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Reference Sector Method

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For the latter case dedicated techniques are required to separate the tropospheric and the stratospheric concentrations. One approach is to use measurements over a clean air region as a background, the so-called Reference Sector Method (see below). In addition, SCIAMACHY’s unique limb/nadir matching capabilities provide a nearly simultaneous stratospheric profile measurement for each nadir measurement. In that respect, SCIAMACHY is clearly superior to its predecessor GOME which obtained measurements of the same species in the UV-VIS range but only in nadir geometry.

5.5.1 Reference Sector Method

The Reference Sector Method, also referred to as Tropospheric Residual Method (Velders et al. 2001, Richter and Burrows 2002, Martin et al. 2002) allows the separation of tropospheric and stratospheric contributions to the total NO2 content under the assumption of a stable – both spatial and temporal – NO2 distribution over the clean, free Pacific Ocean. It is assumed that the stratospheric NO2 distribution is homogeneous with longitude. Then the tropospheric NO2 is primarily the difference between the total column measured over a polluted area and the total column measured over the clean Pacific Ocean. This technique needs no stratospheric profile information and can therefore be applied to generate a consistent GOME – SCIAMACHY tropospheric NO2 data set.

5.5.2 Limb/Nadir Matching

One important area of uncertainty in the determination of the tropospheric column concentration from solar backscatter nadir measurements is the error introduced by estimating the stratospheric column concentration (Boersma et al. 2004). To improve on this topic, it is required to use the measured stratospheric column above the ground scene of interest. SCIAMACHY with its limb/nadir matching measurement mode provides radiances from the same volume of air in limb and nadir geometry since 2002. This allows inference of vertical stratospheric concentration profiles directly over the region of the nadir measurement. Integrating these profiles from the tropopause upwards yields the measured stratospheric column above the target area while the collocated nadir measurement provides the total column amount. The tropospheric column is then – very briefly speaking – determined as the difference between the total and the stratospheric column. An initial application of this approach to derive tropospheric NO2 was presented in Sierk et al. (2006). The method described here is unique in the sense that the information on the stratospheric content is taken directly from the collocated limb measurement and no other assumptions (longitudinal homogeneity) or an estimate of the stratospheric column from a model or from data assimilation are necessary.

5.6 Data Assimilation and Value Added Products

Data assimilation generates synoptic trace gas fields from asynoptic spaceborne measurements. This enables the derivation of interpolated concentration fields, e.g. to separate tropospheric and stratospheric contributions, as well as information about transport mechanisms. Several assimilation schemes are applied to SCIAMACHY measurements to combine stratospheric modelling and nadir column data. They produce results usually referred to as Value Added products.

5.6.1 Tropospheric Trace Gases – the NO2 Example

NO2 permanently resides in the stratosphere and shows significant amounts in the troposphere near source areas. First, the stratospheric and tropospheric parts of the column need to be separated and subsequently, a tropospheric airmass factor needs to be applied to the tropospheric slant column. At KNMI, in collaboration with BIRA/IASB, a data assimilation system was applied to NO2 to derive the stratospheric part of the slant column by assimilation of observed slant columns in a chemistry-transport model (Eskes et al. 2003). This results in a stratospheric analysis consistent with the observations as well as with variations observed in the stratosphere that are due to the atmospheric dynamics and chemical reactions. The tropospheric NO2 slant column is then extracted by subtracting the assimilated stratospheric slant column from the retrieved total slant column.

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