Replacing just 20% of ordinary vehicles with plug-in hybrid electric vehicles (PHEVs) could help reduce levels of atmospheric ozone in certain cities. But the result, from researchers at the University of Texas, US, also shows that the overall impact of PHEV use on air quality is more varied and complicated than previously thought.

Ozone is a secondary pollutant formed by the reactions between volatile organic compounds (VOCs) and nitrogen oxides (NOx) from vehicle emissions in the presence of sunlight. Despite more than 30 years of emissions reductions, some of the more densely populated regions of the US still fail to reach the National Ambient Air Quality Standards (NAAQS) for ozone. The US Environmental Protection Agency recently proposed lowering the maximum 8 hour concentration of ozone from 84 to 75 ppb, so meeting the NAAQS will continue to be a challenge.

Using a regional photochemical model, Michael Webber and colleagues studied how the air quality in the area served by the Pennsylvania–New Jersey–Maryland electric grid would change if 20% of gasoline-powered light duty vehicles, such as cars and trucks, were replaced with PHEVs. In the scenario, the PHEVs would only be used during the day and would be recharged at night using electricity from the grid.

Although the researchers found a decrease in NOx, VOCs and CO emissions from vehicles during the day, they did see an increase in these emissions from electricity-generating units at night. However, the overall ozone concentration decreased by up to 8 ppb in a few highly populated areas. This figure might be further improved if cleaner sources such as plants powered by nuclear or wind energy were used to recharge the PHEVs at night, or if more than 20% of vehicles were replaced with PHEVs.

But the work also showed that ozone concentrations, and those of other pollutants, might actually increase in certain highly localized areas (like those close to the power plant, for example), leading to an overall worsening of air quality. This suggests that the overall impact of PHEVs will be complex.

Webber and colleagues obtained their results using atmospheric chemistry models that take prevailing meteorological conditions into account. Since ozone is formed by complicated photochemistry, it requires light plus a particular mix of emissions from power plant smokestacks and vehicle tailpipes.

"For our calculations, we changed the emissions to reduce them from distributed tailpipes during the day – for vehicles that were replaced with PHEVs – and raised them at night time from power plants," Webber told environmentalresearchweb. "We also incorporated assumptions for PHEV market penetration rate based on available off-peak power."

He added that policymakers in cities wishing to improve their air quality might consider replacing some vehicles with PHEVs.

The team will now continue its research in other regions of the US that have different fuel mixes and different meteorological conditions. Texas is of particular interest because it has air quality problems in dense, urban areas in the eastern half of the state and excess emissions-free, night time wind power in the western half, explained Webber.

"Energy choices are about trade-offs," he added. "PHEVs are not perfect but they might contribute to significant air quality benefits that must be more fully considered."

The work was reported in Environmental Research Letters.