Morphological transformation of soot: investigation of microphysical processes during the condensation of sulfuric acid and limonene ozonolysis product vapors
Authors: Pei Xiangyu , Hallquist Mattias , Eriksson Axel C. , Pagels Joakim , Donahue Neil M. , Mentel Thomas , Svenningsson Birgitta , Brune William , Pathak Ravi Kant 
Published: July 2018

Abstract. The morphological transformation of soot particles via condensation of
low-volatility materials constitutes a dominant atmospheric process with
serious implications for the optical and hygroscopic properties, as well as
atmospheric lifetime of the soot. We consider the morphological
transformation of soot aggregates under the influence of condensation of
vapors of sulfuric acid, and/or limonene ozonolysis products. This influence
was systematically investigated using a Differential Mobility Analyzer
coupled with an Aerosol Particle Mass
Analyzer (DMA–APM) and the Tandem DMA techniques integrated with a laminar
flow-tube system. We hypothesize that the morphology transformation of soot
results (in general) from a two-step process, i.e., (i) filling of void space
within the aggregate and (ii) growth of the particle diameter. Initially, the
transformation was dominated by the filling process followed by growth, which
led to the accumulation of sufficient material that exerted surface forces,
which eventually facilitated further filling. The filling of void space was
constrained by the initial morphology of the fresh soot as well as the nature
and the amount of condensed material. This process continued in several
sequential steps until all void space within the soot aggregate was filled.
And then “growth” of a spherical particle continued as long as vapors
condensed on it. We developed a framework for quantifying the microphysical
transformation of soot upon the condensation of various materials. This
framework used experimental data and the hypothesis of “ideal sphere
growth” and void filling to quantify the distribution of condensed materials
in the complementary filling and growth processes. Using this framework, we
quantified the percentage of material consumed by these processes at each
step of the transformation. For the largest coating experiments, 6, 10, 24,
and 58 % of condensed material went to filling process, while 94, 90, 76,
and 42 % of condensed material went to growth process for 75, 100, 150,
and 200 nm soot particles, respectively. We also used the framework to
estimate the fraction of internal voids and open voids. This information was
then used to estimate the volume-equivalent diameter of the soot aggregate
containing internal voids and to calculate the dynamic shape factor,
accounting for internal voids. The dynamic shape factor estimated based on
the traditional assumption (of no internal voids) differed significantly from
the value obtained in this study. Internal voids are accounted for in the
experimentally derived dynamic shape factor determined in the present study.
In fact, the dynamic shape factor adjusted for internal voids was close to 1
for the fresh soot particles considered in this study, indicating the
particles were largely spherical. The effective density was strongly
correlated with the morphological transformation responses to the condensed
material on the soot particle, and the resultant effective density was
determined by the (i) nature of the condensed material and (ii) morphology
and size of the fresh soot. In this work we quantitatively tracked in situ
microphysical changes in soot morphology, providing details of both fresh and
coated soot particles at each step of the transformation. This framework can
be applied to model development with significant implications for quantifying
the morphological transformation (from the viewpoint of hygroscopic and
optical properties) of soot in the atmosphere.