Plug-In Hybrids Curb Smog

The environmental benefit of switching from gasoline-only cars to plug-in hybrid electric vehicles (PHEVs) isn't only about slowing climate change. Pollutants in car exhaust also create harmful ozone pollution that can cause respiratory and cardiac health problems. To measure how PHEVs might help alleviate ozone issues in urban areas, researchers have modeled how Denver's air quality would change if the whole city adopted electric cars (Environ. Sci. Technol., DOI: 10.1021/es101076c). 

Emitted from vehicle exhaust and other sources, nitrogen oxides (NOx) and volatile organic compounds (VOC) react with sunlight to create ozone, a major cause of smog. The current federal standard for ground-level ozone is 75 ppb, and the Environmental Protection Agency plans to lower it to between 60 ppb and 70 ppb this year. For the past decade, ozone levels in the Denver metro area have regularly exceeded this limit, reaching as high as 90 ppb.

Electric-powered cars would seem like one obvious solution to the ozone problem. But the power to charge the vehicles comes from electricity generation plants, which also emit air pollutants.

Greg Brinkman of the National Renewable Energy Laboratory in Golden, Colo. and colleagues wanted to calculate how much NOx and VOC emissions changed overall, and at what times of day they changed, if Denver residents exchanged their 1.7 million gasoline-only cars for PHEVs. The scientists incorporated Denver's meteorological data for 2006 into a standard model that simulates air quality over time. They then generated several different scenarios by changing the number of PHEVs and the time of day that the vehicles charged up. To account for the new power demand, the researchers assumed that natural-gas-fired power plants would supply the electricity to recharge the PHEVs.

After crunching the numbers, the scientists found that a complete swap of gasoline cars for PHEVs would cut NOx and VOC emissions by as much as 3.5% and 24%, respectively. As a result, peak average ozone concentrations fell by about 2 ppb to 3 ppb, which could shrink the gap between Denver's highest ozone levels and federal limits by about 15% to 30%.

This study's findings are highly specific to Denver, Brinkman says. Still he thinks that the methodology could help guide detailed studies for other cities, while also pointing out that PHEVs not only curb greenhouse gas emissions, but also reduce ozone-related problems.

And highlighting PHEVs' benefits is critical to developing transportation policies that spur the market for greener cars, says Craig Stephan, a former Ford Motor Co. researcher now working with the Argonne National Lab in Illinois: "If you can show that PHEVs are going to improve air quality, that will certainly be a driver for policy that supports the adoption of PHEVs."