Optical Spectrograph And Infrared Imager System Research Articles

Stratospheric ozone profiles retrieved from limb scattered sunlight radiance spectra measured by the OSIRIS instrument on the Odin satellite

Stratospheric ozone density profiles between 15 and 40 km altitude are derived from scattered sunlight limb radiance spectra measured with the Optical Spectrograph and InfraRed Imager System (OSIRIS) on the Odin satellite. The method is based on the analysis of limb radiance profiles in the centre and the wings of the Chappuis‐Wulf absorption bands of ozone. It employs a non‐linear Newtonian iteration version of Optimal Estimation (OE) coupled with the radiative transfer model LIMBTRAN. The derived zonally averaged ozone field for August 2001 is in excellent agreement with the main characteristics of the global morphology of stratospheric ozone, indicating that the limb scatter technique is capable of providing ozone profiles with high accuracy and high vertical resolution on a global scale and a daily basis.

Stratospheric profiles of nitrogen dioxide observed by Optical Spectrograph and Infrared Imager System on the Odin satellite

Vertical profiles of nitrogen dioxide in the 19–40 km altitude range are successfully retrieved over the globe from Optical Spectrograph and Infrared Imager System (OSIRIS) limb scatter observations in late 2001 and early 2002. The inclusion of multiple scattering in the radiative transfer model used in the inversion algorithm allows for the retrieval of NO2 down to 19 km. The slant column densities, which represent the observations in the inversion, are obtained by fitting the fine structure in normalized radiance spectra over the 435–449 nm range, where NO2 electronic absorption is readily observable because of long light paths through stratospheric layers rich in this constituent. Details of the spectral fitting and inversion algorithm are discussed, including the discovery of a pseudo‐absorber associated with pixelated detectors and a new method to verify altitude registration. Comparisons are made with spatially and temporally coincident profile measurements of this photochemically active trace gas. Better than 20% agreement is obtained with all correlative measurements over the common retrieval altitude range, confirming the validity of OSIRIS NO2 profiles. Systematic biases in the number densities are not observed at any altitude. A “snapshot” meridional cross section between 40°N and 70°S is shown from observations during a fraction of an orbit.

Inversion algorithms for recovering minor species densities from limb scatter measurements at UV‐visible wavelengths

Stratospheric ozone can be measured with good global coverage and good vertical resolution by continuous scanning of the limb of the sunlit atmosphere. In the near future there will be several satellite instruments exploiting this limb‐scanning method using the UV‐visible region of the spectrum, e.g., the Optical Spectrograph and Infrared Imaging System (OSIRIS) launched on the Odin satellite in February 2001, and the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography launched on Envisat in March 2002. Envisat also carries the Global Ozone Monitoring by Occultation of Stars instrument, which will measure limb‐scattered sunlight under bright limb occultation conditions. In this paper we present an inversion method to retrieve vertical ozone profiles from limb scatter measurements. The method uses a modified onion‐peeling approach. Multiple scattering is taken into account by a precalculated total to single‐scattering radiance ratios tabulated as a function of wavelength, tangent altitude, and several other relevant parameters. The sensitivity of the retrieval method is studied using the OSIRIS instrument as an example. Constituent retrieval errors are estimated by applying the method to simulated OSIRIS measurements.

Procedures for recovering mesospheric and stratospheric temperatures from OSIRIS scattered-sunlight measurements

Procedures for recovering mesospheric and stratospheric temperature profiles from scattered-sunlight limb radiance measurements made by the OSIRIS (optical spectrograph and infrared imager system) instrument on the Odin satellite are described. We assess various approaches to the problem and show that temperature recoveries based on an analysis of inferred volume-scattering rates are significantly more accurate than those based on direct analysis of the observed limb radiances. A forward model for the OSIRIS instrument is used to test temperature recovery algorithm performance in the presence of realistic instrument noise and to assess the expected accuracy of the OSIRIS temperature recoveries. It is shown that, for radiances measured in the near-infrared region, temperatures can be reliably recovered over the altitude range 40 km to 80 km with an accuracy of better than ±1 K. PACS Nos.: 42.68Mj, 94.10Dy

