Ethanol fuels may increase ground-level ozone production

Nature : Ozone forms when sunlight causes a reaction between nitrogen oxides and hydrocarbons, which are both byproducts of burning fossil fuels. Burning ethanol and gasoline–ethanol blends produces fewer pollutants than burning standard gasolines. However, a study of air quality in São Paulo, Brazil, suggests that the burning of ethanol fuels may actually result in higher ground-level ozone concentrations. Between 2009 and 2011, the consumption of pure gasoline by light vehicles increased from 42% to 68% due to the rising price of ethanol. Alberto Salvo of the National University of Singapore and Franz Geiger of Northwestern University in Evanston, Illinois, used air-quality monitors distributed throughout São Paulo to measure pollutant levels during that period. They found that the fuel-use shift coincided with an average decrease in ozone concentrations of 15 μgm^–3, down from the previous weekday average of 68 μgm^–3. However, there was a significant increase in nitrogen oxide levels. Nitrogen dioxide bonds with hydroxyl, one of the usual sources of ozone, to form nitric acid. When nitrogen oxide levels are high enough, so much hydroxyl is removed from the air that overall ozone production is reduced. Salvo and Geiger note that because all cities have unique air chemistries, the same test in another city might not see the same effect.