Relationship between Coating-Induced Soot Aggregate Restructuring

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Relationship between Coating-Induced Soot Aggregate Restructuring and Primary Particle Number Kaiser Leung, Elijah G Schnitzler, Ramin Dastanpour, Steven Rogak, Wolfgang Jaeger, and Jason S Olfert Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01140 • Publication Date (Web): 29 Jun 2017 Downloaded from http://pubs.acs.org on July 1, 2017

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Environmental Science & Technology

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Relationship between Coating-Induced Soot

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Aggregate Restructuring and Primary Particle

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Number

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Kaiser K. Leung,† Elijah G. Schnitzler,‡,¶ Ramin Dastanpour,§ Steven N. Rogak, § Wolfgang

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Jäger,‡ and Jason S. Olfert*,†

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†

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2G8

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‡

Department of Chemistry, University of Alberta, Edmonton, AB, Canada T6G 2G2

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§

Department of Mechanical Engineering, University of British Columbia, Vancouver, BC,

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Department of Mechanical Engineering, University of Alberta, Edmonton, AB, Canada T6G

Canada V6T 1Z4

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AUTHOR INFORMATION

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Corresponding Author

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*Phone: (780) 492-2341; fax: (780) 492-2200; e-mail: [email protected].

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ACS Paragon Plus Environment

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ABSTRACT

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The restructuring of mono-disperse soot aggregates due to coatings of secondary organic aerosol

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(SOA) was investigated in a series of photo-oxidation chamber experiments. Soot aggregates

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were generated by one of three sources (an ethylene premixed burner, a methane inverted

20

diffusion burner, or a diesel generator), treated by denuding, size-selected by a differential

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mobility analyzer, and injected into a smog chamber, where they were exposed to the photo-

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oxidation products of p-xylene, which partitioned to form SOA coatings. The evolution of

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aggregates from their initial to final morphologies was investigated in situ by mobility and mass

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measurements and ex situ by transmission electron microscopy. At a given initial aggregate

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mobility diameter, diesel aggregates are less dense and composed of smaller primary particles

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than those generated by the two burners, and they restructure to a smaller final mobility

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diameter. Remarkably, the final degrees of restructuring of aggregates from all three sources

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exhibit the same linear dependence on the number of primary particles per aggregate. The

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observed linear relationship, valid for the atmospherically-relevant SOA coating investigated

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here, could allow modelers to predict the evolution of aggregate morphology based on a single

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property of the aggregates.

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TOC GRAPHICS

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Introduction

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Soot aggregates in the atmosphere have a significant warming effect on Earth’s climate,

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second only to that of carbon dioxide,1 due to strong absorption of all visible wavelengths of

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light by their constituent primary particles of elemental carbon.2 Soot aggregates form at their

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sources, which include fossil fuel combustion3,4 and biomass burning,5,6 by collisions between

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primary particles, giving the soot a branched or fractal-like structure.7 Depending on the source

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and the combustion conditions, the primary particles may vary from 5 to 60 nm in diameter,8,9

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and aggregates may comprise tens to hundreds of primary particles.7

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The structural and optical properties of soot aggregates can be affected by internally-

44

mixed non-absorbing species. For example, restructuring of soot aggregates from initially

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branched structures to comparably compact structures can be induced by coatings of many

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species, including water,10–12 heptane,13 ethanol,13 dioctyl sebecate,14 oleic acid,14,15 glutaric

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acid,16 and sulfuric acid.10,17 The evolution of the optical properties of soot aggregates as a result

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of restructuring and/or lensing has recently been investigated, both experimentally18 and

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theoretically.19 For a bare soot aggregate of a given mass, a compact structure results in a greater

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extinction cross section than a branched structure; however, this is due to increased scattering,

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not absorption, so compaction may not necessarily lead to an increased warming effect on

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climate.18 On the other hand, the presence of a coating on the aggregate may result in increased

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absorption even for a compact structure due to lensing, and this enhancement may lead to an

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increased warming effect.20 Thorough knowledge of the morphological evolution of soot induced

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by coatings is necessary to further our understanding of the complex climate effects of soot.

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In the atmosphere, soot aggregates are exposed to many gas-phase species that may

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condense and contribute to coatings. Accordingly, it is important to investigate the


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morphological evolution of aggregates upon coating with mixtures, in addition to neat species.

