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This memo summarizes the results of an analysis which compares the economic, and the air quality and climate impacts, resulting from the use of compressed natural gas (CNG) transit buses to those from modern diesel buses. The purpose of the analysis was to evaluate the potential for life-cycle cost savings to transit agencies from the purchase of new CNG buses instead of new diesel buses, as well as the impact of CNG bus purchases on fleet-wide criteria pollutant and greenhouse gas (GHG) emissions. The analysis of GHG emissions includes both tail-pipe emissions and upstream emissions from production and transport of fuel. The analysis also includes the climate impact of the black carbon portion of tail-pipe PM emissions.
Inputs to the analysis include diesel fuel and natural gas cost projections from the U.S. Energy Information Administration (EIA); transit bus operating and cost data from the American Public Transportation Association (APTA); estimates of tail-pipe nitrogen oxide (NOx), particulate matter (PM), hydrocarbon (HC), carbon monoxide (CO), and methane (CH4) emissions from EPA’s MOVES emissions model; estimates of tail-pipe carbon dioxide (CO2) emissions based on average in-use fuel use; and estimates of up-stream CO2 and CH4 emissions for diesel and natural gas fuel production, from the GREET model.
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