Error analysis for CO and CH4 total column retrievals from SCIAMACHY 2.3 μm spectra A. M. S. Gloudemans, H. Schrijver, O. P. Hasekamp, and I. Aben SRON Netherlands Institute for Space Research, Utrecht, The Netherlands
Abstract. A detailed sensitivity analysis of the Iterative Maximum Likelihood
Method (IMLM) algorithm and its application
to the SCIAMACHY 2.3 μm spectra is presented.
The sensitivity analysis includes a detailed assessment of the impact of
aerosols in the 2.3 μm range.
Results show that near strong aerosol sources mineral
dust and biomass aerosols can have an effect of ~7–10%
on the CH4 total columns retrieved from this wavelength range
if aerosol scattering is neglected in the retrieval algorithm. Similar
but somewhat larger effects are found for CO, but due to the
larger variability of CO
these errors are less important. Away from strong sources much
smaller effects of a few percent are found.
Using CH4 as a proxy for CO and/or including aerosol
information in the retrieval algorithm significantly reduces these errors
for both CO and CH4.
Spectroscopic uncertainties are mostly negligible except for
uncertainties in the CH4 intrinsic line intensities, which can
Application of the IMLM algorithm
to the SCIAMACHY 2.3 μm spectra shows that the
quality of the retrieved CO and CH4 total columns is good, except
for a bias for large instrument-noise errors which is partly due to
remaining calibration issues.
Polarization sensitivity of the SCIAMACHY instrument has a negligible effect
on the retrieved CO and CH4 total columns.
The H2O total
columns, which have to be retrieved simultaneously with CO and CH4
due to overlapping absorption lines, agree
well with H2O total columns from ECMWF data.
This ensures that the fit to the
H2O absorptions is of sufficient quality not to hamper
the retrieved CO and CH4 total columns from SCIAMACHY spectra.
Citation: Gloudemans, A. M. S., Schrijver, H., Hasekamp, O. P., and Aben, I.: Error analysis for CO and CH4 total column retrievals from SCIAMACHY 2.3 μm spectra, Atmos. Chem. Phys., 8, 3999-4017, doi:10.5194/acp-8-3999-2008, 2008.