**The main principle of Sun Photometry/ Solar
Radiometry is to estimate the aerosol optical depth , at a near monochromatic
wavelength for radiation incident normally on the top of the
atmosphere and traveling the entire atmospheric path. When the radiation
is nearly monochromatic and the wavelength lies outside any major
absorption bands, the ground reaching solar flux, is related to the extra-terrestrial
flux at the top of the atmosphere through the Bouguer-Lambert-Beer
law . The ground reaching solar flux depends on the mean and instantaneous
Sun-Earth distances, the normal incidence columnar total optical depth
of the atmosphere at a particular wavelength, and the relative air mass.
The relative air mass term accounts for the increased distance traveled
by the solar radiation incident away from zenith. The airmass term (m)
depends on the solar zenith angle and can be calculated from the knowledge
of the time of observation, the solar declination angle, the ephemeris
transit time (or equation of time) and the geographic co-ordinates of the
observing station (like latitude and longitude). Thus if continuous
measurements of V are made, as a function of time during a day, a plot
of ln (V) against m will show the points lying very close to a straight
line. The slope of the straight line estimated using linear regression
analysis yields the total optical depth and the intercept extrapolated
to meet the ordinate (i.e., for m = 0) can be used as a self-calibration
for the radiometers. This procedure is also widely known as the Langley
technique in literature.It yields the columnar
total optical depth of the atmosphere, which actually is the sum
of optical depths due to molecular extinction and extinction due to aerosols.
By subtracting the molecular contribution, the aerosol optical depth can
be estimated.** |