Aerosol Optical Properties And Direct Radiative Forcing Over Eastern China

By absorbing and scattering solar and infrared radiation,atmospheric aerosol has a significant influence on global climate.Heating or cooling the Earth's atmospheric system depends on the optical and physical chemical properties of the aerosols.Aerosol radiative effects remain one of the largest uncertainties in climate assessments due to the large spatial and temporal variations of aerosol amount and properties and strong interactions with other atmospheric processes.Parameters that describe aerosol properties are necessary for accurately assessing the radiative properties of aerosols.Thus,this paper will help to reveal the long-term seasonal characteristic of aerosol and to improve scientific understanding of the relation of aerosol optical parameters to radiative forcing over the east of China.A combination of ground remote sensing and satellite remote sensing were employed to investigate the aerosol optical properties,radiation forcing and their relationship during long time series.The radiation transfer model was used in simulated radiation forcing.In addition,we also classified the aerosol types based on optical parameters.The results showed that the seasonal variations of aerosol optical depth obtained from ground-based and satellite data are overall similar,with lower values in autumn and winter and higher values in spring and summer.Higher values of optical depth in summer were typically related to hygroscopic growth under high temperature and humidity conditions.The scattering and absorption of light by aerosols in summer were relatively weak,but more light was scattered because of water vapor,which lead to the highest average albedo value of single scattering in summer.The dust particles transported from long distances in spring had a certain influence on the aerosol components.It was more obvious effect in Beijing,which was closer to the dust source area,but a relatively small effect on the Taihu Lake,because Taihu located in a relative downstream area.A large amount of absorptive aerosols(such as black carbon)was increasing during winter,making the absorption of aerosols increased significantly.The comparison of aerosol optical depth between high-resolution satellite data and ground-based data suggested that the satellite retrieval results were in good agreement with the ground-based results.However,satellite retrieval algorithm on a large scale of continental waters like lakes was inaccurate,resulting in thedata over the lakes were missing or overestimated.In terms of the spatial distribution,the high optical depth mainly occured in the southern of Beijing-Tianj in-Hebei,the northern of the Yangtze River Delta region,the border area of the Hunan and Hubei Province,the northern of Henan Province,the northern of Jiangxi Province,and the central of Shaanxi and Shanxi Province.These areas are usually densely populated,economically developed and high-energy consuming areas and industrial zones.The high and low value areas of aerosol optical depth were similar to the column of aerosol mass concentration in four seasons,suggesting that the aerosol level was relatively high in the above areas.The spatial distribution of aerosol fine mode fraction was opposite to that of aerosol optical depth.Aerosols have radiative warming effects on the atmosphere.There were obvious positive correlations between aerosol optical depth and radiative forcing(absolute value),showing that the intensity of extinction was major reason for radiative forcing.However,the radiative forcing was different in the case of the same optical depth,and the difference increased with the increase of optical depth.It means optical depth was not the only factor affecting the aerosol radiative forcing.As shown in result,there was a correlation between the single scatting albedo and the radiative forcing efficiency,indicating the ratio of scattering efficiency to total extinction efficiency could largely influence radiative forcing,especially for the same extinction.The relationship also showed that the contribution of strongly absorbed aerosols to radiation forcing was much greater than that of strongly scattered aerosols.Therefore,it was suggested that strong absorptive aerosols should be took into account when assessing aerosol radiation and climate effects in future study.The three classification methods of ground-based had good applicability,and the results of satellite classification technique were excellent.The classification method based on pollution level and particle size focused on the characteristics of the aerosol clusters.The optical parameters were used in this method were easy to obtain and it was accurately used to describe the aerosol types.Classification methods based on particle extinction characteristics and scale characteristics were usually more stringent,with more emphasis on the individual aerosol types.It was more effective for the identification of a certain type of aerosol burst.Dust aerosol samples increased during spring compared with other seasons.Under the influence of dust mixing,the mixed-large particle type also increased greatly during spring.The main aerosol type over Beijing and Taihu during all the seasons was the mixed-small particle category,followed by the Urban/Industrial category,indicating the pollution sources of Beijing-Tianjin-Hebei and Yangtze River Delta were quite complex.The results of spatial classification showed that polluted dust aerosol types and mixed types dominated in spring.The urban/industrial aerosol types accounted for a high proportion in summer.In fall,biomass-burning aerosols have a higher frequency in the area of Henan and Anhui Province.The high frequency regions of mixed types appear in the north of the Yangtze River,and the high-frequency urban/industrial aerosol types appear in the south of the Yangtze River.The main category of aerosol was mixed aerosol types in winter.In Inner Mongolia region,it was found that aerosol types was dominated by low pollution dust aerosol all the year around.The high value area of clean continental aerosol mainly appears in the forest areas and grasslands.