Burning ethanol in cars can worsen air pollution, adding to the green concerns surrounding biofuels
By Gerard Wynn
Burning biofuels in cars can add to one of the most dangerous forms of city pollution, according to a study in Brazil, published this week.
The study found that ozone levels fell in the city of Sao Paulo when drivers switched from burning ethanol to gasoline in response to higher ethanol prices. In Brazil’s largest city, nearly a half of light vehicles have flex-fuel capability meaning that they can switch between burning gasoline or ethanol or some combination of the two.
Ethanol is one of the most common forms of biofuel, produced by fermenting any biomass high in carbohydrates in a similar process to brewing beer.
Breathing ozone can cause inflammation in the deep lung, and decrease lung function, according to a large literature in a review reported in the American Journal of Preventive Medicine in 2008.
A systematic analysis for the Global Burden of Disease Study, published in the Lancet in 2012, found that ozone pollution was responsible for about 200,000 premature deaths annually.
“We show that ambient ozone concentrations fell by about 20% as the share of bi-fuel vehicles burning gasoline rose from 14 to 76%,” the authors of teh study said, publishing in the journal Nature Geosciences.
Ozone is a major air pollutant produced from chemical reactions between nitrogen oxides (NOX) and volatile organic compounds (VOCs) such as methane and carbon monoxide, from burning fossil fuels or biomass.
The chemistry involved is not simple, however, as ozone production is actually suppressed at higher NOX levels.
Burning gasoline is known to emit NOX, carbon monoxide and hydrocarbons. Burning ethanol emits oxygenated organic compounds, such as acetaldehyde and ethanol.
The authors of the Brazil study concluded that Sao Paulo already had high NOX levels, which meant that adding to these by burning gasoline suppressed ozone pollution.
The study authors stressed that the impact of fuel choice on ozone pollution depended on the atmospheric chemistry of any particular city.
In addition, burning gasoline produced other types of pollutants, including NOX and particulate matter.
Nevertheless, the study findings give another pause for thought regarding the green credentials of biofuels, adding to concerns about their impact on food prices and on tropical deforestation.
In cities where atmospheric NOX is in short supply, called NOX-limited, then the opposite outcome could result from switching to gasoline from ethanol.
One US expert, Sasha Madronich, wrote a comment article on the Brazil study, also published in the journal Nature Geoscience, stressing that the study findings did not represent a thumbs up for burning gasoline.
First, NOX emissions themselves were almost as noxious for human health as ozone, he said, and second, the production of particulate matter which causes heart and lung disease is stimulated by the kinds of heavier volatile organic compounds emitted from burning gasoline.
“The observed reduction in ozone levels should not be taken as evidence that a switch from ethanol to gasoline would improve air quality overall,” said Madronich.
“In São Paolo a switch from ethanol to gasoline use has lowered ground-level ozone concentrations. The net effect of such a switch on air pollution levels more generally remains unclear.”