Evidence From the Limb Infrared Monitor of the Stratosphere for nonlocal thermodynamic equilibrium in the v₂ mode of mesospheric water vapour and the ν₃ mode of stratospheric nitrogen dioxide

The Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) included channels centred at 6.9 μm (1370 cm −1 to 1560 cm −1 ) and 6.2 μm (1560 cm −1 to 1630 cm −1 ) to measure H 2 O and NO 2 , respectively. Since the photochemical lifetime of H 2 O is several months, no diurnal variation was expected, but retrieved concentrations at mid and low latitudes were larger in the daytime than in the nighttime (by 20–30% at 1 mbar). This is attributed to departure from local thermodynamic equilibrium of daytime emission which had not been accounted for in retrievals. It is shown that excess radiance in the lower mesosphere may be due to emission from the H 2 O(020) → (010) hot band and that in the stratosphere pumping of high vibrational states of NO 2 (ν 3 ) by chemical reaction NO + O 3 → NO 2 + O 2 and by absorption of visible and near‐IR sunlight may be the cause. This nonthermal emission from NO 2 is shown to also significantly affect retrieval of H 2 O in the polar night mesosphere and NO 2 in the daytime stratosphere at low latitudes. LIMS nighttime measurements of H 2 O and NO 2 in regions other than the high latitude winter mesosphere should not be affected.