ESTIMATION OF DIESEL PARTICULATE MATTER CONCENTRATIONS IN A SCHOOL BUS USING A FUEL-BASED TRACER: SENSITIVE AND SPECIFIC METHOD FOR QUANTIFYING VEHICLE CONTRIBUTIONS

Particulate matter from diesel exhaust (DPM) is a pollutant of increasing concern, but its concentrations are difficult to measure in the environment. There are no unique chemical characteristics for DPM, so elemental carbon (EC) and black carbon (BC) measurements have historically been used to estimate its concentrations. However, both methods have limitations, and neither is capable of identifying specific sources. That results in significant uncertainties in the inferred DPM concentrations. In this study an intentional fuel-based tracer, iridium, was used to mark the DPM from a school bus\u27s exhaust. Emissions tests established the mass ratio of DPM-iridium in the exhaust. Iridium measurements inside the bus during normal operations could then be used to calculate DPM concentrations without the limitations of methods relying on nonspecific markers. Previous studies have attributed in-bus concentrations of more than 30 microg/cu m to the bus\u27s exhaust, but they relied on nonspecific methods with a sensitivity of approximately 0.1 microg/cu m. In this study the average measured DPM (study bus) concentration was 0.22 microg/cu m--about 1% of previous estimates. Measurement sensitivity was approximately 0.001 microg/cu m--100 times better than that of other methods. The sensitivity and specificity achieved show that this tracer method can resolve major ambiguities and uncertainties in estimates of actual DPM concentrations. The ability to estimate accurately the contributions of different sources to DPM concentrations provides a needed tool to evaluate proposed emission reduction strategies reliably