Ozone is a known pollutant at low levels in the earth’s atmosphere, which causes harmful effects on the respiratory system and sensitive plants.    Ozone forms as hydrocarbons and nitrogen oxides emitted into the air react with sunlight. Two of the largest emitters of these pollutants are vehicles and electricity generating units (EGUs) but as Plug-in Hybrid Electric Vehicles (PHEVs) have risen in popularity over the past decade the positive impact on the environment due to lack of fossil fuel exhaust fumes is offset by increased ozone.

When PHEVs run off battery power they emit no pollutants from their exhaust but the EGU’s which provide electricity to charge batteries do give off pollutants.   New research in Environmental Research Letters shows that charging at night will lead to lower levels of pollution on average.   The work used four U.S. cities and four representative modeling days.

There has been a great deal of debate regarding the best way to charge electric cars. The first scenario in this study was based on charging the car at off-peak times in the night. The second scenario involved charging to maximize battery life (charging just before use and only the amount of charge needed to complete the trip) and the third scenario involved charging the battery when it was a convenient time for the driver (typically just after vehicle use).

The results of the study showed that the overall levels of pollution resulting from EGU emissions associated with charging were lower than the level of pollution resulting from the emissions associated with 20% of gasoline vehicle miles traveled (VMT).

Although night time charging was shown to yield the highest amount of nitrogen oxides, this led to the least amount of ozone on average across all cities and hours modeled as there is no sunlight for the emissions to react with. By the time morning comes, the pollutants are dispersed and diluted by other processes such as the wind.

Lead author Dr Tammy Thompson of MIT said, “The results in general show positive air quality results due to the use of PHEVs regardless of charging scenario with the nighttime charging scenario showing the best results on average by a small margin. 

“This further supports efforts to develop regulation to encourage nighttime charging; an example would be variable electricity pricing. As more of the fleet switches over to PHEVs and a larger demand is placed on the electricity grid, it will become more important that we design and implement policy that will encourage charging behaviors that are positive for both air quality and grid reliability.”

Citation: "Air quality impacts of PHEVs in Texas evaluating three charging scenarios", Environ. Res. Lett 6 024004