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Secondary organic aerosol (SOA), which forms from the photo-oxidation of biogenic and

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anthropogenic volatile organic compounds,21,22 is a mixture of numerous oxygenated organic

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species. Soot aggregate restructuring has been induced by SOA coatings generated from

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isoprene,23 α-pinene,24,25 benzene,26 toluene,26,27 ethylbenzene,26 and m- and p-xylene.26,28 Very

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recently, the morphological and optical properties of burner-generated soot aggregates exposed

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to ambient air in Beijing and Houston were investigated.29 Since fossil fuel combustion is a

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significant source of ambient soot aggregates, it is also important to investigate restructuring of

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engine-generated aggregates, in addition to aggregates generated by typical laboratory burners.

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The evolution of soot aggregates produced by a diesel engine upon photo-oxidation of gas

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emissions has been investigated in the past.30 More specifically, previous studies have

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demonstrated how the properties of the coatings affect restructuring; for example, it was found

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that the extent of restructuring increases with increasing coating mass14,26 and/or surface

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tension.31 However, the question of how the properties of the aggregates themselves, including

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primary particle size and number, dictate restructuring has not been the focus of any previous

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studies.

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Here, we investigate the effects of the soot aggregate properties by systematically

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comparing the coating-induced restructuring of engine- and burner-generated aggregates. We

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describe a series of smog chamber experiments in which SOA coatings derived from p-xylene, a

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representative anthropogenic precursor,32 were used to restructure mono-disperse soot aggregates

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from three sources: a McKenna premixed burner, an inverted diffusion burner, and a commercial

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diesel generator. Adopting a technique introduced by Pagels et al.,10 we characterize the

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morphological evolution of both the coated particles and their soot cores in terms of effective


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densities, dynamic shape factors, and mass-mobility relationships, derived from masses and

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mobilities measured using a centrifugal particle mass analyzer (CPMA) and differential mobility

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analyzer (DMA), respectively. The initial and final morphologies of the aggregates were also

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characterized using transmission electron microscopy (TEM). The primary particle sizes and

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numbers were determined using an indirect method, based on in situ CPMA and the DMA

86

measurements, and a direct method, based on ex situ transmission electron microscopy (TEM)

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samples. The dependence of restructuring on aggregate properties is explored in detail, and the

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atmospheric implications are discussed. Our study provides key insights for modeling the

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evolution of the morphological and, in turn, optical properties of soot aggregates.

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Experimental Section

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Soot generation and treatment. Soot was produced using a McKenna premixed burner

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(Holthuis & Associates), a custom-built inverted diffusion burner, and a commercial diesel

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generator (Yanmar, YDG3700). The respective experimental setups for the treatment and

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injection of soot are shown in Figure 1. The treatment setup for the McKenna burner experiments

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is shown in Figure 1a, and it has been described in detail elsewhere.26 Briefly, ethylene was used

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as fuel, and the equivalence ratio was set to two, achieved with ethylene and air flow rates of

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1.1 and 8.0 L min-1, respectively. Soot was sampled 27 cm above the flame into an ejector dilutor

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(Air-Vac, AVR038H), set to a dilution ratio of three, using 30 psi of nitrogen. The soot particles

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were then treated by passing them through a 25 cm diffusion drier consisting of tubular mesh

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surrounded by anhydrous calcium sulfate, a thermo-denuder set to 300 °C, and a counter-flow

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parallel-plate membrane denuder described elsewhere.26 A representative size distribution is

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shown in Figure S1.


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The treatment setup for the inverted burner experiments is shown in Figure 1b. The

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inverted burner is a successor to the one used by Ghazi et al.33 Methane was used as fuel, and the

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global equivalence ratio was set to 0.76, achieved with methane and air flow rates of 1.3 and

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16.0 L min-1, respectively. In addition to the combustion air, clean filtered air was used for

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dilution, at a flow rate of approximately 40 L min-1. The soot was sampled downstream of the

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dilution phase and directed to the treatment train, which is similar to that used for the McKenna

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premixed burner, with one addition. In control experiments performed with the above treatment,

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OH exposure (without injection of p-xylene) induced restructuring of inverted burner aggregates

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but had no effect on McKenna burner aggregates. This discrepancy indicates that an appreciable

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fraction of trace gas emissions of the inverted burner penetrated the treatment train, and were

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oxidized in the chamber to form SOA. Since our objective is to compare restructuring of soot

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from different sources, it is critical that the SOA coatings are derived solely from p-xylene, so

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the background SOA formation is unsatisfactory. Consequently, we introduced a 91-cm tube

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consisting of tubular mesh surrounded by activated carbon between the thermo- and membrane

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denuders. In subsequent control experiments, OH exposure alone did not cause restructuring of

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inverted burner aggregates, so this treatment is sufficient.

