Environ. Sci. Technol. 2007, 41, 4972-4979
Characterization of Fine Particle and Gaseous Emissions during School Bus Idling J. S. KINSEY* United States Environmental Protection Agency, Office of Research and Development, National Risk Management Research Laboratory, MD E343-02, Research Triangle Park, North Carolina 27711 D. C. WILLIAMS, Y. DONG, AND R. LOGAN ARCADIS U.S., 4915 Prospectus Drive, Suite F, Durham, North Carolina 27713
The particulate matter (PM) and gaseous emissions from six diesel school buses were determined over a simulated waiting period typical of schools in the northeastern U.S. Testing was conducted for both continuous idle and hot restart conditions using a suite of on-line particle and gas analyzers installed in the U.S. Environmental Protection Agency’s Diesel Emissions Aerosol Laboratory. The specific pollutants measured encompassed total PM-2.5 mass (PM e2.5 µm in aerodynamic diameter), PM-2.5 number concentration, particle size distribution, particle-surface polycyclic aromatic hydrocarbons (PAHs), and a tracer gas (1,1,1,2,3,3,3-heptafluoropropane) in the diluted sample stream. Carbon monoxide (CO), carbon dioxide, nitrogen oxides (NOx), total hydrocarbons (THC), oxygen, formaldehyde, and the tracer gas were also measured in the raw exhaust. Results of the study showed little difference in the measured emissions between a 10 min post-restart idle and a 10 min continuous idle with the exception of THC and formaldehyde. However, an emissions pulse was observed during engine restart. A predictive equation was developed from the experimental data, which allows a comparison between continuous idle and hot restart for NOx, CO, PM2.5, and PAHs and which considers factors such as the restart emissions pulse and periods when the engine is not running. This equation indicates that restart is the preferred operating scenario as long as there is no extended idling after the engine is restarted.
Introduction The U.S. Environmental Protection Agency (EPA) has determined that diesel exhaust is a likely human carcinogen that can also contribute to other acute and chronic health effects (1). In addition, children are generally more susceptible to air pollutants such as diesel particulate matter (PM) because their respiratory systems are still developing and they have a faster breathing rate (1). For these reasons, concern has been raised about the exposure of children to PM exhaust pollutants associated with diesel school buses during the commute to and from school. Of particular * Corresponding author phone: (919) 541-4121; fax: (919) 5410359; e-mail:
[email protected]. 4972
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 14, 2007
importance is the exposure of children to idling buses during loading and unloading operations. In these circumstances, the engine tends to run at less than optimum efficiency with limited dispersion of the exhaust pollutants (2). A number of studies has been conducted to assess children’s exposure to diesel pollutants during school bus commutes, some of which address potential exposures during loading/unloading (3-11). In 2002, both Wargo and Brown (10) and Sabin et al. (4) found significantly higher concentrations of black carbon (BC) inside idling buses as compared to those measured in buses while in motion. Gilliam and Reeves (7) also determined similar increases in particle concentration for idling buses after the door was initially opened and also found significant spikes in the PM-2.5 (particles e2.5 µm in aerodynamic diameter) concentration when the buses were first started in the morning. In addition, Behrentz et al. (3) determined the concentrations of BC, particle surface polycyclic aromatic hydrocarbons (PAHs), and NO2 to be 2.5-5 times higher (depending on pollutant) at bus stops as compared to a school loading/unloading zone where idling was limited. Also, in addition to tailpipe pollutants, Hill et al. (6) found the crankcase vent tube to be a major source of PM-2.5 observed at bus stops. On the basis of these and similar data, many regulatory agencies and school districts, including the EPA’s Clean School Bus USA initiative, have issued guidance or regulations limiting the idling of school buses during the loading/unloading of school children (12). A question frequently posed to the EPA and anti-idling advocates is whether restarting school buses will result in higher emissions of diesel pollutants than those attributable to periods of continuous idle. This paper addresses this question by measuring the idle emissions from a limited number of diesel school buses under wintertime conditions. The objective of the study was to test the hypothesis that the benefit of anti-idling, including restart, results in less net emissions than continuous idling.
Experimental Procedures Testing was performed during early March 2005 at the bus yard of the Katonah-Lewisboro School District located in Cross River, NY. The District provided the test site, buses, and fuel used in the study. The District also allowed each test bus to be taken out of service so that it could be evaluated in a more cost-effective manner. A total of six District buses with model years ranging from 1997 (odometer ) 156 669 km) to 2004 (odometer ) 1191 km) were evaluated. The buses were equipped with one of three different models of Caterpillar diesel engine along with a Donaldson Diesel Oxidation Catalyst (DOC) muffler and a Spiracle Crankcase Ventilation Filtration System. Standard pump grade diesel fuel with a sulfur content of 226 ppm (weight) and a cetane index of 44.6 was used during testing. A further description of the test vehicles and fuel used can be found in the Supporting Information. For this study, the Diesel Emissions Aerosol Laboratory (DEAL) was used as the basic sampling platform (13). A special test fixture (Figure 1) was needed, however, to connect the tailpipe of the bus to the DEAL sampling system while providing adequate dilution of the exhaust sample with clean, pollutant-free air. As shown in Figure 1, a tapered fitting was inserted into the bus exhaust pipe and connected to a short-radius 90° elbow and straight section of heated stainless steel pipe containingvariousprobesandsensors.Tracergas(1,1,1,2,3,3,3heptafluoropropane or FM-200) was injected and mixed with 10.1021/es0625024 CCC: $37.00
2007 American Chemical Society Published on Web 06/08/2007
FIGURE 1. Exhaust sampling system. the exhaust flow in the elbow after which the mixture entered the straight working section of the apparatus. In the working section, the volumetric flow rate was determined using a calibrated Annubar and differential pressure cell followed by PM and gas sample extraction with the gas sample directed to the continuous emission monitor (CEM) bench of the DEAL through a heated line. The exhaust temperature was also measured at two locations with the remaining raw exhaust vented to the atmosphere. In the diluter portion of the sampling system (Figure 1), the raw sample flow was mixed with ambient air (test average temperature of 4-14 °C) filtered through a combination high efficiency particulate air (HEPA) and activated carbon filter. A final Teflon filter was also used to remove any carbon particles that might contaminate the diluent air stream. The ratio of dilution air to sample air was generally
