Global distributions of nitric acid from IASI/MetOP measurements

Abstract

Abstract. This paper presents the first global distributions of HNO 3 total columns 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 HNO 3 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 (FORLI) relying on the Optimal Estimation Method. The operational processing of HNO 3 total columns is achieved since March 2008. We show that FORLI-HNO 3 performs well at all latitudes (RMS of the spectral residuals around 2.3×10 −6 W/m 2 sr m −1 ) and provides HNO 3 total columns with on average statistical errors of about 12%, reaching the threshold value of 32% at the equatorial belt. The global distributions of the retrieved total columns for one year (from March 2008 to February 2009) are presented and discussed with emphasis given to seasonal and interhemispheric variations. Local seasonal variations at 6 specific locations are also described and discussed in comparison with MLS volume mixing ratios at 46.5 hPa. The seasonal cycle observedCorrespondence to: C. Wespes (cwespes@ulb.ac.be) in Polar regions is highlighted, with maxima observed in fallwinter and minima during spring-summer. The denitrification inside the Antarctic polar vortex during winter is clearly revealed with unprecedented horizontal resolution: HNO 3 columns decreasing down to about 1×10 16 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 HNO 3 at the vortex edge is also observed around 65-60 • S latitude. Preliminary correlations between IASI derived HNO 3 and O 3 columns inside the polar vortex are presented and discussed.

Polar stratospheric cloud climatology based on CALIPSO spaceborne lidar measurements from 2006 to 2017

“…During most months, the center of the polar vortex is shifted off the pole so that the conventional latitude-altitude cross-sectional monthly means (Fig. 17) blur the sharp gradients in HNO 3 and H 2 O between the interior and "collar" regions (e.g., Wespes et al, 2009) of the vortex that are much more clearly captured in the EqLat-θ cross sections (Fig. 18).…”

Polar stratospheric cloud climatology based on CALIPSO spaceborne lidar measurements from 2006–2017

“…During most months, the center of the polar vortex is shifted off the pole so that the conventional latitude/altitude cross-sectional monthly means (Figure 16) blur the sharp 5 gradients in HNO 3 and H 2 O between the interior and "collar" regions (e.g. Wespes et al, 2009) of the vortex that are much more clearly captured in the EqLat/ cross-sections (Figure 17). Gas-phase HNO 3 and H 2 O are severely depleted by July in the interior of the vortex at EqLat < −75° between 400-500 K potential temperature.…”

Hyperspectral Earth Observation from IASI: Five Years of Accomplishments

“…IASI provides unprecedented information on the global distributions of HNO 3 columns. Preliminary studies have an emphasis on spatial and seasonal variability and on the role of nitric acid in the development of the ozone hole (Wespes et al 2009). Because IASI's sensitivity to HNO 3 is mainly in the UTLS region, there is great interest in using the retrieved distributions to constrain estimates of the production of NO x by lightning.…”