A Multi-City Investigation of the Effectiveness of Retrofit Emissions Controls in Reducing Exposures to Particulate Matter in School Buses

The study identifies two primary sources of in-cabin pollution in conventional school buses: the tailpipe and the engine crankcase. It finds that while diesel particulate filters (DPF) combined with ultra-low sulfur diesel (ULSD) eliminate ultrafine particles, black carbon, and PAH, they do not reduce fine particulate matter (PM2.5) because that pollutant originates from the crankcase. The most effective solution for comprehensive elimination of all measured particles is the combination of a DPF, ULSD, and a Donaldson Spiracle closed-crankcase filtration device.

• Conventional school buses experience significant self-pollution where diesel exhaust enters the cabin primarily through the front door, leading to particulate matter levels that exceed daily and annual PM2.5 National Ambient Air Quality Standards (NAAQS).
• The study identifies two distinct sources of in-cabin pollution: the tailpipe is responsible for ultrafine particles, black carbon, and polycyclic aromatic hydrocarbons (PAH), while the engine crankcase (via the road draft tube) is the dominant source of fine particulate matter (PM2.5) mass.
• The combination of a diesel particulate filter (DPF) and ultra-low sulfur diesel (ULSD) fuel virtually eliminates ultrafine particles, black carbon, and PAH in the cabin, but fails to reduce PM2.5 because the DPF only treats tailpipe emissions and not crankcase emissions.
• The Donaldson Spiracle, a closed-crankcase filtration device, effectively eliminates PM2.5 self-pollution in the cabin by rerouting blow-by emissions back into the engine intake manifold, though it does not reduce ultrafine particles, black carbon, or PAH.
• The most comprehensive solution for eliminating all measured particle species (PM2.5, ultrafine particles, black carbon, and PAH) in the cabin and improving curbside air quality is the combination of a DPF, ULSD, and the Donaldson Spiracle.
• Diesel oxidation catalysts (DOC) provided no clearly ascertainable in-cabin benefit for reducing particulate matter, potentially due to ineffectiveness under idling conditions.
• Compressed natural gas (CNG) buses with rear engines showed significantly lower PM2.5 build-up and limited ultrafine particle pollution compared to conventional diesel buses, as they lack the crankcase emissions source.
• Idling and queuing significantly degrade cabin air quality; in a 3-bus queue, PM2.5 levels exceeded 200 ug/m3 and black carbon reached 65 ug/m3 within ten minutes of opening the doors.
• Simulated drop-off tests indicate that conventional buses leave significant clouds of diesel smoke in schoolyards, whereas the DPF-ULSD combination eliminates measurable ultrafine particle and PM2.5 pollution at the curbside.

APA

Hill, L. B., Zimmerman, N. J., & Gooch, J. (2005). A Multi-City Investigation of the Effectiveness of Retrofit Emissions Controls in Reducing Exposures to Particulate Matter in School Buses. Clean Air Task Force. https://cdn.catf.us/wp-content/uploads/2019/02/21093807/CATF_Pub_School_Bus_Particulate_Matter_Study.pdf

Chicago

Hill, L. Bruce, Neil J. Zimmerman, and James Gooch. A Multi-City Investigation of the Effectiveness of Retrofit Emissions Controls in Reducing Exposures to Particulate Matter in School Buses. Clean Air Task Force, 2005. https://cdn.catf.us/wp-content/uploads/2019/02/21093807/CATF_Pub_School_Bus_Particulate_Matter_Study.pdf.

Wikipedia

{{cite report |last1=Hill |first1=L. Bruce |last2=Zimmerman |first2=Neil J. |last3=Gooch |first3=James |title=A Multi-City Investigation of the Effectiveness of Retrofit Emissions Controls in Reducing Exposures to Particulate Matter in School Buses |publisher=Clean Air Task Force |date=January 2005 |url=https://cdn.catf.us/wp-content/uploads/2019/02/21093807/CATF_Pub_School_Bus_Particulate_Matter_Study.pdf |access-date=23 October 2026 |via=Climate Insights Directory}}

BibTeX

@techreport{hill2005multicity, author = {Hill, L. Bruce and Zimmerman, Neil J. and Gooch, James}, title = {{A Multi-City Investigation of the Effectiveness of Retrofit Emissions Controls in Reducing Exposures to Particulate Matter in School Buses}}, institution = {Clean Air Task Force}, year = {2005}, month = jan, url = {https://cdn.catf.us/wp-content/uploads/2019/02/21093807/CATF_Pub_School_Bus_Particulate_Matter_Study.pdf}, urldate = {2026-10-23}, note = {Indexed by Climate Insights Directory} }