Measuring In-Cabin School Bus Tailpipe and Crankcase PM_2.5: A New Dual Tracer Method

Abstract

Exposures of occupants in school buses to on-road vehicle emissions, including emissions from the bus itself, can be substantially greater than those in outdoor settings. A dual tracer method was developed and applied to two school buses in Seattle in 2005 to quantify incabin fine particulate matter (PM 2.5 ) concentrations attributable to the buses' diesel engine tailpipe (DPM tp ) and crankcase vent (PM ck ) emissions. The new method avoids the problem of differentiating bus emissions from chemically identical emissions of other vehicles by using a fuel-based organometallic iridium tracer for engine exhaust and by adding deuterated hexatriacontane to engine oil. Source testing results showed consistent PM:tracer ratios for the primary tracer for each type of emissions. Comparisons of the PM:tracer ratios indicated that there was a small amount of unburned lubricating oil emitted from the tailpipe; however, virtually no diesel fuel combustion products were found in the crankcase emissions. For the limited testing conducted here, although PM ck emission rates (averages of 0.028 and 0.099 g/km for the two buses) were lower than those from the tailpipe (0.18 and 0.14 g/km), in-cabin IMPLICATIONS PM 2.5 measurements in two Seattle school buses showed average concentrations of 26 and 12 g/m 3 with windows closed and open, respectively. Virtually all PM 2.5 was carbonaceous. Tracer measurements showed that bus selfpollution contributed approximately 50% of total PM 2.5 concentrations with windows closed and 15% with windows open, with over three-quarters of these contributions attributed to crankcase emissions. Maintaining ventilation in buses clearly reduces total PM 2.5 exposures and that from the buses' own emissions. The dual tracer method now offers researchers a new technique for explicit identification of single source contributions in settings with multiple sources of carbonaceous emissions. TECHNICAL PAPER INTRODUCTIONIn-vehicle exposures to vehicle-related emissions of fine particulate matter (PM 2.5 ) and other species inside vehicles have been shown to be high relative to urban outdoor environments. 1,2 Particular emphasis has been given to concentrations of diesel exhaust particulate matter (PM) inside school buses that are attributable to the buses' own exhaust. Such source attribution efforts are difficult in purely observational studies because of the presence of emissions from other vehicles, which are chemically identical to those of the bus, and the presence of chemically similar PM emitted by nonvehicular sources. A method was therefore sought that could allow definitive identification of the emissions of a single bus. Previous studies 3-8 have produced estimates of exhaust PM inside school buses that are attributable to the buses' own exhaust ranging from 0.2 to as much as 20 g/m 3 . With the exception of Ireson et al., 5 these studies inferred bus exhaust contributions from measurements of species emitted by diesel engines and other sources and did not measure the emission rates of the ...

Evaluation of a High Efficiency Cabin Air (HECA) Filtration System for Reducing Particulate Pollutants Inside School Buses

“…Freshly emitted particles from vehicle tailpipes are usually in the nucleation mode with diameter less than 20 nm. Thus, the observed increase in nucleation mode particles inside school buses is likely due to self-pollution as previously observed in other studies. , …”

“…Thus, the observed increase in nucleation mode particles inside school buses is likely due to self-pollution as previously observed in other studies. 14,15 I/O Ratio Reductions. The on-board HECA filtration system effectively reduced particulate pollutants (i.e., UFP, BC, and PM 2.5 ) inside school buses.…”

“…Previous studies have shown that traffic emissions significantly increase concentrations of UFP, BC, and PM 2.5 on local arterial roadways and freeways. − In addition to the pollutants originating from surrounding traffic, school buses’ own exhaust can also penetrate into the bus cabin, the so-called self-pollution. ,− High pollutant levels have been observed not only under driving conditions, but also under idling . Children commuting in school buses may be exposed to even higher pollutant concentrations than regular commuters in passenger cars …”

Experimental observations of the transition pressure drop characteristics of fibrous filters loaded with oil-coated particles

“…Examples of such greasy particles/oil-coated particles are the particles emitted from internal combustion engines or generated in vehicle crankcases, and metal working fluid (MWF) aerosols produced in grinding and milling operations. Several recent studies have indicated that more polycyclic aromatic hydrocarbon (PAH) species were found in vehicle crankcase emissions than in tailpipe emissions, and the crankcase emissions may contribute to in-cabin particulate matter [15][16][17]. There is also evidence showing that an increase in MWF aerosol exposure increases the risk of adverse respiratory health effects, and so needs to be filtered out [18][19][20].…”

Quantification of Sources of Variability of Air Pollutant Exposure Concentrations among Selected Transportation Microenvironments

“…Buses are of interest because they are a common local public transit mode but also because of prior observations that pollutant concentrations can be higher inside buses compared with ambient levels ( 35 , 36 ). Another study estimated that self-pollution of light duty gasoline vehicles (LDGVs) contributed approximately 15% of CO and 30% of PM 2.5 in-cabin exposure concentration ( 37 ).…”