Measurement of black carbon emissions from in-use diesel-electric passenger locomotives in California

Black carbon (BC) emission factors were measured for a California commuter rail line fleet of diesel-electric passenger locomotives (Caltrain). The emission factors are based on BC and carbon dioxide (CO2) concentrations in the exhaust plumes of passing locomotives, which were measured from pedestrian overpasses using portable analyzers. Each of the 29 locomotives in the fleet was sampled on 4e20 separate occasions at different locations to characterize different driving modes. The average emission factor expressed as g BC emittedperkgdiesel consumedwas0.87±0.66 gkg 1 (±1 standarddeviation,n1⁄4362 samples). BCemission factors tended to be higher for accelerating locomotives traveling at higher speeds with engines in higher notch settings. Higher fuel-based BC emission factors (g kg 1)weremeasured for locomotives equippedwith separate “head-end”powergenerators (SEP-HEPs),whichpower thepassengercars,whilehigher time-based emission factors (g h 1) were measured for locomotives without SEP-HEPs, whose engines are continuously operated at high speeds to provide both head-end and propulsion power. PM10 emission factors, estimated assuming a BC/PM10 emission ratio of 0.6 and a typical power output-to-fuel consumption ratio, were generally in line with the Environmental Protection Agency's locomotive exhaust emission standards. Per passengermile, diesel-electric locomotives in this studyemit only 20% of the CO2 emitted by typical gasolinepowered light-duty vehicles (i.e., cars). However, the reduction in carbon footprint (expressed in terms of CO2 equivalents) due to CO2 emissions avoidance from a passenger commuting by train rather than car is appreciably offset by the locomotive's higher BC emissions. © 2015 Elsevier Ltd. All rights reserved.