A vector radiative-transfer model for the Odin/OSIRIS project

A vector radiative-transfer code has been developed that is able to accurately and efficiently calculate radiance and polarization scattered from Earth's limb. A primary application of this code will be towards generating weighting functions, based on calculated limb radiances, for the retrieval of trace gases (O3, NO2, BrO, OClO, and O4) from the optical spectrograph and infrared imaging system (OSIRIS). OSIRIS is a UV–visible instrument on board the Odin satellite that measures limb-scattered light. This model solves the vector radiative-transfer equation using an iterative technique simultaneously in both plane-parallel and spherical-shell atmospheres. OSIRIS simulated limb radiance and polarization and OSIRIS weighting functions are presented along with a discussion of the numerical solution parameters, model intercomparisons and timings, and necessary model improvements. Overall agreement with other models was found to be very good and model speed is comparable to a fast finite-difference code. A set of OSIRIS reference atmospheres have been compiled for use with radiative-transfer models. Each of the 216 atmospheres (18 latitudes × 12 months) include profiles of air, pressure, temperature, ozone, NO2, BrO, and stratospheric aerosols.PACS Nos.: 42.68-w, 94.10Gb

Mesospheric ozone: Determination from orbit with the OSIRIS instrument on Odin

The analysis of the data from the optical spectrograph and infrared imager system (OSIRIS) that will fly on the Odin satellite requires special attention as many of the measurements will be made in regions of the atmosphere that are relatively close to terminator. Under these conditions the photochemical processes in the upper atmosphere that are responsible for much of the oxygen infrared atmospheric band airglow emission are nonstationary. It is this latter aspect that complicates the retrieval of the mesospheric ozone profile from the OSIRIS observations. However, a tomographic analysis technique that has been developed specifically for the Odin project allows accurate recovery of local volume emission rates in the orbit plane. It is shown that the tomographic analysis technique can be combined with nonstationary atmospheric photochemistry models to recover the mesospheric ozone profile near the terminator. PACS Nos.: 94.10Fa, 94.10Rk

The impact of the OSIRIS grating efficiency on radiance and trace-gas retrievals

The optical spectrograph and infrared imaging system (OSIRIS), launched in 2001, is a UV–visible diffraction-grating instrument designed to measure light scattered from the Earth's limb. Laboratory measurements of the OSIRIS diffraction-grating efficiency reveal a sensitivity to polarization including an anomalous structure of width 20–30 nm introduced into light polarized in a direction perpendicular to the grooves of the grating. A vector radiative-transfer model was used to generate synthetic OSIRIS spectra in an effort to examine the effect of this on radiances and trace-gas retrievals. Radiances that included grating effects were found to deviate by nearly 10% from those that did not and also contained the anomalous structure. Performing differential optical absorption spectroscopy (DOAS) on these spectra revealed errors in ozone apparent column densities of up to 80 DU. The size of the error was controlled mainly by the difference in polarization between the two DOAS spectra. Two possible correction methods were investigated. The first was to remove the grating effects by applying a correction factor to the raw radiances calculated using the vector radiative-transfer model. The second was to include the efficiency coefficient spectra in the DOAS fit. PACS Nos.: 42.68Mj, 98.55Qf

A method for recovering stratospheric minor species densities from the Odin/OSIRIS scattered-sunlight measurements

A method for recovering minor species density profiles in the stratosphere from observations made with the OSIRIS (optical spectrograph and infrared imager system) instrument on the Odin satellite is described. The OSIRIS instrument measures limb radiances of scattered sunlight over the spectral range 2800 to 8000 Å, for tangent heights ranging from 10 to 100 km. We describe how the limb spectra may be processed using the DOAS (differential optical absorption spectroscopy) technique to derive apparent column densities for the minor atmospheric constituents O3, NO2, OClO, and BrO. We also show how these column densities, measured over a range of tangent heights, may be inverted using an iterative least-squares technique to determine the local density profiles. The procedures are illustrated using simulated limb radiances generated with a realistic OSIRIS instrument model. PACS Nos.: 42.68Mj, 94.10Dy