Reduction in local ozone levels in urban Sao Paulo due to a shift from ethanol to gasoline use

Ethanol-based vehicles are thought to generate less pollution than gasoline-based vehicles, because ethanol emissions contain lower concentrations of mono-nitrogen oxides than those from gasoline emissions. However, the predicted effect of various gasoline/ethanol blends on the concentration of atmospheric pollutants such as ozone varies between model and laboratory studies, including those that seek to simulate the same environmental conditions. Here, we report the consequences of a realworld shift in fuel use in the subtropical megacity of Sao Paulo, Brazil, brought on by large-scale fluctuations in the price of ethanol relative to gasoline between 2009 and 2011. We use highly spatially and temporally resolved observations of road traffic levels, meteorology and pollutant concentrations, together with a consumer demand model, to show that ambient ozone concentrations fell by about 20% as the share of bi-fuel vehicles burning gasoline rose from 14 to 76%. In contrast, nitric oxide and carbon monoxide concentrations increased. We caution that although gasoline use seems to lower ozone levels in the Sao Paulo metropolitan area relative to ethanol use, strategies to reduce ozone pollution require knowledge of the local chemistry and consideration of other pollutants, particularly fine particles.