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.

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.

Atmospheric methane, the second most important greenhouse gas emitted by human activities, is responsible for approximately one fifth of the changes in the Earth’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 8.2±0.6 and 5.7±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 δD; 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.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз