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Gas standards required by the atmospheric community for monitoring amount fractions of greenhouse gases (GHGs) are predominantly produced using an air matrix that is sourced from relatively clean, whole air (also referred to as natural air), or whole air that has been treated with adsorbents or cryogenically purified of less volatile components (purified air) and compressed. These processes should leave the major components of air (nitrogen, oxygen and argon) at ambient atmospheric levels so as not to introduce matrix effects that could bias measurements. Potential impacts to the minor component amount fractions are process dependent. The production processes have the advantage of being able to fill large cylinders providing greater amounts of calibrating gas for the user. The global production capacity for both whole air and purified air is limited, noting also that specialty gas producers almost uniquely manufacture gas standards by blending high purity gases rather than by adding components to purified air. Standardized requirements for the composition of both purified air and whole air matrix gas, would be an important step to increasing the availability and accessibility to GHG standards. For purified air, potential issues that can, and on occasion have been reported, are significant deviations in the amount fractions of major components in the air, argon amount fraction for example, and large variations in levels of minor components, such as nitrous oxide, carbon tetrafluoride, sulfur hexafluoride, carbon dioxide and the heavy noble gases. For whole air, it is important that its composition is controlled and can be produced with both major and minor components, including water vapour, in well-defined tolerance limits. Additional considerations for whole air matrix gas are that currently, its production is predominantly limited to small-scale, research-oriented facilities, typically utilizing a breathing-air compressor that is no longer manufactured. Standardized specifications for whole air could be expected to stimulate further options for its production. To ensure increased availability of quality-assured gas standards in a suitable matrix, a first step is to define acceptable tolerance limits for the composition of purified air and whole air matrix gas, and the performance of measurement methods required to verify this. The work related to a series of comparisons on carbon dioxide, methane and nitrous oxide standards, where whole air, purified air, and synthetic air matrix gases were compared will be presented, to identify current understanding of composition tolerance limits and current method performance for component quantification and needs for improvement. This will be extended to discussion of extended matrix gas composition requirements for other greenhouse gases, including isotopic analyses. Future activities will focus on working with the atmospheric monitoring community, WMO Central Calibrations Laboratories, National Metrology Institutes, and Specialty Gas Producers in agreeing and publishing acceptable tolerance limits for the composition of air matrix gas, and the performance of measurement methods required to verify this. This output will provide a significant step forward to the final desired outcome of increased availability, accessibility and affordability of quality assured gas standards for a broad range of GHG measurements.
Topic : Theme 2: Accuracy requirements for atmospheric composition measurements across economic sectors, and temporal and spatial scales.
Reference : T2-A15
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