Bilateral comparison of primary reference materials (PRMs) containing methanol, ethanol and acetone in nitrogen

A range of oxygenated volatile organic compounds (OVOCs) are present in the atmosphere as a result of direct emissions and as products of atmospheric oxidation. Long-term measurements are important to understand changes to these emission sources and atmospheric oxidation processes. Accurate and stable traceable gaseous primary reference materials are needed to underpin rigorous quality assurance and quality control at monitoring stations such as those organised by the World Meteorological Organization Global Atmosphere Watch (WMO-GAW) programme. The development of a capability for providing traceable primary reference materials (PRMs) of OVOCs is of paramount importance due to the increasing prevalence of these compounds in the urban atmosphere and also because there is currently no Central Calibration Laboratory (CCL) for these components within the WMO-GAW programme. This EURAMET bilateral comparison demonstrates the measurement compatibility (≤ 3%) for three OVOCs (methanol, ethanol and acetone) at nominally 5 µmol mol⁻¹ between two National Metrology Institutes: the National Physical Laboratory (NPL), UK and the Van Swinden Laboratorium (VSL), the Netherlands. The comparison shows that a gravimetric method for value assignment is applicable to acetone but that a more complicated procedure must be employed to value assign methanol and ethanol due to corrections for adsorption effects, which can be as large as 10 % of the nominal value. This work demonstrates the importance of making appropriate corrections to ensure the accuracy of these reference materials. NPL and VSL used different approaches to make these corrections providing confidence and independent verification. This work supports new calibration and measurement capabilities for methanol, ethanol and acetone in the range of 1 μmol mol⁻¹ –10 μmol mol⁻¹ with expanded uncertainties of 6.9 % (3 %), 7.3 % (3 %) and 1.7 % (2 %) for NPL (VSL), respectively, to be realized in the key comparison database and supports the development of the required traceability infrastructure to underpin long-term global measurements of these OVOCs.