Separation of fine particulate matter emitted from gasoline and diesel vehicles using chemical mass balancing techniques.

Samples of fine particulate matter were collected in a roadway tunnel near Houston, TX over a period of 4 days during two separate sampling periods: one sampling period from 1200 to 1400 local time and another sampling period from 1600 to 1800 local time. During the two sampling periods, the tunnel traffic contained roughly equivalent numbers of heavy-duty diesel trucks. However, during the late afternoon sampling period, the tunnel contained twice as many light-duty gasoline-powered vehicles. The effect of this shift in the vehicle fleet affects the overall emission index (grams pollutant emitted per kilogram carbon in fuel) for fine particles and fine particulate elemental carbon. Additionally, this shift in the fraction of diesel vehicles in the tunnel is used to determine if the chemical mass balancing techniques used to track emissions from gasoline-powered and diesel-powered emissions accurately separates these two emission categories. The results show that the chemical mass balancing calculations apportion roughly equal amounts of the particulate matter measured to diesel vehicles between the two periods and attribute almost twice as much particulate matter in the late afternoon sampling period to gasoline vehicles. Both of these results are consistent with the traffic volume of gasoline and diesel vehicles in the tunnel in the two separate periods and validate the ability for chemical mass balancing techniques to separate these two primary sources of fine particles.

A comparison of sampling and analysis methods for low-ppbC levels of volatile organic compounds in ambient air.

A carefully designed study was conducted during the summer of 1998 to collect samples of ambient air by canisters and compare the analysis results to direct sorbent preconcentration results taken at the time of sample collection. Thirty-two 1 h sample sets were taken, each composed of a "near-real-time" sample analyzed by an autoGC-MS XonTech 930/Varian Saturn 2000 system, and Summa and Silco canisters. Hourly total non-methane organic carbon (TNMOC), ozone, and meteorological measurements were also made. Each canister was analyzed on the autoGC-MS system for a target list of 108 volatile organic compounds (VOCs) and on a manual cryosampling GC-FID system. Comparisons were made between the collection and analysis methods. Because of the low sample loading (150-250 ppbC TNMOC), these comparisons were a stringent test of sample collection and analysis capabilities. The following specific conclusions may be drawn from this study. Reasonable precision (within 15% mean difference of duplicate analyses from the same canister) can be obtained for analyses of target VOCs at low-ppbC concentrations. Relative accuracy between the GC-MS and GC-FID analysis methods is excellent, as demonstrated by comparisons of analyses of the same canisters, if measurements are sufficiently above the detection limits. This is especially significant as the GC-MS and GC-FID were independently calibrated. While statistically significant differences may be observed between the results from canister and near-real-time samples, the differences were generally small and there were clear correlations between the canister results and the near-real-time results. Canister cleanliness limits detection below the EPA Method TO-14 acceptance standard of 0.2 ppbv (0.2-2 ppbC for target analytes).