Abstract
Atmospheric correction is an important part of the data processes for the satellite remote sensing. The optical depth of atmospheric aerosol is an important parameter for this correction. The main aim of the project "The Radiation Characteristics of the Atmospheric Aerosols over China Area" was to deduce the data of aerosol optical depth over all the China. On the ground, five Sun-photometers are used to measure the optical depth from Feb. 1998 to Jan. 1999 in five sites. The Sun-photometer has ten wavebands between 0.4 to 1.0 micrometer, it gives the wavelength dependence of optical depth. This information is used to deduce the size distribution of the atmospheric aerosols. The satellite data include the visible channel of GMS-5 and TOMS are used in this project. Combining with the Sun-photometer data observed at HeFei, the GMS-5's data is used to retrieve the optical depth and the water surface reflectivity over the Chao Lake. The result shows the biggest relative error of the month average optical depth can be controlled fewer than 30%.

1. Introduction
Atmospheric aerosol is an important parameter in atmospheric correction for remote sensing and in research for climate changing. It is also one of the most uncertain parameters for these study. Many measurements have been done in recent years over the world to obtain the detailed information of the aerosol spatial distribution and variation with time. The methods include direct measurement by particle counter (G. Shi et al., 1995) and remote sensing from Sun-photometer and satellite (M. Stettler et al., 1993; R. B. Husar et al., 1997).

For getting the information of atmospheric aerosol in China, the project "The Radiation Characteristics of the Atmospheric Aerosols over China Area" was put into practice. In this project five Sun-photometer were used to measure the Aerosol Optical Depth (AOD) from ground in 1998. These five sites are Shang dian Zi of Beijing (40.65° N, 117.12° E), Wa Li Guan of Qing Hai (36.29° N, 100.90° E), Xing Feng Jiang of Guang Dong (24.05° N, 114.2° E), He Fei of an Hui(31.31° N, 117.16° E) and Dang Xiong of Tibet (30.48° N, 91.1° E). The satellite data of the visible channel of GMS-5 and TOMS are also used in this project to retrieve the AOD all over China.

In this paper, we first discuss the Sun-photometer remote sensing data of AOD, then discuss the method to retrieve the size distribution from these AOD data, in the end we will show the results of AOD retrieved by GMS-5 and TOMS.

II. Ground-Based Remote Sensing
The Sun-photometer we used in this project have ten bands from 400nm to 1000nm. The bandwidth is 10-20nm. All of them are calibrated in Xing Long observatory (HeBei provinces) using Langley method. The linear correlation of the Largley plots are greater than 0.99, some of them greater than 0.999.

2. Data of AOD
After the extinction of the aerosol was measured by the Sun-photometer. The data of AOD were computed using calibrated constants. The results of 550nm AOD are shown in Figure 1 to Figure 5. Because it is difficult to observe in Tibet, we only have the data from late May to early July in Dang Xiong region.


Figure 1: AOD variety over the year in the area of shang Dian Zi

Figure 2: AOD variety over the year in the area of Wa Li Guan

Figure 3: AOD variety over the year in the area of Xin Feng Jiang

Figure 4: AOD variety over the year in the area of He Fei
Figure 1 to Figure 4 show that the value of AOD has an obvious dependence on the season. Normally spring has the highest value of AOD. This phenomenon is associated with the dry climate in spring. In this season, wind brings more dust to atmosphere from ground. In Tibet, the climate can be distinguished by dry and wet season. Although we only observed no more that two months in the area of Dang Xiong, it includes these two seasons. The wet season begins from about 160 Julian day. As can be seen from Figure 5, the value of AOD in dry season is higher and has larger day to day variation than wet season. Many areas over the world also show the AOD dependence on climate such as R. T. Pinker et al (1997).


Figure 5: AOD variety over the period of observation in the area of Dang Xiong