Project at a Glance

Title: Internal combustion engine (ICE) air toxic emissions.

Principal Investigator / Author(s): Durbin, Norbeck, Cocker, Zhu

Contractor: UC Riverside

Contract Number: 02-334A

Research Program Area: Emissions Monitoring & Control

Topic Areas: Mobile Sources & Fuels, Monitoring, Toxic Air Contaminants


The emission rates of gas-phase airborne toxic compounds, as well as particulate matter (PM), have steadily been reduced during the past decade as a result of the introduction of reformulated gasoline and diesel fuel, advances in engine design and fuel metering systems, and the implementation of highly efficient exhaust aftertreatment control devices. As regulatory standards have gotten increasingly stringent, it is important to understand how air toxic emission rates have changed over the years in order to gauge the expected improvement in air quality. In this work, the available literature for studies dealing with the air toxic, PM, and PAH emissions from on-road, off-road, and stationary diesel, gasoline, and natural gas internal combustion engines (ICEs) is reviewed and evaluated. This review focuses on toxic emission studies using typical dynamometer source testing methods, and near-source ambient sampling (e.g., tunnel studies) of ICE emissions.

Gas-phase toxics (formaldehyde, acetaldehyde, acrolein, benzene, and 1,3- butaidene), PAHs, and PM are formed by the incomplete oxidation of hydrocarbons during combustion and are associated with adverse health effects. Formaldehyde, acetaldehyde, acrolein, and 1,3-butaidene all react rapidly with free radicals in the air. The ambient concentrations of benzene, 1,3-butadiene, and acetaldehyde are considerably below the EPA threshold. The ambient concentrations of formaldehyde are at the EPA threshold. Acrolein is the only compound that has ambient concentrations above the EPA threshold.

Both gas-phase toxics and PM emissions from mobile sources have been investigated in a number of dynamometer studies dating back to the 1970s and early 1980s. Data reported here showed large reductions in the airborne toxic and PM emissions from gasoline- and diesel-powered vehicles over the years as a result of improved emissions control technology and the introduction of cleaner fuels. These trends are observed in chassis dynamometer, engine dynamometer, and tunnel studies. It is expected that the emissions of these compounds will continue to be reduced further in the future with the introduction and proliferation of even more advanced technology vehicles.

Of all the engine and vehicle technologies, the catalytic converter provides the greatest emission reductions. For gas-phase toxics, the reductions were about 50 to 80% for oxidation catalysts and 80 to 99%+ for three-way catalyst (TWC) vehicles compared to non-catalyst (NC) vehicles, with conversion efficiencies for today’s modern vehicle reducing toxics by > 98%. For diesel vehicles, a decrease of 69 to 85% in gas-phase air toxic emissions was observed for diesel vehicles equipped with oxidation catalysts (OCs) compared to uncontrolled diesel vehicles. Gas-phase toxics and PM emissions also showed a strong dependence driving cycles.

For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

Stay involved, sign up with CARB's Research Email Distribution List