Long-Term Methane Observations at the Global Atmosphere Watch Site Jungfraujoch with Gas Chromatography and Cavity Ringdown Spectroscopy
M. Steinbacher, C. Zellweger and B. Buchmann
EMPA, Laboratory for Air Pollution/Environmental Technology, Ueberlandstrasse 129, Duebendorf CH – 8600, Switzerland; +41 58 765 4048, E-mail: martin.steinbacher@empa.ch
In situ measurements of atmospheric methane (CH4) at Jungfraujoch started in February 2005 with gas chromatography and flame ionization detection (GC/FID). Ambient air measurements are performed every 24 minutes by analyzing discrete sample volumes. The sample air is dried prior to analysis by means of a Nafion dryer. In December 2009, a Cavity Ringdown Spectrometer (CRDS) for continuous CH4 and Carbon Dioxide (CO2) observations was installed next to the GC/FID. During the first 8 months of operation, the sample air was also dried prior to analysis by means of a Nafion dryer. As the instrument is also capable of measuring H2O, no water vapor removal is used anymore (since August 2010). Thus, CH4 dry air mixing ratios are currently determined by analyzing the fully unaltered humid gas stream and subsequently applying an empirical humidity correction accounting for dilution and pressure broadening effects. This presentation will provide a comprehensive analysis of the more than one year data set of the side-by-side CH4 comparison. Besides a general evaluation of the GC/FID – CRDS agreement, particular attention will be paid to the long-term stability of the CRDS analyzer, the advantages of high time resolution monitoring at remote sites and the issue of sample drying. Figure 1 shows a 4-day time series of the CH4 mixing ratios measured with both techniques giving a first impression that the overall agreement is very good, no significant bias exists and the observed CH4 variability is well captured by both techniques.
Figure 1. 4-day time series of in situ CH4 mixing ratios at Jungfraujoch in June 2010 measured with CRDS and GC/FID. High-resolution data and 1min and 10min aggregates are shown for the CRDS, results of single injections are displayed for the GC/FID.