Burning sugarcane releases ozone-producing nitrogen
Local air pollution and difficult working conditions for cane cutters add to growing concerns about ethanol production in Brazil.
Each year, some 250,000 migrant workers flood into the state of São Paulo to cut sugarcane, most of them from poorer states in northeastern Brazil. Before chopping the cane with large machetes called facão, they burn huge swaths of cane fields to remove dry leaves and drive off snakes and other creatures. The cleared fields are easier to cut by hand, but the massive burns create choking clouds of smoke and ash.
New research in ES&T (DOI: 10.1021/es070384u) shows that burning cane fields also releases large amounts of nitrogen into the air, making air pollution worse in cane-growing regions. The nitrogen is in the form of ammonia and nitrogen oxides (NOx), which react to create ozone. During the harvest period from April to November, a layer of ash covers cars, and NOx levels double in the main sugarcane-growing state of São Paulo, says study leader Arnaldo Cardoso of São Paulo State University.
The equivalent of 35% of the nitrogen applied to cane as fertilizer goes up in smoke, Cardoso says. Some of this comes back down and recycles nitrogen into soil to fertilize plants, but much of it does not. "This may generate impacts such as acid rain, ozone, and changes in the quality of water in rivers and lakes," Cardoso adds. He estimates that annual emissions from the 2.2 million hectares burned in Brazil total about 55,000 metric tons of nitrogen in the form of NOx.
Cardoso notes, based on his group's previous work, that "because this season is also dry, this usually increases the particles in the air." The combination of particles and gaseous emissions can lead to harvest-time ozone levels in the agricultural regions that are "similar to [those in] a big city like São Paulo," he adds. A recent study by a different research group tied sugarcane burning to higher hospital admissions for asthma.
This kind of research is useful, says Mark Delucchi of the University of California Davis, "because it is important to characterize all of the environmental impacts of the bioethanol life cycle in Brazil." The country faces growing concerns about ethanol production, and the workers who harvest cane are believed to be the most vulnerable.
"The working conditions are intense," says Maria Aparecida de Moraes Silva, a sociologist at the State University of São Paulo who has studied labor practices in the cane fields for the past 30 years. Temperatures in the fields can reach 40 °C, and the backbreaking labor is done mainly by young men. "In the 1980s, they were responsible for 6–8 tons of harvest per worker per day," she says. "Now it's 12 tons of cane per day," with pay depending on the day's tonnage.
"My investigations show that the useful life of a worker is 15 years," de Moraes Silva adds. Medical studies have associated burning with high levels of cancer-causing hydrocarbons called PAHs in cane workers' urine as well as increased risk for respiratory cancers and other illnesses, she notes.
Ultimately, the country plans to phase out both burning and manual cane cutting. The São Paulo sugarcane industry union UNICA signed an agreement this year with the state government to end burning there by 2017, ahead of the 2031 target set by state law. That agreement is voluntary on the part of sugar mills, but UNICA says that at least 100 mills have already signed. Burning restrictions will be achieved mainly by mechanizing the harvest, which means burning will no longer be required.
