Filterable Redox Cycling Activity: A Comparison between
Diesel Exhaust Particles and Secondary Organic Aerosol Constituents
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Abstract
The
redox activity of diesel exhaust particles (DEP) collected
from a light-duty diesel passenger car engine was examined using the
dithiothreitol (DTT) assay. DEP was highly redox-active, causing DTT
to decay at a rate of 23–61 pmol min<sup>–1</sup> μg<sup>–1</sup> of particle used in the assay, which was an order
of magnitude higher than ambient coarse and fine particulate matter
(PM) collected from downtown Toronto. Only 2–11% of the redox
activity was in the water-soluble portion, while the remainder occurred
at the black carbon surface. This is in contrast to redox-active secondary
organic aerosol constituents, in which upward of 90% of the activity
occurs in the water-soluble fraction. The redox activity of DEP is
not extractable by moderately polar (methanol) and nonpolar (dichloromethane)
organic solvents, and is hypothesized to arise from redox-active moieties
contiguous with the black carbon portion of the particles. These measurements
illustrate that “Filterable Redox Cycling Activity”
may therefore be useful to distinguish black carbon-based oxidative
capacity from water-soluble organic-based activity. The difference
in chemical environment leading to redox activity highlights the need
to further examine the relationship between activity in the DTT assay
and toxicology measurements across particles of different origins
and composition