Retrieval Of Aerosol Optical Depth By Extended Dark Target Method And V5.2 Algorithm Over Lanzhou Aeras

The aerosol optical depth (AOD) is one of most important parameters of aerosol. It is also an important physical parameter to attribute the atmospheric turbidity. In recent years, it has already been made many progresses in the retrieval of AOD from satellite remote sensing. In the thick vegetation areas, the AOD retrieval has achieved high precision for the low surface reflection in red and blue wave band. But in some areas with high surface reflectance, such as arid, semiarid areas and cities, the retrieval of AOD is still a difficult problem. The main reason is, in these areas, the ground characteristic is very complex. It is extremely difficult to determine the surface index of reflection because of the heterogeneity of surface.The main point in this paper is the retrieval of AOD over land surface of arid, semiarid areas and cities. In this study, we choose the areas around Lanzhou city as the study area, and use the satellite data from Terra (Aqua)/ MODIS. The retrieval methods contain Kaufman extended dark target method and V5.2 algorithm proposed by Robert C. Levy et al. And then, the surface observations of sunphotometer are used to compare with the retrieval result.(1) From the observations of ground-based photometer, we can get the result of the high value of AOD concentrated in winter and spring. In summer, the pollution reduces and air quality becomes good, the AOD becomes relative small.(2) Comparing with the extended dark target method and V5.2 algorithm, we can find that the distribution of AOD is similar. From the distribution charts, the AOD of Lanzhou urban district is relative big. The main cause is the serious pollution in the city. The river-valley topography is disadvantageous to the diffusion of pollutant. The retrieval results in spring season show that there is a relative bigger center of AOD in Xigu district, and at the same time, the AOD of Chengguan district where Lanzhou University located is also big. However the retrieval results are not satisfied in Xinglongshan Mountains with good forest coverage in spring. A few of the retrievals are exceptionally large because the retrieval method is not available on the top of Xinglongshan Mountains with snow or cloud. Over some counties outside Lanzhou, there are high centers of the retrieval AOD corresponding the local high center of air pollution.(3) The comparison analysis between retrievals and sunphotometer observations is carried on. We can find that, in 14-day-retrieval results using the extended dark target method, the retrievals are close to the observations on 27 March 2007, with error -0.03%, and the relative errors are less than 20% in 8 days. The absolute errors are less than 0.10 in the 8 days. The average relative errors are 8.49%. Using V5.2 algorithm in 11-day-retrieval results, the relative errors are less than 20% in 3 days. The absolute errors are less than 0.10 in 4 days. The average relative errors are 24.89%.