Cleaner Cars May Cut Emissions but Increase the Share of Reactive Pollutants

Lower overall vehicle emissions may mask important changes in exhaust chemistry, including shifts toward potentially more reactive or toxic compounds, according to a study by the Chinese Society for Environmental Sciences (CSES), SciTechDaily reported.

Stricter vehicle emissions standards have reduced overall pollution, but researchers say the chemical composition of remaining exhaust also deserves attention.

Stricter vehicle emissions standards are reducing the overall volume of volatile organic compounds (VOCs) released by vehicles.

However, as total emissions decline, the chemical composition of the remaining exhaust is also changing.

An analysis of individual compounds found that oxygenated volatile organic compounds (OVOCs), which include numerous reactive compounds, accounted for a larger share of emissions under stricter standards.

Under certain low-speed testing conditions, one non-plug-in hybrid electric vehicle (HEV) also emitted substantially more OVOCs than a comparable conventional vehicle and produced higher estimated indicators of health risk.

The findings highlight a potential complication for vehicle regulation.

Reductions in total emissions and improvements in fuel economy do not necessarily produce equally large reductions in atmospheric reactivity or toxicity.

As a result, the specific chemical composition of vehicle exhaust may become increasingly important.

Vehicle emissions standards typically regulate total hydrocarbons or broader categories of pollutants, an approach that has substantially reduced tailpipe emissions.

Individual VOCs, however, vary significantly in their ability to contribute to ozone formation, secondary organic aerosol (SOA) production and potential health risks.