  The mass absorption cross-section (MAC) of combustion-generated soot is used
in pollution and emissions measurements to quantify the mass concentration of
soot and in atmospheric modelling to predict the radiative effects of soot on
climate. Previous work has suggested that the MAC of soot particles may change
with their size, due to (1) internal scattering among monomers in the soot
aggregate, (2) the correlation of soot primary-particle diameter with aggregate
size, (3) quantum confinement effects, or (4) a size-dependent degree of soot
graphitization. Here, we report a size-dependent MAC for ex-situ soot sampled
from two commercially available diffusion-flame soot generators, one aviation
turbine engine, and one diesel generator. We also incorporate literature data.
We show that the MAC increases with aggregate size until a plateau is reached
at single particle masses between 4 and 30 fg (approximately 300-650 nm soot
mobility diameter). The smallest particles may have MACs 50% to 80% smaller
than the largest, depending on the source, while the largest particles have
MACs within the range reported by previous measurements on polydisperse
samples. Moreover, we show that models of hypotheses (1), (2), and (3) do not
describe our measurement results, leaving hypothesis (4) as the only remaining
candidate.
