Sources, Composition, Variability and Toxicological
Characteristics of Ultrafine Particles in Southern California
PROJECT SUMMARY
PROBLEM: Current regulatory efforts are focused on reduction of ambient levels of particulate
mass for PM10 and PM2.5, but recent studies have demonstrated that ultrafine (UFP) particles
(tentatively defined as smaller than ~100-200 nm in diameter) are more toxic. Studies that show
individual particles penetrating cellular membranes and causing cell damage, suggest that particle
number rather than particle mass may be a more health relevant metric of ambient particles. The
majority of ambient particle numbers are UFP particles, but, due to their small size, only a small
fraction of ambient PM mass is UF. Unlike PM2.5, for which there is an abundance of continuous
and time-integrated mass (and in several areas chemical speciation) data generated by local and
state agencies, a nationwide network of UFP monitors is not currently in place, and may or may
not become available in the near future. As a result, significantly less is known about the diurnal,
spatial and seasonal characteristics of UFP particles as well as chemical composition in urban and
rural areas of the US. Since these PM characteristics may be important in evaluating the health
effects of UFP relative to coarse PM or PM2.5, it is desirable to have intensive UFP measurements
that will provide insights into the source, composition and health effects of these particles.
PREVIOUS WORK: UFP measurements in the proximity of freeways have been one of the focal
points of the research undertaken by the Southern California Particle Center. Results from these
studies have been extensively cited in the scientific literature and used in legislation at the state
and federal level. More recently, a CARB project granted to our group at USC focuses on the
assessment of the spatial variability of ambient particle number concentrations within two
impacted communities (source and receptor areas of Long Beach and Riverside, respectively).
None of these studies, however, provides information on the larger scale distribution and
characteristics of UFP in the Los Angeles Basin, which is the focus of the proposed project.
OBJECTIVE: The objective of this proposal is to provide the much needed and currently
unavailable or very limited information on the relationships between UFP sources, spatial and
seasonal characteristics, and toxicity in Southern California. Towards this goal, a multidisciplinary
research team will investigate the following research questions:
a. What are the spatial, daily and seasonal differences in UFP number, mass and chemical
composition found in rural and in urban areas of the Los Angeles Basin?
b. What is the seasonal and spatial distribution of UFP toxicity as measured by chemical and
cellular redox activity assays?
c. How are the chemical characteristics of UFP associated with at the above environments
associated with the measured toxicity of UFP?
d. How does UFP PM toxicity differ from that of fine and coarse PM, measured in studies
undertaken by the PIs, and sponsored separately by the recently renewed SCPC?
e. How do the physical, chemical and toxicological characteristics of UFP in the urban and
rural sites of this study compare to those near light- and heavy-duty freeways, airports and
seaports?
f. What are the major primary and secondary sources that of UFP in each site and what is the
seasonal variability? What are the associations of these source contributions with the redox potency of
these particles?