Evidence for CH4 7.6 μm non‐local thermodynamic equilibrium emission in the mesosphere

Abstract

We present unequivocal evidence for CH4 7.6 μm non‐local thermodynamic equilibrium (non‐LTE) mesospheric emissions as measured by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) experiment on board the ESA Envisat satellite. We have employed non‐LTE radiative transfer models, already used and validated in studies of other atmospheric gases, in order to quantify the magnitude and extent of the CH4 non‐LTE deviation in the Earth's atmosphere. The observed daytime enhancement signatures apparent in the measured radiances cannot be attributed to enhanced daytime temperatures and/or CH4 abundance variations and are reproduced successfully with a dedicated CH4 non‐LTE radiative transfer model. We conclude that, depending on the upper stratospheric/mesospheric temperature structure, the departure from LTE might commence at an altitude as low as 45 km, reaching limb radiance deviations of 20% at 60 km and quickly rising up to 60% at 70 km, for the 7.6 μm CH4 lines.