MIPAS used as a nonlocal thermodynamic equilibrium sounder for NO 5.3 μm band

Abstract

The spaceborne MIPAS experiment has been numerically simulated in the 5.3 μm spectral domain, and the possible nonlocal thermodynamic equilibrium (non‐LTE) emissions from NO have been investigated. The interfering non‐LTE contributions from H2O, CO2, and O3 to atmospheric limb radiances have been estimated. The results have shown a negligibly small non‐LTE interference from H2O, CO2, and O3 but a considerable non‐LTE effect from NO in the stratosphere. The inverse problem has been formulated for the retrieval of the NO vertical profile and the vibrational temperatures of the NO lower vibrational states. The optimal estimation technique has been selected as a tool for the evaluation of the retrieval accuracy of NO number density and vibrational temperature vertical profiles. The error matrix calculations have shown that the retrieval accuracy for NO concentration is 30–40% in the altitude region 10–50 km in case of simultaneous retrieval of NO and vibrational temperature vertical profiles but can be improved by 10% if averaging of spectra is performed to achieve low noise. The retrieval accuracy for the vibrational temperatures of the X1/2–1 and X3/2–1 states was estimated to be better than 10 K in the stratosphere, but for the states X1/2–2 and X3/2–2, it appeared to be rather poor. The problem of NO vertical profile retrieval in the non‐LTE atmosphere is discussed. It was shown that in order to obtain a reasonable accuracy of NO retrieval of 20% up to 50 km altitude, it is necessary that the a priori uncertainty of the vibrational temperature of the state X1/2–1 is not higher than 5 K.