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The treatment setup for the diesel generator experiments is illustrated in Figure 1c. A

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series of resistors was used to run the engine at set loads. As shown in Figure S2, an

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approximately 100% load (3.5 kW) resulted in particles with the lowest volatile mass fraction

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(13%). Flexible metal tubing was fastened to the exhaust of the engine, and a sample port was

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drilled 23 cm downstream. The soot particles were then passed through the same treatment train

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as was used for the inverted burner, including the activated carbon denuder.


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After treatment, the soot aggregates were neutralized using an X-ray source (TSI, 3087)

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and classified by mobility diameter using a DMA (TSI, 3081), which was operated with sample

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and sheath flow rates of 1.0 and 10.0 L min-1, respectively. The size-selected, mono-disperse

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aggregates were then injected into the smog chamber, which has been described in detail in the

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past.34,35 The 1.8 m3 cubic chamber is constructed of perfluoroalkoxy film (Ingeniven) and

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equipped with three sets of eight 32 W black-lights. Clean air is provided by a pure air generator

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(Aadco, 737), and the contents of the chamber are circulated by a mixing fan. Ultraviolet

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differential optical absorption spectroscopy was used to measure concentrations of p-xylene by

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passing broadband emission from a deuterium lamp (Ocean Optics, D-2000-S) through the

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chamber to a spectrometer (Ocean Optics, HR-2000+).

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Precursor photo-oxidation and soot restructuring. When soot injection was

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completed, the soot in the chamber was directed, alternately with and without thermo-denuding,

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into parallel systems to measure the mean mobility diameter and mass of the particles. A DMA

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and condensation particle counter (CPC; TSI, 3776) were used to measure the mean mobility

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diameter, by stepping the DMA through a range of diameters and measuring the particle

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concentration at each step with by the CPC. The mean particle mass was measured by stepping a

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CPMA, which classifies particles by their mass-to-charge ratio, through a range of particle

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masses and measuring the particle concentration with another CPC (TSI, 3771). When the DMA

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voltage was set to select, for example, singly-charged 150 nm aggregates from the diesel

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generator, McKenna burner, and inverted burner, about 5, 22, and 38%, respectively, of the

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classified particles were estimated to be larger doubly-charged aggregates. The transmission of

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multiply-charged particles reduces the ability to resolve the mass of singly charged particles in

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the CPMA; as shown by Radney et al., the mass can be better resolved by using a second


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neutralizer (TSI, 3077) upstream of the CPMA.36 Soot was monitored continuously for

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approximately one hour after injection to confirm that there was no background change in

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mobility diameter or mass. Then p-xylene (Fisher, 99.9%) was injected into the chamber to give

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a concentration of approximately 1 ppm. This concentration, which is too high to be

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atmospherically representative, is required to probe a wide range of coating masses; also, it has

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been shown that different initial precursor concentrations lead to the same extent of restructuring

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at a given coating mass,27 so the effects reported here, particularly at low coating masses, are

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likely relevant to atmospheric conditions. Precursor concentrations higher than 1 ppm have been

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used, for example, in studies of the optical properties of SOA.37 Hydrogen peroxide (Sigma, 30%

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w/w in water) was then injected into the chamber to yield hydroxyl radicals by photolysis with

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UV radiation,38 which was applied continually throughout the experiment.

