Date of Award
Master of Science (MS)
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.
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.
Chukwuto, Humphrey Chigozie, "Investigating Cloud Condensation Nuclei Activity And Particle Growth Of Aging Diesel Exhaust Particles" (2017). Theses and Dissertations. 2187.