Over the years, vehicular traffic has increased multifold causing an associated increase in the total emissions from transportation sources. These vehicular emissions affect two human occupied environments most significantly: human occupied regions near and around the roads including residences, offices, schools, hospitals etc., and indoor vehicle compartments that act as individual microenvironments trapped inside the high concentration zone. A detailed literature review indicated five major gaps in the knowledge base related to pollutant characteristics from the vehicular exhaust and pollutant behavior inside buses which are addressed in this research.
• Characterization of indoor air pollutant behavior: The literature review did not yield any elaborate study results on indoor pollutant behavior trends and/or models for public transport bus compartments. Comparative studies on the behavior of indoor-outdoor pollutant relationships and concentration variation at different locations inside the bus were also limited in scope.
• Indoor air quality in buses and factors influencing the indoor air quality (IAQ): No comprehensive study has been reported in the literature in which simultaneous measurement of multiple gaseous contaminants and particulate matter (PM) inside buses have been carried out that have focused on the identification of important explanatory variables of indoor air quality.
• Measurement of fine particulates: The current work reported in the literature is limited to particulate measurements for sizes more than 2.5 micrometer (µm) and most of the studies focus on PM10 (particle matter less than 10 µm in diameter) concentrations. Very few studies have measured the number of particles/unit volume and compared the associated abundance of particles to their respective mass concentrations.
• Characterization of exhaust emission behavior: Most of the work reported has a limitation on the number of vehicles studied and fails to elaborate the interaction between influencing variables. Very few studies have reported emission comparisons for biodiesel and ultra-low sulfur diesel (ULSD) operated fleets and limited models are available to study fleet emission behavior and instantaneous emission concentrations with respect to engine performance and fuel.
• Impacts of alternative fuels on indoor air quality have not been studied extensively.
This research has tried to lessen these knowledge gaps in the field and is the first to attempt an extensive real time monitoring and measurement of numerous operational and traffic variables that could have an effect on the emissions and the air quality of public transport buses. A comprehensive emission testing protocol was developed for characterizing emission characteristics from Toledo Area Regional Transportation Authority (TARTA) public transport buses, and over 120 buses were tested in engine idling and on-road operation modes. This research was also the first to attempt a comprehensive indoor air quality study spanning thirteen months of data collection involving the monitoring and measurement of multiple indoor gaseous pollutants and ultra-fine particulate number and mass concentrations.
The emission protocol identified important influencing factors that affect vehicular emissions during real-world operating conditions. Emission comparisons for TARTA buses showed that although B20 biodiesel in comparison to ULSD fuel emitted higher concentrations of nitric oxide (NO) and nitrogen dioxide (NO2) for 300 series (Bluebird) fleet, and lower carbon monoxide (CO) concentration for both 300 and 500 series (Thomas) fleets, other factors such as engines’ operating conditions, preventative maintenance history, vehicle operation at different engine loads and engine operating temperatures had a larger influence on emission behavior. Regular engine idling mode and higher engine temperatures were found to reduce vehicular emissions most significantly (up to 30-42%) while performing preventative maintenance reduced emission concentrations by 15-20%. Emission models for seven TARTA fleets were developed for six gases – oxygen (O2), carbon monoxide, carbon dioxide (CO2), nitric oxide, nitrogen dioxide and sulfur dioxide (SO2). These models explained an average of 90% of the emission data for each pollutant. Instantaneous models also developed for the six pollutants based on real-time on-road test data explained an average of over 80% the variability in the pollutant emissions. Engine temperatures, exhaust temperature, accelerator pedal position, % engine load, and engine rpm were the most important variables affecting the concentrations of the pollutants studied (90% of the models had p<0.001 for each of the variables).
The indoor concentrations of carbon dioxide, carbon monoxide, sulfur dioxide and nitrogen oxides (NOx) were found to be independent of the fuel used in the bus. Variation of all the pollutants studied were dependant on the route traveled, and additionally carbon dioxide was also found to be greatly affected by passenger ridership. Higher concentrations for all the pollutants were observed during the morning pullout and in periods of heavy traffic (around 9 am). The indoor concentration of fine particulates was found to be identical in both the B20 and ULSD buses suggesting minimal effect of fuel on the particulate concentrations. The effect of the fuel used in the bus was observed only during large periods of idling with the doors and windows opened, but during an average run, TARTA buses do not continuously idle for long periods with the windows opened. The indoor fine particulate levels were primarily a result of just-outside (roadside) concentrations and passenger activity. Over 95% of the indoor particulates have diameter less than 1 µm. PM1.0 mass was determined to be comprised of over 40% particles less than 0.40 µm, 25% particles between 0.40-0.50 µm and 35% particles between 0.50 and 1.0 µm in diameter. These pose the highest risk to humans as they can travel deep inside the lungs. The regression models developed using a combination of vehicular, traffic, ambient meteorology and in-vehicle comfort parameters, and ambient concentration explained approximately 72-81% of the hourly indoor mass and number concentrations of fine and ultra-fine particulates.
In conclusion, this dissertation demonstrates a feasible strategy for developing a comprehensive emission inventory and indoor air quality database to study the indoor and ambient impacts of public transport buses. The dissertation also presents a new analysis procedure for identifying the potential impact of each influencing variable from a comprehensive multi-variable environmental database developed from an experimental procedure. This research is also the first to attempt a comprehensive study to characterize the ultra-fine particulate (PM<1.0 µm) behavior inside public transport buses and to develop predictive models for multiple indoor pollutants including fine and ultra-fine particulate mass and number concentrations. The research was also able to characterize the emission and indoor air pollutant behavior for public transport fleets and identify the important influencing variables affecting the overall air quality.