Atmospheric Chemistry and Physics Discussions
www.atmos-chem-phys-discuss.net
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2008
10.5194/acpd-8-9697-2008
http://www.atmos-chem-phys-discuss.net/8/9697/2008/
http://www.atmos-chem-phys-discuss.net/8/9697/2008/acpd-8-9697-2008.html
http://www.atmos-chem-phys-discuss.net/8/9697/2008/acpd-8-9697-2008.pdf
9697
9729
2008-05-27
FRESCO+: an improved O<sub>2</sub> A-band cloud retrieval algorithm for tropospheric trace gas retrievals
P. Wang
wangp@knmi.nl
P. Stammes
R. van der A
G. Pinardi
M. van Roozendael
Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands
BIRA-IASB, Belgian Institute for Space Aeronomy, Brussels, Belgium
The FRESCO (Fast Retrieval Scheme for Clouds from the Oxygen A-band)
algorithm has been used to retrieve cloud information from measurements
of the O<sub>2</sub> A-band around 760 nm by GOME, SCIAMACHY and GOME-2.
The cloud parameters retrieved by FRESCO are the effective cloud
fraction and cloud pressure, which are used for cloud correction in
the retrieval of trace gases like O<sub>3</sub> and NO<sub>2</sub>.
To improve the cloud pressure retrieval for partly cloudy scenes,
single Rayleigh scattering has been included in an improved version of
the algorithm, called FRESCO+. We compared FRESCO+ and FRESCO effective
cloud fractions and cloud pressures using simulated spectra and
one month of GOME measured spectra. As expected, FRESCO+ gives more
reliable cloud pressures over partly cloudy pixels. Simulations and
comparisons with ground-based radar/lidar measurements of clouds shows
that the FRESCO+ cloud pressure is about the optical midlevel of the
cloud. Globally averaged, the FRESCO+ cloud pressure is about 50 hPa
higher than the FRESCO cloud pressure, while the FRESCO+ effective
cloud fraction is about 0.01 larger.
<br><br>
The effect of FRESCO+ cloud parameters on O<sub>3</sub> and NO<sub>2</sub>
vertical column densities (VCD) is studied using SCIAMACHY data
and ground-based DOAS measurements. We find that the FRESCO+
algorithm has a significant effect on tropospheric NO<sub>2</sub>
retrievals but a minor effect on total O<sub>3</sub> retrievals.
The retrieved SCIAMACHY tropospheric NO<sub>2</sub> VCDs
using FRESCO+ cloud parameters (v1.1) are lower than the tropospheric
NO<sub>2</sub> VCDs which used FRESCO cloud parameters (v1.04), in particular
over heavily polluted areas with low clouds. The difference between
SCIAMACHY tropospheric NO<sub>2</sub> VCDs v1.1 and ground-based MAXDOAS
measurements performed in Cabauw, The Netherlands,
during the DANDELIONS campaign is about
−2.12×10<sup>14</sup> molec cm<sup>−2</sup>.
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