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 Abstract

  Atlanta Commute Vehicle Soak and Start Distributions and Engine Starts per Day, Impact on Mobile Source Emission Rates (64 pp, 2.09 MB) (EPA/600/R-07/075) April 2007

Image: Atlanta Commute Vehicle Soak and Start Distributions and Engine Starts per Day, Impact on Mobile Source Emission Rates
The Georgia Institute of Technology School of Civil and Environmental Engineering research team analyzed the 2004 second-by-second vehicle activity data obtained from vehicles in the Atlanta Commuter Choice Value Pricing Initiative, otherwise known as Commute Atlanta.

The on-board monitoring equipment installed in each participating Commute Atlanta vehicle records each second of vehicle activity. Therefore, each engine event (engine on, engine off) is time-stamped to provide the start and end times of each vehicle trip. The times of each trip end can be used to directly quantify the number of engine starts per day, the start times for each trip, and the durations of soak times between trips.

The soak time affects exhaust start and exhaust running emissions. A vehicle is considered to be soaking if its engine is not running. Soak time is the length of time between the engine turn off time and engine start time. Cold soak time in MOBILE6 (an emission rate model) determines the percentage of vehicles that have been soaking for a given amount of time, for each hour of the day, prior to an engine start. The hot soak time distributions represent the proportion of vehicles experiencing a hot soak of a given duration at each hour of the day. For the purpose of the analyses presented in this report, the research team followed the same criteria EPA used to develop soak and start time distributions and engine starts per day in the upgraded emission rate model, MOBILE6.2.

The objective of this research is to develop gasoline vehicle soak and start time distributions and engine starts per day for EPA light-duty vehicle and light-duty truck classes using vehicle trip data collected in the 13-county Atlanta metropolitan area during the calendar year 2004. Given the equipment and methods used in the Commute Atlanta vehicle activity, data can be linked to general household demographic parameters (household income, household size, and vehicle ownership) and vehicle characteristics (vehicle type and model year). Most of the vehicles in this study (80–85 percent) are not shared, meaning that most vehicle activity can also be linked to individual driver characteristics (age and gender).

The research team developed weekday and weekend soak and start time distributions and engine starts per day by demographic and vehicle characteristics parameters, using the same hour-of-day intervals and soak-time groups used in MOBILE6.2.

After developing the soak and start time distributions and engine starts per day from Commute Atlanta trip data, the research team conducted MOBILE6.2 emission rate modeling to assess the potential impacts on the regional emission inventory for the 13-County Atlanta metropolitan area. The research team also examined potential emissions benefits from changing soak and start time distributions by applying the Commute Atlanta MOBILE6.2 external data files to three other areas, including Gaston and Mecklenberg Counties in North Carolina, and York County in South Carolina.

The research team found that the start and soak distributions observed in Atlanta were significantly different from the default distributions currently used in the MOBILE6.2 emission rate model. The team also found that the use of the observed Atlanta distributions has a significant impact on predicted emission rates and inventories generated using these emission rates. By applying Commute Atlanta soak and start time distributions and engine starts per day in MOBILE6.2, engine start volatile organic compound (VOC) emission rates are predicted to be 17.4 percent lower and hot soak VOC emission rates are predicted to be 27.3 percent lower. These significant reductions can account for an 8.3 percent reduction in predicted on-road VOC emissions in the 13-county Atlanta metropolitan area.

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Sue Kimbrough


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