Variability in Hydrocarbon Speciation Measurements at Low Emission (ULEV) Levels

As vehicle tailpipe emission levels decrease with improvements in emission control technology and reformulation of gasolines, exhaust hydrocarbon levels begin to approach the levels in ambient air. Hydrocarbon speciation at these low levels requires high sensitivity capillary gas chromatography methods. In this study, a mixture of “synthetic” exhaust was prepared at two concentration levels (approximately 5 ppm C and 10 ppm C), and was analyzed by the widely-used Auto/Oil Air Quality Improvement Research Program (AQIRP) Phase II (gas chromatography) speciation method with a sensitivity of 0.005 ppm C for individual species. The mixture at each concentration level, along with a sample of ambient air, was analyzed a total of 20 times on 10 separate days over a 2½ week period. Concentrations of total hydrocarbons (HCs) and individual species (using the AQIRP library) were measured; averages and standard deviations were calculated. The variation in the measurement of individual species was dependent in part on the concentration levels of the species and also on the degree of chromatographic separation. For each set of speciation data, the specific reactivity (SR), in g O₃/g NMOG was calculated. The SRs for the synthetic exhaust at the two concentrations differ by only 2.1 % This study, using low level mixtures of synthetic exhaust, suggests that the AQIRP Phase II method is suitable to measure specific reactivities at ULEV levels. This study takes into account only the variability in the analytical measurement itself, and does not address the variability associated with sample collection and sample handling.