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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - ISOMET (Atmospheric content of the most abundant of 12CH4 isotopologues from ground-based and satellite infrared solar observations and development of a methane isotopic GEOS-Chem module.)

Teaser

Summary

Atmospheric methane, the second most important greenhouse gas emitted by human activities, is responsible for approximately one fifth of the changes in the Earthâ€™s balance energy since the beginning of the industrialization (~1750). Methane concentrations have reached a new high of 1859 ppb in 2017 with a renewed rise in the last decade after a period of stabilization between 1999 and 2006. To this day, the source(s) and/or sink(s) responsible of the latest increase remain(s) unexplained.
Depending on the emission process, heavy molecules of methane (13CH4 and CH3D), called isotopologues, are emitted along methane with specific emission ratios. Despite their small relative abundances of ~110 and ~60 000 respectively, they give information on the methane concentration in the atmosphere and its evolution. Determining the isotopic ratio of atmospheric methane is therefore a unique tracer of its budget. In a context of an urgency for climate change mitigation, understanding the evolution of a major greenhouse gas is of crucial importance.
In the course of the project, retrieval strategies have been developed to derive, for the first time, the abundance of 13CH4 and CH3D from observations performed at ground-based stations. These approaches proved successful for a broad range of atmospheric conditions (urban,remote, polar), at three sites part of the present project.
Time series spanning up to 30 years were obtained, giving an unprecedented view of the evolution of methane and its isotopologues. The Jungfraujoch time series illustrated here clearly show that the various isotopologues have undergone contrasted evolutions with time, especially CH3D which was almost constant over about 20 years. Even if CH4 and 13CH4 show closer behaviors, a careful look reveals some subtle differences, such as trends for the 2010-2018 time frame which are statistically different, with an annual increases of 8.2Â±0.6 and 5.7Â±0.7 ppb, for CH4 and 13CH4, respectively.
Both isotopologue data sets have been compared with satellite measurements performed by the ACE-FTS instrument since 2004. The comparison involved the lower stratosphere and indicated an excellent agreement in terms of phase, amplitude of the seasonal cycle (with max/min in Feb./Aug.) and abundance. These successful comparisons give good confidence in the ground-based products and confirm the ability to distinguish a meaningful stratospheric signal. The decadal trends affecting the data sets were investigated, and insignificant rates of change were derived, demonstrating that the increase affecting the abundance of both isotopologues over the recent years is driven by their accumulation in the troposphere.
The second figure shows the evolution of the CH3D/CH4 (or Î´D; unitless) at two participating sites of the project. Yearly averages are shown for years with sufficient temporal sampling. It is interesting to note dissimilar evolutions, with a significant increase at Eureka (of about +1.5 per year), i.e., a relative enrichment in CH3D at that site, and a significant decrease at Jungfraujoch (of about -1 per year).
After two years, the project has led to of original results, revealing interesting features that will undoubtedly motivate further work. Interpretation of these results will likely require to expand the data sets to more sites, including in the Southern hemisphere, and to perform comparison with dedicated model simulations.

Work performed

During the second year, the CH3D, CH4 and Î´D time series have been produced for five NDACC FTIR stations distributed around the world. As to the second isotopologue, 13CH4, the retrieval strategy has been further developed and finalized using observations from Toronto (urban wet site) and Jungfraujoch (dry remote site). A 5-window strategy has been applied to the Jungfraujoch observations, allowing producing simultaneous 13CH4, CH4 and Î´13C time series for more than 30 years.
Information content and uncertainty budgets have been consistently established for both isotopologues, showing in both cases a good vertical sensitivity allowing the determination of independent tropospheric and stratospheric time series. The random and systematic uncertainties both amount to 2-3% for 13CH4, with the temperature found to be the dominant contribution. For CH3D, the situation is a little less favorable, with corresponding uncertainties amounting to 4.5 and 6.5%, respectively.
Comparison with in situ surface measurements and GEOS-Chem model data has been performed for 12CH4 which is needed to compute the Î´ time series. This investigation involved the four data sets available at Reunion Island. It demonstrated a good agreement between the model and the measurements. More information is available in Zhou et al. (ACP, 18, 2018).
Regarding the stratospheric methane, ACE-FTS and FTIR lower stratospheric time series have been compared for Jungfraujoch, showing a very good agreement for both isotopologues. The trends were found to be non-significant for the four data sets (at 2-sigma). These comparisons give good confidence in the stratospheric FTIR time series produced for both isotopologues and they suggest that the trend evolution of the total columns do not result from changes in the stratospheric abundance of the two target gases.
Overview of results
â— Retrieval strategies have been developed for both isotopologues, they are applicable to sites spanning broad atmospheric conditions (humidity)
â— Information content analyses indicated that independent time series can be derived for the troposphere and the stratosphere, for 13CH4 and CH3D
â— Random and systematic uncertainty budgets were established, showing that the abundance of the two isotopologues can be retrieved with a precision of 2-3% (13CH4) and 4-5% (CH3D); the systematic uncertainties amount to 2-3% and 6-7&, respectively.
â— For CH3D, simultaneous time series (CH3D, CH4, Î´D) have been produced for five NDACC sites, the longest data set (Jungfraujoch) documents the evolution of 13CH4 over more than 30 years
â— For 13CH4, simultaneous time series (13CH4, CH4 and Î´13C) have been produced for the Jungfraujoch, here again, more than 30 years of data are available; production of the time series for the other sites is pending.
â— Comparisons between ground-based FTIR and satellite stratospheric time series for CH3D and 13CH4 showed a very good agreement in terms of abundance, seasonal modulation and trend
â— Further comparisons have been performed, involving several data sets (surface in situ, remote-sensing and a GEOS-Chem 2x2.5Âº global simulation) available for CH4 at Reunion Island, showing here again a good agreement
The dissemination of the results was ensured through:
â— Regular participation to symposia (eight over the project duration)
â— Active participation to the Researcher\'s Night, an event organized such as to connect a broad public with researchers
â— Radio interview on BXFM 104.3 Bruxelles
â— Four papers have been published to date.

Final results

This project allows for first long term measurements of heavy methane from infrared ground-based solar observations. By reaching out to various atmospheric composition measurement, this project raises interest in the question of atmospheric methane increase through a new and innovative angle. Improving our knowledge in the budget of the second most important greenhouse gas through international collaborations and with different research communities is at the forefront of climate change studies.