Ozone levels are relatively high in São Paulo, with hourly concentrations above 75 and 125 µg m⁻³, respectively, being 2.7 and 5.3 times more likely than for PM₁₀ in our sample. Light transportation is a key contributor to air pollution in this gridlocked metropolis, with large public health implications. In 2011, 40% of the city’s six million active light-duty vehicles—probably accounting for over one-half of all light-vehicle distance travelled—possessed bi-fuel capability. This capability allowed consumers to choose between gasoline (an E25 or E20 blend) and ethanol E100 at the pump, as both fuels were ubiquitous among São Paulo’s retailers.

In recent years, government-controlled gasoline prices held steady whereas market-set sugarcane ethanol prices tracked the significant swings in the world price of sugar. Large fluctuations in the relative price of ethanol between 2009 and 2011 led to large-scale switching out of ethanol and into gasoline as ethanol prices soared, and back to ethanol when prices dropped, as evidenced by aggregate shipments reported by wholesalers for the state of São Paulo, as well as revealed-choice surveys of consumers. For perspective, wholesaler reports suggest that the unblended (pure) gasoline component shifted between 42% and 68% of total gasoline-plus-ethanol light-vehicle distance travelled.

This empirical setting provides a rarely observed opportunity to examine whether urban air pollution was impacted by emissions that transitioned between gasoline and ethanol—both combustion and evaporation. São Paulo city currently features clogged roads but limited industrial activity and residential heating. Electricity generation is mostly hydroelectric. The shifts in the fuel mix occurred over relatively short time windows during which meteorological conditions and vehicle usage, including ridership of public transport, were broadly similar. These fuel mix shifts were a response to exogenously varying relative prices, and to a temporary change in the gasoline blend mandate, not to concerns over air quality; furthermore, evidence established herein indicates that the relative price variation did not significantly impact road traffic. Such characteristics, together with the existence of extended air quality, weather and vehicle traffic monitoring networks, make São Paulo a unique natural laboratory for studying the impact of gasoline versus ethanol fuel combustion on urban air pollution.

—Salvo and Geiger

Salvo and Geiger used fuel sales data and 14,000 consumer surveys to predict the magnitude of the fuel shift from ethanol to gasoline and back over the course of 30 months. They next determined the change in pollutant concentrations based on the predicted fuel mix, meteorology data and traffic levels across 600 miles of roads and discovered that ambient ozone concentrations fell by about 20% as the share of bi-fuel vehicles burning gasoline rose from 14 to 76%.

They cautioned that although gasoline use seems to lower ozone levels in the São 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.

Scientific studies using “big data” can help take policymakers a long way in figuring out how to mitigate air pollution given local conditions, the researchers said. The empirical analysis of atmospheric pollutants, traffic congestion, consumer choice of fuel and meteorological conditions can provide an important tool for studying other large cities, such as Chicago, New York, London and Beijing. Previous studies mainly have consisted of computer simulations of atmospheric chemical reactions based on tailpipe emissions studies.

Resources

• Alberto Salvo & Franz M. Geiger (2014) “Reduction in local ozone levels in urban São Paulo due to a shift from ethanol to gasoline use,” Nature Geoscience doi: 10.1038/ngeo2144

• Sasha Madronich (2014) “Atmospheric chemistry: Ethanol and ozone,” Nature Geoscience doi: 10.1038/ngeo2168