Improvements to the retrieval of tropospheric NO2 from satellite – stratospheric correction using SCIAMACHY limb/nadir matching and comparison to Oslo CTM2 simulations

[摘要] Satellite measurements of atmospheric trace gases have provedto be an invaluable tool for monitoring the Earth system.Whenthese measurements are to be used for assessing troposphericemissions and pollution, as for example in the case of nadirmeasurements of nitrogen dioxide (NO₂), it is necessary toseparate the stratospheric from the tropospheric signal.

The SCIAMACHY instrument offers the unique opportunity tocombine its measurements in limb- and nadir-viewing geometries intoa tropospheric data product, using the limb measurements of thestratospheric NO₂ abundances to correct the nadirmeasurements' total columns.

In this manuscript, we present a novel approach to limb/nadir matching,calculating one stratospheric NO₂ value fromlimb measurements for every single nadir measurement, abandoning global coverage for the sake of spatial accuracy. For comparison, modelled stratospheric NO₂ columns from the Oslo CTM2 are alsoevaluated for stratospheric correction.

Our study shows that stratospheric NO₂ columns from SCIAMACHYlimb measurements very well reflect stratosphericconditions. The zonal variability of the stratospheric NO₂field is captured by our matching algorithm, and the quality of theresulting tropospheric NO₂ columns improves considerably.Both stratospheric datasets need to be adjusted to the level ofthe nadir measurements, because a time- and latitude-dependentbias to the measured nadir columns can beobserved over clean regions. After this offset is removed, thetwo datasets agree remarkably well, and both stratospheric correctionmethods provide a significant improvement to the retrieval oftropospheric NO₂ columns from the SCIAMACHY instrument.

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