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Clean Air

Study reveals a drop in the concentration of one of the major air pollutants in the São Paulo Metropolitan Region released by the exhaust of alcohol-fueled cars

EDUARDO CESARSão Paulo’s residents are breathing air that is less polluted. A recent study by scientists at the Institute of Astronomy, Geophysics and Atmospheric Sciences, at the University of São Paulo (IAG/USP) revealed that, in the last 30 years, the concentration of acetaldehyde in the atmosphere of the São Paulo Metropolitan Region has decreased considerably. This pollutant, which is part of the aldehydes group, is released mainly through the exhaust of ethanol-fueled vehicles. In addition to irritating mucous membranes, eyes and airways and triggering asthma attacks, aldehydes are potentially carcinogenic. They also contribute to global warming. According to the findings of the research carried out at USP, the drop in the acetaldehyde concentration is mainly due to two factors: improvements in automotive engine technology and public policies implemented in Brazil in recent decades aimed at controlling vehicular pollution, notably the Motor Vehicle Emission Control Program (PROCONVE).

The study’s findings are somewhat surprising, because in recent years there has been a substantial increase in the number of ethanol-(or alcohol-) fueled vehicles. Launched in 2003, cars with flex technology, which can be fueled with gasoline, ethanol or a mixture of the two, already constitute most of the national fleet, with over 20 million in circulation. Gasoline sold at gas stations, for its part, is mixed with alcohol at refineries at a ratio of 75% gasoline and 25% alcohol. “Despite an increase in the number of vehicles using ethanol, a renewable biofuel made ​​from sugarcane, there has been no increase in the level of acetaldehyde in the air of Greater São Paulo. Quite the opposite. Our measurements, conducted between 2012 and 2013, showed an average acetaldehyde concentration of 5.4 parts per billion (ppb) in the atmosphere, while a study done in 1986 showed that this value was about three times higher, or 16 ppb,” says Thiago Nogueira, a chemist and IAG postdoctoral student who led the study. Parts per billion is a measure of concentration used to measure chemicals when the solutions are much diluted.

An automotive technology that is the key to curbing any increase in aldehydes is the catalytic converter. This piece of equipment is located near the car’s exhaust and is used to treat the gases generated by the combustion process before they are released into the environment. All current vehicles, in order to meet PROCONVE’s standards, leave the factory fitted with three-way catalytic converters, so-named because they help to reduce the amount of the three major air pollutants released by automobiles: carbon monoxide (CO), nitrogen oxides (NO2) and volatile organic compounds, a group that includes aldehydes.

“Without catalytic converters it would be impossible to reduce the presence of aldehydes or any other vehicular pollutant in the atmosphere,” says Henrique Pereira, a mechanical engineer and a member of the technical committee for engines of SAE Brazil (Society of Mobility Engineers). “Catalytic converters convert the harmful gases from burning fuel, including aldehydes, into compounds less detrimental to human health and the environment.” According to Pereira, although they are critical to improving air quality, catalytic converters would not be as efficient were it not for improved engines. “For the catalytic converter to achieve its best performance, the car’s engine needs to receive an ideal mixture of fuel and air. In this regard, electronic injection was a mandatory innovation for proper performance of the engines and, consequently, of the catalytic converters,” says Pereira. Electronic fuel injection, a component replacing the carburetors in cars built before the 1980s, prepares an ideal combination of fuel and air for the engine.

The combustion chamber, where explosions of the air-fuel mixture occur, was another structure improved by automakers to limit the amount of pollution emitted. “By improving the efficiency of burning this mixture, cars decrease their emissions of pollutants. This helped to reduce aldehyde emissions from ethanol-powered cars,” says Alfredo Silvio Castelli, a chemical engineer and director of the Brazilian Automotive Engineering Association (AEA).

The engine development took place largely because of program approvals and stricter environmental laws. Aiming to reduce and control air pollution from mobile sources, in 1986 the National Environment Council (Conama) launched PROCONVE, which established deadlines and emission ceilings on domestic and imported cars. “Before the regulation of emissions in Brazil with PROCONVE, automakers designed their models with two main factors in mind: engine performance and fuel consumption. Emissions were not a priority,” says Pereira. “With PROCONVE, emissions control has become a priority in designing new vehicles.”

