Date of Award

January 2017

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Chemical Engineering

First Advisor

Frank Bowman

Abstract

Soot particles are an essential atmospheric constituent because of their effects on human

health, regional and global climate. The impact of soot on climate and human health depends on

their ability to interact with water vapor. Freshly emitted soot particles are hydrophobic but the

chemical and physical transformations (aging) they undergo while airborne can make them hydrophilic

enough to retain water and act as nuclei for cloud formation. The aging of soot is a

complicated process and represents a significant uncertainty in quantifying the net radiative forcing

of soot particles. In this study, the cloud condensation nuclei (CCN) activity and the secondary

organic aerosol formation of diesel exhaust aged in the presence of one or more of the following:

UV-radiation, α-pinene, and ammonium sulfate seed particles were investigated. Four

sets of experiments (diesel exhaust, diesel exhaust + α-pinene, diesel exhaust + ammonium sulfate

seed particle, and diesel exhaust + α-pinene + ammonium sulfate seed experiments) were

conducted. The changes in size distribution, number concentration and CCN activity of the aged

diesel exhaust particles were monitored with Scanning Mobility Particle Sizer (SMPS) and

Cloud Condensation Nuclei Counter (CCNc).

Results from dark experiments with diesel exhaust alone reveal no significant secondary

organic aerosol (SOA) formation, nor CCN activation for fresh and aged diesel exhaust particles,

as indicated by decreases in particle mass with time (mass decreased by a factor of 0.3), near

zero kappa (κCCN ∼ 0) and activation ratio (< 1%) values at all times, and large critical diameters.

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Aging of diesel exhaust particles in the presence of UV- radiation in the diesel exhaust experiments

resulted in a significant increase in mass (increased by factor of 1.0-1.4), kappa value

(ranging from 0.03-0.08), activation ratio (ranging from 0.1-0.75), and a substantial decrease in

critical diameter after 200 minutes of aging.

The addition of 39 ppbv of α-pinene (diesel exhaust + α-pinene experiments) further enhanced

CCN activation and SOA formation of diesel exhaust particles exposed to UV radiation.

ΚCCN values (0.08-0.15) and particle mass (increased by a factor 1.4 – 1.7) increased further by

88% and 21%, respectively, when compared to the diesel exhaust experiments. Also, the addition

of α-pinene reduced the particle diameter required for activation.

Experiments with a mixture of diesel exhaust and ammonium sulfate seed particles under

UV and dark conditions investigated the effect on ammonium sulfate seed particles on SOA formation

and CCN activation. Over the course of an experiment, particle mass in dark experiments

decreased by a factor of 0.9-0.8, while particle mass in the UV experiments increased by a factor

of 1.1-1.2. The result shows that SOA formation was higher for diesel exhaust experiments than

when a mixture of diesel exhaust and ammonium seed particle were exposed to UV radiation.

ΚCCN of the aged particles, which seems to be correlated with the ammonium sulfate fraction and

the % increase in particle mass, was found to range from 0.02-0.3 for UV experiments.

The addition of 39 ppbv of α-pinene to the mixture of diesel exhaust and ammonium sulfate

had a significant effect on particle mass (increased by 60%) but no discernible effect on κCCN

when compared to diesel exhaust +ammonium sulfate experiments.

The results from this study underscore the importance of exposure to sunlight (UV), mixing

with water-soluble particles, such as ammonium sulfate, and SOA precursors such as gas phase diesel exhaust and α-pinene, in transforming hydrophobic soot into more CCN-active particles.

The increase in hygroscopicity and CCN activity of aged diesel particles is expected to reduce

their atmospheric lifetime.

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