Atmospheric Measurement Techniques | 2019
An improved total and tropospheric NO 2 column retrieval for GOME-2
Abstract
Abstract. An improved algorithm for\nthe retrieval of total and tropospheric nitrogen dioxide ( NO2 )\ncolumns from the Global Ozone Monitoring Experiment-2 (GOME-2) is presented.\nThe refined retrieval will be implemented in a future version of the GOME\nData Processor (GDP) as used by the EUMETSAT Satellite Application Facility\non Atmospheric Composition and UV Radiation (AC-SAF). The first main\nimprovement is the application of an extended 425–497\u2009nm wavelength fitting\nwindow in the differential optical absorption spectroscopy (DOAS) retrieval\nof the NO2 slant column density, based on which initial total\n NO2 columns are computed using stratospheric air mass factors (AMFs).\nUpdated absorption cross sections and a linear offset correction are used for\nthe large fitting window. An improved slit function treatment is applied to\ncompensate for both long-term and in-orbit drift of the GOME-2 slit function.\nCompared to the current operational (GDP\xa04.8) dataset, the use of these new\nfeatures increases the NO2 columns by\n ∼1 – 3×1014 \u2009molec\u2009cm 2 and reduces the slant column error\nby ∼24 \u2009%. In addition, the bias between GOME-2A and GOME-2B\nmeasurements is largely reduced by adopting a new level 1b data version in\nthe DOAS retrieval. The retrieved NO2 slant columns show good\nconsistency with the Quality Assurance for Essential Climate Variables\n(QA4ECV) retrieval with a good overall quality. Second, the STRatospheric\nEstimation Algorithm from Mainz (STREAM), which was originally developed for\nthe TROPOspheric Monitoring Instrument (TROPOMI) instrument, was optimised\nfor GOME-2 measurements to determine the stratospheric NO2 column\ndensity. Applied to synthetic GOME-2 data, the estimated stratospheric\n NO2 columns from STREAM shows good agreement with the a priori truth.\nAn improved latitudinal correction is introduced in STREAM to reduce the\nbiases over the subtropics. Applied to GOME-2 measurements, STREAM\nlargely reduces the overestimation of stratospheric NO2 columns over\npolluted regions in the GDP\xa04.8 dataset. Third, the calculation of AMF\napplies an updated box-air-mass factor (box-AMF) look-up table (LUT)\ncalculated using the latest version 2.7 of the Vector-LInearized Discrete\nOrdinate Radiative Transfer (VLIDORT) model with an increased number of\nreference points and vertical layers, a new GOME-2 surface albedo\nclimatology, and improved a priori NO2 profiles obtained from the\nTM5-MP chemistry transport model. A large effect (mainly enhancement in\nsummer and reduction in winter) on the retrieved tropospheric NO2 \ncolumns by more than 10\u2009% is found over polluted regions. To evaluate the\nGOME-2 tropospheric NO2 columns, an end-to-end validation is\nperformed using ground-based multiple-axis DOAS (MAXDOAS) measurements. The\nvalidation is illustrated for six stations covering urban, suburban, and\nbackground situations. Compared to the GDP\xa04.8 product, the new dataset\npresents improved agreement with the MAXDOAS measurements for all the\nstations.