Atmospheric transmittances for the AVHRR channels

Vertical path atmospheric transmittances from the surface to the top of the atmosphere have been computed for the five channels of the Advanced Very High Resolution Radiometer (AVHRR). Gaseous absorption computed from a line-by-line transmittance model and both molecular and aerosol scattering are included. Contributions from all gases with any significant absorption in each channel are listed together with transmittance spectra for the more important absorbers in each channel. The transmittance in channels 1 and 2 is dominated by aerosol scattering which is dependent on both aerosol type and concentration. For channel 3 water vapor line absorption is the principal component with water vapor continuum absorption and aerosol scattering having a smaller but significant affect. The absorption due to other minor atmospheric constituents (i.e. CH₄, N₂O and N₂) is also significant in this channel. For channels 4 and 5 water vapor continuum absorption dominates. Other atmospheric constituents having a significant absorption in these channels are CO₂, HNO₃, and the chlorofluorocarbons (CFCs) Fll and F12. The latter two only have a small effect on both channels at present but as the concentration of CFCs is increasing at ~ 5%/year this will increase significantly with time.