Accurate Measurements of Hydrocarbons in the Atmosphere (AMOHA): Three European intercomparisons

Abstract

[1] The Accurate Measurements of Hydrocarbons in the Atmosphere (AMOHA) project was proposed to evaluate and improve current gas chromatographic methods used across Europe to determine concentrations of C2-C9 nonmethane hydrocarbons (NMHCs) in ambient air. Four international intercomparisons of increasing complexity were carried out over a period of 4 years. These were designed to evaluate the analytical performance of a considerable number of European laboratories making frequent ambient NMHC measurements, to identify the major problems encountered in these analyses, and to recommend solutions for these problems. Calibration gas standards prepared gravimetrically and certified by the National Physical Laboratory (NPL) in the UK were used in all of the intercomparisons to eliminate the variability that would have been introduced by laboratories using their own calibration gas mixtures. The results presented here are for the first three of the AMOHA intercomparisons. A four-component synthetic test gas mixture was used for the first intercomparison, a 30-component synthetic test gas mixture for the second intercomparison, and both a pressurized ambient air sample and a 30-component synthetic test gas mixture for the third intercomparison. The results of the 12-14 participating laboratories showed generally good agreement with the gravimetric concentrations of the synthetic NMHC mixtures, although some considerable differences also occurred. The analytical results on the pressurized air samples were similar, but with a greater scatter of values. Most importantly, there were distinct improvements in the measurement quality achieved in this project when compared to previous European intercomparisons. This emphasized the importance of using high-quality NMHC multicomponent gas calibration standards when making accurate measurements of NMHCs in air. Despite the generally good results, some compounds, such as isobutene, butadiene, methyl pentanes, and trimethyl benzenes, gave considerable analytical problems in some laboratories. The most frequent causes of these problems were found to be interference by coelution with other compounds, breakthrough of C2 compounds during sample preconcentration, and adsorptive losses of C7-C9 compounds in the transfer lines before the analytical instrumentation. Calibration with multicomponent NMHC standards was demonstrated to provide significantly more accurate results than the frequently used method of calibration with just a single hydrocarbon species, such as n-butane.

European hydrocarbon intercomparison experiment AMOHA part 4: Canister sampling of ambient air

“…Similar results were achieved during three AMOHA intercomparison measurements compared to the first four tasks of NOMHICE [ Slemr et al , 2002; Apel et al , 2003]. Although some of the participants had substantial problems in analyzing the complex test mixtures, others achieved good results demonstrating that reliable analyses are feasible.…”

“…The pressure in the canisters was about 200 kPa (abs), which is sufficient for at least one analysis per canister. The techniques used by the participating laboratories are described in Table 2 of the previous study on AMOHA [ Slemr et al , 2002]. The detection limits stated there were below 20 pptC for most participants and compounds, corresponding to generally better than 4 pptv for C 5 compounds.…”

“…However, deviations of individual results from the reference values indicated problems in the analysis of particular compounds by particular laboratories. During three stages of the European AMOHA (Accurate Measurements of Hydrocarbons in the Atmosphere) project [ Slemr et al , 2002], between twelve and fourteen participating European laboratories analyzed synthetic NMHC test mixtures containing from four to thirty components and a compressed whole air sample. The gradually increasing complexity enabled the investigators to identify several error sources.…”

Mirror image hydrocarbons from Tropical and Boreal forests

“…It should be noted, however, that the measurement of enantiomeric monoterpenes represents an increase in analytical difficulty. Since it has already been shown through non-enantiomeric VOC measurement intercomparisons that such determinations can be subject to large errors (Apel et al, 1994(Apel et al, , 1999(Apel et al, , 2003Slemr et al, 2002), particular care must be taken with these measurements in the future. Accurate knowledge of monoterpenes will be essential for the accurate modelling of present and future atmospheric hydrocarbon emissions by forests, and recognise the sophistication of interactions between flora, fauna and the atmospheric environment.…”

An automated monitoring system for VOC ozone precursors in ambient air: development, implementation and data analysis

“…Because the use of a single calibration substance instead of a multicomponent one might be controversial [15], some laboratory tests have been conducted to compare the concentrations certified from two QC standards (US NIST, UK NPL) to the ones obtained from our calibration method. The differences are lower than 5% for the majority of compounds and between 5 and 10% for 1,3,5-and 1,2,4-trimethylbenzenes, indicating the reliability of our calibration method.…”