Global distributions of nitric acid from IASI/MetOP measurements
1Spectroscopie de l'Atmosphère, Chimie Quantique et Photophysique, Université Libre de Bruxelles (U.L.B.), Brussels, Belgium
2UPMC Univ. Paris 06, CNRS UMR8190, LATMOS/IPSL, Paris, France
3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
4Department of Physics and Atmospheric Science, Dalhousie University Halifax, Canada
5Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA
Abstract. This paper presents the first global distributions of HNO3 acquired by the Infrared Atmospheric Sounding Interferometer (IASI) instrument, launched onboard the MetOp platform in October 2006. IASI is an infrared nadir-looking Fourier transform spectrometer providing atmospheric radiance spectra at 0.5 cm−1 spectral resolution, from which temperature and infrared absorbing gas concentration profiles are retrieved with global Earth coverage twice a day. A first analysis of the IASI measurements in terms of information content demonstrates the possibility of retrieving a total column for HNO3 at all latitudes with a maximal sensitivity in the middle stratosphere. The retrievals are performed from IASI spectra in the atmospheric window using a fast radiative transfer model and inversion software relying on the Optimal Estimation Method. The operational processing of HNO3 is achieved since March 2008. The global distributions of the retrieved total columns for 10 months (from March to December 2008) are presented and discussed with emphasis given to seasonal and interhemispheric variations. Local trends at 6 specific locations are also described and discussed in comparison with MLS volume mixing ratios at 46.5 hPa. The seasonal cycle observed in Polar regions is highlighted, with maxima observed in fall and minima during spring-summer. The denitrification inside the Antarctic polar vortex during winter is clearly revealed with unprecedented horizontal resolution: HNO3 columns decreasing down to about 1×1016 molecules.cm−2 are observed, which is consistent with the lower values of temperature observed between 50 and 15 hPa (~20–25 km) and the resulting formation and sedimentation of polar stratospheric clouds. During the same period, the collar region of high quantities of HNO3 at the vortex edge is also observed around 65–60° S latitude. Preliminary correlations between IASI derived HNO3 and O3 columns inside the polar vortex are presented and discussed.