New Fuels = New Emissions Problems?
Researchers at UC Berkeley have produced an extensive body of knowledge about "tail-pipe" emissions generated from vehicles that burn conventional fossil fuels. As these fuels are replaced with alternatives in the hope of creating cleaner vehicles, new atmospheric effects are starting to be observed. Exactly what they will be, and their long-term consequences on air quality, require much work, researchers say.
Federal policies encouraging ethanol production have caused its use to grow rapidly in recent years (though its total share of the transportation fuel budget remains at two to three percent). But much remains to be understood about the emission consequences of widespread ethanol use, notes Robert Harley, Professor of Civil and Environmental Engineering, who has carried out extensive analysis of gasoline- and diesel-powered car and truck emissions.
"Ethanol evaporates more readily and causes some mixture issues when combined with gasoline-derived hydrocarbons," he says. "That is going to have effects that we need to study more."
The Caldecott "Lab"
Harley and his group have done an unusually detailed analysis of long-term trends of tailpipe emissions, by studying 10-plus years of data that they have collected from a unique local resource: the three-bore Caldecott Tunnel, which carries more than 2,000 vehicles per lane per hour at peak times on trips between the Bay Area and the inland counties to the east. While there have been major reductions in emissions from gasoline-powered vehicles, some diesel-based emissions are rising, Harley notes.
The tunnel is a useful "laboratory," Harley explains. "Probably the biggest strength is the large sample size and that it is reasonably random." Also, the air inside the tunnel is not affected by meteorological changes and emissions from other sources. And because samples are collected under similar conditions year in and year out, it is easy to compare them over time. (There are some drawbacks: it does not include heavy-equipment vehicle emissions, for example, or emissions produced during stop-and-go driving and cold starts.)
The Caldecott data are further enhanced by the fact that heavy-duty trucks are restricted from one bore, enabling researchers to observe how much they contribute to the overall emissions burden by comparing mixed use lanes with those that only carry cars and light-duty trucks.
However, in most settings, it is not possible to obtain such high-quality data. To make up for those shortcomings, air monitoring results are typically supplemented with models that extrapolate emissions levels from mileage and trip data. But in many places, notably the developing world, where many sustainability issues are most pressing, travel data are hard to come by.
Harley has developed an alternative, an "inventory" system, which estimates emissions based on actual amounts of fuel sold. Because a set amount of fuel produces a set amount of emissions, the amount of fuel sold can be translated into pollutants relatively easily.
"We're trying to make estimates in developing countries at the front lines of these sustainability questions, and this is a better approach," Harley notes.
"Another big concern in terms of sustainability is the secondary reactions in the atmosphere away from the tailpipe. These are very complex reactions," he said.
The puzzle of the "weekend effect"
The most common reactions are photolytic (sunlight-driven) changes in the chemical composition of the atmosphere. As reductions in certain emissions are achieved, there is a possibility that those reactions will be affected, and not always in a desirable way. That is the suspected explanation for the so-called "weekend effect" rise in ozone levels observed in some urban areas.
The explanation had come down to two possibilities: weekend trips being bunched closer together in the middle of the day versus the two separate rush hours on a work day, or something else.
Analyzing weigh-in-motion traffic counts and data from the Caldecott Tunnel, Harley and his group found evidence that the weekend effect was due to a reduction in truck traffic, which caused diesel emissions to fall, resulting in less NOx, which, though harmful, also can suppress ozone. However, Harley suggests that if NOx were reduced even more, through tighter controls, the problem could be eliminated. This is one of many questions requiring more study.
Harley is currently working on a research project funded by the University of California Transportation Center to study the effects of new control technologies on diesel engines, the next logical target for air emission reductions.
Diesel fuel consumption grew at three times the rate of gasoline consumption in the 1990s, he notes. NOx from gasoline engines dropped by a factor of about two in the same time period. By 2000, diesel produced half the on-road NOx emissions.
New requirements for diesel trucks go into effect in the U.S. with the 2007 model year. There are numerous implications: starting with how well the new technology will accomplish its goals, which is the focus of Harley's study. The research team is also creating models to project trends in photochemical reactions and other possible side effects 10 years into the future.
Selected links to transportation emissions research by Harley:
Ongoing Research Projects, includes Air Pollution Modeling, Motor Vehicle Emission Trends, and a Fuel-Based Approach to Estimating Emissions.
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Last updated: June 12, 2006