols.4、The impact of local anthropogenic emissions to aerosol optical and radiative properties in Sichuan Basin:To investigate the influence of local anthropogenic emission to the aerosol optical and radiative properties,the AOD and ADRF of the Sichuan Basin during a severe haze pollution episode from 1^st to 7^th January 2017 were simulated by using WRF-Chem model.The result showed that the AOD was reduced by 85%and the ADRF at the surface was significantly decreasing over the western Sichuan Basin without the anthropogenic emission.During the haze pollution episode,the variations of AOD and ADRF over the western Sichuan Basin were attributed to the anthropogenic emission,especially the most severe pollution period.