Greenhouse Gases I

MIPAS on Envisat measured a large number of greenhouse gases (GHGs) in the upper troposphere and stratosphere during its mission lifetime from July 2002 to April 2012. Among these greenhouse gases are H2O, O3, N2O, CH4, CFC-11, CFC-12, HCFC-22, CCl4, and SF6. We have analysed the trends and shorter-scale variability of the GHGs as a function of altitude and latitude and have related their variations to the seasonal cycle, QBO impact and, in some cases, to solar variability and ENSO. We have also derived the linear trends over the 10-years mission lifetime depending on altitude and latitude. All GHGs show a similar trend pattern with a dipole structure in the two hemispheres. We trace this pattern back to a shift of the stratospheric mean circulation to the South that is manifested as a shift of the latitudinal positions of the subtropical mixing barriers. In order to overcome the complication in trend assessment due to the large variability of trends in the stratosphere, we have developed a method to derive the trend at the entry point to the stratosphere. We discuss the implication of greenhouse gas trends varying with altitude for the vertically non-resolving monitoring of greenhouse gases.

The Tropospheric Monitoring Instrument (TROPOMI) was successfully launched as single payload of ESA’s Sentinel-5 Precursor (S5P) mission on October 13th, 2017. The instrument observes Earth reflected radiances from the ultraviolet to the shortwave infrared spectra range with daily global coverage and with a spatial resolution of 7x7 km². The telluric absorption around 2.3 μm provides information on the total column amount of methane where the operationally data are processed with the RemoTeC full-physics retrieval algorithm, developed by SRON. In this work, we present recent results on the S5P methane product inferred from nearly one year of TROPOMI measurements. We discuss the use of different data bases for molecular absorption cross sections of water vapor and methane including the recent data of  ESA’s SEOM-Improved Atmospheric Spectroscopy Databases (IAS) project. Moreover, for selected TCCON sites we apply our retrieval algorithm to radiometric measurements of the Fourier Transform Spectrometers at the ground site and compare the results with the TCCON and TROPOMI methane product to better understand differences between the data products. Finally, the comparison of the methane S5P product with the proxy methane product from the Japanese GOSAT satellite complements the study. 

The RAL Remote Sensing Group has developed an optimal estimation scheme to retrieve global height-resolved information on methane from IASI using the 7.9 micron band and produced a v1 mission dataset for MetOp-A (R. Siddans et al., 2017, https://doi.org/10.5194/amt-10-4135-2017). This scheme has subsequently been improved through use of pre-retrieved temperature, water vapour and surface spectral emissivity from IASI, MHS and AMSU.

The methane band at 3.7 microns is also observed by IASI. At this wavelength, the terrestrial Planck function is more sensitive to temperature than at 7.9 microns, and on the dayside of the orbit there can also be a significant surface-reflected solar component to top-of-atmosphere spectral radiances. The 3.7 micron band therefore offers the potential to add information on methane in the near-surface layer, where temperature contrast between the surface and atmosphere is low, which limits sensitivity in the 7.9 micron band.

In this paper, we present results from a v2 mission dataset, based on the improved 7.9 micron scheme, as well as results from simulations and test retrievals combining the 3.7 and 7.9 micron bands in comparison to the established 7.9 micron retrieval scheme. An update is also provided on the status of IASI methane processing via the RAL-RSG MetOp NRT chain.

The recent launch of Copernicus Sentinel-5P (S5P), with its TROPOspheric Monitoring Instrument (TROPOMI) and the future launch of Sentinel 5 (S5), with its Ultraviolet, Near Infrared, Shortwave Infrared (UVNS) instrument will allow for daily global measurements of methane (amongst other atmospheric constituents). In order to enhance the understanding of its sources and sinks and to monitor trends of atmospheric abundances. S5P and S5 aim to aid in the global documentation of methane localized sources, which may have been missed up until now due to lack of, or sparse measurements. Determining the nature of these sources (i.e. whether the source is biological in origin, such as wetlands, or non-biological, such as industrial burn off), could provide additional context to S5P and S5 retrievals, and greatly improve knowledge of the global methane budget. It has been shown (Schwietzke et al., 2016) that the nature of methane sources can be determined through a metric known as δ¹³C, which is the ratio of its primary isotopologues, ¹²CH₄ (~98% of atmospheric methane) and ¹³CH₄ (~1.1%).  

In this study, we investigate the potential for calculating the δ¹³C ratio from S5P and S5 retrievals, through Information Content analysis techniques (Rodgers, 2000). This study is based on synthetic spectra, calculated from a global ensemble of atmospheric conditions, designed to simulate a wide range of atmospheric and surface conditions over the course of a year. We then perform retrievals of ¹²CH₄ and  ¹³CH₄  on these synthetic spectra using the RemoTeC algorithm, assuming the SWIR3 band from S5P/TROPOMI (2305 – 2385 nm) and the SWIR1 band from S5/UVNS (1590 – 1675 nm), where TROPOMI only contains the SWIR3 band, and S5/UVNS contains both SWIR1 and SWIR3. We investigate both bands since SWIR3 contains the most spectral lines, but SWIR1 has higher SNR. For spectroscopy we used the HITRAN 2012 database.

