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The SCIAMACHY instrument onboard the ESA satellite ENVISAT allows measurements of variousatmospheric trace gases, such as NO2.A unique feature of SCIAMACHY is that measurements are made alternately in limb and nadirmode. The limb measurements provide an opportunity for directly determiningstratospheric column densities (CDs), which are needed to extract tropospheric CDs from the totalCD measurements performed in (quasi simultaneous) nadir geometry. Here we discuss the potential and limitations of SCIAMACHY limb measurementsfor estimating stratospheric CDs of NO2 in comparison to asimple reference sector method, and the consequences for the resultingtropospheric CDs.A direct, absolute limb correction scheme is presented thatimproves spatial patterns of tropospheric NO2 column densities at highlatitudes, but resultsin artificial zonal stripes at low latitudes. Subsequently, a relativelimb correction scheme is introduced that successfully reducesstratospheric artefacts in the tropospheric data product withoutintroducing new ones. This relative limb correction scheme is rather simple,robust, and, in essence, based on measurements alone. The effects of the different stratospheric estimation schemes on troposphericCDs are discussed with respect to zonal and temporaldependencies. In addition, we define error quantities from the nadir/limbmeasurements that indicate remaining systematic errors as a function oflatitude and day. Our new suggested stratospheric estimation scheme, the relative limbcorrection, improves mean tropospheric slant CDssignificantly, e.g. from −1×1015 molec/cm2 (using areference sector method) to ≈0 in the Atlantic ocean, and from+1×1015 molec/cm2 to ≈0 over Siberia, at50° N in January 2003–2008

Retrieval of tropospheric column densities of NO&lt;sub&gt;2&lt;/sub&gt; from combined SCIAMACHY nadir/limb measurements

Retrieval of tropospheric column densities of NO<sub>2</sub> from combined SCIAMACHY nadir/limb measurements