EPA should improve methods for estimating pollutants in vehicle emissions

provided by National Research Council of the National Academies

he US Environmental Protection Agency (EPA) should take stronger steps to revise the computer model it uses to estimate the amount of volatile organic compounds, nitrogen oxides, and other harmful pollutants emitted from motor vehicles, says a new report by the National Research Council of the National Academies. Moreover, EPA should begin using additional data sources and tools as soon as possible to more accurately predict vehicle emissions and the pollutants they produce.

"EPA uses these estimates to develop regulations and programs for protecting air quality," said Armistead Russell, chair of the committee that wrote the report and professor of environmental engineering, Georgia Institute of Technology, Atlanta. "These estimates need to be as accurate as possible so that pollution control programs, which cost many millions of dollars, are effective in protecting the environment and public health. EPA should develop and draw on a broader array of tools and data to improve these important predictions."

The model, known as Mobile Source Emissions Factor Model or MOBILE, was originally developed in 1978 to estimate the amounts of emissions from motor vehicles. Since then, MOBILE has become a central tool used by environmental and transportation agencies to assess national, state, and local programs aimed at controlling vehicle emissions. However, studies indicate that the model is underestimating vehicle emissions of volatile organic compounds - which form ozone that is damaging to public health and the environment and that estimates of nitrogen oxides and fine particles in vehicle emissions are inaccurate.

The current version of the model was developed using test data from several categories of vehicles, such as passenger cars and minivans, and incorporates factors that affect emissions, including average speeds, fuel characteristics, and vehicle ages. However, only a selected number of vehicles are tested, and those likely to have higher emissions, such as older vehicles with malfunctioning exhaust systems, are not adequately represented in the model. In addition, other factors that affect emissions, such as how the vehicle is being driven and maintained, are not fully accounted for in the model.

EPA should conduct studies immediately to identify and reduce disparities between the model's estimates and actual vehicle emissions, the committee said. Evaluations should incorporate outdoor air measurements, random roadside testing, as the state of California is doing, and direct tailpipe measurements from on-road vehicles equipped with emissions-monitoring devices. Other approaches, such as monitoring tailpipe emissions at inspection stations and remote sensing - a method used to determine pollution levels in exhaust while vehicles are being driven also should be used for evaluating the model.

The committee noted that EPA has made many technical improvements in preparing a new version of MOBILE, and commended the agency for addressing a number of past criticisms, including the need for better documentation and opening up the process to stakeholders and the public. However, upgrades to MOBILE should go beyond those already planned by EPA for completion this year, and should be used in conjunction with other models in a "tool kit" that could serve a broad range of purposes. Each modeling component could be tailored for specific applications. These tools also must be peer-reviewed and evaluated for accuracy.

EPA should work more closely with the US Department of Transportation and state agencies to complete within one year a long-range plan that examines new approaches to emissions modeling, the committee said. Thus far, EPA has not worked adequately with other agencies to improve planning and coordination for future model development. For example, the planning process should examine data that will be needed for monitoring and controlling fine particulate matter, which has recently fallen under national and state regulation.

The committee noted that its findings should not be a basis for eliminating MOBILE. In addition, it recommended the following specific actions to improve the MOBILE model:


  • Incorporate new data into MOBILE more quickly. New information about emissions- control measures - such as the effectiveness of vehicle inspection and maintenance programs and of oxygenated fuels as been available for several years, but still is not reflected in the model.
  • Improve estimates of fine particles in exhaust. The modeling tool that estimates particulate matter emissions should be substantially upgraded and evaluated against the results of field studies. Such a capability is particularly important for new national and state regulations concerning particulate matter.
  • Provide more-accurate estimates of diesel truck emissions. Data indicate that emissions from these vehicles are underestimated in the current model. Estimates should be improved quickly because states are developing pollution-control plans based on MOBILE predictions, the committee said. The model should be refined to include a broad range of engine technologies and draw on data that indicate when and where trucks are most likely to be on the road.
  • Review predictions on the effectiveness of emissions-control inspection programs and equipment. Many states require inspections to detect when a vehicle's emissions-control systems are malfunctioning. The MOBILE model assumes that these programs help reduce emissions, but existing data do not support the level of reduction estimated by the model. Moreover, the model predicts that new technologies, such as indicator lights to signal equipment failures, will significantly reduce vehicle emissions. However, very little information is available to assess whether motorists actually repair their vehicles in response to these prompts.
  • Determine the number of high-emitting vehicles on the road, their emission rates, and travel patterns. These vehicles are thought to represent a substantial fraction of on-road emissions. In addition, vehicles that leak gas, or have fuel systems that allow large amounts of gas to evaporate, should be identified.

In addition, the committee also called for monitoring of emissions from off-road sources such as farm equipment, motorized lawn and garden tools, and recreational vehicles. EPA should develop a plan for collecting and analyzing those data and improve its capabilities in this area within one year of upgrading the MOBILE model, the committee said.

The study was funded by EPA and the US Department of Transportation. The National Research Council is the principal operating agency of the National Academy of Sciences and the National Academy of Engineering. It is a private, nonprofit institution that provides science advice under a congressional charter.