Stricter vehicle emissions standards are dramatically reducing the overall amount of volatile organic compounds (VOCs) released by petrol cars, but new research suggests that the pollution left behind may be more chemically harmful than previously thought.
A study published in Environmental Science and Ecotechnology found that as vehicle standards became tighter, oxygenated volatile organic compounds (OVOCs), a group of chemicals that contribute to air pollution and pose health risks, accounted for a growing share of exhaust emissions.
Researchers from the Chinese Research Academy of Environmental Sciences, the Chongqing Academy of Ecology and Environmental Sciences and Peking University tested seven petrol vehicles, including six conventional cars and one non-plug-in hybrid. The vehicles were tested on a dynamometer under different starting and driving conditions, with researchers analysing individual chemicals in the exhaust.
The results showed that total VOC emissions from conventional vehicles during low-speed cold starts fell by about 75%, from 287.8 milligrams per kilometre (mg/km) for China IV vehicles to 71.8 mg/km for China VI models. However, OVOCs increased from around 20–22% of total VOCs to about 35%.
The hybrid vehicle produced particularly notable results under certain conditions. During low-speed hot starts, it emitted 25.4 mg/km, compared with 13.3 mg/km from a comparable conventional vehicle. During low-speed cold starts, its estimated non-cancer health-risk indicator was 69% higher, while its estimated cancer risk was nearly twice as high.
Researchers believe frequent engine starting and stopping in hybrids may contribute to the problem. When a petrol engine restarts, the exhaust system and catalytic converter can be relatively cool, making it harder to remove some pollutants efficiently. Low-speed driving can make these conditions more pronounced.
Chemicals including acrolein, benzene, toluene and xylene were among those identified as important contributors to pollution or potential health risks.
The researchers stressed that the findings do not mean hybrid vehicles are inherently more polluting than conventional cars. Only one hybrid was tested, so the results cannot be used to represent the entire hybrid fleet.
Instead, the study highlights a limitation of measuring vehicle pollution solely by its total mass. As overall emissions decline, the researchers argue, regulators may need to pay greater attention to the specific chemicals being released and their ability to form ozone, secondary particles or cause health effects.
Future regulations could therefore combine total-emission limits with controls on particularly reactive or toxic compounds. The researchers also suggest improvements such as faster catalyst heating and hybrid powertrain strategies designed to reduce incomplete combustion during repeated engine restarts.
The findings point to a broader challenge for cleaner transport: reducing the amount of pollution from vehicles remains essential, but understanding exactly what is left in the exhaust may become just as important.
The full research can be read here.
Photo: Christopher John
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