Vibrationally excited ozone in the middle atmosphere

Daytime mesospheric limb emission spectra of ozone in the 4.8 μ m regime are analyzed with respect to vibrational excitation and relaxation processes. The data, which was obtained by the MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) instrument on board ESA's Environmental Satellite, is simulated by means of a non-local thermodynamic equilibrium (non-LTE) model utilizing 3 -abundance, temperature, and pressure data from simultaneous retrievals in other spectral regions. The vibrational states of ozone depart from LTE due to the absorption of radiation from the lower atmosphere and due to the production of excited states in the O + O₂ + M → O₃ (v) + M recombination reaction. The energy flow into the ozone molecule as well as the collisional relaxation are highly uncertain. Model calculations that assume ozone formation at energies larger than 5000 cm - 1 underestimate the measured radiances by a factor of 2-3 in the 50-75 km altitude regime, if the nominal relaxation scheme is assumed. Agreement between measured and modeled radiances is achieved, if the collisional rates for the transformation of hot band stretching to bending quanta are reduced by about a factor of three, or if the quasi-nascent distribution of ozone favors vibrational states in the 3000 cm^-1 region.