Abstract
Abstract. Improved line positions and intensities have been generated for the 7.6 µm spectral region of nitric acid. They were obtained relying on a recent reinvestigation of the nitric acid band system at 7.6 µm and comparisons of HNO3 volume mixing ratio profiles retrieved from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) limb emission radiances in the 11 and 7.6 µm domains. This has led to an improved database called MIPAS-2015. Comparisons with available laboratory information (individual line intensities, integrated absorption cross sections, and absorption cross sections) show that MIPAS-2015 provides an improved description of the 7.6 µm region of nitric acid. This study should help to improve HNO3 satellite retrievals by allowing measurements to be performed simultaneously in the 11 and 7.6 µm micro-windows. In particular, it should be useful to analyze existing MIPAS and IASI spectra as well as spectra to be recorded by the forthcoming Infrared Atmospheric Sounding Interferometer – New Generation (IASI-NG) instrument.
Highlights
Optical remote sensing of nitric acid in the infrared range can be performed using the three strongest band systems of this species, namely the {ν5, 2ν9}, {ν3, ν4}, and ν2 band systems located near 11, 7.6, and 5.8 μm respectively
Focusing on the spectral ranges covered by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument (Fischer et al, 2008) that was operational on board the ENVISAT satellite (Endemann, 1999) in the years from 2002 to 2012, Flaud et al (2006 and references therein) created a HNO3 linelist covering the 600–1800 cm−1 region with the aim to provide the best and most consistent possible set of line parameters for this molecular species
We selected MIPAS measurements acquired in three different days of the years 2002 and 2003
Summary
Optical remote sensing of nitric acid in the infrared range can be performed using the three strongest band systems of this species, namely the {ν5, 2ν9}, {ν3, ν4}, and ν2 band systems located near 11, 7.6, and 5.8 μm respectively. We selected MIPAS measurements acquired in three different days of the years 2002 and 2003 (still measurements acquired with the MIPAS full spectral resolution of 0.025 cm−1) The results of these additional tests show that, the observed differences between the average HNO3 VMR retrieved from the 11 and the 7.6 μm regions amount to a maximum of 1.5 % in the height range from 15 to 30 km. The blue and the red lines represent the average residuals computed as the difference between the average observation and the average simulation using, respectively, the MIPAS-OLD and the MIPAS-2015 HNO3 linelists From these figures we can clearly appreciate improvements in the residual spectra achieved with the new HNO3 linelist. Average MIPAS-observed spectrum (black) in the 1329.5–1331.5 cm−1 spectral region, and simulated spectra with the MIPAS-OLD (magenta line) and the MIPAS-2015 (orange line) HNO3 linelists. The blue and the red lines are the residuals obtained with the MIPAS-OLD and the MIPAS-2015 linelists respectively
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