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3 AND
4 BANDS OF NITRIC ACID (HNO3) AT 7.6~µm FOR ATMOSPHERIC STUDIES.
A. PERRIN, J. M. FLAUD, Laboratoire Inter Universitaire des Systemes Atmosphériques, CNRS, Université Paris EST-Créteil, 61 Av du General de Gaulle, 94010 Créteil Cedex France; M. RIDOLFI, M. CARLOTTI, Dipartimento di Chimica Fisica e Inorganica (DCFI), Universiy of Bologna, Viale del Risorgimento, 4 - 40136 - Bologna, Italy.
Nitric acid (HNO3) plays an important role as a reservoir molecule of the NOx (nitrogen oxides) species in the stratosphere. The three strongest infrared bands of nitric acid are located at 11~µm (
5 and 2
9 bands), 7.6~µm (the
3 and
4 bands at 1326 and 1303~cm-1) and 5.8~µm (
2 band). Although two times weaker than those located at 7.6 and 5.8~µm, the 11~µm region is the only one which is used for nitric acid retrievals in the atmosphere by several satellite instruments like MIPAS (Michelson Interferometer for Passive Atmospheric Sounding on ENVISAT) or ACE-FTS (ACE Fourier transform spectrometer on SCISAT). This is because the available spectroscopic parameters for HNO3 in the HITRAN and GEISA databases are of very good quality in this spectral region. Of the two remaining bands, the 7.6~µm one is only partly masked by water, and therefore can be used also for nitric acid retrievals in the upper stratosphere. Moreover, because of their large difference in band intensity, combining measurements at 11~µm and 7.6~µm could maximize informations on the vertical distribution of HNO3 in the atmosphere. However at 7.6~µm the spectroscopic parameters available in the HITRAN and GEISA databases are are not so good. Indeed, these parameters originate from a list generated more than 20 years ago . The low quality of the list at 7.6~µm prevents HNO3 retrievals and severely affects the retrievals of several species absorbing in the 7.5-7.7~µm region, like SO2. This work is a new and more accurate investigation of the line positions and intensities for the
3 and
4 bands of nitric acid located at 1326.187 and 1303.074~cm-1). For this task, we used new infrared laboratory data combined with a new theoretical model. Examples showing substantial improvements will be given.