i Detection of Smoke and Dust Aerosols Using Multi-sensor Satellite ...

The BT of the 11 µm band (band 31) and the BTD between the 11 µm band and the
3.7 µm band (band 20) (Ackerman et al., 2002) are used to separate smoke aerosol from
cloud, as well as the reflectance at the 1.38 µm channel, a spectrum window of water
vapor useful for detecting high cirrus cloud (Gao et al., 1993; Gao et al., 2002).
The normalized ratio of two blue bands, (R3 - R8) / (R3 + R8), and the single
reflectance of band 8 (412 nm) are proposed to filter out the noise pixel (mainly water
pixels and coastline pixels). Band 8, the shortest wavelength band within MODIS, is
strongly sensitive to the smoke aerosol because of Rayleigh scattering. The average
geometrical radii of smoke particle are in the range of 0.01 - 0.05 µm, largely satisfying
the Rayleigh scattering limit (Wong and Li, 2002; Remer et al., 2002). Rayleigh
scattering is wavelength dependence. Its intensity is inversely proportional to the fourth
power of the wavelength. As a result, this ratio of two blue bands is very sensitive to
aerosol. Two examples with smoke plumes are shown in Fig. 3.6. The normalized ratio of
two blue bands for smoke plumes in both images is relative higher than vegetated and
water areas, and the ratio values increases with the plume intensity.
For the convenient expression in this dissertation, the single reflectance of certain
band is expressed with R (reflectance) plus band number. The BT of certain band is
denoted with BT plus CW. The normalized ratio of two bands has the formula like Ratio
(Ra, Rb) = (Ra - Rb) / (Ra + Rb), where a and b are band number. The BTD between two
bands is given in BTD (CWa, CWb) = BTCWa –BTCWb, where CWa and CWb are CW
of band a and band b.
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