Since the program’s introduction, the aldehyde limit has been dropping. During phase one, there was no standard for organic compound emissions made up of a combination of carbon, oxygen and hydrogen atoms. As of phase two of PROCONVE in 1992, cars were required to leave the plant emitting a maximum of 150 milligrams (mg) of the substance per kilometer. Five years later, in phase three, this value dropped to 30 mg and now in phase five, the limit is 20 mg per kilometer. “PROCONVE definitely helped to improve the air quality in cities. Today, vehicles manufactured in Brazil, known as light vehicles, a group that includes cars, utility vehicles and pickup trucks, are subject to the same emissions standard as that of the United States,” says Castelli. Cars running on ethanol used to emit a larger amount of acetaldehyde than the flex cars manufactured today,” he says. “The study is interesting because it shows the current atmospheric conditions in the São Paulo Metropolitan Region,” says Francisco Emilio Nigro, an engineer and professor at USP’s Polytechnic School (Poli/USP) and a technical adviser to the São Paulo State Department of Economic Development, Science and Technology.

Localized concentration
The study conducted at IAG-USP measured not only the concentration of acetaldehyde in the air of São Paulo, but also the concentration of formaldehyde—another type of aldehyde, emitted mainly by diesel-or gasoline-fueled vehicles—nitrogen oxides (NO and NO2 ) and ozone (O3). The samples were collected on the roof of the IAG building on the USP campus in the Butantan neighborhood, the area west of São Paulo, between June 2012 and May 2013. In comparing these samples to readings from previous studies, the concentration of formaldehyde increased from 5.4 ppb three decades ago to 8.6 ppb today.

The work also compiled measurements of the aldehyde concentration found in tunnels in the city of São Paulo conducted over the past 20 years. “The advantage of taking measurements at these sites is that they isolate the pollutants emitted by vehicles from those released by other sources such as industrial or fires, and also the pollutants formed by photochemical reactions in the atmosphere,” says Nogueira. Concentrations were compared in three main tunnels in São Paulo: Presidente Jânio Quadros, Maria Maluf, and the Ring Road. The data show a marked reduction in acetaldehyde, from 60 ppb in the Jânio Quadros tunnel in 2004 to 13.3 ppb in the same tunnel in 2011. As far as formaldehyde is concerned, it dropped from 50 ppb to 10.3 ppb.

The complete results of the research are described in an article published in the September 2014 online edition of the journal Fuel. The importance of studying the atmospheric concentration of volatile organic compounds such as formaldehyde and acetaldehyde, as well as nitrogen oxides is that they indirectly affect climate. Both are substances that, under ideal conditions of temperature and solar radiation, undergo photochemical reactions, giving rise to secondary pollutants such as ozone. “This pollution often exceeds the air quality standards of the São Paulo Metropolitan Region,” says Maria de Fátima Andrade, an IAG professor and coauthor of the study. “There is a relationship between the primary and secondary compounds involved in burning ethanol, and although they have reduced the primary pollution, they have not improved the secondary,” she says. The primary ones are aldehydes, carbon monoxide, nitrogen oxides, sulfur oxides and hydrocarbons, and ozone is one of the secondary compounds. According to Andrade, ozone formed in the troposphere (the layer closest to the surface of the earth) contributes to local warming, unlike the ozone found in the stratosphere (a higher level in the atmosphere), which absorbs solar radiation and prevents most ultraviolet rays from reaching earth.

“Despite the reduction in some of the ozone precursor compounds such as acetaldehyde, the concentration of this gas has not diminished in recent years in São Paulo. It is important to study why this is so. Could it be because of the difference between volatile organic compounds and nitrogen oxides? And what role does ethanol play in ozone production in São Paulo? These are questions that still need to be answered,” says Andrade. The study finding a decrease in the acetaldehyde concentration in São Paulo started in 2011and was done as a four-year thematic project, coordinated by Professor Andrade at IAG.

1. Narrowing the uncertainties on aerosol and climate changes in São Paulo State – nuances-Sps (nº 2008/58104-8); Grant mechanism Research Assistance – Research Program on Global Climate Change – Thematic Project; Principal investigator Maria de Fátima Andrade (USP); Investment R$2,083,587.98 and $1,314,236.24 (FAPESP).
2. Assessment of vehicular emissions of organic and inorganic compounds from the combustion of biofuels and their contributions to air quality in the São Paulo Metropolitan Region (nº 2011/18777-6); Grant mechanism Scholarship in Brazil – Regular – Postdoctoral; Principal investigator Maria de Fatima Andrade (USP); Grant recipient Thiago Nogueira (USP); Investment R$267,841.44 (FAPESP).

Scientific article
NOGUEIRA, T. O. et al. Formaldehyde and acetaldehyde measurements in urban atmosphere impacted by the use of ethanol biofuel: Metropolitan Area of São Paulo (MASP), 2012-2013. Fuel. v. 134, p. 505. 13. out 2014.