New analysis of the ν5+ ν9− ν9 hot band of HNO 3

Abstract

Nitric acid which is an important NO x atmospheric reservoir molecule exhibits a strong absorption in the 11 μm spectral region. Since this region, which corresponds to an atmospheric window, is one of the most commonly used for the retrieval of HNO 3 in the atmosphere it is essential to have the best possible corresponding spectral parameters. Updates of these spectral line parameters were recently performed in the last versions of the atmospheric databases. They concern the line positions and intensities not only of the two interfering cold bands ν 5 and 2 ν 9 but also of the ν 5+ ν 9− ν 9 hot band. This hot band exhibits indeed a sharp and strong Q branch at 885.425 cm −1 which is clearly observable in atmospheric spectra and is used for the retrievals. However, in spite of these recent updates, it proved that the spectral parameters of the ν 5+ν 9−ν 9 HNO 3 hot band are not accurate enough to reproduce accurately the observed atmospheric HNO 3 absorption in ATMOS spectra. The present paper is dedicated to a more accurate analysis of this hot band using new laboratory high-resolution (0.002– 0.005 cm −1 ) Fourier transform spectra. As a consequence, new and more precise line positions and line intensities (about 35% weaker than in HITRAN2K) were derived leading to a significant improvement in the simulation of atmospheric spectra.