Indirect Influence of Humidity on Atmospheric Spectra Near 4 μm

We give the first demonstration that humidity has indirect effects on atmospheric spectra near 4 μm, through the influence of collisions with H₂O on both the collision-induced band of N₂ and the wings of the CO₂ lines. This is shown by comparing computed and measured values of atmospheric transmissions and outgoing radiances. The usual assumption that collisions with H₂O and dry air have equal effect on the N₂ and CO₂ absorptions leads to significantly underestimated absorptions for humid atmospheres. This bias is considerably reduced when the influences of H₂O on the N₂ and CO₂ contributions to the spectra are taken into account using proper spectroscopic models. This opens perspectives of increased accuracy near 4 μm where the atmosphere is relatively transparent, for astronomical observations of outer objects from ground as well as for retrievals of the Earth land/sea surface temperatures from radiances recorded from space.