ACTRIS non-methane hydrocarbon intercomparison experiment in Europe to support WMO GAW and EMEP observation networks

Abstract. The performance of 18 European institutions involved in long-term non-methane hydrocarbon (NMHC) measurements in ambient air within the framework of the Global Atmosphere Watch (GAW) and the European Monitoring and Evaluation Programme (EMEP) was assessed with respect to data quality objectives (DQOs) of ACTRIS (Aerosols, Clouds, and Trace gases Research InfraStructure Network) and GAW. Compared to previous intercomparison studies the DQOs define a novel approach to assess and ensure a high quality of the measurements. Having already been adopted by GAW, the ACTRIS DQOs are demanding with deviations to a reference value of less than 5 % and a repeatability of better than 2 % for NMHC mole fractions above 0.1 nmol mol⁻¹. The participants of the intercomparison analysed two dry gas mixtures in pressurised cylinders, a 30-component NMHC mixture in nitrogen (NMHC_N₂) at approximately 1 nmol mol⁻¹ and a whole air sample (NMHC_air), following a standardised operation procedure including zero- and calibration gas measurements. Furthermore, participants had to report details on their instruments and assess their measurement uncertainties. The NMHCs were analysed either by gas chromatography–flame ionisation detection (GC-FID) or by gas chromatography–mass spectrometry (GC-MS). For the NMHC_N₂ measurements, 62 % of the reported values were within the 5 % deviation class corresponding to the ACTRIS DQOs. For NMHC_air, generally more frequent and larger deviations to the assigned values were observed, with 50 % of the reported values within the 5 % deviation class. Important contributors to the poorer performance in NMHC_air compared to NMHC_N₂ were a more complex matrix and a larger span of NMHC mole fractions (0.03–2.5 nmol mol⁻¹). The performance of the participating laboratories were affected by the different measurement procedures such as the usage of a two-step vs. a one-step calibration, breakthroughs of C₂–C₃ hydrocarbons in the focussing trap, blank values in zero-gas measurements (especially for those systems using a Nafion^® Dryer), adsorptive losses of aromatic compounds, and insufficient chromatographic separation.