On the influence of underlying elevation data on Sentinel-5 Precursor TROPOMI satellite methane retrievals over Greenland

Abstract

Abstract. The Sentinel-5 Precursor (S5P) mission was launched on October 2017 and has since provided data with high spatio-temporal resolution using its remote sensing instrument, the TROPOspheric Monitoring Instrument (TROPOMI). The latter is a nadir viewing passive grating imaging spectrometer. The mathematical inversion of the TROPOMI data yields retrievals of different trace gas and aerosol data products. The column-averaged dry air mole fraction of methane (XCH4) is the product of interest to this study. The daily global coverage of the atmospheric methane mole fraction data enables the analysis of the methane distribution and variation on large scales and also to estimate surface emissions. The spatio-temporal high-resolution satellite data are potentially particularly valuable in remote regions, such as the Arctic, where few ground stations and in-situ measurements are available. In addition to the operational Copernicus S5P total-column averaged dry air mole fraction methane data product developed by SRON, the scientific TROPOMI/WFMD algorithm data product v1.5 (WFMD product) was generated at the Institute of Environmental Physics at the University of Bremen. In this study we focus on the assessment of both S5P XCH4 data products over Greenland and find that spatial maps of both products show distinct features along the coast lines. Anomalies up to and exceeding 100 ppb are observed and stand out in comparison to the otherwise smooth changes in the methane distribution. These features are more pronounced for the operational product compared to the WFMD product. The spatial patterns correlate with the difference between the GMTED2010 digital elevation model (DEM) used in the retrievals and a more recent topography data set indicating that inaccuracies in the assumed surface elevation are the origin of the observed features. These correlations are stronger for the WFMD product. In order to evaluate the impact of the topography dataset on the retrieval we reprocess the WFMD product with updated elevation data. We find a significant reduction of the localized features when GMTED2010 is replaced by recent topography data over Greenland based on ICESat-2 data. This study shows the importance of the chosen topography data on retrieved dry air mole fractions. The use of an accurate and as up-to-date as possible DEM is advised for all S5P data products as well as for future missions which rely on DEM as input data. A modification based on this study is planned to be introduced in the next version of the WFMD data product.

Integrated Methane Inversion (IMI 1.0): a user-friendly, cloud-based facility for inferring high-resolution methane emissions from TROPOMI satellite observations

“…Validation of Version 1.3.0 showed a global mean bias of −2.7 ppb relative to ground-based measurements from 19 sites in the Total Column Carbon Observing Network (TCCON; Wunch et al, 2011a;Qu et al, 2021), but global bias is of no consequence for regional inversions because it is effectively corrected through the boundary conditions. Of more concern are spatially variable biases (regional biases), caused mainly by aliasing of surface albedo errors into the methane retrieval (Lorente et al, 2021) but also by scattering-induced surface reflectance errors (Barré et al, 2021) and errors in surface altitude (Hachmeister et al, 2022). Qu et al (2021) quantified a nominal TROPOMI regional bias of 6.7 ppb in Version 1.3.0 as the standard deviation of station-to-station biases between TROPOMI and the 19 TCCON sites, and a similar analysis for Version 2.2.0 shows a regional bias of 5.6 ppb (Lorente et al, 2021).…”

The importance of digital elevation model accuracy in XCO2 retrievals: improving the OCO-2 ACOS v11 product

“…Problems especially persist in the NASADEM+ over high northern and southern latitudes. Through this analysis and other evaluations of the Copernicus DEM (Hachmeister et al, 2022;Li et al, 2022;Maresová et al, 2022;Carrera-Hernández, 2021;Guth and Geoffroy, 2021;Karlson et al, 2021;Fahrland et al, 2020) there is substantial evidence to suggest that the Copernicus DEM is the most globally continuous and accurate global DEM that fits the requirement of the OCO-2 ACOS retrieval algorithm for a globally consistent, void-free, and publicly available surface terrain model. We show that changes to the DEM in the OCO-2 ACOS retrieval algorithm have significant impacts on the bias-corrected X CO2 product, which is a direct result of the inclusion of the dpfrac term in the OCO-2 bias correction (see Fig.…”

“…Consequently, inaccuracies in the DEM can propagate into the a priori surface pressure and yield inaccuracies in X CO2 . Furthermore, Hachmeister et al (2022) have demonstrated the importance of the DEM in TROPOMI retrievals of X CH4 , showing that this dependence is not unique to OCO-2 or to retrievals of X CO2 .…”

Spaceborne Observations of Lightning NO2 in the Arctic

“…Other sources of NO 2 include oil and gas activities associated with the Mackenzie Valley in Arctic Canada, Prudhoe Bay/Kuparuk in North Alaska, and Yamal gas pipeline/Urengoy gas fields in Russia . The NO 2 spikes along the Greenland coast may be false signals caused by retrieval complications due to the complicated topography of the region . Contrary to NO 2 pollution, TROPOMI observations north of 70° N show low background NO 2 (4.4 ± 1.3 μmol m –2 ), which is 45% lower than the mean NO 2 between 60° N and 65° N.…”

“…While the 78 The NO 2 spikes along the Greenland coast may be false signals caused by retrieval complications due to the complicated topography of the region. 79 Contrary to NO 2 pollution, TROPOMI observations north of 70°N show low background NO 2 (4.4 ± 1.3 μmol m −2 ), which is 45% lower than the mean NO 2 between 60°N and 65°N.…”