Comparison of an Alternative Particulate Mass Measurement with Advanced Microbalance Analysis 2004-01-0589
The regulated level of particulate mass for 2007 heavy duty diesel on-road engines is 0.01 g/bkhp-hr. Measurement of this low level of particulate by weighing is costly and time consuming. The weighing method must measure 100 μg or less of particulate on a filter that weighs about 100 mg with a resolution of ± 2.5 μg or better. This means that the microbalance and sampling handling procedure must be accurate within ±25 ppm by mass or ±1/40,000. It requires a microbalance with 0.1 μg precision housed in a special environment. Moreover, the weighing method involves a lengthy process. The filter must be equilibrated, and then pre- and post-weighed, usually with repeat measurements. An alternative to gravimetric analysis is a thermal mass analyzer that measures the semi-volatile organic fraction (SOF), as well as soot and sulfate fractions of the particulate matter (PM) collected on a cleaned quartz filter. The calibration of the thermal mass measurement is discussed in detail. At very low emission levels, the separation of the SOF and soot signals presents problems that are addressed. Use of quartz as filter media raises concerns about gaseous hydrocarbon adsorption inducing a falsely increased SOF reading. Methods of addressing these concerns are described and results presented. Finally, results of gravimetric analysis using high efficiency filters are compared with the results of thermal mass analysis of particulate matter collected from various light-duty diesel vehicles, some equipped with diesel particulate filters.