Atmospheric absorption in the visible range is significant and is a combination of specific absorption bands (mainly from O2 and H2O at the red end) and a general increasing absorption towards the blue end, from ozone , Rayleigh scattering (which is why the sky is blue) and scattering from aerosols. None of these are sufficient to completely absorb the light however but the effects do have to be corrected for. (Spectra should be corrected as though the observation was made at the top of the atmosphere) You can see a typical atmospheric absorption curve here
http://www.astrosurf.com/buil/atmosp...nsmission2.png
as measured by Christian Buil using his method described here
http://www.astrosurf.com/buil/atmosp...ansmission.htm
Cheers
Robin