Stricter Emissions Rules Cut Total Car Pollution but Shift Its Chemical Mix, Study Finds

Researchers from the Chinese Research Academy of Environmental Sciences, the Chongqing Academy of Ecology and Environmental Sciences, and Peking University found that while stricter Chinese vehicle standards have sharply cut total emissions of volatile organic compounds (VOCs) from gasoline cars, the chemical makeup of what remains is shifting toward oxygenated VOCs (OVOCs), a group that includes many reactive or toxic chemicals. The findings were published in the journal Environmental Science and Ecotechnology.

The team tested seven gasoline direct-injection vehicles, including six conventional internal combustion vehicles and one non-plug-in hybrid electric vehicle (HEV), under cold- and hot-start conditions using the Worldwide harmonized Light vehicles Test Cycle. During low-speed cold starts, total VOC emissions from conventional vehicles fell 75% between the older China IV standard and the newer China VI standard, from 287.8 to 71.8 milligrams per kilometer. But across the cold-start cycle, OVOCs rose from 20%-22% of VOC emissions to 35% of the total.

The hybrid vehicle showed a further trade-off: during low-speed hot starts, it released 25.4 milligrams per kilometer of OVOCs, nearly twice the 13.3 milligrams per kilometer from a comparable conventional vehicle. Under low-speed cold-start conditions, the hybrid's estimated health-risk hazard index was 69% higher and its estimated cancer risk was almost double that of the conventional vehicle, which the researchers linked to the hybrid's repeated engine restarts, incomplete combustion, lower exhaust temperatures and reduced efficiency of its catalytic converter at those points.

The researchers stressed that findings from a single hybrid vehicle cannot be applied to an entire fleet, and said the results should not be taken to mean hybrid vehicles are inherently more polluting. They said larger studies covering more vehicle models are needed to determine whether the same patterns hold more broadly, and that regulations based only on total VOC mass could miss such shifts in reactive or toxic compounds.