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Restructuring, or the collapse of the soot particles, is described in terms of mass-mobility

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exponent, effective density, and shape factor. The mass-mobility exponent, , describes how

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the particle mass, , scales with mobility diameter, , according to

=

163 164

(1)

where is the mass-mobility prefactor.4 For an irregular particle, the mass-mobility exponent is

less than three. The effective density, , of a particle is the mass of the particle divided by its

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mobility-equivalent volume, (6 )/( ). In the following discussion, the effective densities

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of coated and coated-denuded particles are denoted as and , respectively. The dynamic

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shape factor, , is calculated as [ ( )]/[ ( )], where is the Cunningham slip

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correction factor for a certain diameter, and ve is the volume equivalent diameter, which is the

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hypothetical diameter of a spherical particle with the same volume. Assuming there are no


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internal voids (i.e. empty regions within the particle that are isolated from the bulk gas phase39),

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ve is

6 =

)

! − # + '(

$%&

(2)

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where and $%& are the material densities of soot and SOA, respectively, and and

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are the masses of the initial soot aggregates (before the injection of p-xylene) and the coated

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particles (after photo-oxidation), respectively. Again, the superscript denotes that neither type of

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particles was thermo-denuded after sampling from the chamber. The difference −

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gives the mass of the SOA coating. The material density of soot was taken as 1.8 g cm-3,40 and

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the density of the p-xylene-derived SOA was taken as 1.46 g cm-3.41 For spherical particles,

178 179

will be equivalent to , and the shape factor will be unity. The shape factors of coated and coated-denuded particles are denoted as and respectively.

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Restructuring was also probed using TEM. To this end, mobility-classified particles were

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collected on TEM grids using a thermophoretic particle sampler (TPS). Images were produced

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by a Hitachi H7600 transmission electron microscope operated at 80 kV. Images were analyzed

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by an open-source automated TEM image processing program, which uses the pair correlation

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method (PCM).42,43 This program enables measuring of the average primary particle diameter

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and various morphology parameters of aggregates.

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Results and Discussion

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Initial aggregate morphology. The mass-mobility relationships for the initial aggregates

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are depicted in Figure 2a. The mass-mobility exponents for aggregates from the three sources are

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all significantly less than three, indicating that their effective densities decrease with increasing

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mobility diameter. This trend is well-known and has been previously observed for soot generated


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by laboratory burners26,33 and diesel engines.3,4 The mass-mobility relationships of the diesel,

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inverted burner, and McKenna burner aggregates determined here are broadly consistent with

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earlier measurements (see Figure S3).3,4,33,44 At mobility diameters of 100 and 150 nm,

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aggregates from both burners are considerably denser than those from the diesel generator.

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At a given mobility diameter, the mass of soot aggregates can be related to the diameters

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of the primary particles constituting the aggregates generated by each of the three sources.

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Primary particle diameters were investigated both indirectly, using mass and mobility

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measurements, and directly, using TEM microscopy. If the aggregate mobility diameter is taken

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to be equivalent to the projected-area equivalent diameter (determined by TEM),45,46 then the

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surface-area equivalent primary particle diameter, * , can be estimated from the following

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relationship:47 *

202

)

,- . +*

( ),- ' = 6

(3)

where +* and 0 are parameters dictated by the formation of and collisions between primary

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particles. The values of +* and 0 ‒ 1.13 and 1.1, respectively ‒ were optimized based on mass-

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mobility and TEM data for two engine sources (diesel and gasoline direct injection) over a wide

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range of operating conditions, and they are expected to be an estimate for other soot sources.48

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The estimated * values for each experiment are listed in Table S1, and they are compared to

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the ex situ, but direct, results of the TEM image analysis obtained by the PCM program.42 We

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note that small relative differences in primary particle diameter between the two methods lead to

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more significant differences in primary particle number. In general, the primary particles

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produced by the diesel generator were estimated to have smaller diameters (15.6 nm for 100 nm

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aggregates) than those produced by the inverted and McKenna burners (27.5 and 28.5 nm for


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100 nm aggregates, respectively). Similarly, direct TEM observations of 100 nm aggregates

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indicate that the diesel generator produces the smallest primary particles: for the diesel generator,

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the mean * is 18.0 nm (with a 95% confidence interval of 13.7–22.3 nm); for the McKenna

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and inverted burners, the mean * values are 24.6 and 24.5 nm, respectively (with respective

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95% confidence intervals of 21.8–27.4 and 21.3–27.7 nm). The average number of primary

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particles, 1, is the ratio of aggregate mass to primary particle mass, the latter of which is derived

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from * and the material density of soot, and listed in Table S1. Based on TEM observations,

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aggregates initially having a 100 nm mobility diameter produced by the diesel generator

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comprise about 46 primary particles, whereas those produced by the McKenna and inverted

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burners comprise about 29 and 30 primary particles, respectively. This difference explains why

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aggregates from the diesel generator are the least dense; the greater number of primary particles

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accommodates a more branched structure.

