Most emission inventory calculations use a computer model to estimate the fleet’s emissions. California uses the EMFAC (EMission FACtors) emissions model; the rest of the US uses the EPA MOVES (Motor Vehicle Emission Simulator) model.

Both models use a combination of fleet size, fleet average emissions, vehicle speeds (EMFAC), and fleet average mileage activity to calculate a mass of emissions.

Fleet size and activity are linked with vehicle age; mean emissions generally involve age, initial emission certification levels, and emission level classifications. In both models, vehicle activity levels are most often uniformly applied across all of the vehicle emission level classifications, with the underlying assumption that all vehicles, including high emitters, have the same age-adjusted travel fractions.

Modern vehicle fleet emissions are dominated by a relatively small number of high emitting vehicles, resulting in a skewed emission distribution, which is best described by a gamma distribution. High emitters occur as a result of breakage, mal-maintenance (performance modifications, rechipping, and/or after-treatment removal), and at end of a vehicle’s useful life.

For example, recent on-road emission measurements in California and other locations have shown that the 99^th percentile is responsible for more than a third of the carbon monoxide (CO) and hydrocarbon (HC) emissions and more than 15% of the nitric oxide (NO) emissions on a fuel-specific basis, and these imbalances continue to grow each year. These percent contributions assume that the fuel consumption is proportional to the fleet fraction sampled. However, these percentages could increase or decrease if the vehicles represented in the 99^th percentile consume significantly less or more fuel than the fleet average, potentially adding an additional compounding skewed distribution to the calculation.

One segment of the on-road fleet that is believed to drive more miles and consume more fuel than the average fleet vehicle is livery (for-hire) vehicles, such as taxis and limousines. … Livery fleets have changed significantly in the past decade, with many city fleets now featuring large numbers of hybrid and alternatively fueled vehicles. There has been little recent research into the emission durability of these new fleets, and in this paper, we examine some recent and historical emission data from the Seattle and Chicago area to obtain a picture of how current livery vehicle emissions compare to the rest of the fleet and examine their importance to fleet emissions.

—Bishop et al.

In their study, Bishop et al. used data collected at three US sampling sites: (1) in Washington State on the taxi and shuttle loop at the Seattle−Tacoma International Airport; (2) a long-term light-duty site in California located on I-10 between Santa Monica and downtown Los Angeles; and (3) a long-term light-duty site in Illinois located in the northwest suburbs of Chicago. The researchers used the West LA data as a baseline for comparison with the high-mileage fleet measurements.

They used remote vehicle exhaust sensors to collect emissions measurements, and recorded a video image of the license plate of each vehicle. The transcribed plate was used to obtain non-personal vehicle information, including make, model year, vehicle identification number, and fuel type from the state registration records of California, Illinois, and Washington.

SeaTac Toyota Prius taxis mean grams of pollutant per kilogram of fuel emissions (●) by model year compared to Toyota Prius model emissions measured on-road at the WLA measurement site (△). Uncertainties are standard error of the means determined from the daily samples for the west LA data and by bootstrap resampling techniques for the SeaTac measurements. Credit: ACS, Bishop et al. Click to enlarge.

Among their findings:

• Hybrid taxis (Prius) at Sea-Tac showed large increases in carbon monoxide, hydrocarbon, and oxide of nitrogen emissions in their fourth year when compared to similar vehicles from the general population.

• In Chicago, the observed, on-road taxi fleet also had significantly higher emissions and an emissions share that was more than double their fleet representation.

Fleet emission inventory calculations generally rely on some type of age-weighted activity factor, usually model year, for generating the totals. High-mileage vehicles, such as taxis and limousines, are not generally considered to be numerous enough to change these weighting factors, because, for example, in King County, Washington, taxis only account for about 0.6% of the registered vehicles (2000/325000). If, however, as observed in Chicago, their emission contribution is about twice their fleet percentage and, in addition, they consume more fuel than the county average, then their emission contribution could grow to be 2−5% of the total.

In areas where these high-mileage fleets are in higher concentrations, such as airports and downtown business centers, their emission contribution could produce much larger inventory errors if not taken into account. As tailpipe emissions continue to decrease, emission distributions will become more skewed, likely increasing the importance of higher emission high-mileage fleets and their contribution to vehicle emission inventories.

—Bishop et al.

Resources

• Gary A. Bishop, Donald H. Stedman, Daniel A. Burgard, and Oscar Atkinson (2016) “High-Mileage Light-Duty Fleet Vehicle Emissions: Their Potentially Overlooked Importance” Environmental Science & Technology doi: 10.1021/acs.est.6b00717