Master´s thesis defense by Laxmi Narayan Paudel – Københavns Universitet

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Master´s thesis defense by Laxmi Narayan Paudel

Title: Methane concentration measurements from individual small ice cores samples

Abstract (English): Methane (CH4) is the second most important greenhouse gas after carbon-dioxide (CO2). Its concentration in the atmosphere in the past can be measured directly by analysing the air entrapped in ice cores. Since the 1970’s, a variety of techniques are established to extract the air trapped in bubbles from ice cores and to measure the methane concentration in the extracted air. Traditionally the gas chromatography and mass-spectrometry are used to conduct the measurements of absolute methane concentration. In recent times, Cavity Ring Down Spectrometry (CRDS) is implemented as an analytical method providing relative concentration of the methane. Gas analysis in ice core samples involve dry and wet gas extraction methods. The usage of the extraction method is dependent on the sensitivity of a particular gas species to dissolution. Methane concentration is possible to measure using wet extraction. The sampling could be conducted in discrete, semi-continuous and continuous flow analysis (CFA) manner with low to ultra-high resolution respectively. However, the continuous methods cannot fully compete yet in the reproducibility and accuracy of the discrete methods due to dissolution issue and yielded calibration uncertainty (Stowasser et al., 2012). The only discrete measurements could provide absolute values of methane concentration and be supportive to the calibration of the ultra-high resolved CFA data as well.
Here is presented a discrete method to measure methane concentration in the trapped air from an ice core sample. The trapped air is extracted by wet extraction method and delivered to a water vapour trap to remove an excess water vapour. The dried air sample is then pumped into the cavity of CRDS analyser (Picarro G1101i). The Picarro analyser can measure the methane concentration and water vapour simultaneously. Thus, this new method overcomes the disadvantages of cross-talk between water vapour and methane in established discrete method and in recent continuous measurements (Rella, 2010).
In order to verify the working state of the built set-up I present the results of methane concentration measurements in the samples of ice core from Central Greenland (EuroCore, 72° 34’ 27.89” N, -38°27’19.84” W, 3,216 meters above sea level) covering depth from 149.05 to 160.05 meters. At this point, the accuracy of the conducted measurements is shown to be incomparable to the established discrete methods for methane measurement. For reliable measurements, the set-up needs to be improved and important correction factors such as dilution effect and solubility of methane in melt water must be applied.

Thomas Blunier
Diana Vladimirova

Jens Olaf Pepke Pedersen, DTU