Abstract. An improved algorithm for
the retrieval of total and tropospheric nitrogen dioxide (NO2)
columns from the Global Ozone Monitoring Experiment-2 (GOME-2) is presented.
The refined retrieval will be implemented in a future version of the GOME
Data Processor (GDP) as used by the EUMETSAT Satellite Application Facility
on Atmospheric Composition and UV Radiation (AC-SAF). The first main
improvement is the application of an extended 425–497 nm wavelength fitting
window in the differential optical absorption spectroscopy (DOAS) retrieval
of the NO2 slant column density, based on which initial total
NO2 columns are computed using stratospheric air mass factors (AMFs).
Updated absorption cross sections and a linear offset correction are used for
the large fitting window. An improved slit function treatment is applied to
compensate for both long-term and in-orbit drift of the GOME-2 slit function.
Compared to the current operational (GDP 4.8) dataset, the use of these new
features increases the NO2 columns by
∼1–3×1014 molec cm2 and reduces the slant column error
by ∼24 %. In addition, the bias between GOME-2A and GOME-2B
measurements is largely reduced by adopting a new level 1b data version in
the DOAS retrieval. The retrieved NO2 slant columns show good
consistency with the Quality Assurance for Essential Climate Variables
(QA4ECV) retrieval with a good overall quality. Second, the STRatospheric
Estimation Algorithm from Mainz (STREAM), which was originally developed for
the TROPOspheric Monitoring Instrument (TROPOMI) instrument, was optimised
for GOME-2 measurements to determine the stratospheric NO2 column
density. Applied to synthetic GOME-2 data, the estimated stratospheric
NO2 columns from STREAM shows good agreement with the a priori truth.
An improved latitudinal correction is introduced in STREAM to reduce the
biases over the subtropics. Applied to GOME-2 measurements, STREAM
largely reduces the overestimation of stratospheric NO2 columns over
polluted regions in the GDP 4.8 dataset. Third, the calculation of AMF
applies an updated box-air-mass factor (box-AMF) look-up table (LUT)
calculated using the latest version 2.7 of the Vector-LInearized Discrete
Ordinate Radiative Transfer (VLIDORT) model with an increased number of
reference points and vertical layers, a new GOME-2 surface albedo
climatology, and improved a priori NO2 profiles obtained from the
TM5-MP chemistry transport model. A large effect (mainly enhancement in
summer and reduction in winter) on the retrieved tropospheric NO2
columns by more than 10 % is found over polluted regions. To evaluate the
GOME-2 tropospheric NO2 columns, an end-to-end validation is
performed using ground-based multiple-axis DOAS (MAXDOAS) measurements. The
validation is illustrated for six stations covering urban, suburban, and
background situations. Compared to the GDP 4.8 product, the new dataset
presents improved agreement with the MAXDOAS measurements for all the
stations.
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