The Gas Metrology Group and the NOAA ESRL have been collaborating on the analysis of atmospheric gases. Within the last year the NOAA ESRL has been designated by the World Metrology Organization (WMO) to provide traceable gases to the WMO climate science community under the BIPM Mutual Recognition Arrangement. Under this designation it is important to demonstrate comparability to other NMIs through Key Comparisons and/or bilateral comparisons. As most of the Key Comparisons have been accomplished prior to their designation, NIST and NOAA have begun a series of bilateral comparisons to demonstrate comparability.
The Gas Metrology Group has undertaken a major climate change program that involves collaborations with key stakeholders in the United States and internationally. NIST has developed a partnership with NOAA Earth System Research Laboratory (ESRL) (formerly Climate Monitoring and Diagnostic Laboratory) in Boulder CO to produce new 'northern continental air' gas mixture Standard Reference Materials (SRMs) for atmospheric monitoring and to intercompare NIST gas mixture SRMs with NOAA Reference gas mixtures to provide the measurement traceability required by NOAA to maintain and calibrate the world's largest global monitoring network for atmospheric greenhouse gas concentrations. Internationally, the Gas Metrology Group is collaborating with the National Institute of Water and Atmospheric Research (NIWA) in Wellington, New Zealand, to develop a 'southern oceanic air' SRM. The Gas Metrology Group is also collaborating with other National Metrology Institutes to develop gas standard mixtures to support the World Meteorological Organization (WMO) Global Atmospheric Watch (GAW) measurement and monitoring programs for volatile organic compounds, including hydrocarbons, aromatics, and terpenes and monoterpenes.
Additional Technical Details
Since the late 1980s, the Gas Metrology Group and NOAA ESRL have collaborated, on a limited bases, on analysis of chlorofluorocarbons (CFCs). Under the new designation of the NOAA ESRL, this collaboration gains new urgency. Over the past year NIST and NOAA have begun new work on these important compounds. As a beginning NIST has obtained highly clean air samples of six key halocarbon gases, and providing purity analysis of these gases (Table 1). Preparation of NIST primary standards has begun. These will form the basis for traceable gas standards that would be compared between NIST and NOAA and other WMO laboratories. Cylinders for evaluation have been filled by NOAA at Niwot ridge using special procedures to stabilize the CFC gases. These cylinders, prepared by NOAA, will be assessed by NIST to demonstrate comparability and stability, with a plan to produce a new SRM containing certified CFCs at atmospheric levels in 2015. The GMG at NIST has been collaborating with other NMIs to develop hydrocarbon standards at the low nmol/mol levels to support the WMO/GAW-VOC measurement community. Several comparisons have been accomplished between NPL, VSL, KRISS and NIST. NIST has developed a suite of hydrocarbons covering a concentration range of 50 pmol/mol to 250 pmol/mol (ppt) (Table 2). One PSM from the shite has been analyzed by several NMIs, NOAA, INSTARR at the University of Colorado, University of Miami, and NCAR. This standards research, development,and comparison data have been published in several articles 'Hydrocarbon Gas Standards at the pmol/mol Level to Support Ambient Atmospheric Measurements' and 'International Comparison of a Hydrocarbon Gas Standard at the pmol/mol Level.'
The GMG has also developed a 17 component monoterpenes and aromatic VOC mixture that was studied for stability during 2009 (Table 3). This mixture is currently being analyzed by other NMIs. The results of the gas standard development and stability studies to date has been written up in an article titled 'Stability Assessment of Gas Mixtures Containing Terpenes at Nominal 5 nmol/mol in Treated Aluminum Gas Cylinders,' were published in Analytical and Bioanalytical Chemistry in 2010. Research over the past three years has resulted in determining a suitable cylinder/treatment package that has shown stability of monoterpenes in the gas phase. The results of this research were published in Analytical Chemistry in 2013: 'Stability Assessment of Gas Mixtures Containing Monoterpenes in Varying Cylinder Materials and Treatments.' In 2013, the WMO appointed NIST as the Central Calibration Laboratory (CCL) for monoterpenes. NIST began to supply certified standards for monoterpenes to the WMO community in the fall of 2013.
NIST has produced new suites of primary gas standards for carbon dioxide, methane, carbon monoxide, and nitrous oxide in air at atmospheric levels. The primary standards are designed to have the lowest possible uncertainty, as our comparisons with NOAA will be at the highest level of accuracy. This required the lowest possible levels of uncertainty on the purity analysis of source air as well as the source pure gas.
- The primary standard suite for methane is complete and shows an unprecedented level of consistency and uncertainty, and agrees extremely well with prior suites. NOAA has analyzed one of the NIST primary standard mixtures (PSMs) in a bilateral comparison and the agreement between the laboratories is within 2.5 nmol/mol (ppb) (0.13 % relative). A paper on the development of the methane PSM suite has been published in Analytical Chemistry, Vol.84 (8) pp. 3802-3810, 2012. A bilateral comparison with NPL in 2013 for methane on one of the candidate SRMs showed identical results.
- The nitrous oxide suite has been prepared and final verification completed, demonstrating unprecedented levels of consistency and uncertainty. NOAA (the WMO CCL for N2O) and Scripps Institute of Oceanography (SIO) (who hold the AGAGE scale) have analyzed one of the NIST primary standard mixtures (PSMs) in a trilateral comparison. The agreement between the NOAA and NIST is within 0.07 nmol/mol (ppb) (0.02 % relative) and 0.20 nmol/mol (0.06 % relative) between SIO and NIST.
- The carbon dioxide suite has been completed and was used to certify new SRMs 1720 and 1721.
- The atmospheric-level carbon monoxide suite was completed, however, carbon monoxide growth has been observed in the cylinders. Further research is underway to determine the cause of this growth.
NIST and NOAA are preparing and certifying a new material, SRM 1720 Northern Continental Air. NOAA ESRL has completed preparation of the SRM candidates at Niwot Ridge, west of the NOAA ESRL Boulder Colorado laboratory in the Rocky Mountains. This location has been shown to exhibit global atmospheric levels of the major global warming gases in winter. A total of 30 cylinders were filled at this remote site in February and March of 2010. These cylinders have been analyzed at NOAA ESRL for methane, carbon dioxide, and nitrous oxide. They are currently being assessed by NIST for similar analytes against our new primary standards. We anticipate our analyses to be complete in March 2015. A certified value for the global warming gases will be applied by NIST with NOAA results being given for those needing to use the WMO CCL scale for reporting data.