HATS Standards Program

Accurate measurements of trace constituents in the atmosphere measured by the HATS Division begins with our knowledge and ability to accurately prepare gas calibration standards. Various halocarbons including chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), methyl halides, halons, chlorinated solvents, and other gases are prepared gravimetrically as primary standards in our laboratory. The concentrations of most of these standards are at part per trillion (ppt or pmol/mol) levels. These standards are then intercalibrated with secondary and tertiary gas mixtures that are used throughout the various projects within the NOAA and also other federal, state, and private laboratories world-wide. The secondary and tertiary standards normally contain dried real air that is collected using a pumping system located at Niwot Ridge, CO.

The process of preparing a gas standard begins with the preparation of the compressed gas cylinder that will contain the gas mixture. The cylinders are received valved, cleaned, treated with a chemical vapor deposition process which forms a barrier between the gases and the internal surface of the cylinder, and filled with nitrogen. The nitrogen is analyzed for contamination using gas chromatography with electron capture detection and then analyzed for moisture before use. A cylinder is then evacuated and weighed on one of two weighing balances. An enclosed double-pan balance with a readability of 0.001g designed specifically for weighing gas cylinders is used for weighing small cylinders. A single-pan floor balance with a readability of 0.1g is used for weighing large cylinders. The minor gas to be added to a cylinder is prepared using one of the two described methods. A micro-balance with a readability of 1 x 10^-7g is used for weighing a glass capillary tube and a balance with a readability of 1 x 10^-5g is used for weighing a stainless steel cylinder used in method 2. Blended air is normally used as the diluent gas. The air is analyzed for oxygen content before use and the molecular weight of the gas is calculated. The air is purified using several purifiers attached to the manifold. The cylinder is again weighed after both the minor and diluent gases are added to the cylinder and a mixing ratio is calculated using mole fractions based on the weight of the gases. After the contents of the cylinder is mixed, it is then ready to be analyzed and used as a calibration standard or an aliquot of the mixture can then be used to prepare additional standards at lower mixing ratios. Several dilution steps must be performed to produce gas standards with mixing ratios at the part per trillion level. Two or three dilutions may be required. The final suite of calibration standards may be single or multi-component standards. Several compounds may be added during the preparation of the parent mixture or added during a dilution step.

We also prepare standards at ambient atmospheric levels and higher for organizations which include trace gas research within our lab's mission. Our clients have included the Canadian Department of Energy (DoE) , the Atmospheric Environment Service (AES) of Environment Canada, German Forschungszentrum Jülich GmbH. (KFA), US National Center for Atmospheric Research (NCAR), US National Aeronautics and Space Administration (NASA), Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO), Methyl Bromide Coalition, Alternative Fluorocarbons Environmental Acceptability Study (AFEAS), University of Colorado, Harvard University, and many other organizations.

For more information on pricing and availability, please contact James W. Elkins(+1 303 497 6224; jelkins@cmdl.noaa.gov ).

Halocarbons & other Atomspheric Trace Species 325 Broadway R/E/CG1 Boulder, CO 80303 For more information contact: James W. Elkins