Global distributions of nitric acid from IASI/MetOP measurements

C. Wespes,C. Clerbaux,D. Hurtmans,P. F. Coheur,M. L. Santee,R. V. Martin

doi:10.5194/acp-9-7949-2009

Abstract

Abstract. This paper presents the first global distributions of HNO3 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 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 (FORLI) relying on the Optimal Estimation Method. The operational processing of HNO3 total columns is achieved since March 2008. We show that FORLI-HNO3 performs well at all latitudes (RMS of the spectral residuals around 2.3×10−6 W/m2 sr m−1) and provides HNO3 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 observed in Polar regions is highlighted, with maxima observed in fall-winter 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.

Highlights

HNO3 is the dominant form of oxidized nitrogen in the atmosphere, usually abbreviated as NOy (NOy=NOx+oxidized products, with NOx=NO+NO2)
In order to evaluate the methodology of the FORLI-HNO3 processing described above and in particular the choice of global profile scaling for HNO3, we have performed a series of full profile retrievals at representative latitudes using the line-by-line, computationally more demanding, Atmosphit software described elsewhere (Barret et al, 2005; Coheur et al, 2005)
The FORLI-HNO3 (Fast Operationnal Retrievals on Layers for Infrared Atmospheric Sounding Interferometer (IASI)) software, a fast radiative transfer model coupled to an inversion software relying on the Optimal Estimation Method, has been developed to retrieve HNO3 total columns in near-real time

Summary

Introduction

HNO3 is the dominant form of oxidized nitrogen in the atmosphere, usually abbreviated as NOy (NOy=NOx+oxidized products, with NOx=NO+NO2). Nitric acid is mainly present in the stratosphere at polar latitudes where only a small fraction (about 10–15% on a column basis) is in the troposphere In the stratosphere, it contributes, along with other oxidized forms, in sequestering NOx, thereby reducing catalytic ozone destruction processes. (Solomon, 1999; Brasseur and Solomon, 2005) It is removed from this layer either by photochemistry or by sedimentation of Polar Stratospheric Cloud (PSC) particles containing HNO3 during the cold polar nights. This process causing stratospheric denitrification influences chlorine chemistry and is an important factor governing the ozone hole development in polar springtime (Solomon, 1999; Santee et al, 1999, 2004; Jin et al, 2006, and references therein)

Methods

Results

Conclusion