A method to partition the measured atmospheric CO2 based on oxygen measurements

Valentino, Francesco Luca (2007). A method to partition the measured atmospheric CO2 based on oxygen measurements (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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The atmospheric greenhouse gas CO2 has been increasing since the 18thcentury. To predict future CO2 levels it is necessary to understand the global carbon cycle. Systematic measurements of the atmospheric CO2 together with atmospheric oxygen measurements 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 performed with different analysis techniques.
An analyzer system for continuous O2 and CO2 measurements was developed and installed in a large airfreight container with automated instruments for measurement of atmospheric gases and trace compounds, operated on a monthly basis onboard a Lufthansa International Airways during long-distance flights within a EU founded project named CARIBIC (Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrument Container). The analysis principle for oxygen is based on the fuel cell technique whereas the common infrared absorption technique is used for carbon dioxide. Aspects of experiments regarding the optimal performance, reproducibility and data evaluation as well as measurements are discussed. Several problems were experienced during the flights, in particular for the oxygen determination which revealed variations in the order of about 150 per meg, still significantly above the expected range, and are therefore difficult to be interpreted. Flight CO2 measurements exhibited variations linked to the cruise altitude changes, to stratospheric air intrusion and to mixing of air masses carrying different CO2 concentrations.
Within the European projects AEROCARB and CARBOEUROPE IP 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 profiles at Griffin Forest, UK, were analyzed by isotope ratio mass spectrometry.
The observations on flask samples collected at the Jungfraujoch station show, since the year 2003, an enhancement of the oxygen trend which amounts to about 45 per meg/yr with a corresponding CO2 increase of around 2.4 ppm/yr. This enhancement is also comparable with that observed at the Puy de Dôme station where oxygen, since mid 2002, has decreased with a rate of about 50 per meg/yr whilst the CO2 increase was of around 1.7 ppm/yr but exhibiting a higher variability compared to the Jungfraujoch CO2. The apparent slopes calculated from correlation plots between detrended CO2 and δO2/N2 records as well as between corresponding trends are significantly larger than the observed terrestrial exchange and fossil fuel emission slopes indicating a strong oceanic influence.
Observations from a four years campaign of regular aircraft flights over the Griffin forest show decreasing oxygen trends which amount to about 49±9 per meg/yr with a corresponding CO2 increase rate varying from 2.9±1 ppm/year to 1.9±0.9 ppm/yr according to the sampling altitude. For the period 2004.2-2006.9 the seasonal co-variances of O2 and CO2 indicate an oceanic influence on the δO2/N2 trends or the predominance of natural gas as fuel, which is rather unrealistic.
The cycle amplitudes decrease with the height for CO2 while for δO2/N2 they show a different behavior decreasing from 800 m to 1100 m and than remaining constant at the higher sampling heights. The land biogenic exchange accounts for more than 70% of the δO2/N2 seasonal cycle at 800 m and for 27% to 46% at the higher altitudes. Vertical δO2/N2 and CO2 profiles do not show only typical distinct summer-winter structures with regular monotonic decrease (or increase) with the sampling height. They rather exhibit contrasting shapes and their O2:CO2 exchange ratios vary from -1.0±0.1 and -4.5±1.2 mol O2/mol CO2.
A one-year time series of atmospheric CO2 measurements from Bern, Switzerland, together with the atmospheric tracers 222Rn, δAr/N2, δO2/N2, δ29N2, δ34O2 and stable isotopes of CO2 is also presented. 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 tCKm−2month−1. Nighttime δ13C and δ18O of CO2 show a considerable variance throughout the year 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 -1.69 mol O2/mol CO2. Furthermore, δAr/N2measurements showed that artifacts like thermal fractionation at the air intake are relevant for high precision measurements of atmospheric O2.

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