Gas Concentration Standards Supporting Atmospheric Monitoring

Summary

Monitoring of Earth’s atmosphere for trace and greenhouse gases relies upon a system of high accuracy gas concentration standards.  Greenhouse gases concentration standards disseminated by NOAA, recognized worldwide, and linked to the International System of Units by NIST form the basis for effective comparison of atmospheric observing data taken at observatories globally.  NIST works in close collaboration with NOAA to ensure the integrity and accuracy of these widely-used standards.

Description

NIST develops suites of SI-traceable, amount-of-substance, primary gas standard mixtures (PSMs) for key trace and greenhouse gas species to provide traceability to the International System of Units for a range of user communities.  Standard Reference Materials (SRMs), certified to the PSMs, are developed and disseminated to researchers and scientist to achieve consistency in their atmospheric measurements through SI traceability.

For the greenhouse gases, the requirements represented by the Data Quality Objectives (DQO) for monitoring of the global atmosphere set by the World Meteorological Organization’s (WMO) Global Atmospheric Watch (GAW) are an important user community need.

WMO Data Quality Objectives for key Greenhouse Gases

GHG	Data Quality Objective
Carbon Dioxide	± 0.1 µmol/mol (390 µmol/mol)
Methane	± 2 nmol/mol (1840 nmol/mol)
Nitrous Oxide	± 0.1 nmol/mol (325 nmol/mol)
Carbon Monoxide	±2 nmol/mol (150 nmol/mol)

NIST has recently developed new standards that provide uncertainties that are equivalent to, or lower than, on a relative basis, the WMO DQO requirements when considering all sources of error.  These SRMs required that NIST develop a new suite of gravimetric primary gas mixtures linked to the SI, through the kilogram.  The resulting uncertainties are an order-of-magnitude smaller than achieved previously. The purity and isotopic composition of starting materials were determined with unprecedented sensitivity, demanding a variety of new methods and analyses, and extensive scrutiny of the experimental results.

Substantially improved mass measurements also were necessary, which required the use of advanced, automated, high-precision mass balances.  Previous mass weighing’s were made manually which required hands-on time and added significant uncertainty to the results.  Using enclosed and automated weighing systems, the group realized high-precision mass determinations with tenfold lower uncertainties.  Also, the SRM gases were obtained by sampling in remote locations (Niwot Ridge, Colorado in collaboration with NOAA’s Global Monitoring Division and Baring Head, New Zealand) where pristine samples of the atmosphere could be acquired.  On-site visits to these remote sampling sites were performed and sampling procedures were scrutinized by NIST staff.  

In all respects, the work was far from routine and clearly advanced the state-of-the-art.  It should be noted that this work addressed development of the suites of methane and nitrous oxide primary standards, technical challenges caused by interactions of carbon dioxide with gas cylinder walls, investigation of the methane historical record and descriptions of the resulting Northern Continental (SRM 1720) and Southern Hemisphere (SRM 1721) gas standards.

 

The automated cylinder handling carousel located above the top-loading balance that comprise the major automated gravimetric measurement system components used for gas concentration standard value assignment.

WMO’s data quality objectives are baseline values used globally by the atmospheric greenhouse gas measurement community to report measurements reported to the WMO and are the basis for comparison among data sets obtained from the observing stations of GAW’s global monitoring network.  This network has been established and is supported by many nations.  The consistency of these data is achieved largely through linkage of observing protocols and concentration (amount of substance) standards for CO₂, CH₄, and N₂O. These greenhouse gas concentration scales are established and disseminated by NOAA’s Earth System Research Laboratory, Global Monitoring Division (GMD) acting as WMO’s Central Calibration Laboratory for dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O).  Observations of greenhouse gas concentrations made by the U.S. and other nations are primary data products providing information about change in concentrations of greenhouse and other trace gases in the global atmosphere.  NIST collaborates closely with GMD to provide linkage to the International System of Units (the SI), as the final step in bolstering international recognition of these data sets.

Major Accomplishments

• Developed CH₄ in dry whole air suite of ambient level PSMs with a combined uncertainty of < 0.04 % relative uncertainty in PSMs
• Developed CO₂ in dry whole air suite of ambient level PSMs with a combined uncertainty of < 0.03 % relative uncertainty in PSMs
• Developed N₂O in synthetic air (O₂/N₂)suite of ambient level PSMs with a combined uncertainty of < 0.03 % relative uncertainty in PSMs
• Developed N₂O in synthetic air (Ar/O₂/N₂)suite of ambient level PSMs with a combined uncertainty of  < 0.03 % relative uncertainty in PSMs
• Participated in ambient level CH₄ in air CCQM key comparison K-82.  NIST results were within 2 nmol/mol of key comparison reference value and were within the uncertainty bounds of the key comparison reference value.