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Evolution of aggregate morphology. To generate atmospherically-relevant coatings,

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we exposed p-xylene, a representative anthropogenic aromatic hydrocarbon, to hydroxyl radical,

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which initiated photo-oxidation and led to the formation of semi-volatile oxygenated organic

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compounds. Since the concentration of hydroxyl radical was constant, the reaction of p-xylene

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was governed by pseudo-first-order kinetics, and the product concentrations increased

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exponentially. Once the products saturated the gas phase, they began partitioning onto the soot

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aggregates as SOA, leading to exponential growth in particle mass. To facilitate comparisons

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between aggregates from different sources and of different initial mobility diameters, we divide

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mobility diameters and masses by their initial values to give diameter and mass growth factors,

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Gfd and Gfm, respectively. Typical trends in diameter and mass growth factors are shown in

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Figure S4 for initially 100 nm aggregates from both the inverted burner and the diesel generator.


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As shown in Figure S4b, the mass of the coated-denuded aggregates was constant throughout the

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experiment, indicating that the thermo-denuder temperature (300 °C) was high enough to

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evaporate all the SOA coating. (The thermo-denuder likely also affected the physical properties

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of the SOA coatings before evaporation; in a future study, we will investigate the role of SOA

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coating viscosity, in particular, under a variety of relative humidity conditions.) As shown in

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Figure S4a, the diameter of the coated-denuded aggregates from the generator decreases more

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than that of the aggregates from the burner, a first indication that the morphological evolution of

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soot aggregates from different sources varies.

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The evolution of aggregate morphology occurs in three stages that can be identified, for

244

example, in Figure 3, which depicts effective densities and shape factors versus mass growth

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factor for initially 250 nm aggregates from the two burners. In the first stage, the particles

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become denser as coating mass increases, but the core soot aggregates retain their initial

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morphologies. This induction period is not captured by an earlier exponential model derived

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from experiments with a lower coating mass resolution.14 In the second stage, commencing at a

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mass growth factor of about 1.5, the soot aggregates begin to restructure under the influence of

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the SOA coating. The final stage commences at a mass growth factor of about five, and the

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effective density of the coated-denuded soot plateaus at approximately 0.6 g cm-3, which is

252

considerably lower than the material density of soot. The shape factors of the coated particles

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continue to decrease with increasing coating mass before reaching unity at a mass growth factor

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of approximately 10, indicating that a thickly-coated spherical particle is then present. Despite

255

the initial difference in the shape factors of the coated-denuded aggregates from the two burners,

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they both plateau at approximately 1.8, which implies that the restructured soot is not spherical,

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consistent with the final effective density.


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In one experiment, significantly greater coating masses were applied to initially 150 nm

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aggregates from the inverted burner. As shown in Figure S5, a mass growth factor of over 30 is

260

reached. As discussed above, at a mass growth factor of approximately 10, the shape factor of

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the coated particles plateau, and the effective density ceases to increase. As the mass growth

262

factor continues to increase, the shape factor is of course constant, but the effective density

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exhibits a slight decrease, because the mass fraction of the soot, which is denser than the SOA

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coating, decreases significantly. The asymptote corresponds to the material density of the SOA

265

coating.

266

To compare the evolution of soot aggregates from all three sources, we must consider

267

smaller aggregates, because too few 250 nm aggregates are produced by the diesel generator to

268

allow classification and injection into the smog chamber. Accordingly, trends in effective

269

densities and shape factors versus mass growth factor are shown in Figure 4 for initially 100 nm

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aggregates from the three sources. As noted above for the 250 nm aggregates, restructuring

271

began at a mass growth factor of about 1.5. For aggregates from the two burners, restructuring

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continued until the mass growth factor reached about three, at which point the effective densities

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and shape factors of the coated-denuded particles plateaued. In contrast, the aggregates from the

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diesel generator continued to restructure until the mass growth factor reached a value of

275

approximately six. The initial effective densities of aggregates from the McKenna and inverted

276

burners are similar (~ 0.6 g cm-3), whereas that of the aggregates from the diesel generator is

