The Global C02Measuring Network

It is clear that such unanticipated discrepancies between results from different measuring laboratories are a major obstacle for high-precision merging of data sets. The merging is highly desirable from the point of view of maintaining adequate spatial monitoring of global trends and for identification of regional source/sink changes from atmospheric inversion techniques. From this perspective, we conclude this chapter by outlining an international calibration strategy that aims to overcome identified shortcomings in the present methods. The strategy is also aimed at providing frequent, low-cost access to a constantly monitored international calibration scale, which is currently not an option, particularly for new laboratories from developing countries. It grows out of the IAEA CLASSIC strategy, and we call it here GLOBALHUBS. While CLASSIC specifically targets CO, stable isotopes, GLOBALHUBS is seen as providing calibration for the majority of long-lived trace gas species in ambient air.

Since high freight costs and long delays are the major constraints on circulaton of highpressure standards for round-robin exercises, GLOBALHUBS is structured around four geographically distributed "HUB" Laboratories (e.g., USA, Europe, Japan, Australia), see Figure 5. Here, the Australian HUB is allocated a special preparation (PREP) role based on its ability to prepare high-pressure standards and high-quality, low-pressure subsamples from high-pressure cylinders, to quickly assess regulator effects on CO,, to produce state-of-the art precision measurements on a wide range of key trace-gas species using unusually small sample sizes and its established pioneering role in operational intercali-brations with CMDL/INSTAAR, as well as Japanese, French, German, Canadian and New Zealand laboratories. The USA CMDL laboratory is allocated a special calibration (CAL) role reflecting both its current status as the WMO Central C02 Calibration laboratory (with absolute manometric standards), also and its potential to implement the results of the GLOBALHUBS comparisons into the GLOBALVIEW globally- consistent trace gas data sets.

A common HUB scale is maintained by a variety of approaches:

1. An upgraded CLASSIC rotation between the HUB laboratories is conducted at least once per year. With upgraded and certified regulators, CO,, SI3C, SlsO, CH4, N20, CO, H2, etc. can be established to high precision (e.g., CO, ~±0.01 ppm, S"C ~±0.01%o) with respect to the CORE laboratory scale for air standards covering the full range of anticipated clean air values. The CLASSIC highpressure cylinders ("circulators") are accompanied by a range of pure-C02 standards for the isotope measurements. The CLASSIC rotation, though relatively cumbersome and expensive, provides a long standard lifetime (decades for the air standards, and many decades for C02 isotope standards). It also provides precise detector response information from both air and pure C02 standards.

2. It introduces "oscillator" exchanges between the PREP labora tory and the other three HUB laboratories. The containers are high-quality, electropolished, four-liter stainless steel filled at 1 -4 bar pressure by decanting from high-pressure cylinder air standards comprising CO,-free air (CO, stripped from ambient Southern Hemisphere marine boundary-layer air)

plus ~ 360 ppm of GS20 (or equivalent, with near-ambient

CO, isotopic ratios). The oscillator air standards provide moderate to high precision (e.g., CO, ~ ±0.01 to 0.03 ppm,

<5I3C ~±0.01 to 0.03%o, depending on required sample size), and have moderate frequency (~4 per year). Possible complications related to high-pressure regulators are avoided.

3. It maintains/upgrades flask air-sharing (ICP) programs for Cape Grim samples and introduces new ICP programs where they become possible. This is seen as a verification step. It uses exact sample methodology and has high frequency (2-4 per month), but with lower precision (e.g., CO, ~ ± 0.1 ppm, SI3C ~ ± 0.03%o).

4. The PREP laboratory prepares and provides each HUB laboratory with calibrated high-pressure cylinders of southern hemisphere marine boundary-layer air (GLOBALHUB or GH tanks), plus oscillator-type containers and the technology to decant into these for frequent provision to regional laboratories to propagate and maintain the HUB scale.

5. The HUB structure permits rapid assessment and dissemination of communityapproved calibration scale adjustments (e.g., arising from new manometric or gravimetric determinations via the CAL laboratory) or of new methods (e.g., "continuous flow" technology for SI3C; D. Lowe, NIWA, NZ, personal communication).

Initial funding is required to establish the HUB capability in existing advanced laboratories, and to secure their long-term involvement. Once the HUB scale is established quite modest regional funding can maintain operation and access. The current strategy is to seek endorsement from WMO and IAEA, and a commitment to continue their roles for planning, assessment, and dissemination of results, with particular encouragement to laboratories from developing countries. Coordinated establishment costs and regional operating costs are being sought from international funding bodies with a charter to support atmospheric composition/climate change research.

We speculate here on the improvements to atmospheric inversion studies of air-sea and air-land carbon fluxes that might flow from more effective global calibration strategies for C02, <5I3C, and 02/N2. A realistic ambition for the precision of year-to-year and large spatial-scale differences over the next 5 years for more than 100 station networks, i.e., using merged data from different measurement laboratories, is close to an order of magnitude improvement (0.2-0.02 ppm, 0.2-0.02%o for CO,, S13C, respectively). For 02/N2, perhaps 20-50 sites might contribute to similar precision improvements. With parallel improvements in atmospheric transport and surface flux parameterization, surface fluxes on current GCM grid scales may be improved from current levels of ~ 1 Gt C year-1 to better than 0.1 Gt C year-1.

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