Cars and trucks, particularly heavy-duty diesel vehicles, are thought to be the main vehicular pollution sources. This needs rethinking, a new study shows, as two-stroke scooters, ridden by hundreds of millions across southern Europe, Asia, Africa and the Middle East (Figure 1), are the leading source of some of the most harmful traffic pollutants.
Two-stroke scooters emit 100s to 1000s times more light aromatics and carbonaceous particles than any other type of light-duty and heavy-duty vehicles. These particles, whether directly emitted from the exhaust or formed in the atmosphere via oxidation reactions, are notorious for their adverse respiratory and cardiovascular health outcomes, increasing the mortality and morbidity risks. The reasons for these elevated emission factors are that (1) two-stroke engines require addition of lubricant oils to the fuel, (2) some of the fuel/air mixture passes straight through the engine without burning, and (3) these vehicles generally use a low air/fuel ratio improving drivability while producing higher carbon monoxide, light aromatics and primary carbonaceous particle emissions.
Figure 1: Fraction of fuel consumed by two-stroke scooters in several locations worldwide.
Among the aromatics, benzene is of particular concern due to its carcinogenicity. The researchers found levels in the raw two-stoke scooter exhaust as high as 300,000 mg m3 or 146 ppm. The EU annual mean limit for the protection of human health is 5 mg m3, while the US National Institute for Occupational Safety and Health recommends that workers wear special breathing equipment when exposed to benzene at levels exceeding 1 ppm for 15 min. Waiting in traffic behind an idling two stroke scooter may therefore be highly deleterious to health, the authors conclude.
Despite the difficulty of making a precise estimation of a relative contribution to vehicular aromatics and particle emissions from two-stroke scooters, the researchers suggested that these vehicles fall into the super-polluting category, especially as a considerable number of scooters are in operation in some regions without any form of emissions control. They estimate for example that in some Asian cities such as Bangkok, two-stroke scooters could contribute on average ~70% (up to 97%) of all roadside primary particles even though they account for only ~10% of fuel consumption.
There are even predictions that scooters will emit more volatile organic compounds than all other vehicles in Europe combined by 2020. Yet, the European regulations for scooters are older and less stringent than for other types of vehicle, dating back to 2002 rather than 2013 for cars, the study notes. Given the alternative technologies available (four-stroke and electric scooters), restrictions on two-stroke scooters, already implemented in China since the late 1990s, could be a cost-effective way to improve air quality in many cities around the globe.
Dr. Imad El Haddad is research scientist at the laboratory of atmospheric chemistry, energy department, Paul Scherrer Institute, Villigen, Switzerland. http://www.psi.ch/