probing rbc-containing particle morphology with a...

Probing rBC-containing Particle Morphology with a Single Particle Soot Photometer (SP2)

A.J. Sedlacek, III1, E.R. Lewis1, T.B. Onasch2,3, A.T. Lambe2,3, P. Davidovits3, and L. Kleinman1 1Brookhaven National Laboratory; 2Aerodyne Research Inc.; 3Boston College

Scattering(particle diameter: ∼165 nm - 325 nm)

Incandescence(rBC diameter: ∼60 — 600 nm )

‣ Probe coating thickness: optical and BC mass equivalent diameters‣ Examine temporal profiles of the scattering and incandescence signals

Incident Laser

Probing rBC Mixing State with SP2Schwartz et al., 2006; Moteki & Kondo, 2007, Subramanian et al., 2010

Motivation (Sedlacek et al., 2012 GRL)

Boston College Experiments - 2012

Asymmetric Amplitudes Interpreted as rBC Inclusions Near Surface

particle enters laser beam

coating material at core/coating interface vaporizes

vaporized material fractures particle early in laser

Denuded BC incandescences & pure scatters continue through laser beam

Incandescence

direction of particle➠

Near-Surface rBC-Containing Particles Correlated with Biomass Burn tracers

Dependence of Scattering Signal on Acquisition Parameters

Conclusions• Negative lagtime signals observed by Sedlacek et al. (2012) reproduced in

laboratory.

• Lagtime distributions suggest differing particle morphology for RB+NaCl and RB+DOS systems.

• Striking similarity in negative lagtime signal structure for RB+DOS coagulation and condensation suggest similar particle morphology.

• Incandescence lagtime and scattering signal structure is strongly dependent upon laser power and sample flow rate.

• SP2 lagtime analysis can provide useful information on rBC-containing particle morphology. Complement chemical composition measurements.

Abstract

AcknowledgementsThis research was performed under sponsorship of the U.S. DOE under contracts: DE-AC02-98-CH10886 (BNL), DE-SC0006980 (Boston College), DE-FG02-05ER63995 (ARI) and NSF grant number ATM-0525355 (Boston College) and ATM 0854916 (Aerodyne Research Inc.)

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To answer some of the questions posed by the original work, a series of laboratory experiments were carried out as part of the ASR-sponsored Boston College Black Carbon Study-3 (BC3).

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Laser Power (mA)

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Pos /Neg Ratio

Ratio of positive-to-negative lagtimes as a function of sample flow and laser power reveals systematic dependence on these parameters.

Implications of Present Study• Positive lagtimes for coagulated particles could be misinterpreted as a core-

shell particle resulting in an overestimate of light absorption.

• Dependence of lagtime distribution on laser power and sample flow warrant caution when interpreting rBC mixing state.

For the BC3 experiments, four systems were examined:

• Coagulation of regal black (RB) with Dioctyl Sebacate (DOS) solid + liquid

• Coagulation of RB with Sodium Chloride (NaCl) solid + solid

• Coagulation of RB with dry Ammonium Sulfate (AS)

solid-solid

• Condensation of RB with DOS.

RB + DOS coagulation

1

Preliminary Analysis of Near-Surface rBC-containing DOS/AS Systems

July 12, 2012

Background: Exploitation of the incandescence and scattering signals collected by the Single-Particle Soot Photometer (SP2) has been shown to provide mixing state information on refractory BC-containing particles. (Schwarz et al., 2008; Subramanian et al., 2010) Such data has greave value in ongoing efforts at quantifying the contribution of black carbon direct radiative forcing. A central tenet of this SP2-based mixing state analysis is the assumption that an rBC-containing particle has a core-shell configuration with the rBC as it core. When such a particle enters the laser beam of the SP2, the rBC core absorbs energy, which it initially dissipates though evaporation of the non-refractory coating, and then subsequently through incandescence. This behavior allows measurement of the time difference between the peak of the scattering signal and the incandescence signal (together with the assumed core-shell geometry) that can, in turn, be used to provide an estimate of the coating thickness; the greater the lagtime the greater the coating thickness (upper and middle panel of Figure 1).

