Abstract
Abstract. The scattered sunlight measurements made by the Optical Spectrograph and InfraRed Imaging System (OSIRIS) on the Odin spacecraft are used to retrieve vertical profiles of stratospheric aerosol extinction at 750 nm. The recently released OSIRIS Version 5 data product contains the first publicly released stratospheric aerosol extinction retrievals, and these are now available for the entire Odin mission, which extends from the present day back to launch in 2001. A proof-of-concept study for the retrieval of stratospheric aerosol extinction from limb scatter measurements was previously published and the Version 5 data product retrievals are based on this work, but incorporate several important improvements to the algorithm. One of the primary changes is the use of a new retrieval vector that greatly improves the sensitivity to aerosol scattering by incorporating a forward modeled calculation of the radiance from a Rayleigh atmosphere. Additional improvements include a coupled retrieval of the effective albedo, a new method for normalization of the retrieval vector to improve signal-to-noise, and the use of an initial guess that is representative of very low background aerosol loading conditions, which allows for maximal retrieval range. Furthermore, the Version 5 data set is compared to Stratospheric Aerosol and Gas Experiment (SAGE) III 755 nm extinction profiles during the almost four years of mission overlap from 2002 to late 2005. The vertical structure in coincident profile measurements is well correlated and the statistics on a relatively large set of tight coincident measurements show agreement between the measurements from the two instruments to within approximately 10% throughout the 15 to 25 km altitude range, which covers the bulk of the stratospheric aerosol layer for the mid and high latitude cases studied here.
Highlights
Stratospheric aerosol has been measured from space for decades by solar occultation, and the long-standing heritage and accuracy of these aerosol extinction measurements, in particular from the Stratospheric Aerosol and Gas Experiment (SAGE) series of instruments (SAGE, 1979 to 1981; SAGE II, 1984 to 2005; SAGE III, 2002 to 2005), has provided an essential understanding of the stratospheric aerosol layer, the impact of volcanic eruptions
A number of satellite instruments capable of making spectral observations of limb scattered sunlight have been in operation including the Optical Spectrograph and InfraRed Imaging System (OSIRIS), SAGE III (Rault, 2005; Rault and Loughman, 2007) and the SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY (SCIAMACHY) (Bovensmann et al, 1999) and Global Ozone Monitoring by Occultation of Stars (GOMOS) (Bertaux et al, 2010)
Comparisons of coincident measurements made by SAGE III at 755 nm between 2002 and 2005 show excellent agreement to within approximately 10 % over the bulk of the stratospheric layer from about 15 to 25 km
Summary
Stratospheric aerosol has been measured from space for decades by solar occultation, and the long-standing heritage and accuracy of these aerosol extinction measurements, in particular from the Stratospheric Aerosol and Gas Experiment (SAGE) series of instruments (SAGE, 1979 to 1981; SAGE II, 1984 to 2005; SAGE III, 2002 to 2005), has provided an essential understanding of the stratospheric aerosol layer, the impact of volcanic eruptions. The limb scatter measurement technique has a distinct benefit over occultation in terms of sampling; vertical profiles can be obtained repetitively for tangent points in the sunlit hemisphere providing nearly global coverage within a relatively short time period. The local time of the ascending node of the Odin orbit is 18:00 h This configuration provides measurements of the sunlit summer hemisphere, global measurements during equinox, and no coverage of the mid to high latitude winter hemisphere. We provide the details of several important improvements to this retrieval algorithm; these are used in the production of the first OSIRIS stratospheric aerosol data product for public release, Version 5.
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