Airmass Factor Calculator
The column retrieved from a DOAS measurement depends on the amount of the absorber present, but also on the light path through the atmosphere. The latter depends on many factors including viewing geometry and the vertical distribution of the absorber. Usually, the effect of light path is expressed as airmass factor (AMF) which is defined as the ratio between the retrieved slant column (SC) and the atmospheric vertical column (VC): AMF = SC / VC.
For overhead sun and a nadir viewing instrument, an airmass factor of 2 would result if all photons coming from the sun pass through the absorber, are reflected on the ground and travel back through the absorber to the instrument. In the same geometrical approximation, the light path at 30° solar zenith angle (SZA) would be 1 + 1 / cos(30°).
In the real atmosphere, many photons are scattered and depending on surface albedo, solar zenith angle and wavelength, the sensitivity of the measurement and thus the airmass factor can deviate significantly from the geometrical approximation.
The airmass factors have been computed with the radiative transfer model SCIATRAN using the following settings:
The presence of aerosols might change the results significantly by enhancing visibility in and above the aerosol layer and decreasing sensitivity below. The same is true for clouds, only much more so.
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