ARB Research Seminar

This page updated March 25, 2014

Understanding the Volatility of Primary Organic Aerosol Emitted from Light-Duty Vehicles

Photo of Michael J. Kleeman, Ph.D.

Michael J. Kleeman, Ph.D., University of California, Davis

March 20, 2014
Cal EPA Headquarters, 1001 "I" Street, Sacramento, CA

Video: 1. 2.
Research Project


This presentation explores the volatility of primary organic aerosol (POA) emitted from light-duty gasoline-powered vehicles at atmospherically relevant concentrations. Measurements were made using state-of-the-science instrumentation while carefully controlling for exhaust dilution factor, temperature, humidity, and concentration of background particles in the dilution air. A 2-component model for aerosol volatility was used to interpret the results from the current study and re-interpret results from past measurements.

Realistic experimental conditions using a fleet of typical California gasoline-powered vehicles produced exhaust particle concentrations <5 μg/m3 that were 75-80% elemental carbon (EC) and 20-25% POA. Real-time measurements show that the highest EC emissions occurred during the cold-start portion of the test and/or during periods of hard acceleration. POA emissions could be classified into two basic categories: (i) semi-volatile material with a volatility distribution similar to motor oil and (ii) low-volatility material attributed to fuel combustion products. The POA emissions were generally more volatile during the cold start portion of the driving cycle and less volatile after the engine and exhaust system reached operating temperature. The prevalence of carbonyl species in the POA suggests that these species may be basic building blocks that are transformed into non-volatile POA as the vehicle exhaust ages. Carbonyl emissions increased with humidity suggesting an aqueous production pathway.

The 2-component model for aerosol volatility explains the experimental measurements made in the current study with R2>0.9. The 2-component model predicts that approximately half the vehicles in the test fleet had POA dominated by semi-volatile motor oil while the other half of the vehicles had POA dominated by non-volatile fuel combustion products. The same 2-component model applied to previous experimental measurements of "high emitting vehicles" finds that POA emissions from these vehicles are likely dominated by motor oil. Based on these results, we recommend that the 2-component volatility framework for POA should be incorporated into future regional modeling applications after sufficient measurements have been made to characterize the on-road fleet in California.

Speaker Biography

Michael J. Kleeman, Ph.D., is a Professor of Civil and Environmental Engineering at the University of California at Davis. Professor Kleeman's research is focused on measurements and modeling of urban and regional air pollution problems. Dr. Kleeman has published over 90 peer-reviewed research articles on topics ranging from ozone production from agricultural sources to climate effects on airborne particulate matter to characterization & source apportionment of atmospheric ultrafine particles.

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