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90 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING
2.5.19 Comparison of GOME NO2 retrievals analysed by different scientific
groups
Participating scientists: Walburga Wilms-Grabe, Steffen Beirle, Randall Martin, Andreas Richter,
Michel van Roozendael, and Thomas Wagner
Abstract In order to improve nonuniform GOME NO2 products of differing analysis tools applied
at several institutes, NO2 SCDs have been compared for selected GOME orbits. The results show
principal agreement between retrievals of IASB in Brussels, IUP Bremen and IUP Heidelberg, but in
some extent clear discrepancies to retrievals of Dalhousie University in Halifax.
Figure 2.53: NO2 SCDs of one GOME orbit crossing north-east of the United States and the Pacific,
analysed for 03 February 1996 (left) and 06 February 2000 (right) by Heidelberg (blue), Bremen
(black), Brussels (green) and Halifax (red)
Background Several scientific groups are occupied
with the analysis of GOME trace gas measurements.
In dependence of the used retrieval
tools, the resulting trace gas distributions differ
from each other, though they were analyzed for
the same location and time. Therefore, a comparison
study is performed in order to assess the
discrepancies and to evaluate the reliability of
GOME products. The study is concentrated on
NO2, an important trace gas for both the troposphere
and stratosphere. The assessment of NO2
retrievals can help all GOME data users to choose
the most appropriate data set and particularly
helps the provider of evaluated GOME data to
unify the different existing analyzing algorithms.
Funding See satellite group overview.
Methods and results For the whole GOME
measurement period, two GOME orbits per year
have been selected as case studies: one orbit in
the winter time (February) and one in the summer
time (July). The chosen orbits are all located
above nearly the same region, including high polluted
areas in north-east of the United States and
clean areas above the Pacific. This choice allows
to compare NO2 retrievals for the complete orbit,
but also to consider separately polluted and
non-polluted parts of the orbit. In order to obtain
really basic information about reasons for the
variability of GOME NO2 products, in a first step
pure SCDs were compared.
The figure shows two examples of several NO2 retrievals.
The principal agreement of all participating
groups is existing. However, all selected
orbits indicate better agreement between the NO2
distributions of Bremen, Brussels and Heidelberg
and more obvious deviations in the NO2 results
of Halifax. The deviations of Halifax are related
as well to the absolute SCD values (positive offset)
as to the scattering of the data. In the figure,
the differing scatter patterns of Halifax can
be seen clearly in the region between −50 ◦ and
−10 ◦ latitude. Correlations between NO2 results
of Bremen, Brussels and Heidelberg lie around
0.97 and 0.99 in both polluted and unpolluted regions.
Correlations between data of one of these
groups and data of Halifax are around 0.97 in
polluted areas and about 0.55 in the less NO2enriched
region between −50 ◦ and −10 ◦ latitude.
Outlook/Future work Level 2 data of the
DLR have to be included in the comparison study.
Also VCDs and air mass factors have to be considered,
the number of selected orbits has to be
extended.
Main publication Publication is planned later
on in an advanced stage of the comparison study.