277

lower (~ 0.4 g cm-3). Despite the difference in initial effective densities, the coated diesel

278

aggregate effective densities increase less rapidly to approximately 1.4 g cm-3 than those of the

279

other aggregates. When restructuring is complete, the coated-denuded aggregates from the

280

McKenna and inverted burners reach effective densities of approximately 0.75 g cm-3, while the


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diesel aggregates plateau at an effective density of 0.6 g cm-3. The shape factors of the coated

282

aggregates from the burners converge to unity at a mass growth factor of approximately four,

283

while that of the diesel aggregates converge at mass growth factors closer to 10.

284

Despite the above differences between 100 nm aggregates from the burners and the diesel

285

generator, the evolution of initially 100 nm aggregates from the diesel generator is remarkably

286

similar to that of initially 200 nm aggregates from the burners, as depicted in Figure S6. In both

287

cases, aggregates cease restructuring at a mass growth factor of six, and coated particles reach

288

sphericity at a mass growth factor of about 10. Interestingly, the 100 nm diesel aggregates and

289

200 nm inverted burner aggregates comprise similar numbers of primary particles, 70 and 67,

290

respectively (see Table S1), suggesting that restructuring is related to primary particle number.

291

This notion is explored in greater detail below.

292

Final aggregate morphology. The compaction of coated-denuded aggregates can be

293

visualized using representative TEM samples collected before and after the photo-oxidation

294

stage of a typical experiment, shown in Figure 5a-b for initially 100 nm inverted burner

295

aggregates. As shown in Figure 5c, the final diameter growth factor, the ratio of the final

296

restructured aggregate mobility diameter to the initial aggregate mobility diameter, decreases for

297

all three sources as a function of initial mobility diameter. However, at a given initial mobility

298

diameter, the degree of restructuring varies significantly with the source of the soot. For

299

example, at an initial mobility diameter of about 100 nm, aggregates from the diesel generator

300

collapse appreciably (Gfd = 0.92), but those from the burners do not (Gfd = 0.99). Diesel and

301

inverted burner aggregates initially 100 nm in mobility diameter comprise about 70 and 22

302

primary particles, respectively; these estimates, based on the indirect method described above

303

(see Eq. 3), suggest that in general an aggregate comprised of more primary particles


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304

accommodates more restructuring. If we compare initially 100 nm diesel aggregates and initially

305

200 nm inverted burner aggregates, as above, there is a striking similarity between the final

306

diameter growth factors (0.92 compared to 0.90) and primary particle numbers (70 compared to

307

67).

308

Motivated by the above comparisons between aggregates comprising roughly the same

309

number of primary particles, we consider here whether there is a systematic relationship between

310

the final degree of restructuring and primary particle number. A plot of the final diameter growth

311

factor as a function of the number of primary particles per aggregate (see Figure 5d) reveals an

312

approximately linear relationship across the range of primary particle numbers within the data

313

set, which is most relevant to ambient soot; this relationship may reflect the dilation symmetry of

314

the fractal-like aggregates. If separate fits are performed for each source, all three are nearly

315

identical, indicating that the source of the soot is important only to the extent that it dictates the

316

size and number of primary particles. Note that the linear fits cannot be used for extrapolation, in

317

the event the aggregates are composed of more than about 150 primary particles. Furthermore,

318

aggregates with similar numbers of primary particles collapse to a similar extent at any given

319

coating ratio, even before reaching the final morphology; for example, at mass growth factor of

320

about five, 100 nm diesel aggregates and 200 nm inverted burner aggregates both collapse by

321

about 6% of their initial mobility diameters. Consequently, if modelers know only the coating

322

mass and the number of primary particles per aggregate for a given ambient sample, which can

323

be derived from mass-mobility relationships that are known for many types of soot,33,48, they can

324

predict the evolution of aggregate morphology. Since the extinction cross section of soot

325

aggregates increases with compaction,18 the above fits may equip modelers to make better

326

estimations of the evolution of the optical properties of soot aggregates.49


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327

We note that the final coated-denuded aggregates from the three sources were not

328

spherical. Consider the final mass-mobility exponents of aggregates from the McKenna burner,

329

inverted burner, and diesel generator, as shown in Figure 2b: 2.63, 2.68, and 2.23, respectively.

