Non-LTE state distribution of nitric oxide and its impact on the retrieval of the stratospheric daytime no profile from MIPAS limb sounding instruments Original Research Article

The non-LTE vibrational, rotational and spin state distribution of NO(ν = 1,J,Ω) has been calculated. The daytime vibrational (ν = 1) state population was found to depart from LTE even at stratospheric altitudes due to the pumping of the (ν = 1) state by the photolysis of NO2. The rotational and spin degree of freedom is in non-LTE above 110 km. Non-LTE limb radiance spectra of the fundamental band at 5.3 μm as will be recorded by the MIPAS-B ballon borne experiment or, in the near future, by the MIPAS-ENVISAT space borne experiment (Fischer and Oelhaf, 1996) have been simulated. A retrieval strategy that takes into account this problem has been developed for the MIPAS-B instrument. Here, the stratospheric non-LTE state distribution is retrieved simultaneously with the NO volume mixing ratio (vmr) profile. Instead of multi-parameter retrieval of the state distribution in terms of a vibrational temperature profile, only one parameter describing the impact of the NO2 photolysis is retrieved additionally to the vmr profile. The feasibility of this approach for the application to MIPAS-ENVISAT data is discussed.