Vehicle Driver & Atmospheric Factors in Light-Duty Vehicle Particle Number Emissions

Made possible by the collection of on-board tailpipe emissions data, this research identifies road and driver factors that are associated with a relatively understudied tailpipe pollutant from lightduty vehicles: ultrafine particle number emissions. High emission events (HEE) of ultrafine particle number (PN) emissions occurred most frequently at locations with steep upgrades or locations that required moderate to rapid accelerations (>3 mph/s). The analysis revealed that less than 2% of the time driving was responsible for almost a third of all ultrafine particles emitted along the designated 17-mile test route for a sample of 22 drivers. Variables identified in a generalized linear model as significant to PN emissions include measures of engine speed (RPM), driver behavior (speed and acceleration rates), and road geometry (grade). These factors account for approximately 61% of the variability measured. Few modal emissions models estimate PN emissions; however, this research has revealed that the same predictor variables used to model gas pollutants are significant predictors of PN emissions. Therefore, the addition of PN emissions estimates to existing models would require little effort if these relationships were developed with larger datasets. This project also documented additional challenges related to on-board PN data collection related to temperature, humidity, and background PN concentrations. Finally, a large amount of the variation in light-duty PN emissions remains unexplained, suggesting a need for more comprehensive on-board datasets, including data on particle size distribution. UVM TRC Report # 14-006