3. Retrieval of the Aerosol Size Distribution
The optical properties of aerosol are determined by the aerosol size distribution, refractive index, shape of the particle etc. it is very difficult to retrieve all these parameters from the data of AOD. But we can assume that the atmospheric aerosol is composed of four types of aerosols as recommended by WCP-55 (1983). They are dust like, water soluble, oceanic and soot. The parameters of these four types are assumed well known, and their size distribution accords to Log-normal function. Ten the AOD of k(k=1,10)band can be calculated by the formula:


where ni means total number of particles of i(i=1, 4) type, fi(r) means size distribution of i types.

According our assumption, we know well the parameters of the four types of aerosol, so we can use MIE theory to calculate QEX(). Then in formula (1) only n is unknown. Because we have known the AOD of ten bands, we can have ten formulas with only four unknowns. It is enough to retrieve the total number of particles of each type. The size distribution of the aerosol can be obtained by adding these four types together.

III. Satellite Remote Sensing
When illuminated by the Sun, aerosol particles can scatter a fraction of the solar radiation back to space. The radiation received by satellite includes the information of aerosol. So satellite can be used to retrieve aerosol distribution. As we can see, although ground based remote sensing can get the aerosol properties, it is a hand work to set enough ground-based equipment to get the aerosol properties over the world, especially over ocean, desert and other hard environment. Satellite have a high spatial and temporal resolution, it is widely used to map aerosol distribution, especially over ocean (Y. J. Kaufman, 1997; T. Nakajika, 1997).

The main aim of the project "the radiation Characteristics of the atmospheric Aerosols over China area" was to deduce the data of atmospheric optical depth over all the China. So besides using Sun-photometer in five sites as discussed above, we will retrieve the data of AOD by GMS-5 and TOMS data.

1. GMS-5 Remote Sensing
Assuming the Earth's surface is Lambertian surface with a reflectance Rs, the measured reflectance of the visible channel can be expressed as


where TA(m0) and TA(m) stand for the total transmittance of the atmospheric from the Sun to the surface and from the surface to the instrument, respectively. RA is the atmospheric reflectance. The factor 1/(1-Rs RA) expresses the multiple reflections between the surface and the atmosphere.

We can see from equation (2), RM is related to the reflectance of the surface and the components of atmosphere. For one channel retrieval, we must know one of them to retrieve another. Water surface has low and relative steady reflectance in visible channel. It is more convenient to retrieve the AOD over water than over land (Y. J. Kaufman, 1997). So we want to retrieve the AOD data over twenty-five lakes first. Chaoi Lake is one of the lakes.

We have the AOD data of He Fei observed by Sun-photometer, a site not far from Chao lake. First, using the data of AOD as atmospheric correction, the reflectance of the lake is retrieved. The result is shown in Figure 6. Then assuming the reflection of the lake is known, the AOD over the lake can be retrieved by satellite measured reflectance. The comparison between the month averaged AOD (550nm) observed by Sun-photometer and retrieved from satellite reflectance is shown in Figure 7.


Figure 6: Reflectance of Chao lake over the year ovserved by GMS-5 visible channel
Figure 6 shows the reflectance of the lake change with the season. Late spring has a peak of the year. The reason is the wavelength range of GMS-5 visible channel is 0.5mm-0.9mm. High reflectance of chlorophyll in near infrared makes the lake have a high surface reflectance. As can be seen from Figure 7, the biggest relative error between the months averaged AOD retrieved by GMS-5 and observed by Sun-photometer is no more than 30%.


Figure 7: Comparison of AOD observed from Sun-photometer GMS-5
2. TOMS Remote sensing
Because of the requirement of low and nonvariable surface reflectance, the method of using satellite visible channel to retrieve AOD is limited to being over water. In UV band, the surface reflectance is low and nearly constant over both water and land. So in recent years some authors have tried to detect aerosol using TOMS data (J. R. Herman et al., 1997).

Mostly these works use a spectral contrast method. The term used in this method is called Aerosol Index (AI). For Earth Probe (EP) TOMS, AI can be expressed as


where Imeas is the backscatter radiance measured by TOMS at a given wavelength, Icalc. Is the radiance calculated using a Radiative transfer model for a pure rayleigh atmosphere.