Understanding the carbon cycle through atmospheric carbon dioxide and oxygen observations

Uglietti, Chiara (2009). Understanding the carbon cycle through atmospheric carbon dioxide and oxygen observations (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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The present-day concentration of atmospheric carbon dioxide (CO2 ) is higher than in the past 800000 years and continues to rise. Constant emissions of CO2 and other greenhouse gases would influence the global climate system during the next decades and centuries. It is therefore essential to gain knowledge on the processes involving CO2 exchanges among the atmosphere, the biosphere and the ocean in order to estimate the carbon sources and sinks and to fill the gaps in the comprehension of the global carbon cycle. Therefore greenhouse gases and other atmospheric compounds are constantly monitored in several locations around the world. Moreover long-term records obtained with high precision measurements are necessary in order to study the evolution and to budget the sources and sinks of atmospheric CO2. Furthermore atmospheric oxygen can be used as carbon cycle tracer and its measurements provide an additional and independent tool to characterize the partitioning of anthropogenic CO2 between the terrestrial biosphere and the ocean.
In addition, in nature during various biogeochemical processes the relative abundance of isotopes is altered. Changes in the isotopic composition of atmospheric CO2 ( 13 C/ 12 C ratio) help to distinguish various processes as biosphere-atmosphere exchange as well as fossil fuel input.
In the atmospheric distribution of CO2 and O2 several components can be distinguished: First, the horizontal distribution as function of latitude and longitude. These annual variations are related to the annual mean space distribution of sources and sinks. Then the variations in time which include a long-term trend, the seasonal cycle and short-term variations (weekly and daily changes).
The seasonal oscillation is influenced by the latitude: The northern hemisphere represents a higher land to ocean area ratio than the southern hemisphere therefore, the former leads to predominantly driven terrestrial biospheric activity whereas the latter shows a more ocean driven seasonality. The seasonality in the Southern hemisphere is weaker and in opposite phase than the corresponding signal in the northern hemisphere.
Finally, the vertical gradient is important to investigate variations of the atmospheric composition with altitude. This is important for understanding the mechanisms of the vertical mixing and transport of the atmospheric compounds from the ground towards the high altitude sites and to the free troposphere.
In this thesis the results of high precision atmospheric carbon dioxide and oxygen observations from the High Altitude Research Station Jungfraujoch (Switzerland) as well as from other several European atmospheric measurement sites are presented and studied in order to assess the carbon sources and sinks at the European continental scale. In particular, within the European project CARBOEUROPE IP since 2000 glass flasks from Jungfraujoch and from the mountain site Puy de Dôme (France) were sampled constantly and analyzed by isotope ratio mass spectrometry at the University of Bern. The CO2 measurements are part of the Swiss GCOS (Global climate observing system) office program supporting long-term records. Moreover since 2005 a continuous in situ measurement system for CO2 and O2 was established at Jungfraujoch. The comparison between the continuous records and the corresponding flask samples is relatively good and precise for CO2, but poorer in O2 since the detection of small variations in the atmospheric O2 signal is challenging because of its large atmospheric background. Nevertheless real time measurements can be used to better validate the flasks data and vice versa and they provide useful information on the short-term variations (weekly, diurnal and hourly variations).
In addition at Jungfraujoch since December 2007 beside the normal sampling procedure a dedicated δO2 /N2 flasks intercomparison was performed every two weeks filling simultaneously flasks coming from three laboratories, Bern (UBE), Jena (MPI) and Groningen (CIO). In general the results of this intercomparison experiment were not satisfying because of the poor agreement, especially in δO2 /N2. Nevertheless an intercomparison approach is significant in order to fix problems which cannot be discovered by measuring flasks in one laboratory only. Moreover the intercomparison is important to ensure a good quality control on the measured values.
CO2 and O2 at Jungfraujoch reveal the typical seasonality due to the biospheric activity since in summertime the vegetation takes up CO2 and releases O2. Therefore, atmospheric CO2 concentrations reach a minimum in summertime and a maximum at the end of the winter. On the contrary, atmospheric O 2 is higher in summer and lower during winter time.
The seasonal cycle of the Jungfraujoch CO2 and O2 records was computed and compared with a number of CarboEurope sites (flasks measurements) revealing dissimilarities between the sites mainly due to their location (in the vicinity of the ocean or close to cities or at high altitude sites.).
In addition a trend is superimposed on the CO2 and O2 seasonal cycle. The increasing CO2 trend and corresponding decrease of O2 are predominantly due to anthropogenic influences from fossil fuel combustion and land management. Moreover, the annual growth rate is associated with the natural carbon and oxygen sources and sinks (e.g. the ocean plays an important role in the oxygen cycle).