330

These values are significantly greater than the initial mass-mobility exponents, but they are still

331

well below three, indicating that indeed these aggregates were non-spherical when restructuring

332

ceased. Further restructuring could occur if the surface tension of the coating increased; for

333

example, coatings of glycerol (64 mN m-1 at 20 °C) have been shown to result in final mass-

334

mobility exponents of three for aggregates generated by the McKenna burner.31 Consequently,

335

the linear relationships reported here, although accurate for the representative anthropogenic

336

SOA coating, could change for coatings of higher surface tension. In the atmosphere, the surface

337

tension of SOA may increase via uptake of water at high relative humidity or other

338

atmospherically-relevant species, such as sulfuric acid.50,10 Furthermore, photo-chemical aging

339

and the attendant increase in the oxygen-to-carbon ratio of the SOA, may also increase the

340

surface tension and will certainly increase the hygroscopicity.51

341 342

ASSOCIATED CONTENT

343

Supporting Information

344

The Supporting Information is available free of charge on the ACS Publications website at DOI:

345

___.

346

Figures of representative size distributions, volatile mass fractions and initial effective

347

densities of diesel aggregates, representative time series of diameter and mass growth factors,

348

trends in effective density and shape factor for very thickly-coated inverted burner aggregates,


17


Page 18 of 31

349

comparisons between 100 nm diesel aggregates and 200 nm inverted burner aggregates; table of

350

primary particle diameters and numbers.

351

AUTHOR INFORMATION

352

Corresponding Author

353

*Phone: (780) 492-2341; fax: (780) 492-2200; e-mail: [email protected].

354

Present Address

355

¶

356

Notes

357

The authors declare no competing financial interest.

358

ACKNOWLEDGMENTS

359

This project was undertaken with the financial support of the Government of Canada through the

360

Federal Department of the Environment and the Natural Sciences and Engineering Research

361

Council of Canada (NSERC). We thank René Zepeda for assisting in the use of the TPS. E.G.S.

362

thanks NSERC and Environment Canada for scholarships. W.J. holds a Tier I Canada Research

363

Chair in Cluster Science.

364

REFERENCES

365

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512 513

Figure 1. Experimental setups during injection of soot aggregates generated by (a) the McKenna

514

premixed burner, (b) the inverted diffusion burner, and (c) the commercial diesel generator and

515

(d) during sampling. TD: thermo-denuder; CPMD: counter-flow parallel-plate membrane

516

denuder; DMA: differential mobility analyzer; CPC: condensation particle counter; CPMA:

517

centrifugal particle mass analyzer.

518


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519 520

Figure 2. Mass-mobility relationships for soot aggregates from the three sources (a) before and

521

(b) after restructuring. Dashed curves show linear least-squares fits to the natural logarithms of

522

aggregate mass and mobility; the data points are weighted by their uncertainties, calculated from

523

experimental precision and instrumental bias. The data point at about 50 nm is the same in both

524

plots; the mass and mobility diameter were measured directly from the diesel generator treatment

525

train, and no photo-oxidation experiment was performed, because the number concentration after

526

classification was too low to counteract losses to the chamber walls during injection. Since very

527

little restructuring was observed for initially 70 nm diesel aggregates, initially 50 nm diesel


27


528

aggregates are assumed to undergo no change in either mass or mobility. The constant C in the

529

fit functions (Eq. 1) has units of fg nm–7 .

Page 28 of 31

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531 532

Figure 3. Trends in (a) effective density and (b) shape factor with increasing mass growth factor

533

for coated and coated-denuded burner-generated aggregates initially about 250 nm in mobility

534

diameter.

535


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536 537

Figure 4. Trends in (a) effective density and (b) shape factor with increasing mass growth factor

538

for coated and coated-denuded aggregates, initially about 100 nm in mobility diameter, generated

539

by all three sources.

540


30

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541 542

Figure 5. TEM images of initially 100 nm inverted burner soot aggregates (a) before and (b)

543

after restructuring due to SOA coating, and final diameter growth factor as a function of (c)

544

initial mobility diameter and (d) number of primary particles per aggregate, estimated using the

545

indirect method; the uncertainty in the number of primary particles is detailed in the SI


31

Relationship between Coating-Induced Soot Aggregate Restructuring

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