"It's not green in terms of air pollution," said study author Mark Jacobson, a Stanford University civil and environmental engineering professor. "If you want to use ethanol, fine, but don't do it based on health grounds. It's no better than gasoline, apparently slightly worse."
The science behind why ethanol might increase smog is complicated, but Jacobson said part of the explanation is that ethanol produces more hydrocarbons than gasoline. Ozone is the product of hydrocarbons and nitrogen oxide cooking in the sun.
Also, ethanol produces longer-lasting chemicals that eventually turn into hydrocarbons that can travel farther. "You are really spreading out pollution over a larger area," he said.
And finally, while ethanol produces less nitrogen oxide, that can actually be a negative in some very smoggy places. When an area such as Los Angeles reaches a certain high level of nitrogen oxide, that excess chemical begins eating up spare ozone, Jacobson said.