Abstract
Abstract. Limb measurements provided by the SCanning Imaging Absorption spectrometer for Atmospheric CHartographY (SCIAMACHY) on the ENVISAT satellite allow retrieving stratospheric profiles of various trace gases on a global scale, among them BrO for the first time. For limb observations in the UV/VIS spectral region the instrument measures scattered light with a complex distribution of light paths: the light is measured at different tangent heights and can be scattered or absorbed in the atmosphere or reflected by the ground. By means of spectroscopy and radiative transfer modelling these measurements can be inverted to retrieve the vertical distribution of stratospheric trace gases. The fully spherical 3-D Monte Carlo radiative transfer model "Tracy-II" is applied in this study. The Monte Carlo method benefits from conceptual simplicity and allows realizing the concept of full spherical geometry of the atmosphere and also its 3-D properties, which is important for a realistic description of the limb geometry. Furthermore it allows accounting for horizontal gradients in the distribution of trace gases. In this study the effect of horizontally inhomogeneous distributions of trace gases along flight/viewing direction on the retrieval of profiles is investigated. We introduce a tomographic method to correct for this effect by combining consecutive limb scanning sequences and utilizing the overlap in their measurement sensitivity regions. It is found that if horizontal inhomogenity is not properly accounted for, typical errors of 20% for NO2 and up to 50% for OClO around the altitude of the profile peak can arise for measurements close to the Arctic polar vortex boundary in boreal winter.
Highlights
While nadir observations provide knowledge of the total column density and low vertical resolution information about profiles for strong absorbers only (Bhartia et al, 1996; Hoogen et al, 1999), measurements in limb geometry provide further opportunities to extract height resolved profile information
The retrieval is performed applying either 1-D or 2-D box air mass factors (AMFs) in order to investigate if the 2-D box AMF concept is changing the profile retrieval to the direction expected by the considerations in Sect. 4.1: This is awaited because of the improved description of the reality in the forward model for situations where the distribution of the considered trace gas along the line of sight (LOS) is inhomogeneous
The two step approach (DOAS, radiative transfer modelling (RTM) combined by optimal estimation) allows to investigate the effects of spectroscopy and radiative transfer on profiles separately
Summary
While nadir observations (i.e. space borne instruments looking perpendicularly to the surface of the Earth) provide knowledge of the total column density and low vertical resolution information (about 10 km) about profiles for strong absorbers only (Bhartia et al, 1996; Hoogen et al, 1999), measurements in limb geometry (i.e. tangential view with respect to the Earth surface) provide further opportunities to extract height resolved profile information. This is achieved by measuring backscattered light from air masses at different tangent heights. More instrumental details can be found in Bovensmann et al (1999)
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