A standard solution for atmospheric science

Scientists at the National Physical Laboratory (NPL) have produced the world’s first synthetic air reference standard which can be used to accurately measure levels of carbon dioxide and methane in the atmosphere. Paul Brewer, Principal Research Scientist at NPL explains In order to measure the concentration of greenhouse gasses in the air, most notably carbon dioxide (CO₂) and methane (CH₄), we need to have a reliable standard with which to measure them against. If we are to measure mass we must all agree what a kilogram is by establishing a primary standard. If someone sells us a kilogram of gold, to ensure the mass is accurate, we can check this against a working standard which is traceable to the international prototype kilogram (maintained by the International Bureau of Weights and Measures in Sèvres, France). The same goes for measuring the concentration of components in air. If primary standards are established which are stable over time and traceable to the SI, then we can use these to measure how the gasses in the atmosphere are changing by comparing them to these standards. This allows us to monitor atmospheric changes in greenhouse gas levels – over time or between locations. Being able to do so is a vital component in our understanding of global climate change as well as monitoring issues like air quality in specific locations. However the current method for producing these standards is demanding and can only be done in a single location because real air is sampled. As demand grows for reliable standards, the current producer is struggling to meet demand. Recent research led by NPL has found an alternative – creating the gas standard in a lab using a technique that can be universally reproduced. The bulk of demand for gas standards comes from atmospheric monitoring stations around the world. Many of these are part of The Global Atmosphere Watch (GAW) program of the World Meteorological Organization (WMO). This is a major source of data that contributes to our understanding of the atmosphere’s natural and anthropogenic change. This programme helps to improve the understanding of interactions between the atmosphere, the oceans, and the biosphere. More than 65 countries currently host and operate the GAW's global or regional measurement stations. There are also contributing stations that furnish additional data. Various environmental research organisations around the world also maintain their own networks for monitoring greenhouse gas concentrations in specific regions. These organisations need to regularly check that their instruments are measuring correctly. In order to do so, they require a reliable primary standard to check their instruments against and make adjustments if necessary. It is important these measurements are accurate and the data can be reliably compared between stations. Reference standards are also important for manufacturers of gasses and gas calibration mixtures which are supplied for scientific, medical and industrial purposes. These organisations need reliable and easily accessible standards to calibrate measurement tools to ensure the prepared gas is in the correct ratios. The manufacturers of the measurement tools used by both environmental monitors and gas companies also rely on standards to produce and calibrate their technology. An essential part of the quality assurance system implemented by the WMO is the designation of a Central Calibration Laboratory (CCL) that maintains the Primary Standard (PS). The CCL for CO₂ and CH₄ is the Earth System Research Laboratory of the National Oceanic andAtmospheric Administration (NOAA) in Boulder, Colorado. The standards are captured from the local atmosphere and kept in high-pressure cylinders. Manometric systems – systems which measure pressure – are used to assign absolute values to the primary standards. They have been the subject of intensive research to determine their accuracy and lifetime. These are used to create secondary standards using non dispersive infrared instruments, which are distributed to those responsible for the instruments that measure greenhouse gasses around the world, and used to calibrate them. A new improved measurement technique for atmospheric monitoring – cavity ring-down spectroscopy (CRDS) – has resulted in a dramatic increase in the number of atmospheric measurements taken from monitoring stations. As the requirement for data that is comparable to the WMO scale increases, there is a corresponding increase in the demand for comparable reference standards. Supplying the demand for reference standards comparable to the WMO scale is becoming an issue due to the logistics challenges of preparing the standards and shipping them around the world. An alternative could be offered from standards prepared gravimetrically – i.e. by weighing the gas in the cylinder – that are traceable to the International System of Units (SI). Alongside an infrastructure to disseminate such standards, this could offer a means of broadening availability. These could overcome the cost and complexity of sampling air under global background conditions which can only be carried out at remote locations. NPL has developed a solution, producing a synthetic standard which can be used to calibrate carbon dioxide and methane measuring instruments. The standard is comparable to the WMO scale and can be quickly produced in a laboratory and distributed, meeting growing demand. The results are described in a paper published in Analytical Chemistry. Rather than sampling air directly, NPL created the sample in the laboratory by carefully blending a mix of gaseous components found in air. However preparing reference standards synthetically presents a significant challenge. Industrially produced carbon dioxide has a different isotopic distribution to that of atmospheric air, which measurement instruments read differently. By using high accuracy gravimetry, the team was able to prepare a gas mixture that accurately replicated the natural occurring isotopic carbon dioxide. The samples were tested using NPL’s world leading measurement equipment and expertise, which demonstrated that the synthetic standard was comparable with the NOAA standard and suitable for use with the international measurement scale for atmospheric monitoring. The research has demonstrated that air standards comparable to the WMO scale can be prepared synthetically with an isotopic distribution matching that in the atmosphere. The methods used can be replicated, leading to widespread availability of standards for globally monitoring these two high impact greenhouse gasses. Standards are a critical problem in greenhouse gas measurement. Developing high accuracy reference standards of carbon dioxide and methane with international comparability, and traceability to the SI, will solve the pressing supply issue for the international atmospheric monitoring community and for gas companies, and will help improve our understanding of how greenhouse gases affect the atmosphere. More information The full paper can be viewed here: http://pubs.acs.org/doi/abs/10.1021/ac403982m Author Paul Brewer is Principal Research Scientist at the National Physical Laboratory (NPL)