Studies of the global carbon cycle using atmospheric oxygen and associated tracers

This thesis presents research into the global carbon cycle using measurements of atmospheric composition made at the Commonwealth Scientific and Industrial Research Organisation's (CSIRO) Global Atmospheric Sampling LABoratory (GASLAB). The focus is on high precision measurement of atmospheric 021N2 and its application to deduction of carbon fluxes due to surface exchange and atmospheric transport processes. A key theme is the use of multiple species (gas concentrations and isotopomer ratios) constraints to enhance both the interpretation of atmospheric data and the diagnosis of experimental artefacts. Behaviour of the linked carbon and oxygen cycles in the contemporary atmosphere is examined on three timescales, in each case addressing important but unresolved scientific issues: ‚Äö The long term trend in atmospheric 02/N2 constrains the partitioning of uptake of anthropogenic CO2between the oceans and the land biosphere. The partitioning is deduced here by determination of trends at.Cape Grim, Tasmania, based on 5 years of biweekly flask sampling and by reconstruction of a 23-year record using archived air. The results favour a small net global biospheric sink, implying significant oceanic and terrestrial (after allowance for land clearing) carbon uptake between 1978 and 2001. ‚Äö More than forty years of atmospheric CO2monitoring at Mauna Loa, Hawaii has revealed strong correlation in interannual variability (IAV) of CO 2growth rate with the El Nirio Southern Oscillation (ENSO). GASLAB multi-species measurements during the 1990s showed correlation of ENSO and global IAV in most of the measured species. They include CO 2 . established tracers of terrestrial carbon exchange (021N2 and 8' 3C) and other species (H2, CO and CH4) whose atmospheric budgets are not as obviously linked to CO 2 . A multi-species analysis implicates biomass burning as a major influence on IAV in all species. ‚Äö Seasonal cycling in composition of background Southern Hemisphere air is investigated by ground-based flask sampling of the marine boundary layer at Cape Grim and from aircraft-based vertical profiling of the troposphere to altitudes of 6-8 km above Cape Grim. Seasonal variations in the vertical 0211‚Äö12 gradient are useful as a constraint of vertical mixing rates. Measurements of CO2, 02/N2 and other relevant tracers are used to explore the relative contributions of multiple processes (atmospheric transport, terrestrial and air-sea exchange) to seasonal signals. Measurement of 021N2 to the precision required for global carbon cycle, studies is a major challenge. Extensive consideration is therefore given to the technical aspects of this measurement program, especially in relation to the mass spectrometric analytical technique and to gas handling procedures. Numerous causes of significant 02/N2 artefacts were identified, in some cases shedding light on artefacts previously observed, but not understood, for other species (e.g. CO2).