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 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. 
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