Abstract
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.
Highlights
Nitrogen dioxide (NO2) is an important trace gas in the Earth’s atmosphere
Based on a tropospheric NO2 climatology and the Global Ozone Monitoring Experiment-2 (GOME-2) cloud product, STRatospheric Estimation Algorithm from Mainz (STREAM) calculates weighting factors for each satellite pixel to define the contribution of initial total columns to the stratospheric estimation; potentially polluted pixels are weighted low instead of being totally masked out in the Global Ozone Monitoring Experiment (GOME) Data Processor (GDP) 4.8 spatial filtering method, cloudy observations at medium altitudes are given higher weights because they directly provide the stratospheric information, and the weights are further adjusted in a second iteration if pixels suffer from large biases in the tropospheric residues
NO2 columns retrieved from measurements of the GOME2 aboard the MetOp-A and MetOp-B platforms have been successfully applied in many studies
Summary
Nitrogen dioxide (NO2) is an important trace gas in the Earth’s atmosphere. In the stratosphere, NO2 is strongly related to halogen compound reactions and ozone destruction (Solomon, 1999). The total NO2 slant columns depend on the viewing geometry and on parameters such as surface albedo and the presence of clouds and aerosol loads They are converted to initial total NO2 vertical columns through division by a stratospheric air mass factor. The GDP 4.8 algorithm applies a modified reference sector method, which uses measurements over clean regions to estimate the stratospheric NO2 columns based on the assumptions of longitudinally invariable stratospheric NO2 layers and of negligible tropospheric NO2 abundance over the clean areas. The STRatospheric Estimation Algorithm from Mainz (STREAM) method (Beirle et al, 2016) has been developed for the TROPOMI instrument and was successfully applied on GOME, SCIAMACHY, OMI, and GOME-2 measurements. We show an end-to-end validation of the tropospheric NO2 dataset using ground-based multiple-axis DOAS (MAXDOAS) datasets with different pollution conditions (Sect. 7)
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