Morphological transformation of soot: investigation of microphysical processes during the condensation of sulfuric acid and limonene ozonolysis product vapors

摘要

The morphological transformation of soot particles via condensation oflow-volatility materials constitutes a dominant atmospheric process withserious implications for the optical and hygroscopic properties, as well asatmospheric lifetime of the soot. We consider the morphologicaltransformation of soot aggregates under the influence of condensation ofvapors of sulfuric acid, and/or limonene ozonolysis products. This influencewas systematically investigated using a Differential Mobility Analyzercoupled with an Aerosol Particle MassAnalyzer (DMA–APM) and the Tandem DMA techniques integrated with a laminarflow-tube system. We hypothesize that the morphology transformation of sootresults (in general) from a two-step process, i.e., (i) filling of void spacewithin the aggregate and (ii) growth of the particle diameter. Initially, thetransformation was dominated by the filling process followed by growth, whichled to the accumulation of sufficient material that exerted surface forces,which eventually facilitated further filling. The filling of void space wasconstrained by the initial morphology of the fresh soot as well as the natureand the amount of condensed material. This process continued in severalsequential steps until all void space within the soot aggregate was filled.And then growth of a spherical particle continued as long as vaporscondensed on it. We developed a framework for quantifying the microphysicaltransformation of soot upon the condensation of various materials. Thisframework used experimental data and the hypothesis of ideal spheregrowth and void filling to quantify the distribution of condensed materialsin the complementary filling and growth processes. Using this framework, wequantified the percentage of material consumed by these processes at eachstep 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 toestimate the fraction of internal voids and open voids. This information wasthen used to estimate the volume-equivalent diameter of the soot aggregatecontaining internal voids and to calculate the dynamic shape factor,accounting for internal voids. The dynamic shape factor estimated based onthe traditional assumption (of no internal voids) differed significantly fromthe value obtained in this study. Internal voids are accounted for in theexperimentally derived dynamic shape factor determined in the present study.In fact, the dynamic shape factor adjusted for internal voids was close to 1for the fresh soot particles considered in this study, indicating theparticles were largely spherical. The effective density was stronglycorrelated with the morphological transformation responses to the condensedmaterial on the soot particle, and the resultant effective density wasdetermined by the (i) nature of the condensed material and (ii) morphologyand size of the fresh soot. In this work we quantitatively tracked in situmicrophysical changes in soot morphology, providing details of both fresh andcoated soot particles at each step of the transformation. This framework canbe applied to model development with significant implications for quantifyingthe morphological transformation (from the viewpoint of hygroscopic andoptical properties) of soot in the atmosphere.