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This is Scientific American’s 60-Second Science. I’m Karen Hopkin. This will just take a minute.

Some things are best done after dark--setting off fireworks, telling scary stories and charging your plug-in hybrid¹ electric vehicle, because a new study shows that topping off your car battery at night generates less ozone². The work appears in the journal Environmental Research Letters.

Plug-in electric hybrids³ give off less exhaust than gas guzzlers. And when they’re running on battery power, they generate no fumes⁴ at all. But there’s a catch. The power plants that make the electricity you use to recharge the battery produce plumes⁵ of pollution. But how much ozone gets produced depends on when you reach for that plug.

Electric power plants that burn fossil fuels release hydrocarbons⁶ and nitrogen oxides. When sunlight hits those compounds, ozone forms. At night, however, well, it’s dark. And the compounds disperse⁷ in large part before the sun comes up.

Scientists modeled how much ozone would be generated if the cars were charged at three different times: overnight, just before they’re driven or just after. And they found that drawing electricity from the grid⁸ after-hours is best. So to drive green, plug in, lay down and get some ozone-free Zs.

Thanks for the minute. For Scientific American’s 60-Second Science, I’m Karen Hopkin.