The AMOHA project aimed at improvement of NMHC measurement techniques based on sampling ambient air in stainless steel canisters. The project consisted of RTD and intercomparison parts. In the RTD part each step of the sampling and analysis procedure has been investigated, problem areas have been identified and recommendations for the best measurement practice had to be made. In the intercomparison part the analytical proficiency of up to 15 European laboratories has been tested in four annual intercomparisons of increasing complexity.
Procedures for cleaning canisters, sampling of ambient air, sample preconditioning, separation of individual NMHC compounds, their identification and quantification, preparation of stable standards and testing of NMHC gas mixtures have been investigated in the RTD part. In the intercomparison part we carried out four NMHC intercomparisons using synthetic 4 component, 30 component, 30 component plus pressurised ambient air test samples in the 1st, 2nd, and 3rd intercomparisons, respectively, and ambient air measurements in the 4th intercomparison. The results of each intercomparison were discussed at a workshop in order to obtain feedback on the measurement problems of the participants and to demonstrate that the problems can be overcome. The improved measurement practice has been described in standard operating procedures (SOPs).
Currently used electropolished stainless steel canisters proved to be superior for NMHC measurements to silica lined and aluminum canisters, although the latter might offer advantages when other more polar volatile organic compounds are to be measured. Aluminum high-pressure cylinders performed better in terms of NMHC stability for pressurised ambient air sample than stainless steel high-pressure cylinders. Reactions of ozone with NMHCs in the ambient air sample influenced only the most reactive NMHCs (butadiene and isoprene), because of rapid destruction of ozone within the sampling pump and the canister. Several preconditioning procedures (drying and CO2 removal) were successfully tested. NMHC separation on new GC columns was investigated and an alternative was found to the almost exclusively used Al(2)O(3) WCOT columns. A device for automatic air sampling into canisters has been successfully developed.
In general, the results of the first three intercomparisons showed a substantial improvement of the performance of the European laboratories against the similar intercomparisons made in 1992 and 1994. Part of this improvement was due to the availability of stable and accurate calibration gas standards prepared gravimetrically by NPL. Further development of the analytical techniques and the training in the course of AMOHA intercomparisons might also have contributed. Simultaneous NMHC measurements made in ambient air in the last intercomparison (sampling and analysis) showed a good agreement between almost all participating laboratories. However, comparison with an on-line NMHC monitor suggested that there had been some contamination of the air samples with alkenes, which deserves further investigation.
The AMOHA workshops after each intercomparison have been unable to identify the best NMHC measurement practice. Techniques used by one laboratory were frequently deemed unreliable by another one and vice versa. Consequently, the general feeling among the participants was that the standard operating procedures (SOPs) should emphasise the need for a validity tests for each analysis step, instead of describing the best analytical technique (on which no consensus had been achieved).
Slemr, J., F. Slemr, R. Partridge, H. D´Souza, N. Schmidbauer, Accurate measurements of non-methane hydrocarbons in air (AMOHA): Three European intercomparisons, J.Geophys. Res., submitted.
Accurate measurements of non-methane hydrocarbons (NMHCs) in ambient air are one of the major prerequisites to designing measures for the control of ozone precursor emissions and monitoring the efficiency of this control. NMHC measurements in ambient air are difficult because a large number of individual substances (30 and more) have to be identified and quantified at mixing ratios of usually less than 1 ppbv (10-9 volume ratio) in the presence of interfering substances. Previous intercomparisons in Europe and North America showed large differences between laboratories when measuring NMHCs in synthetic test and ambient air samples.
Funding SchemeCSC - Cost-sharing contracts
TW11 0LW Teddington