Browsing by Author "Seppälä, Eemi"

Methane(CH4) is a powerful greenhouse gas and even though the CH4 concentrations in the atmosphere have been increasing rapidly since the year 1750, there still remains large uncertainties in the individual source terms to the global CH4 budget. Measuring the isotopic fractions from various CH4 sources should lead to new knowledge on the processes involving CH4 formation and emission pathways. Nowadays stable isotope measurements for various CH4 sources are quite routinely made, but radiocarbon measurements have for long been too expensive and time consuming. For this reason a new CH4 sampling system for radiocarbon measurements was developed at the Laboratory of Chronology of University of Helsinki. The system allows sampling directly from the atmosphere or from different environmental sources using chambers. To demonstrate the functionality of the system it was tested and optimized in various laboratory experiments and in the field. The laboratory measurements showed that before combustion of CH4 to carbon dioxide(CO2), ambient carbon monoxide(CO) and CO2 can be removed from the sample gas by a flow for more than 10 hours with a flowrate of 1 l/min. After the CO and CO2 removal the CH4 in the sample gas is combusted to CO2. The combustion efficiency for CH4 was 100% with a flowrate of 0.5 l/min. After CH4 is combusted to CO2 it is then collected to molecular sieves and can be later on analyzed using accelerator mass spectrometer. The laboratory measurements, however, showed that due to adsorption of nitrogen(N2) to the molecular sieves, the 1g of molecular sieve material used in molecular sample sieve tubes was not sufficient for low concentration samples where the sampling times are very long. In the field, CH4 was collected from the atmospheric ambient air at Hyytiälä SMEAR II station, Juupajoki, Finland, and from tree and soil chambers. The radiocarbon content of the atmospheric CH4 was 102.27 ± 0.02 percent Modern Carbon (pMC) and 101.40 ± 0.02 pMC. These values were much lower than the expected values, indicating a large spatial and temporal variability. The CH4 collected from chambers closed around tree-stems had a radiocarbon content had of 113.60 ± 0.37 pMC, which was slightly higher than the 108.71 ± 0.37 pMC measured from soil chambers located in the nearby Siikaneva peatland. This indicated that a larger amount of CH4 emitted from peatland surface was recently fixed near the soil surface and a larger amount of the CH4 emitted from tree-stem surface was from older origin transported via roots from the deeper depths of the soil. There is, however, a possibility that the lower radiocarbon content from the peatland surface emitted CH4 was due to a significant contribution from old CH4 fixed before bomb effect, and which is diffused from deeper depths of the soil. This would explain the results from the autumn campaign where the radiocarbon contents were 91.84 ± 0.03 during nighttime and 104.26 ± 0.03 pMC during daytime. These results also indicated that during the daytime more of the emitted CH4 is fixed near the surface of the peatland soil. One additional CH4 sample was collected in January 2019 from the atmospheric ambient air at Kumpula, Helsinki, Finland using a significantly larger molecular sample sieve. This sample had a radiocarbon content of 52.40 ± 0.21 pMC. The old carbon in the sample originated from a fossil methane used in earlier laboratory experiments and indicated that the regeneration process for the larger sample sieve was incomplete. Overall the system functions very well, while collecting samples from environmental chambers, as the CH4 concentrations are left to build-up before collecting the sample. For atmospheric samples, for which the sampling times are higher, the sample sieve size and the regeneration time and temperature will have to be further investigated. In the future, more measurements of the radiocarbon content for individual CH4 sources are needed to provide better knowledge on the CH4 pathways. This portable system allows an efficient way to collect CH4 samples for radiocarbon analyzes from various locations.