Plug-in hybrid electric vehicles (PHEVs) can make a significant contribution towards reducing ozone levels. That's according to a study by US researchers who modelled the effect of different charging scenarios for PHEVs on ozone levels over the state of Texas, US.

Many regions of Texas fail to meet national air quality standards for ozone. Tammy Thompson and her colleagues from the University of Texas wanted to find out what effect the introduction of PHEVs would have on the levels of this pollutant.

A previous study had looked at night-time charging of PHEVs using electricity generated by coal-fired power stations. Although the researchers found that the overall ozone concentration would decrease by up to 8 ppb in a few highly populated areas, 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.

"We wanted to expand on this and look at different charging scenarios, and also model the Texas power grid to take into account the different sources of electricity," Thompson told environmentalresearchweb.
The researchers examined the impact of replacing 20% of gasoline-powered vehicle miles travelled (VMT) with electric VMT by the year 2018 in four major Texas cities. This involved first modelling the Texas power grid and the different electricity generation units that will be online in 2018 – including nuclear, coal, natural gas, biomass and wind energy plants.

Using this data, the researchers then modelled the different levels of ozone precursors, such as nitrogen oxides (NOx), that would be emitted by these power stations when the PHEVs are being charged. They used three different charging scenarios – night-time charging; charging to maximize battery life; and charging to maximize driver convenience. "Using this data to then model ozone levels is not straightforward because ozone is formed via a photochemical reaction when NOx, carbon monoxide and volatile organic compounds (VOCs) react in the atmosphere in the presence of sunlight," said Thompson. "For our modelling, we selected four days in 2018. These days were selected to represent four varying wind patterns, each blowing the emissions from power plants into or away from the cities being modelled."

The calculations showed that day-time ozone levels were reduced in most cases and were mainly due to the fact that there were fewer gasoline-powered cars on the roads. The different charging scenarios all produced similar amounts of ozone, with the night-time charging scenario deemed the most environmentally friendly.

"Ozone levels drop during the night because sunlight drives the production of ozone," said Thompson. "But Texas has more installed wind power than any other state and that wind blows strongly at night, which opens up the prospects for emissions-free charging of some portion of electric vehicles."

However, Texas experiences many episodes of high ozone concentrations and Thompson warns that "it is going to take a lot more than just a few PHEVs to significantly change the ozone levels over Texas for the better".

The researchers published their research in Environmental Research Letters (ERL).