Recently Sedlacek et al. (2012) observed negative lagtimes in aerosol particles containing rBC (bottom panel of Figure 1). Such behavior had not previously been reported in ambient conditions. For example, in one time period studied during the Aerosol Lifecycle IOP last summer at BNL, the fraction of rBC-containing particles with negative lagtimes was greater than 60%, as shown in Figure 2

where the distribution of measured lagtimes are plotted as a function of rBC mass equivalent diameter, Dme,rBC. The occurrence of a negative lagtime would require that the rBC incandesced before the peak of the scattering signal, a situation seemingly not possible with the core-shell structure. To explain this behavior, Sedlacek et al. proposed that the rBC was near the surface in such particles, and that upon absorption of energy from the laser the particle fragmented because of the inability of the coating

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Figure 1. SP2 incandescence and scattering signals for thinly coated (top), thickly coated (middle), and near-surface rBC-containing particles (bottom).

Figure 2. Lagtimes for a plume containing negative lagtimes as a function of rBC mass-equivalent diameter Dme,rBC

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Episode B characterized by negative lagtimes

Episode A Characterized by positive lagtimes

Conclusions from Sedlacek et al.:

• Negative lagtimes interpreted as rBC inclusion near the surface of particle.

• Correlation of near surface BC with biomass burning. Observed in the present study, CARES, and StormVEx (Wallops wildfire).

Research questions: • How common are near-surface rBC-containing particles?

• Are near surface rBC inclusions more likely from BB?

• Can this technique uniquely probe rBC-containing particle morphology?

• What is the radiative forcing impact of these particles?

Striking similarity between lagtime distributions for RB+DOS coagulation and condensation suggests similar particle morphology. Signals also similar to that observed by Sedlacek et al., (2012).

Particle Formation Through Coagulation

Use of Negative Lagtime Signals to Quantify rBC Mixing State• Negative lagtime scattering signal unambiguously defines center of laser

beam, greatly simplifying reconstruction of unperturbed particle diameter via normalized derivative method. (Moteki and Kondo, 2008)

• Analysis is currently underway on the refinement of the interpretation of negative lagtime signals

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t=1113 mindmean=219 nm

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C /dLogDp

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dmean: 172 nm dmean=172 nm

By varying the sample flow and laser power (diode injection current) positive lagtime scattering signals can be shifted to negative lagtime signals

RB + NaCl coagulation

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Pos/Neg ratio: 0.23laser: 3000 mAflow: 360 cc/min

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Negative mode observed to evolve. Note similarity in negative mode structure immediately following mixing with that observed for RB+NaCl system following overnight mixing.

RB + DOS coagulation

RB + NaCl coagulation

Positive scattering signal structure is also found to evolve for RB+DOS system. This evolution hints at possible morphological changes with the RB+DOS system: RB diffusion in DOS?

Particle Formation Through Condensation (RB + DOS)

Lagtime distributions contain both positive and negative modes.

Examination of Individual Positive Scattering Signals

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Incandescence

RB+NaCl (τ > 17 hr)

Incandescence

RB+NaCl (τ< 1 hr)

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RB+DOS (τ >18 hr)

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RB+DOS (τ <1 hr)

Particle formation through Coagulation and Condensation

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Mass concentration ratio

C2H4O2+/Org (m/z=60)

C3H5O2+/Org (m/z=73)

During the DOE-sponsored Aerosol Lifecycle (ALC) field campaign episodes were encountered where large fractions of rBC-containing particles were characterized with negative lagtimes.

regal black

dioctyl sebacateammonium sulfate

sodium chloride

SP2SP-AMSSMPSCPC

filter

diffusion dryer filtersteel drum

(55 gal)

A significant source of the uncertainty associated with refractory black carbon (rBC) radiative forcing is due to uncertainty in the rBC-containing particle chemical composition and morphology. While mass spectrometry is now routinely utilized to provide in situ, online analysis on the chemical composition of these particles, morphology is still limited to microscopy. One class of instrumentation that has recently shown promise towards addressing this gap is the Single Particle Soot photometer (SP2).

Using the SP2 lagtime method, Sedlacek et al., (2012) interpreted the existence of a particle scattering signal after rBC incandescence as evidence of rBC inclusions at or near the surface of their non-refractory hosts. In an effort to further evaluate this initial interpretation, elucidate the origins of these signal conditions, and to explore the utility of this methodology for studying rBC particle morphology, a series of laboratory-based experiments were carried out as part of the ASR-sponsored Boston College Black Carbon study (BC3).

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Episode B

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probing rbc-containing particle morphology with a...

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