The Optical Properties And Global Radiative Forcing Simulation Of Black Carbon And Dust Aerosols

Radiative properties and global distribution of black carbon and dust aerosols are analyzed byusing data in GADS (Global Aerosol Data Set). Meanwhile, by use of the modified radiationtransfer model, the global radiative forcing of black carbon and dust aerosols is simulated and thereasons may lead to uncertainty in the radiative forcing are discussed, thus some meaningfulconclusions are achieved.The main work of this study can be summarized as follows:1. By use of GADS, the complex refraction index, extinction coefficient, single scatteringalbedo and asymmetric factor of different black carbon and dust aerosols are analyzed. Bycombing the above results with concentration data of black carbon and dust aerosols, theoptical depth of black carbon and dust aerosols in winters and summers is calculated, then thecharacteristics of spatial and temporal distribution of global optical depth is analyzed. The resultsshow: (1) The global optical depth distribution of black carbon and dust aerosols is closelyrelated to human activity areas, and it is mainly expressed as absorption to solar radiation,presenting the main characteristics of backscattering; (2) The four main dust areas are in NorthAfrica, Mid-Asia, Western Australia and Western North America. The dust aerosols performweek absorption to the radiation with wavelength less than 8μm and strong absorption in 8-11μm.For wavelengths less than 0.8μm, the dust aerosols mainly perform the properties of forwardscattering.2. Using the radiation transfer model BSTAR5C/CCSR/NCC modified by Zhang et al., theradiative forcing of black carbon and dust aerosols in clear sky is estimated. The effects onradiative forcing in clear sky of land albedo and sun altitude by virtual of a serial of sensitivityexperiment are discussed and analyzed the change of mean in latitudinal direction of blackcarbon aerosols. The results show: (1) The global mean radiative forcing at the top of tropospherein winters caused by black carbon aerosols is 0.085 W/m~2, while 0.155 W/m~2 in summers. Theglobal mean land radiative forcing in winters is -0.37 W/m~2, while -0.626 W/m~2 in summers. Thepositive radiative forcing at the top of troposphere and absolute value of negative radiativeforcing on the ground of black carbon aerosols linearly increase with the cosine of solar zenithangle. Land albedo greatly influences the magnitude and distribution of radiative forcing causedby black carbon aerosols. (2) The dust aerosols can absorb and scatter solar radiation andlongwave radiation of land. The global mean radiative forcing at the top of troposphere in winterscaused by dust aerosols is -0.07 W/m~2, while -0.02 W/m~2 in summers. The global mean landradiative forcing in winters is -0.85 W/m~2, while -0.98 W/m~2 in summers. For shortwave radiative forcing, land albedo may play an important role in the magnitude and distribution ofradiative forcing caused by dust aerosols. The radiative forcing at the top of troposphere andabsolute value of negative radiative forcing on the ground of dust aerosols linearly increase withland albedo. Solar zenith angle greatly influences the radiative forcing at the top of troposphereand shortwave radiative forcing on the ground caused by dust aerosols. For the dust aerosols withthe same concentration, their influences on the thermal and dynamic structure of land-atmospheresystem are different in different seasons.3. Radiative forcing of black carbon and dust aerosols in cloudy sky is estimated, usingISCCP cloud cover data and Canadian cloud diagnose scheme of J.Li. Compared to radiativeforcing in clear sky, the influences of cloud on radiative forcing at the top of troposphere and onthe ground are analyzed. The results show: (1) Middle and low cloud mainly play the effect ofcloud on radiative forcing caused by black carbon aerosols. Cloud does not change the sign ofradiative forcing of black carbon on the ground, but reduce the negative radiative forcing on theground. (2) For the shortwave radiative forcing in clear sky caused by dust aerosols at the top oftroposphere, cloud weakens the direct radiative forcing caused by dust aerosols over the landwith higher albedo while strengthen the direct radiative forcing caused by dust aerosols over theland with lower albedo. Cloud markedly not only weakens regional average of shortwaveradiative forcing on the top of troposphere, but also for longwave radiative forcing, but theeffects are smaller than shortwave radiative forcing. Also, cloud can reduce radiative forcingcaused by dust aerosols on the ground, but the effects are smaller than those on the tropopause.