For sunphotometers that do not employ constant temperature detectors, a major source of variation and hence uncertainty arises due to inadequate temperature compensation for the response. Furthermore, for sunphotometer that do not employ auto-tracking or peak hold features, another major source of uncertainty arises due to imperfect pointing. For measurements reported here, these are not expected to contribute to absolute errors in the measurements of aerosol optical thickness. Aerosol optical thickness is highly variable in space and time and an accurate estimate is made by measuring the direct solar radiance at the ground by the use of a calibrated sunphotometer. Solar radiation reaching the ground is attenuated by absorption and scattering by atmospheric molecules as well as particles.
Molecular absorption is avoided by an appropriate choice of band passes.
Calibration is important and is performed at high altitudes where the sunphotometer is above most of the atmospheric water vapor. Since the aerosol growth rate is tied to the amount of water in the atmosphere, the aerosol optical thickness at the mountain sites especially in the early mornings can be expected to be a constant. Under such conditions the response of the sunphotometer during a rapid change in airmass yields a fictitious exoatmospheric response which is the calibration coefficient. The calibration procedure is known as the Langley plot method. Calibration can also be performed by comparing responses with a better calibrated instrument.