Atmospheric oxygen and associated tracers from flask sampling and continuous measurements: Tools for studying the global carbon cycle

Sturm, Patrick (2005). Atmospheric oxygen and associated tracers from flask sampling and continuous measurements: Tools for studying the global carbon cycle (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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Atmospheric oxygen measurements, together with carbon dioxide, are a useful tool to deduce and disentangle carbon fluxes due to surface exchange and atmospheric transport processes. This thesis presents measurements of atmospheric oxygen and associated tracers and discusses experimental artifacts that have to be considered for high precision O2 measurements. Different analysis techniques for O2 measurements have been established and tested. Within the European project AEROCARB flask samples of atmospheric air from the high-altitude site Jungfraujoch, Switzerland, the mountain site Puy de Dôme, France, as well as from aircraft-based vertical profiling at Griffin Forest, UK, were analyzed for O2/N2, CO2 and δ13C by isotope ratio mass spectrometry.
The four-year records of Jungfraujoch and the three-year records of Puy de Dôme show distinct seasonal cycles and superimposed long-term trends in the measured parameters. At Jungfraujoch the seasonal variations are about two times smaller than at Puy de Dôme. The
seasonal O2:CO2 correlation gives at both sites slopes of about 2 mol O2/mol CO2. Stable carbon isotope ratios of source CO2 showed depleted values in wintertime and isotopically enriched values in summer.
At the Griffin Forest site, the peak-to-peak amplitude of the seasonal cycle of O2/N2 decreases from 171 per meg at 800 m to 113 per meg at 3100 m. Furthermore the seasonal cycle is shifted from low to high altitudes with a lag of about 1 month. The same features are observed for CO2 with a decrease in the peak-to-peak amplitude of the seasonal cycle from 17.6 ppm at 800 m to 11.4 ppm at 3100 m. The vertical profiles show decreasing O2/N2 ratios in summer and increasing O2/N2 ratios in wintertime with increasing sampling height, due to surface exchange of oxygen with the land biosphere. The O2:CO2 exchange ratio for selected vertical profiles varies between −1.5 and −2.4 mol O2/mol CO2.
Technical aspects of the mass spectrometric method and gas handling procedures played a key role in achieving the required precision for O2 measurements. In particular, glass flask storage drift due to permeation of atmospheric gases through polymer seals proved to be a crucial issue for δO2/N2 flask sampling programs.
Continuous observations of CO2, 222Rn, O2/N2, and stable isotopes of CO2 at Bern, Switzerland, shed light on diurnal and seasonal patterns of the carbon cycle in an urban atmosphere. There is considerable variance in nighttime δ13C and δ18O of source CO2 throughout the year, however, with generally lower values in winter compared to summertime. The O2:CO2 oxidation ratio during the nighttime build-up of CO2 varies between −0.96 and −1.54. Concurrent Ar/N2 measurements showed again the importance of artifacts like thermal fractionation for precise measurements of O2/N2. Using the correlation from short term fluctuations of CO2 and 222Rn we estimated a mean CO2 flux density between February 2004 and April 2004 in the region of Bern of 95 ± 39 tC km−2 month−1.
Additionally, analyzer systems for continuous O2 and CO2 measurements were developed. In the framework of the European CarboEurope-IP project O2 and CO2 will be measured continuously at Jungfraujoch. Atmospheric O2 is measured by paramagnetic as well as fuel cell techniques. A second O2 and CO2 analyzer system dedicated for making measurements during long distance flights in a passenger aircraft was developed in the context of the European CARIBIC project. Our instrument is based on electrochemical cells for O2 analysis and a NDIR analyzer for CO2 measurements.
In summary, this thesis contributes to the observational database for atmospheric O2 and CO2 measurements and reveals important technical issues for accurate measurements of atmospheric composition.

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