Espic, Christophe (2022). Radiocarbon analysis of atmospheric methane: A new setup to unravel regional methane sources. (Thesis). Universität Bern, Bern

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Abstract

Summary Methane (CH4) is the second most important anthropogenic greenhouse gas after carbon dioxide (CO2). The understanding of the past and future evolution of CH4 abundance in the atmosphere is a key factor in a context of climate change. Unfortunately, the complexity of natural and anthropogenic CH4 sources and sinks is a major hindrance to unraveling the present and future role of atmospheric CH4 on the climate. A tool proposed for the distinction between biogenic and fossil CH4 sources is to measure the radiocarbon content (14C) of their emissions. As living plants and animals continuously exchange carbon with the environment through the processes of respiration and photosynthesis, their CH4 emissions contain present-day 14C levels. Conversely, fossil sources of CH4 such as natural gas seepage or fossil fuel burning are devoid of 14C, as their related CH4 emissions originate from very old organic matter containing no more 14C. Hence, the comparison of the 14C content of atmospheric CH4 of a polluted site and of free tropospheric air should allow the apportionment of regional biogenic and fossil sources of CH4, which should in turn help developing effective mitigation strategies. Unfortunately, such measurements are expensive and time-consuming, as CH4 needs to be extracted and purified from large amounts of air. Hence, very few measurements of atmospheric 14CH4 have been published so far and none of them could lead to a source apportionment of regional CH4 sources. The present thesis carries two main objectives. The first is technical, with the development of a setup allowing the preparation of pure atmospheric CH4 samples for 14C analysis. The second is scientific, with the collection, pretreatment and 14C measurement of air samples in Switzerland, the results of which are used to test the feasibility to apportion local and regional CH4 sources with the radiocarbon technique. The procedure leading to atmospheric 14CH4 results is as follows: an air sample is first collected in the field by filling a bag with 60 L air using a small pump. The sample is then connected to the methane preconcentration and purification setup in the laboratory, which is composed of two parts: a preconcentration line, where CO2 and most of bulk air are removed from the sample; and a purification setup, which uses a preparative gas chromatography technique to extract and purify CH4 from all other carbon-containing gases. Pure CH4 is then combusted to CO2, recovered in a small glass ampoule and its 14C content is measured with accelerator mass spectrometry. The main advantages of this setup are: efficiency, as its compactness limits the operation time and allows to prepare 3–4 samples per day; reliability, as the purity of CH4 is guaranteed by a chromatic purification; and flexibility, as CO2 is also recovered and the purification can be used as a standalone, offering the possibility to analyze high CH4 concentration samples. We conducted biweekly 14CH4 and 14CO2 analyses of atmospheric air samples collected at three strategic sites in Switzerland. The 14CH4 results obtained from free tropospheric air collected at the Jungfraujoch Research Station confirm that 14CH4 releases from nuclear power plants kept increasing the global 14C content of atmospheric CH4 since the early 1970s, and address the lack of published 14CH4 values of clean background air since the early 2000s. Unfortunately, nuclear power plants discharging 14C are responsible for a very large scatter of 14CH4 values at the two other sampling sites, precluding any attempt to apportion fossil and biogenic sources of CH4. Since atmospheric 14CO2 is much less sensitive to 14C emissions from nuclear power plants, 14CO2 measurements allowed to estimate recently added fossil fuel CO2 at Beromünster (rural site) and Bern (urban area). As expected from their respective geographical situation, the fossil fuel CO2 component is significantly larger in the city, because the sampling site is close to fossil sources such as the road traffic and domestic heating exhausts. The preponderant influence of 14C emissions from nuclear activities on atmospheric 14CH4 levels motivated the conduction of a field study of atmospheric 14CH4 and 14CO2 near the Gösgen nuclear power plant during a revision period, when substantial 14C discharges were expected. The air samples collected and analyzed reveal an extremely large impact of sporadic 14C releases on atmospheric 14CH4 contents, with measured values up to 1800 times natural background levels at a distance of 6 km from the nuclear power plant. Although the Gösgen nuclear power plant emits little amounts of 14CO2 on average, considerable 14CO2 enhancements were also observed during the dispersion of the emission plume, emphasizing the large temporal variability of this 14C source. Unfortunately, the influence of 14C emissions from nuclear power plants on atmospheric 14CH4 and 14CO2 levels at a specific location are still simulated by supposing constant emissions in atmospheric studies. Thus, the observations made during the Gösgen study bring two key findings: First, they explain the very large scatter of 14CH4 values measured in Switzerland, which prevents from any meaningful 14C source apportionment of atmospheric CH4 in regions of nuclear activities under current conditions. Second, they indicate that when not correctly accounted for, sporadic discharges from nuclear power plants create significant biases in the estimation of the fossil fuel CO2 component at a sampling site. Finally, we demonstrate the flexibility of the new setup with the participation in a laboratory intercomparison, which aimed at using the 14C method to apportion the biogenic and fossil fractions of blends of biogas and natural gas. The purification setup was very useful at separating individual subfractions of the gas mixtures and analyzing their respective 14C content, which provided some interesting additional information.

Item Type:	Thesis
Dissertation Type:	Cumulative
Date of Defense:	12 January 2022
Subjects:	500 Science > 540 Chemistry
Institute / Center:	08 Faculty of Science > Department of Chemistry, Biochemistry and Pharmaceutical Sciences (DCBP)
Depositing User:	Hammer Igor
Date Deposited:	03 Jul 2025 06:58
Last Modified:	03 Jul 2025 22:25
URI:	https://boristheses.unibe.ch/id/eprint/6354

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