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Report on the 1^st International UV/vis Limb Scattering Workshop,

Bremen, Germany, April 14-15, 2003

Christian von Savigny (csavigny@iup.physik.uni-bremen.de),
Kai-Uwe Eichmann (eichmann@iup.physik.uni-bremen.de),
Heinrich Bovensmann (heinrich.bovensmann@iup.physik.uni-bremen.de)
John P. Burrows (burrows@iup.physik.uni-bremen.de),

Institute of Environmental Physics, University of Bremen, Bremen, Germany

Introduction

Satellite-borne remote sensing instruments operating in the UV/visible spectral range to study the chemical composition of the Earth's atmosphere have traditionally been of two types: (a) Nadir viewing spectrometers (e.g., TOMS (Total Ozone Mapping Spectrometer) [McPeters et al., 1998] and GOME (Global Ozone Monitoring Experiment) [Burrows et al., 1999]) providing almost global observations of total column amounts of ozone and other minor constituents on a daily basis, and (b) solar occultation instruments (e.g., SAGE (Stratospheric Aerosol and Gas Experiment) [McCormick et al., 1989] and POAM (Polar Ozone and Aerosol Measurement) [Lucke et al., 1999]) capable of providing vertical profiles of typically O₃, NO₂, H₂O and aerosol extinction with high vertical resolution (1-2 km). Each of these observation techniques has its disadvantages. The nadir viewing instruments cannot provide vertical profiles with a vertical resolution better than about 8 km and the occultation instruments only measure 15 - 30 profiles per day and for a limited range of latitudes only. The limb scattering observation technique, where the instrument line of sight follows a slant path tangent through the atmosphere and limb-scattered solar radiation is measured, allows to retrieve vertical profiles of several minor constituents with high vertical resolution as long as the sunlit of the Earth is observed. Thus, global coverage is combined with high vertical resolution.

In recent years several space-borne limb scattering instruments were launched to remotely sense the Earth's atmosphere. These include the SOLSE/LORE (Shuttle Ozone Limb Sounding Experiment/Limb Ozone Retrieval Experiment) [McPeters et al., 2000] flown on the space shuttle mission STS-87 in 1997, its re-flight SOLSE - 2 on the shuttle Columbia that was tragically lost in January 2003, OSIRIS (Optical Spectrograph and InfraRed Imaging System) [Llewellyn et al., 1997] on the Swedish-led Odin satellite launched in February 2001, SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) [Bovensmann et al., 1999] on ESA's ENVISAT launched in March 2002, as well as NASA's SAGE III on the Russian Meteor-3M satellite.

It should be noted that the limb-scattering technique has also been applied by several satellite-borne spectrometers in the past. For instance, limb scattering measurements with the UV spectrometer on SME (Solar Mesosphere Explorer) were used to retrieve mesospheric ozone profiles [Rusch et al., 1984] and NO₂ profiles [Mount et al., 1984] in the upper stratosphere since the early 80s. Yet, the greatly enhanced computing power makes profile retrievals possible for extended altitude ranges and other minor constituents, since spherical radiative transfer models - taking multiple scattering into account - can be employed.

A total of 48 scientists from 7 countries attended the workshop that was dedicated to the STS-107 shuttle crew, whose responsibilities also included the operation of the SOLSE - 2 limb-scattering experiment. The workshop consisted of three sessions: (1) Instruments, (2) Inversion algorithms and radiative transfer, and (3) Retrievals and first scientific results.

Instruments

All of the recently launched Earth orbiting instruments capable of performing limb scattering observations were represented at the workshop. Overviews were given on the SOLSE/LORE and SOLSE-2 (R. McPeters), OSIRIS (E. Llewellyn), SAGE III (D. Rault), SCIAMACHY (J. Burrows, S. Noel), as well as the future OMPS mission (Ozone Mapping and Profiles Suite) (J. Larsen) on a NPOESS satellite (National Polar-Orbiting Operational Environmental Satellite) - scheduled for launch in 2008. The main technical features of the these instruments are listed in Table 1.

There was a general consensus that the two most important problems all limb scattering instruments have to deal with to a certain extent are external straylight and the limb pointing accuracy. External straylight is an issue mainly in the visible and NIR spectral ranges where the limb scattered radiance decreases by up to 5 orders of magnitude when going from 10 km tangent height to 90 km tangent height. Thus, only a minute amount of straylight from the lower atmosphere may significantly contaminate the limb measurements at higher tangent heights. Pointing is a potential problem, since very precise knowledge of the satellite attitude and mirror positions etc. are required. For example, an error in the knowledge of the satellite's orientation of only 1/100 degree translates to a tangent height error of about 0.5 km. The experiences from the existing limb scattering instruments will hopefully lead to significant improvements of the next generation limb scattering instruments.

Table 1: Earth orbiting limb scattering sensors

Instrument	Platform	Launch date	Spectral range	Spectral resolution	Tangent height range	Vertical resolution approximatively
LORE	Space shuttle	1997 & 2003	322, 350, 603, 675, 760 nm	filter radiometer	0 - 75 km	1 km
SOLSE	Space shuttle	1997 & 2003	530 - 850 nm 270 - 423 nm	0.7 nm 0.35 nm	0 - 75 km	1 km
OSIRIS	Odin	Feb 2001	280 - 800 nm	1 nm	7 - 70 km	2 km
SAGE III	Meteor-3M	Dec 2001	290 - 1020 nm	1.4 - 2.5 nm		1 km
SCIAMACHY	ENVISAT	Feb 2002	220 - 2380 nm	0.2 - 1.5 nm	0 - 100 km	3 km
OMPS	NPOESS	2008	290 - 1000 nm	1.5 - 40 nm	0-60 km	3 km

Inversion Algorithms and Radiative Transfer

The existing inversion algorithms for retrievals of vertical minor constituent profiles from measurements of limb scattered radiation can be roughly classified into two categories, although all of them rely on differential absorption signatures between spectral regions where the absorption cross-sections of the species of interest differ.

The first type of algorithm follows a DOAS (Differential Optical Absorption Spectroscopy) approach and exploits the high frequency structure of absorption cross sections, while removing the slowly varying component of the cross sections and the measured limb spectra. These algorithms are well suited for weakly absorbing species, such as NO₂ [Sioris et al., 2003], BrO and OClO, but they can also be applied to stronger absorbers, such as O₃. Retrievals of stratospheric O3 and NO₂ density profiles from OSIRIS limb scattering observations (C. Haley) and from SCIAMACHY measurements (A. Rozanov, C. Sioris) using DOAS type algorithms were presented at the workshop.

The second type of retrieval algorithm exploits the absolute absorption of solar radiation in strong absorption features of atmospheric constituents, e.g., the differential absorption between the center and the wings of the Chappuis, Huggins or Hartley bands of O₃. These algorithms usually require only several narrow spectral windows and they are, therefore, computationally more efficient and well suited for operational mass data processing. Yet, their applicability is limited to strongly absorbing species. A first algorithm was designed by Flittner et al. [2000] for O₃ profile retrievals from SOLSE/LORE observations, and it is now employed in a slightly modified way for operational analysis of OSIRIS observations [von Savigny et al., 2003].

The determination of minor constituent profiles from limb scattering observations requires radiative transfer (RT) modeling. RT models that accurately account for all relevant physical processes are therefore a necessary prerequisite of profile retrievals from limb scattering observations. Several pseudo-spherical and spherical RT models were presented at the workshop (C. McLinden, A. Rozanov, J. van Gent), including a 3D Monte Carlo model (C. von Friedeburg)

Limb scattering retrievals are generally based on different homogeneity assumptions. Two presentations addressed the retrieval errors associated with violations of these homogeneity assumptions: (a) the impact of inhomogeneous surface reflectance on ozone profile retrievals from limb scattering observations (D. Flittner), and (b) the impact of horizontal inhomogeneities of atmospheric trace constituent fields on the retrieval of vertical profiles (C. McLinden).

As mentioned above, inaccurate pointing knowledge is one of the most important sources of retrieval errors all limb scattering instruments are affected by. Fortunately, tangent heights can also be retrieved from the limb measurements themselves with an accuracy of at least ± 2 km. All of the employed pointing retrieval algorithms are based on the so-called "knee", i.e. a maximum in UV limb radiance profiles due to absorption by O₃ occurring in the upper stratosphere/lower mesosphere depending on wavelength. Different generalisations of the standard "knee" method using a continuous wavelength range rather than a single UV wavelength are presently employed for tangent height retrievals (J. Kaiser, C. Sioris). Unfortunately, tangent height information and the ozone density profile cannot be retrieved reliably at the same time.

