During the second year, the CH3D, CH4 and δD 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 δ13C 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 δ 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
● Retrieval strategies have been developed for both isotopologues, they are applicable to sites spanning broad atmospheric conditions (humidity)
● Information content analyses indicated that independent time series can be derived for the troposphere and the stratosphere, for 13CH4 and CH3D
● 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.
● For CH3D, simultaneous time series (CH3D, CH4, δD) have been produced for five NDACC sites, the longest data set (Jungfraujoch) documents the evolution of 13CH4 over more than 30 years
● For 13CH4, simultaneous time series (13CH4, CH4 and δ13C) 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.
● 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
● Further comparisons have been performed, involving several data sets (surface in situ, remote-sensing and a GEOS-Chem 2x2.5º global simulation) available for CH4 at Reunion Island, showing here again a good agreement
The dissemination of the results was ensured through:
● Regular participation to symposia (eight over the project duration)
● Active participation to the Researcher's Night, an event organized such as to connect a broad public with researchers
● Radio interview on BXFM 104.3 Bruxelles
● Four papers have been published to date.
