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 mu m, through the influence of collisions with H2O on both the collision-induced band of N-2 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 N-2 and CO2 absorptions leads to significantly underestimated absorptions for humid atmospheres. This bias is considerably reduced when the influences of H2O on the N-2 and CO2 contributions to the spectra are taken into account using proper spectroscopic models. This opens perspectives of increased accuracy near mu m 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. Plain Language Summary We demonstrate that the presence of water vapor in the atmosphere enhances the absorption contributions of nitrogen and carbon dioxide near mu m. Properly taking this indirect influence into account largely improves the agreement between measured and calculated spectra. This opens the route for an improved use of this wavelength region in retrievals of land and sea surface temperatures from space-borne recordings of the outgoing radiation.