Retrieval of tropospheric NO₂ columns from SCIAMACHY combining measurements from limb and nadir geometries

Satellite measurements of atmospheric trace gases have proved to be an invaluable tool for monitoring the Earth system. When these measurements are to be used for assessing tropospheric emissions and pollution, as for example in the case of nadir measurements of nitrogen dioxide (NO2), it is necessary to separate the stratospheric from the tropospheric signal. The SCIAMACHY instrument offers the unique opportunity to combine its measurements in limb and nadir viewing geometries into a tropospheric data product, using the limb measurements of the stratospheric NO2 abundances to correct the nadir measurements' total columns. In this manuscript, we present a novel approach to limb/nadir matching, calculating one stratospheric NO2 value from limb measurements for every single nadir measurement, abandoning global coverage for the sake of spatial accuracy. As a comparison, modelled stratospheric NO2 columns from the Oslo CTM2 are evaluated as stratospheric correction, and both datasets are confronted with the originally used reference sector method. Our study shows that stratospheric NO2 columns from SCIAMACHY limb measurements very well reflect stratospheric conditions. The zonal variability of stratospheric NO2 is captured by our matching algorithm, and the quality of the resulting tropospheric NO2 columns improves considerably. Modelled stratospheric NO2 columns from the Oslo CTM2 agree remarkably well with the measurements. Both datasets need to be matched to the level of the nadir measurements, however, because a time and latitude dependent bias between both stratospheric datasets and the measured nadir columns can be observed over clean regions. After accounting for this systematic bias between SCIAMACHY nadir observations and the stratospheric columns, both new stratospheric correction methods provide a significant improvement to the retrieval of tropospheric NO2 columns from the SCIAMACHY instrument.