Method for Modeling Driving Cycles, Fuel Use, and Emissions for Over Snow Vehicles Jiangchuan Hu, † H. Christopher Frey,* ,† Gurdas S. Sandhu, † Brandon M. Graver, † Gary A. Bishop, ‡ Brent G. Schuchmann, ‡,§ and John D. Ray ∥ † Department of Civil, Construction, and Environmental Engineering, North Carolina State University, Campus Box 7908, Raleigh, North Carolina 27695-7908, United States ‡ Department of Chemistry and Biochemistry, University of Denver, Denver, Colorado 80208, United States § SGS Environmental Testing Corporation, 2022 Helena St., Aurora, Colorado 80011, United States ∥ National Park Service, Air Resources Division, Denver, Colorado 80225, United States * S Supporting Information ABSTRACT: As input to a winter use plan, activity, fuel use, and tailpipe exhaust emissions of over snow vehicles (OSV), including five snow coaches and one snowmobile, were measured on a designated route in Yellowstone National Park (YNP). Engine load was quantified in terms of vehicle specific power (VSP), which is a function of speed, acceleration, and road grade. Compared to highway vehicles, VSP for OSVs is more sensitive to rolling resistance and less sensitive to aerodynamic drag. Fuel use rates increased linearly (R 2 > 0.96) with VSP. For gasoline-fueled OSVs, fuel-based emission rates of carbon monoxide (CO) and nitrogen oxides (NO x ) typically increased with increasing fuel use rate, with some cases of very high CO emissions. For the diesel OSVs, which had selective catalytic reduction and diesel particulate filters, fuel-based NO x and particulate matter (PM) emission rates were not sensitive to fuel flow rate, and the emission controls were effective. Inter vehicle variability in cycle average fuel use and emissions rates for CO and NO x was substantial. However, there was relatively little inter-cycle variation in cycle average fuel use and emission rates when comparing driving cycles. Recommendations are made regarding how real-world OSV activity, fuel use, and emissions data can be improved. ■ INTRODUCTION Over snow vehicles (OSVs), including snow coaches and snowmobiles, are the major winter transportation mode at Yellowstone National Park (YNP). A snow coach is a multipassenger vehicle designed or modified to operate over snow or ice. Temporary YNP winter use plans were adopted by the National Park Service starting in 2003 to regulate visiting OSVs. 1−6 To support development of the most recent YNP Supplemental Winter Use Plan and Environmental Impact Statement, assessments of fuel economy and emission rates of OSVs were conducted. 7−9 In-use OSV emissions were measured using remote sensing in 1998, 1999, and 2005 and a portable emissions measurement system (PEMS) in 2005 and 2006. 10−12 The measured vehicles actually operated in the park but were not selected to represent a fleet distribution. The lower emitting gasoline OSVs measured in 2006 were, on average, 5 years newer than those measured in 2005. Differences in results for measured vehicles from different study years are, at least in part, a result of differences in engine fuel delivery and emissions control. Differences in snow conditions, ambient temperature, and driving cycles among the field studies may also lead to variability in the comparisons. These measurements were conducted during real-world operations which may be specific to the observed driving cycle. To enable comparisons between vehicles, there is a need to be able to estimate cycle average rates for each vehicle based on a common cycle. Fuel use and emission rates of passenger cars, trucks, and buses are found to be highly associated with instantaneous engine load, which is affected by driving cycle. 13−17 A driving cycle is typically represented in terms of second-by-second speed, acceleration, and road grade. For highway vehicles, these three factors are used to estimate vehicle specific power (VSP). 18,19 Key coefficients in calculating VSP are related to changes in kinetic energy, changes in potential energy, rolling resistance, and aerodynamic drag. Since OSVs operate on snow and use tracks instead of wheels, the rolling resistance term is expected to differ. Since OSVs typically operate at relatively low speeds, aerodynamic drag may not be an important factor. Received: March 19, 2014 Revised: June 12, 2014 Accepted: June 19, 2014 Published: June 19, 2014 Article pubs.acs.org/est © 2014 American Chemical Society 8258 dx.doi.org/10.1021/es501164j | Environ. Sci. Technol. 2014, 48, 8258−8265