The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
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The AERONET ground-based sunphotometer makes one 
measurement every 15 minutes. AOT of 7 channels (0.34, 0.38, 
0.44, 0.50, 0.67, 0.87 and 1.02pm) can be derived from the 
direct solar radiation measurements (with only part channels at 
some sites) with the precision of 0.01-0.02 (Eck et al, 1999). 
The ground-based and satellite products used covered the 
period from Jan. 2005 to May 2007. 
2.2 Spatio-temporal Matching Up Method 
The spatio-temporal sampling properties of satellite and 
ground-based data are different. Thus, proper sampling window 
should be determined according to the measurement interval of 
ground-based data, space resolution of satellite product and 
aerosol variation and uniformity over time and space. The 
averages of effective aerosol pixels in the space window and 
effective ground-based measurements in the time window make 
one match-up to be involved into the validation dataset. 
Based on former studies (Choku et al, 2002; Remer et al, 2002, 
2005; Chen & Yang, 2005), ground-based data were averaged 
within ±0.5h (or ±1.0h) centered at the satellite observation 
time, and those with measurements not less than 2 (or 4) were 
included into the validation dataset. 
Satellite data were averaged within a radius of 25km (or 45km) 
centered at the AERONET site location, and those with 
effective pixels not less than 5 (or 9) after data elimination for 
land, turbid water, and cloud contamination were included into 
the validation dataset. 
2.3 Validation Channel Selection 
Although MODIS and AERONET provide AOT for several 
channels, only channels centered at 658nm (675nm for 
AERONET) and 860nm (870nm for AERONET) are most 
nearest for direct comparison. 
Former research indicated that the natural logarithm of AOT 
can be well fitted as the quadratic polynomial of natural 
logarithm of wavelength, with the fitting error of about 
0.01-0.02 (Eck et al, 1999). Thus, the 550nm AOT at 
AERONET site was estimated with equation (1) to validate the 
MODIS AOT at 550nm. Wavelengths of 440, 675, 870 and 
1020nm were selected for fitting. 
lnx a (X,)=ao+ a i lnX,+a 2 (lnX.) 2 ( 1 ) 
where x a (À) = AOT at wavelength X 
ao, a b a 2 = polynomial coefficients 
3. RESULTS AND DISCUSSION 
Using 25km radius and ±0.5h as the sampling window, there 
were 162 match-ups. Excluding those with ground-based 
measurements less than 2 and satellite pixels less than 5, 32 
matched data were left, in which ground-based data were from 
sites of “Gosan SNU” (29 match-ups) and “Shirahama” (3 
match-ups) with the averaged AOT at 550nm of 0.35. Figure 2 
displays scatter diagrams of the match-ups, in which dashed 
lines represent the upper and lower limits of expected 
uncertainties of MODIS AOT product (over ocean, 
At=±0.05±0.05t for 550nm (Tanre et al, 1997) and 
At=±0.03±0.05t (Remer et al, 2002) for 660 and 860nm), and 
the red is the linear fitting line. Table 1 lists the corresponding 
error statistics. 
The R 2 of linear fitting between MODIS and AERONET was 
0.68, 0.65 and 0.56 for MODIS 550, 658 and 860nm channels 
respectively. There was relatively large dispersion at low AOT, 
and the R 2 of these 3 channels was increased to 0.77, 0.74 and 
0.67 when data with AOT less than 0.05 were discarded. 
The RMSE was 0.15, 0.13 and 0.10 for MODIS 550, 658 and 
860nm channels respectively. The mean relative errors all 
exceeded 70%, but were reduced to 37.8%, 30.8% and 30.0% 
without AOT less than 0.05. 
The percentages of retrievals within the expected uncertainties 
were 62.5% for 550nm channel (only 46.9% for 
At=±0.03±0.05t) and 46.9% for channels of 658 and 860nm. 
There were relatively large disparities compared with the 
results of Remer et al.(2005) using MODIS aerosol products 
from Aug. 2000 to Aug. 2002 and AERONET data from 132 
sites globally distributed ( level 1.5 product, 2052 match-ups 
with averaged AERONET 550nm AOT of 0.18): 62%, 66% and 
70% respectively for 550, 660 and 870nm3 *. But, in that 
research, the percentages were only 56%, 53% and 60% over 
Asia-Pacific waters (57 match-ups with averaged AERONET 
550nm AOT of 0.21). It indicates that the retrieval performance 
is relatively poor in this region compared with that of global. 
When data with AOT less than 0.05 were discarded, the 
percentages were increased to 66.7% for 550nm (50.0% for 
At=±0.03±0.05t) and 51.7% for 658 and 860nm channels. If 
the uncertainties were At=±0.05±0.05t for 658 and 860nm 
channels, the percentages were increased to 59.4% and 65.6% 
separately. 
Figure 2. AOT match-up scatter diagrams at 550nm, 658nm and 
860nm for 25km radius and ±0.5h 
The result of “Shirahama” was better with RMSE of 0.03, 0.04 
and 0.04, and mean relative error of 8.0%, 13.2% and 20.9% 
respectively for 550, 658 and 860nm channels compared with 
others. But, it lacked statistical meaning because of the little 
data amount. For spring (Mar. to May, 22 match-ups), the linear 
*At=±0.03±0.05t, sampling window was 5x5 pixels and ±0.5h, 
same data elimination method as in this paper.