Aerosol Optical Thickness Retrieval By Using The Synergy Of TERRA/AQUA MODIS Data

Aerosol has a significant impact on global climate and environment. Aerosol Optical Depth (AOD) is a very important parameter to describe the radiation attenuation caused by the aerosol. It can be used to indicate the atmospheric turbidity, evaluate the level of the atmospheric pollution. With the development of economy, atmospheric pollution is increasingly serious in China. Therefore, the study of aerosol has become a hot issue in atmospheric science and some related fields.AOD retrieval with the ground-based remote sensing has the advantages of the high precision and high time resolution, and its disadvantage is that point data cannot reflect the variation characteristics on the land surface. Satellite remote sensing can make up for the weakness of ground-based remote sensing. With the satellite data, AOD can be retrieved quickly in a wide range of areas. There are two very important problems in the process of AOD retrieval, they are how to determine the reasonable aerosol type and how to separate the contribution of the land surface and atmosphere from the radiation received in the satellite. The region of Taihu in China was chosen as the experimental area. In this paper, we explore to use the MODIS data of TERRA and AQUA to retrieve AOD over the land. The main research results of this paper are as follows:(1) The optical properties of aerosol and clouds(OPAC) was be used to simulate, the simulated results shows that the relative humidity don’t influence the refractive index of the non-absorbing aerosol particles, and the refractive index of the absorbing aerosol particles change with the relative humidity. Therefore, there is a flaw in this method which describes the absorption and scattering characteristic of the absorbing aerosol particles with a single refractive index.(2) The aerosol type of Taihu is the moderate absorbing type of MODIS aerosol algorithm. To identify the suitable for Taihu from the predefined five aerosol types in the OP AC, We analyzed the relative humidity data of Taihu between2009and2012, and determined to compare the predefined five aerosol types over the land in the relative humidity of70%with the moderate-absorbing type in the MODIS aerosol algorithm. The results show that the average continental type is the mostly close to the moderate absorbing type, so this article selects the average continental type in the OPAC to retrieve AOD over the land. (3) To evaluate the land reflectivity is one of the most important issue in the process of AOD retrieval. But this new algorithm doesn’t directly evaluate the land reflectivity in this paper. The algorithm is based on the fact that the ratio K of the land surface reflectance at different times in the same area has nothing to do with the wavelength, assuming that AOD is a constant within a short period of time, then it can be retrieved from the atmospheric radiative transfer model which was established and resolved with the assistance of the surface reflectance obtained from two MODIS images data of TERRA and AQUA satellites.(4) The comparison between the computed results by using the synergetic algorithm and the AEROENT data shows that the algorithm has high precision. The two bands of0.47μm and0.66μm have45sets of data, the average relative error of the computed results is23.8%and25.4%, the correlation coefficient is0.718and0.735respectively. This synergetic algorithm has the advantage that it don’t evaluate the land surface reflectivity in the process of AOD retrieval, the land surface reflectivity and AOD was calculated together as two variables, so it can effectively expand the applicable scope of the aerosol algorithm of the land.