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Diesel exhaust is a major source of combustion particles that contribute to poor air quality nationwide. Since almost all school buses are operated with diesel engines, diesel engine exhaust can thus also be a source of concern, specifically with regard to exposure to children. Diesel particulate matter (DPM) is a complex and unhealthy mixture of inorganic and organic carbon particles with adhered toxic substances and metals. The purpose of the study was to investigate the causes of school bus self-pollution and to document in-cabin diesel particulate matter exposures in buses retrofit with a variety of available particulate matter emissions control combinations. This is one of the first studies to report on the in-cabin benefits of retrofit technology. To date, our testing has been conducted on school bus fleets in three U.S. cities – Chicago, IL and Atlanta, GA in 2003 and in Ann Arbor, MI in 2004. Retrofit combinations tested included:
- Conventional buses on conventional fuel
- Conventional bus with ultra-low sulfur diesel fuel (ULSD)
- Bus with diesel oxidation catalyst (DOC) and conventional fuel
- Bus with Spiracle and ULSD fuel
- Bus with diesel particulate filter (DPF) and ULSD fuel
- Bus with DPF, Spiracle and ULSD fuel
- Bus with DOC, Spiracle and ULSD fuel
- Bus with DPF, ULSD and Enviroguard
- Compressed natural gas (CNG) bus
During all bus runs, a lead car with identical instrumentation was used as a control to characterize ambient air in the roadway in front of the bus. Actual school bus routes were followed in largely quiet residential neighborhoods with few nearby diesel sources thereby minimizing the confounding influence of sources of diesel emissions other than the bus itself. Measured parameters included: 1) fine particulate matter (particles 2.5 microns and less), 2) ultrafine particles (extremely small particles smaller than 0.1 microns) and 3) black carbon (elemental carbon soot) and particle-bound polycyclic aromatic hydrocarbon (PAH).
Tests conducted on conventional buses (common yellow school buses with the engine in the front and without emissions controls devices) along actual bus routes found that diesel exhaust routinely penetrated the school bus cabins from the tailpipe and the engine compartment through the front door of the bus. Over the course of the bus routes, particulate matter built up to levels multiple times that of outdoor ambient conditions above the daily and annual particulate matter (PM2.5) NAAQS. Particle emissions rarely were found to seep into conventional school buses through other pathways such as closed windows, the back door or from the engine compartment. During queuing – where buses are parked closely end-to-end with front doors open--we observed rapid build up of particulate matter within the bus cabin.
Ultrafine particles, black carbon and particle-bound PAH measured in the cabins of the buses during bus routes, idling, and queuing were traced directly to the tailpipe of the buses. In contrast, however, fine mass (PM2.5) concentrations were dominated by particulate matter emissions from the crankcase vented under the hood of the bus through the "road draft tube." Crankcase emissions proved to be an extremely strong source of PM2.5 in the school bus.
A number of emissions controls combinations were tested following the assessment of cabin air quality on the conventional buses. The application of a diesel particulate filter (DPF) and ultraflow sulfur diesel fuel (ULSD) virtually eliminated ultrafine particles, black carbon, and PAH pollutants in the cabin. Surprisingly, the DPFs did not measurably reduce fine particle mass (PM2.5) in the cabin – not due to a lack of particle removal efficiency--but instead as a result of the strong crankcase PM 2.5 source under the hood of the bus. To control the strong PM2.5 concentrations remaining after application of DPF-ULSD retrofit, several experiments were performed including: 1) adding extension tubing to the road draft tube shunting emissions toward the back of the bus away from the door, 2) installation of a Fleetguard Enviroguard filter, and 3) installation of a Donaldson Spiracle, a closed-crankcase filtration device. In the first experiment, the extension tubing had showed a limited PM2.5 reduction in the cabin. In the second experiment, the Enviroguard demonstrated no measurable PM2.5 benefit. The device. designed to reduce oil spillage in the roadway from the crankcase--releases strong postfiltration PM2.5 emissions in the engine area close to the bus doorway where they enter the bus cabin. In the third attempt to abate the crankcase emissions, we found that the Spiracle eliminated the PM2.5 self-pollution in the cabin but did not result in improvements in ultrafine particles, black carbon or PAH. The Spiracle reroutes the crankcase emissions back into the intake manifold of the engine, ultimately directing them through the exhaust system and away from the engine compartment, where they can be removed by tailpipe filtration devices.
A bus equipped with a diesel oxidation catalyst (DOC) showed cabin levels of ultrafine particles, black carbon and PAH that were similar in magnitude to those observed in conventional buses. Thus, we found it difficult to ascertain whether a DOC provided any in-cabin benefit. This may be for a variety of reasons including: 1) inability of the methodology to determine small changes, 2) confounding by variable wind directions relative to the two emissions sources and the cabin door, 3) potential ineffectiveness of DOC under idle conditions. Our testing did not examine benefits that may occur for other pollutants with the DOC such as hydrocarbons, CO and nitrogen oxides (NOx). Furthermore, how particulate matter levels outside the bus (e.g. in a schoolyard during idling, drop off, or pick up) are affected by the DOC were not fully investigated warrants further research.
A compressed natural gas (CNG) bus – with a rear engine--showed little build up of PM2.5 in the cabin and mean levels were largely the same as outdoor ambient. However, the CNG bus showed evidence of limited ultrafine particle self-pollution at a few bus stops but at much lower levels compared to the conventional bus.
Combinations of both tailpipe and crankcase emissions control devices were also tested including the DPF-ULSD-Spiracle, and DOC-Spiracle. The DOC-Spiracle combination eliminated only one parameter – PM2.5 mass, presumably due to the Spiracle alone. The DPF-ULSD-Spiracle combination resulted in elimination of all measure parameters on the bus – ultrafine particles, black carbon, PAH and PM2.5.
In addition to cabin air quality, air quality outside school buses is also a factor in children's exposure to diesel exhaust. In a Connecticut test, ambient air quality measurements were measured adjacent to a New Haven elementary school yard to gauge the impact of buses during student drop off and pick up. Significant increases in PM2.5 and ultrafine particulate matter levels were observed adjacent to the school yard when uncontrolled conventional buses left the school after dropping off children leaving a cloud of diesel smoke in their wake. Because retrofit buses were unavailable for comparison at the time in New Haven, we simulated school bus drop off scenario with retrofit buses in tests conducted in all three cities. These tests show that the DPF-ULSD combination eliminated all PM2.5 and ultrafine particulate matter at the curbside outside of the bus. CATF has prepared video clips graphically superimposing changing pollutant levels over a digital video image of the bus at drop off. These videos vividly demonstrate the benefits of the retrofits (see www.catf.us/projects/diesel/resources/videos/.)
In conclusion, this research suggests that the combination of DPF, Spiracle and ULSD results in a comprehensive elimination of all particle species measured and is the most effective solution for addressing school bus cabin air quality as well as improving conditions outside of schools. In addition, the closed crankcase filtration device proved to be an extremely cost-effective initial step to improve cabin air quality in school buses we tested.
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