A Study of the Impact of Diesel Buses on Downtown Boulder

Josue Rene Hernandez Pedroza, a Fulbright scholar from Panama, just completed his MS in Mechanical Engineering at CU Boulder. I directed his thesis research in air quality. The research was motivated by the fact that the Regional Transportation District conducts more than 1,000 diesel bus trips per day in Boulder, CO (and counting trips to and from both bus stations in Boulder, the total is approximately 1400). Boulder residents commissioned this study of the air quality impact of these bus trips. 

Traffic-related air pollution, and specifically from emissions from diesel engines, is responsible for many of the most dangerous contaminants for human being welfare and health. A high density of diesel vehicles in operation represents a serious hazard for people living or spending several hours within the surrounding areas. In Colorado, the Boulder Downtown Bus Station is located in the middle of a residential and commercial district, with a high density of people living, working and visiting the downtown area. A high concentration of diesel buses traveling in downtown Boulder may present a significant concern for air quality and health. The objective of this research was to provide data about the effects of diesel buses on the air quality level in the downtown region of Boulder, Colorado. In this study, measurements were taken in Summer and Winter over the course of a week in each season to understand different seasonal emission patterns. Bicycle rides were carried out to measure pollutants on-road, in which buses and vehicles were chased to capture fresh emissions. Computational modeling was used to characterize the effects of buses to the total traffic-related emissions on Boulder Downtown. 

Sampling set-up on balcony in downtown Boulder.

Many studies have reported the adverse health effects of diesel combustion engine emissions; diesel exhaust is considered carcinogenic to humans. This research examined the major pollutants produced by diesel-powered engines within the city of Boulder, Colorado: particulate matter, black carbon, and nitrogen oxides. Ozone was also studied due to its secondary formation from nitrogen oxides. Summer and Winter measurements were taken next to the Boulder Downtown Bus Station, for one week in each season. It was not possible to estimate bus emissions due to variability in the number of vehicles and buses traveling in the study corridor. Results showed limited association between pollutant levels and traffic patterns, and followed regional pollution and seasonal trends. Only Winter particle number and nitrogen oxides followed traffic trends. NOx was elevated in winter, and Black carbon contributed 12-34% of the total fine particulate matter.

Bicycle rides were carried out to capture fresh emissions on-road. Black carbon concentration peaked when chasing certain buses, while emissions from other buses were low. Passenger vehicle emissions were not reflected in the black carbon levels. Bicycle ride pollutant averages were lower than averages from stationary measurements, while bicycle ride peak values were higher than peak values in stationary measurements for black carbon and particulate matter. Ozone was always higher on-road.

With the use of MOVES and R-Line, the impact of buses on the concentration of pollutants was analyzed. Emission rates were estimated with MOVES in Summer and Winter by running simulations with and without buses. The R-Line dispersion model was used to estimate the concentration of pollutants at the Boulder Downtown Bus Station and the street right in front of the station. Results showed the fraction of NOx and PM that could be attributable to buses was between 24-40% and 16-45%, respectively. These values would have been lower if traffic-related emissions from additional surrounding streets would have been included. Bus emissions due to the idling period represented a significant part of pollutants emitted. Therefore, the bus station contributed significantly to the total traffic emissions in Boulder Downtown. 

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