Underpinning atmospheric monitoring to tackle climate change
The measurement of greenhouse gases is pivotal to understanding changes in the Earth's climate. National and international legislation is aimed at reducing greenhouse gas emissions and requiring their measurement in the atmosphere. Long-term observations based on accurate and stable standards ensure that data meets the requirements of World Meteorological Organization (WMO) compatibility goals and environmental policy makers, as well as academic and regulatory users.
Research at NPL has led to significant advances in the development of high accuracy, SI traceable, gaseous reference materials of CO2, CH4, N2O and CO. We have been driving the uncertainty of the reference materials towards the WMO compatibility goals and monitoring their stability. Improvements have been achieved by optimising passivation chemistry used in cylinder treatment, reducing the uncertainty in the gravimetry of the matrix components and making high-accuracy quantification of target impurities in the matrix gas. In addition, a capability to characterise the isotopic composition of the CO2 in the reference materials has been developed to account for measurement biases introduced by instrumentation detecting only certain isotopologues. Knowledge of the CO2 composition is crucial for addressing commutability issues from preparing synthetic reference materials and also for assigning the correct atomic weight for the calculation of gravimetrically prepared mixtures.
This work has provided the framework for new research priorities focused on developing gaseous reference materials of CO2 and N2O for underpinning measurements of stable isotopes to infer their origin in the atmosphere. A new infrastructure is proposed that will deliver international CO2 reference materials with traceability to the VPDB primary standard, to meet the increasing demand. New international gaseous N2O reference materials will also be developed with stated uncertainties. The research will develop new field-deployable spectroscopy and initiate SI traceability of the international CO2 isotope ratio scale by re-measuring the absolute isotope ratios via gas-source isotope ratio mass spectrometry.
Developing a measurement infrastructure to enable better air quality
More accurate measurements of air pollutants such as sulphur dioxide, oxides of nitrogen, carbon monoxide and benzene must be performed to fulfil the requirements of the EC Directive on Ambient Air Quality and Cleaner Air for Europe (Directive 2008/50/EC). They are also essential to understand population level exposure, improve air quality models and emission inventories, to discern long‑term trends in concentrations and to enforce air quality and vehicle emission legislation. This is essential for the timely evaluation of air pollution mitigation policies, and to improve our understanding of the influence of anthropogenic emissions on the climate system.
Our research focuses on developing novel dilution systems for generating accurate reference materials of reactive components at trace amount fractions. Producing static reference materials in high pressure cylinders using novel passivation chemistry will ensure long-term stability with a particular focus on nitrogen dioxide where the measurement challenges and impact on society are the greatest. We are developing capabilities for the direct measurement of NO2 using newly-available selective techniques and direct calibration. We are working towards a full characterisation and minimisation of impurities in reference standards such as water vapour and reactive nitrogen compounds, such as nitric acid, which increase uncertainty and decrease long‑term stability.
In 2016, NPL was recognised for its outstanding international contribution to underpinning amount-of-substance measurements of nitrogen oxides by being appointed by the World Meteorological Organisation as Central Calibration Laboratory to provide reference standards of nitrogen monoxide to the Global Atmosphere Watch Network.
Disseminating traceability for volatile organic compounds
Volatile organic compounds (VOCs) play a key role in the chemical mechanisms that lead to the photochemical generation of ozone, which can have harmful effects on ecosystems and human health and control the oxidation capacity of the troposphere. The need to prevent or reduce these effects on the public and the environment has led to requirements for stable and accurate gas reference materials to facilitate the traceable calibration of analytical instrumentation. NPL provides 30-component ozone precursor gas reference materials that contain all the hydrocarbon compounds listed in the European Directive covering ambient ozone measurements (Directive 2002/3/EC). The 30 component gas standard is recognised by the World Meteorological Organisation as their primary standard.
Our research focuses on improving the stability and accuracy of these reference materials and addressing challenging requirements, such as underpinning emissions of biogenic carbon into the atmosphere. The most abundant components in these emissions are isoprene, α-pinene, β-pinene, Δ3-carene, camphene and myrcene. These components play a central role in the oxidation processes in the atmosphere and in the formation of particles.
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