The trend analysis revealed a mean annual CO2 and O2 growth rates at Jungfraujoch of 1.8 ± 0.5 ppm/yr and -20.3 ± 12.8 per meg/yr. These values are in accordance with other recent studies and with the mean global annual increase rate calculated for CO2 based on the CMDL/NOAA sites (i.e. Mauna Loa). However, each year reveals high variability in the growth rates both for CO2 and O2. In particular at Jungfraujoch, in the period 2003-2005 the O2 growth rate decreased to a mean value of - 36 per meg/yr and the mean CO2 growth rate was estimated to be 2.3 ppm/yr.
One mechanism proposed to explain this trend differences is related to the oceanic influence over the carbon and oxygen cycles. CO2 and O2 are not totally coupled in the oceans because CO2 ocean’s uptake is normally driven by the carbonate buffering system. On the contrary air-sea oxygen exchange is more related to the solubility and biological pumps.
Actually the oceans play an important role in the global climate through their ability to absorb and release heat. Moreover the partitioning of CO2 and O2 between the ocean and the atmosphere is also sensitive to the temperature of the ocean (solubility pump): Warmer waters tend to hold less gas generating an outgassing towards the atmosphere, while colder waters cause a net uptake of gases.
Therefore the decrease in the O2 trend might be related to the changes in the sea surface temperatures (SST) of the ocean. Moreover the North Atlantic Ocean is considered the largest ocean sink for atmospheric CO2 in the Northern Hemisphere. Several studies revealed a relation between the North Atlantic sink variations and the North Atlantic Oscillation (NAO). During a negative phase of NAO the CO2 uptake of the ocean decreases and thus more CO2 remains in the atmosphere. On the contrary during positive phases of NAO the oceanic CO2 uptake increases. Our results for the period 2003 – 2005 are in agreement with the NAO and SST variations.
Another useful tracer to account for the oceanic influence on the oxygen seasonality is the Atmospheric Potential Oxygen (APO). APO is an additional and powerful tool to constrain the carbon budget because it provides a sensitive tracer for oceanic air-sea gas exchange because it is invariant respect to land biospheric processes. The mean amplitude of the resulting APO seasonal cycle was estimated to be around 21 ± 9 per meg and 45 ± 14 per meg for Jungfraujoch and Puy de Dôme respectively. In both stations air-sea exchange fluxes contribute at about 35% to the oxygen seasonal cycle. Therefore even at remote sites, like Jungfraujoch the influence of the ocean on the seasonal variations can be detected. Moreover the trend variability for the APO reveals the same behavior of O2.
Another important aspect is the analysis of the correlation between CO2 and O2 which is especially important because CO2 and O2 are inversely coupled during photosynthesis and respiration processes and during the fossil fuel combustion process. The oxidation ratio was calculated on the Jungfraujoch and Puy de Dôme data, computing the slope by geometric mean regression and expressed in unit of mol O2 per mol CO2. The resulting ratio is -2.17 ± 0.12 (R2 = 0.8) for Jungfraujoch and -1.88 ± 0.08 (R2 = 0.5) for Puy de Dôme. These values are relatively high compared to the land photosynthesis and respiration O 2 :CO2 exchange ratio of -1.1 mol/mol and to -1.4 mol/mol generally assumed value for fossil fuel combustion. These higher values for the O 2 /CO2 ratio support the hypothesis of a strong oceanic component contributing to the oxygen seasonal cycle at Jungfraujoch and therefore influencing the observed oxygen trend. Nevertheless further studies should be done to better understand the interaction of O2 and CO2 and assigning those variations to fossil fuel emissions, biological and oceanic processes.
The thesis is divided in six main chapters as follows:
In Chapter 1 a general introduction on the global carbon cycle is given. The different compartment where carbon is stored and exchanged is described. Moreover the important role of oxygen in the partitioning of anthropogenic carbon dioxide is discussed.
Chapter 2 explains the different laboratory techniques used to measure CO2, O2 and the isotopic composition of CO2 performed at the High Altitude Research Station Jungfraujoch and in the urban site of city of Bern.
In chapter 3 the results of four years (January 2005 – June 2009) of continuous measurements of CO2 and O2 performed at Jungfraujoch and the comparison with corresponding weekly flask samples is presented and discussed.
Chapter 4 investigates simulations of transport of CO2 towards Jungfraujoch using backward trajectories. By simulating different CO2 concentrations and the combined trajectories it is possible to define different source and sink areas over Europe. Trajectories are a useful tool to simulate and classify the air masses arriving at Jungfraujoch and thus explaining some particular features occurring in the CO2 record (very high peaks in the records). It is also important to compare CO2 with different atmospheric trace gases such CO and CH4 in order to improve air mass transport studies.
Chapter 5 presents the flasks air sample measurements performed at Jungfraujoch (in the period October 2000 – June 2009) and Puy de Dôme (in the period August 2001 – June 2009) within the CarboEurope integrated project. A trend analysis was presented and discussed and the seasonal cycles of a number of CarboEurope measurement stations were compared.
Chapter 6 illustrates and discusses the atmospheric composition measurements performed in the city of Bern (sampling intake is located on the roof of the University of Bern) and presents a comparison with the results obtained at Jungfraujoch.

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