We find that for both spectral bands, assuming a non-scattering atmosphere through the “proxy” technique (Parker et al., 2011), that there is typically enough information content to calculate the δ¹³C ratio (Degrees of Freedom of Signal > 1). This is true over a wide range of atmospheric conditions, including tropical, arid and desert regions, however high latitude regions show lower information content due to high solar zenith angles. Retrieval errors of less than 10‰ can be achieved with sufficient spatial and/or temporal averaging, with the SWIR1 band showing lower retrieval errors than the SWIR3 band. Additional investigations are performed to establish the sensitivity of the retrieved trace gases to expected systematic errors in the prior knowledge of the RemoTeC algorithm.  

Sentinel-5P was successfully launched in October 2017 with on-board its single payload instrument TROPOMI. TROPOMI is a push-broom spectrometer measuring various trace gases amongst which methane. Methane is the second most important anthropogenic greenhouse gas after CO₂, but on a per molecule basis methane is a much more potent GHG compared to CO₂. In addition, because of its much shorter lifetime in the atmosphere reduction of its emissions have already effect on the short term, making it an interesting target for climate change mitigation action which has been recognized by many policy makers.

The potential of detecting 'point sources' of methane and quantifying its emissions from space has been demonstrated by studies on SCIAMACHY and GOSAT data. With TROPOMI we aim to make a next step in this field as TROPOMI CH₄ measurements have unprecedented high spatial resolution (7 x 7 km²) combined with daily global coverage. This is very important as we can only use cloud-free observations which effectively means we can only use a few percent of the data. In this presentation we will show a few first examples of methane 'point sources' observed with TROPOMI including preliminary estimates of emissions. One of the events is a major accidental gas blow-out in a Gas & Oil facility.

 Spatially- and temporally-resolved measurements of atmospheric CO2, CH4, and other GHGs can provide an integrated constraint on the net exchange of these gases between the surface and the atmosphere. If these atmospheric measurements are integrated into a comprehensive atmospheric GHG monitoring system, they could form the basis of a Measurement, Reporting and Verification (MRV) approach that complements the bottom-up inventories used to track anthropogenic emissions. They could also provide timely insight into changes in the natural carbon cycle as it evolves in response to climate change. Estimates of XCO2 and XCH4 from space-based observatories could play a crucial role in this atmospheric carbon monitoring system by detecting emission hot spots and providing improved estimates both natural and anthropogenic fluxes on urban to regional scales. The principal advantage of this approach is that it can yield frequent measurements at high spatial resolution over most of the globe, including areas that are too geographically or politically inaccessible to support ground-based stations. The principal challenge has been the need for unprecedented precision and accuracy in the XCO2 and XCH4 estimates used to quantify surface sources and sinks. Great progress has been made in these areas by the EVISAT/SCIAMACHY, GOSAT/TANSO-FTS and OCO-2 missions. These sensors are now yielding estimates of XCO2 with single sounding random errors between 0.1 and 0.3% (0.4 to 1.2 ppm) and systematic biases between 0.25 and 0.5% (1 to 2 ppm) over most of the globe. These results are being used to quantify natural CO2 sources and sinks on regional scales, to detect CO2 gradients across large urban areas and in selected cases, to quantify emissions from large, coal-fired power plants. For XCH4, these systems have demonstrated single sounding random errors are near 13 ppb and systematic biases are between 0.2 and 0.4% (4 and 7 ppb). While these experiments clearly demonstrate the potential value of space-based CO2 and CH4 measurements, additional advances are needed to meet the increasingly demanding requirements for precision, accuracy, resolution, and coverage. This presentation summarizes the current state of the art, near-term plans and the prospects for an operational, space-based constellation designed to quantify CO2 and CH4 fluxes on urban to national scales.

Contributions from ESA and MAG experts
Major international institutions

As part of the European Copernicus Programme, the European Commission (EC) and the European Space Agency (ESA) together with the support of Eumetsat and the European Centre for Medium-range Weather Forecasts (ECMWF) are considering to further develop the first generation Copernicus Space Component to include measurements for fossil CO₂ emission monitoring. The greatest contribution to the increase in atmospheric CO₂ comes from emissions from the combustion of fossil fuels and cement production. Current uncertainties associated with their emission estimates at national and regional scales may translate into ill-informed policy decisions and limitations in assessing the effectiveness of CO₂ emission strategies.

Satellite and in-situ atmospheric measurements, in addition to bottom-up inventories, would enable the transparent and consistent quantitative assessment of CO₂ emissions and their trends at the scale of megacities, regions, countries, and the globe as well. Such a capacity would provide the European Union with a unique and independent source of information, which can be used to inform on the effect of policy measures, and to track their impact en-route towards decarbonizing Europe and meeting national emission reduction targets. Further, there would be potential synergies at international level with observation systems under discussion with other third parties.

This presentation will provide an overview of the CO₂ monitoring mission objectives, the observational requirements on CO₂ and auxiliary measurement capabilities. It also provides a status update of activities and dedicated studies currently undertaken to prepare for the implementation of the space component of this monitoring system.