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Colloquium

Department of Atmospheric Science

Tues., Nov. 28, 3:10 pm, EN6085

Atmospheric formation of secondary organic fine particles from combustion sources: Tractors, Trees, and Traffic

Dr. Shantanu Jathar

Colorado State University

Abstract

Fine particulate matter, roughly half of which is composed of organic particles, offsets half of the warming associated with greenhouse gases and that from incomplete combustion has been linked to more than seven million deaths globally each year. Combustion sources such as motor vehicles, wildfires, and cookstoves emit gas-phase organic compounds that photochemically react in the atmosphere to form secondary organic particles. Despite the abundance, there are large uncertainties surrounding the sources, formation pathways, and environmental impacts of organic particles. In this talk, I will discuss findings from three laboratory/numerical studies where we have examined the atmospheric formation of organic fine particles from a selection of combustion sources: a representative off-road diesel engine, wildfires in the Western United States, and mobile sources in southern California.

In the first study, we investigated the photochemical formation of secondary organic particles from an off-road diesel engine under varying engine load, fuel, and emissions control combinations using a flow reactor. We found that the secondary organic particle formation was a factor of 10 higher than direct emissions of primary particles at idle-like loads, regardless of the presence of an aftertreatment system. Furthermore, the majority of the secondary particles formed could be linked to emissions and oxidation of high molecular weight organic compounds that are typically not included in atmospheric models. In the second study, we performed experiments at the Fire Sciences Laboratory in Missoula, MT to determine the day- and night-time formation of secondary organic particles from six different types of wildfires using an environmental chamber. We found some formation of secondary organic particles (average secondary-to-primary ratio of 0.7) with modest changes in particle composition (average 30% increase in oxygen-to-carbon ratio of organic particles), although the observations varied substantially from fire to fire. Finally, in the third study, we simulated the atmospheric evolution of organic particles from mobile sources in Southern California using a regional chemical transport model. Mobile sources and the aromatic precursors in their tailpipe emissions were the single most important source of organic particles in the urban areas of Southern California and gasoline-powered sources contributed thirteen times more organic particle mass than diesel-powered sources.

In summary, atmospheric processing of gas-phase combustion emissions leading to the formation of secondary organic particles is an important contributor to the atmospheric aerosol burden and studying the formation, evolution, and fate of secondary organic particles is essential to understanding the impact of aerosols on climate and human health. 


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