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Description of a formaldehyde retrieval algorithm for the Geostationary Environment Monitoring Spectrometer (GEMS)

Abstract. We describe a formaldehyde (HCHO) retrieval algorithm for
the Geostationary Environment Monitoring Spectrometer (GEMS) that will be
launched by the Korean Ministry of Environment in 2019. The algorithm
comprises three steps: preprocesses, radiance fitting, and postprocesses.
The preprocesses include a wavelength calibration, as well as interpolation and
convolution of absorption cross sections; radiance fitting is conducted
using a nonlinear fitting method referred to as basic optical absorption
spectroscopy (BOAS); and postprocesses include air mass factor calculations
and bias corrections. In this study, several sensitivity tests are conducted
to examine the retrieval uncertainties using the GEMS HCHO algorithm. We
evaluate the algorithm with the Ozone Monitoring Instrument (OMI) Level 1B irradiance/radiance data by
comparing our retrieved HCHO column densities with OMI HCHO products of the
Smithsonian Astrophysical Observatory (OMHCHO) and of the Quality Assurance
for Essential Climate Variables project (OMI QA4ECV). Results show that OMI
HCHO slant columns retrieved using the GEMS algorithm are in good agreement
with OMHCHO, with correlation coefficients of 0.77–0.91 and regression
slopes of 0.94–1.04 for March, June, September, and December 2005. Spatial
distributions of HCHO slant columns from the GEMS algorithm are consistent
with the OMI QA4ECV products, but relatively poorer correlation coefficients
of 0.52–0.76 are found compared to those against the OMHCHO products.
Also, we compare the satellite results with ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS)
observations. OMI GEMS HCHO vertical columns are 9 %–25 % lower than
those of MAX-DOAS at Haute-Provence Observatory (OHP) in France, Bremen in
Germany, and Xianghe in China. We find that the OMI GEMS retrievals have
less bias than the OMHCHO and OMI QA4ECV products at OHP and Bremen in
comparison with MAX-DOAS.
