Road dust: Rural vehicles emit more pollutants than urban ones

January 18, 2000

On top of looming tougher pollution rules for SUVs and other popular vehicles, an environmental engineer at Washington University in St. Louis now has devised a new measurement that could make both car owners and manufacturers nervous. It's called ppvm (pollutant per vehicle mile). It is a measurement of the total particulate matter emissions a vehicle makes per mile traveled. While ppvm won't appeal to the auto owner the way that mpg, rpm or mph do, it's likely to have a future impact on air pollution measures and standards nationwide.

Jay R. Turner, D.Sc., assistant professor of chemical engineering and civil engineering at Washington University and director of the university's Air Quality Laboratory, has performed an ambitious study of vehicular emissions in the St. Louis region. Emission measurements were taken for particles smaller than 2.5 micrometers in diameter, which is the size range for a standard issued by the U.S. Environmental Protection Agency (EPA) in 1997. Results from an urban interstate site and a nearby rural Illinois site that Turner surveyed indicate that an average urban vehicle, whether a motorcycle or diesel truck, emits between 30 to 40 milligrams of particulate matter per mile traveled; an average rural vehicle emits between 200 to 300 milligrams ppvm traveled. So much for fresh country air.

"We think there is much more heavy diesel traffic outside the city and there are greater road dust emissions in rural areas because of the proximity to open land, and those account for higher rural readings," Turner explains. Road dust is more than the simple dirt a vehicle stirs up as it moves along the road. Besides dirt from soil, road dust also contains the suspended fine particulate matter created from tail pipe emissions. It is a major significant component of vehicular air pollution that the medical profession and the EPA are paying strict attention to these days. Upper respiratory illnesses, cardiovascular diseases, such as arrhythmia, and cancer increasingly are being linked to road dust and other vehicular particulate matter. The acidity of the matter, its heavy metal composition and the sheer volume of minuscule particles suspended in the air all make your automobile particulate matter air pollution a potential public health threat.

Turner and his students used particulate matter samplers that pull air through filters; they measured the mass of the filters before and after the sampling. They concentrated on Interstate 40, which runs through the heart of St. Louis, and Interstate 55 in Madison County, Illinois, approximately 30 miles northeast of St. Louis. Periodically, from January to April 1998, the team painstakingly counted and classified vehicles at both sites over six- to eight-hour time spans, taking five-minute readings every 15 minutes while they collected the particulate matter samples. They determined that the rural site averaged about 1,300 vehicles per hour; the urban varied widely from 7,500 to 10,000 vehicles per hour.

"The numbers indicate that a single vehicle stirs up a considerable amount of particulate matter, more so than what people might think," says Turner. "When you then consider that you can multiply this daily value by the many thousands of miles vehicles travel on the roads each day, you get a clearer view of what role the automobile potentially plays in air pollution."

Turner's results were published in a fall 1999 issue of the Journal of Air and Waste Management Association. The U.S. EPA funded part of the study. The data he has collected plus his analysis of U.S. EPA mathematical models that predict air particulate matter emission rates will help environmental agencies and municipalities better sample their roads and address their particulate matter air quality challenges.

It's more than your car's tail pipe that contributes to vehicular air pollution.

"Emissions come from the tail pipe, from brake wear, tire wear and suspended dust from along the roads," says Turner. "Our research tends to focus on diffuse emissions, those that are difficult to characterize because they don¹t come from a single point, say, the tail pipe. What we essentially do is look at the net sum of emissions coming from vehicles and compare them to the EPA model that estimates the individual components and we add them up to see if the real world data conform to what the models predict."

Faulty program

Turner found a discrepancy in the EPA model, or its application, regarding road dust. The model uses a default value - a theoretical generic number - to account for the amount of road dust particulate matter on roadways because it is not always possible to measure the actual amount of suspendable dirt on the road. These measurements are done by sweeping. But to get an absolutely perfect measurement of roadside dust, the study area has to be swept clean with high-performance vacuums. This is next to impossible to do with interstate highways because of the large traffic flow.

"We consistently found that the EPA models with generic input parameters over-predicted the emissions we found in the field, and that the one input the models are most sensitive to is the amount of suspendable road dust on the roadway," Turner says. "This forces a re-thinking of how to use the EPA models or how to make them better."

The EPA issued a new standard for particulate matter emissions in 1997, and many areas of the country will likely violate those standards. Thus, these communities will need to do particulate matter inventories to set goals for reducing emissions.

"We are telling state and local agencies that they have to find a way to get locally generated data for certain critical inputs to the models, much as we did, and if they have to rely on default values in the models, they may well be subject to large uncertainties in their estimates," he says. "If you construct a faulty program, you won't have the best solution."

Washington University in St. Louis

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