Driving in São Paulo

The arguments over the environmental credentials of biofuels continue to rage. Just recently, two environmental engineers at the University of Illinois at Urbana-Champaign, US, reported in a paper in Earth’s Future that growing corn as food is better for the environment than converting it into fuel.

They came to this conclusion after developing a new approach for studying the benefits and impacts of agriculture in the US, which calculates an overall monetary value for the social, economic and environmental effects. This revealed that growing corn for food generates a social and economic worth of $1492 per hectare, whereas converting it to biofuel leads to a loss of $10 per hectare.

Following on, a paper in Nature Communications now hits back with new evidence that burnishes biofuels’ environmental credentials. In it, a team of economists, physicists and chemists report that greater use of bioethanol is associated with lower emissions of ultrafine airborne particles.

In controlled experiments, scientists had already shown that ethanol emits lower amounts of ultrafine particles when burnt than gasoline, but no one had investigated whether this was replicated in the real world. Alberto Salvo at the National University of Singapore, who led the team, realized that the Brazilian city of São Paulo might offer an ideal location for conducting just such a real world study.

One reason for this is that São Paulo conducts regular monitoring of airborne particles, producing a readily-available dataset that stretches back several years, although many other cities conduct similar monitoring. More crucially, São Paulo is home to a large number of flex-fuel cars that can run on pure ethanol or gasoline, or any blend of the two.

What is more, the price of gasoline is controlled by the state and is thus fairly stable, whereas the price of ethanol depends on the price of sugarcane, as the major ethanol feedstock in Brazil, which tends to vary. As a consequence, drivers in São Paulo can switch between ethanol and gasoline depending on which is more expensive. If ethanol does emit lower airborne particles in a real world setting, then the levels of airborne particles in São Paulo should increase when gasoline is cheaper and decrease when ethanol is cheaper, as drivers switch between the two fuels. So Salvo and his team decided to see whether they could spot such a relationship.

Unfortunately, they didn’t have actual figures for consumption of gasoline and ethanol, but they did have figures showing how the cost of ethanol and gasoline had fluctuated over a period of several years. Using a model based on actual consumer behavior, they could calculate how these changing prices should have affected the consumption of gasoline and ethanol.

They decided to focus on a period from January to May 2011, when the price of ethanol rose to become more expensive than gasoline and then fell back down to become less expensive. Their model confirmed that this change in prices would have led to an increase in the consumption of gasoline at the expense of ethanol and then a decrease. Comparing this with the records of airborne particles, they found that variations in the concentration of ultrafine particles, meaning less than 50nm in size, matched exactly the predicted variations in gasoline and ethanol consumption. The concentrations rose by a third as ethanol consumption fell and then fell as consumption subsequently rose.

The researchers didn’t see a similar close relationship with the concentrations of larger airborne particles. This makes sense, because passenger flex-fuel vehicles are the main source of ultrafine particles, whereas larger particles come from other sources, such as heavy vehicles that mainly use diesel rather than gasoline or ethanol.

In addition to burnishing biofuels’ environmental credentials, however, this study also demonstrates the full complexity of the issue. Because in a 2014 paper in Nature Geoscience the same researchers used the same environmental monitoring data from São Paulo to show that bioethanol was associated with higher emissions of ozone.

The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.