Retrievals and First Scientific Results

The second day of the workshop was almost entirely spent on first scientific results and validation of limb scattering retrievals. The majority of the contributions dealt with minor constituent profile retrievals. Examples of minor constituent profile retrievals are shown in Figure 1. Successful stratospheric O₃ profile retrievals were performed from OSIRIS (C. Haley, S. Brohede), SOLSE/LORE (R. Loughman), SAGE III (D. Rault), and SCIAMACHY (A. Rozanov, C. von Savigny). The 2002 SH ozone hole split event received special attention, and stratospheric profiles of O₃, NO₂, BrO and OClO within and outside the polar vortex were shown (A. Rozanov, C. Sioris, C. von Savigny) highlighting the capability of limb instruments to globally observe the vertical structure of atmospheric trace constituents on a daily basis. Furthermore, the retrieval of CH₄ and H₂O profiles in the UTLS region from SCIAMACHY measurements is presently under development (K.-U. Eichmann).

(A)	(B)
(C)	(D)
Figure 1. a): Ozone profile retrieval from SOLSE-2 limb scattering observations on Jan. 23, 2003 (green line with corrected tangent height registration), and comparison with a co-located ECC sonde launched from Santa Cruz/Teneriffe [Courtesy R. Loughman]. b): Stratospheric ozone number density (in molecules / cm3) field along an entire orbit retrieved from OSIRIS limb measurements on Oct. 2, 2002 (Courtesy E. Llewellyn]. Note that this is about one week after the Antarctic vortex split into two parts in late September 2002; the secondary ozone hole has already almost disappeared. c): Stratospheric ozone number density (in molecules/cm3) profile retrieved from SAGE III limb measurement on January 29, 2003 (Latitude: 70º N, longitude: 2º W) and comparison with co-located SAGE III occultation measurement [Courtesy D. Rault]. d) BrO volume mixing ratio field (in ppt, black lines) retrieved from SCIAMACHY limb measurements inside and outside the Antarctic polar vortex [Courtesy A. Rozanov]. The white solid lines are contours of modified potential vorticity.

Apart from minor constituent profile retrievals another important application of UV/vis/NIR limb scattering measurements are aerosols and clouds. Due to the long slant paths through the atmosphere in limb geometry, several atmospheric phenomena can be studied, which cannot be observed with nadir viewing instruments. These comprise tropospheric, stratospheric, as well as mesospheric aerosols. Tropospheric water and ice clouds are easily discernible in the vis/NIR spectral range (D. Degenstein), where the additional scatterers lead to an enhanced limb radiance signal. Also presented were retrievals of stratospheric aerosol extinction profiles and aerosol particle size estimates from OSIRIS limb scattering observations (D. Gattinger). This is a particularly difficult task, since the limb radiance contribution from stratospheric sulphate aerosols is generally quite small, especially under the very clean stratospheric conditions of the past years. Furthermore, accurate retrievals of stratospheric aerosol information requires accurate characterization of the external straylight contamination.

Apart from stratospheric background aerosol, polar stratospheric clouds (PSCs) can be detected and their particle sizes can be estimated. Information of the chemical composition of PSCs can most likely not be inferred from UV/vis/NIR limb scattering observations, since the characteristic spectral absorption features of PSC constituents only occur at longer wavelengths.

In the mesosphere polar mesospheric clouds (PMCs) are accessible by limb scattering experiments and first qualitative and quantitative results are available from both OSIRIS and SCIAMACHY. The observations also allow the estimation of PMC particle sizes.

Conclusions

Considering the fact that the limb scattering instruments have been orbiting the Earth only for a few years at the most, the variety and quality of available data products is extremely promising. They include minor constituents, whose profiles can be measured from the UT/LS region partly up to the middle and upper mesosphere, aerosols such as stratospheric sulphate aerosols, PSCs, PMCs and cirrus clouds. Yet, it must be recognized that for many data products algorithm refinement and data product validation is still in progress.

In conclusion, the limb scattering instruments have the potential to greatly contribute to our understanding of a variety of atmospheric processes and to provide a long-term archive or a continuation thereof of the atmospheric composition.

The International Limb Scattering Workshop held at Bremen was the first of a hopefully long-lasting series of workshops. J. Stegman volunteered to organise the second limb scattering workshop tentatively scheduled to take place at MISU on October 10-13, 2004 in Stockholm, Sweden.

References

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Burrows, J.P., et al., The Global Ozone Monitoring Experiment (GOME): Mission Concept and First Scientific Results, J. Atmos. Sci., 56, 151-175, 1999.

Flittner, D.E., et al., O₃ profiles retrieved from limb scatter measurements: Theory, Geophys. Res. Lett., 27, 2061-2064, 2000.

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