2. CARBON CYCLE

2.1. OVERVIEW

The overall goal of the work by the Carbon Cycle Group (CCG) is to improve our understanding of what determines the atmospheric burden of the major trace gases involved in the carbon cycle: CO₂, CH₄, and CO. Concern about global climate change, and possible future management of the problem, is the driving force behind the work. The anthropogenic influence on all three gas species is large, but natural cycles are involved as well.

Two methods were employed from the start of the Geophysical Monitoring for Climatic Change program, the forerunner of CMDL. Continuous measurements in remote clean air locations, namely the four CMDL observatories, and the collection of weekly or bi-weekly discrete flask samples in pairs, also at remote clean air locations. Initially the samples were analyzed only for CO₂. Methane was added in 1983, CO in 1988, isotopic ratios of CO₂ (in collaboration with the University of Colorado/Institute for Arctic and Alpine Research (CU/INSTAAR)) in 1990.

Information on the sources and sinks of the trace gases is obtained from their rates of increase and from their spatial distributions. The link between sources and observed mixing ratios is provided by numerical models of atmospheric transport operating in both two and three dimensions. Since we are working "backwards" from observed concentrations, this problem is in the class of so-called inverse problems. The greatest limitation is sparseness of data, especially in regions close to important sources and sinks. Therefore the Carbon Cycle Group has gradually expanded the spatial coverage of the cooperative air sampling network, as well as added isotopic ratio measurements since different sources/sinks may be characterized by distinctive isotopic "signatures."

To overcome the limitation of having only measurements from the marine boundary layer, remote from many important source areas, two new approaches were initiated. One is to continuously measure a number of chemical species and atmospheric physical properties at different heights on very tall towers. Mixing ratios in the continental planetary boundary layer are highly variable and their interpretation is more difficult, requiring much more auxiliary data, than the "traditional" marine air samples. The second new approach is to obtain discrete air samples from low-cost airplanes in automated fashion from the boundary layer up to about 8 km altitude. These samples are then sent back to the laboratory in Boulder for analysis. The use of this method, especially over North America, will be greatly expanded to provide significant regional-scale constraints on the budgets of carbon species.

The global air samples also provide a unique resource for narrowing the uncertainties of additional atmospheric problems. In collaboration with the Nitrous Oxide And Halocompounds (NOAH) group of CMDL, the species N₂O (a greenhouse gas) and SF₆ were added to the suite of flask measurements, and other species are under investigation. The development of a method to routinely measure the ¹³C/¹²C ratio of CH₄, in collaboration with INSTAAR, is at an advanced stage.

Since the global coverage of our sampling is unmatched, CMDL plays an active role in bringing together the measurements from many different laboratories around the world. Towards this end, measurements of field samples as well as reference gas standards are intercompared. The link with the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Australia is particularly strong in this regard. For CO₂ and CO, calibrated reference gases are provided under the auspices of the World Meteorological Organization (WMO). At the "product" end, a common database for CO₂ has been assembled without significant calibration or methodological discrepancies, incorporating the measurements of a number of laboratories, called GLOBALVIEW-CO₂. Its intended use is for three-dimensional (inverse) modeling. Plans are to maintain and enlarge the database, as well as assemble similar ones for CH₄, isotopic ratios, etc.

Data records and monthly means can be obtained for each site from the CMDL World Wide Web page (www.cmdl.noaa.gov); the ftp file server's "pub" directory (ftp.cmdl.noaa.gov), from the WMO World Data Center for Greenhouse Gases (Tokyo), and from the Carbon Dioxide Information Analysis Center (Oak Ridge, Tennessee).

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