Development of a merged NO2 climate data record from GOME-2 instruments

Kurzfassung

As part of the EUMETSAT Satellite Application Facility on Atmospheric Composition Monitoring (AC SAF) and the ESA Climate Change Initiative (CCI) Precursors project, a consistent and harmonized long-term climate data record on nitrogen dioxide (NO2) is generated from the Global Ozone Monitoring Experiment-2 (GOME-2) A, B and C measurements. In this study, an improved prototype algorithm for the retrieval of total and tropospheric NO2 columns from GOME-2 is presented and evaluated. The AC-SAF prototype GOME-2 NO2 retrieval employs an extended 425-497 nm wavelength fitting window, updated absorption cross sections, and two additional polarisation functions to reduce slant column errors and increase the signal-to-noise ratio. In addition, an improved slit function treatment and a reprocessed Level 1B version 6.3 (and later versions) are applied in the GOME-2 NO2 slant column retrieval, which corrects for incomplete removal of contamination in calibration key data and compensates for long-term and in-orbit drifts. The stratospheric estimation is implemented using the STRatospheric Estimation Algorithm from Mainz (STREAM) with an improved latitudinal correction. The tropospheric NO2 vertical column is converted using an updated box air mass factor look-up table and ancillary input datasets. The retrieved NO2 columns from the improved prototype algorithm show better consistency among three different GOME-2 sensors and reduced retrieval errors over a long-term period compared to the current operational (GDP 4.8) dataset and Quality Assurance for the Essential Climate Variables (QA4ECV) products. Based on the evaluation of the AC-SAF prototype GOME-2 NO2 retrieval performance, we proceed to develop NO2 climate data records (CDRs) by combining GOME-2A, B and C Level 2 orbital NO2 products to create one merged Level 3 product from 2007 to the present. Estimation of the random and systematic uncertainty is included in the NO2 CDR product to assess the temporal evolution of uncertainties and the stability of data records. The merged AC-SAF GOME-2 NO2 CDR will serve as useful data for analysing long-term trends and interannual variabilities of NO2 columns in different regions. Furthermore, this data will be included as a morning sensor input database for the generation of CCI+ precursors multi-mission NO2 CDR.