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Optical Properties of Airborne Soil Organic Particles Daniel P. Veghte, Swarup China, Johannes Weis, Libor Kovarik, Mary K. Gilles, and Alexander Laskin ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.7b00071 • Publication Date (Web): 08 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017

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ACS Earth and Space Chemistry

Optical Properties of Airborne Soil Organic Particles Daniel P. Veghte1, Swarup China1, Johannes Weis2,3, Libor Kovarik1, Mary K. Gilles2, Alexander Laskin4,* 1

William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, USA. 2 Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. 3 Department of Chemistry, University of California, Berkeley, California 94720, USA. 4 Department of Chemistry, Purdue University, West Lafayette, IN 47907-2084 USA. Manuscript in Preparation for: ACS Earth and Space Chemistry

*Correspondence: [email protected]

Keywords: Airborne soil organic particles, optical constants, electron energy loss spectroscopy, scanning transmission electron microscopy, refractive index

1 ACS Paragon Plus Environment


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Abstract:

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organic particles (ASOP). The Chemical composition of ASOP include macromolecules such as

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polysaccharides, tannins, and lignin (derived from degradation of plants and biological

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organisms), which determine light absorbing (brown carbon) particle properties. Optical

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properties of ASOP were inferred from the quantitative analysis of the electron energy-loss

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spectra acquired over individual particles using transmission electron microscopy. The optical

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constants of ASOP are compared with those measured for laboratory generated particles

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composed of Suwanee River Fulvic Acid (SRFA) reference material, which is used as a

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laboratory surrogate of ASOP. The chemical composition of the particles was analyzed using

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energy dispersive x-ray spectroscopy, electron energy-loss spectroscopy, and synchrotron-based

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scanning transmission x-ray microscopy with near edge x-ray absorption fine structure

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spectroscopy. ASOP and SRFA exhibit similar carbon composition, with minor differences in

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other elements present. When ASOP are heated to 350 °C their absorption increases as a result of

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pyrolysis and partial volatilization of semi-volatile organic constituents. The retrieved refractive

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index (RI) at 532 nm of SRFA particles, ASOP, and heated ASOP were 1.22-0.07i, 1.29-0.07i,

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and 1.90-0.38i, respectively. Retrieved imaginary part of the refractive index of SRFA particles

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derived from EELS measurements was higher and the real part was lower compared to data from

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more common optical methods. Therefore, corrections to the EELS data are needed for

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incorporation into models. These measurements of ASOP optical constants confirm that they

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have properties characteristic of atmospheric brown carbon and therefore their potential effects

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on the radiative forcing of climate need to be assessed in atmospheric models.

The impact of water droplets on soils has recently been found to drive emissions of airborne soil

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Introduction: Atmospheric aerosols affect the Earth’s climate directly by scattering and absorbing solar

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and terrestrial radiation. Aerosol optical properties depend on the chemical composition, sizes,

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shapes, mixing states, and refractive indices of individual particles as well as aerosol

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concentrations.1-6 An additional indirect effect of aerosols occurs when they modify optical and

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microphysical properties of clouds where particles act as ice and cloud condensation nuclei.7

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Light absorbing particles in the troposphere are of particular interest due to their warming

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effect.6 The two major classes of carbonaceous absorbing particles are black carbon (BC) and

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brown carbon (BrC). While BC dominates the absorption, BrC contributes to between 18-56% of

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the carbonaceous absorption of solar radiation globally depending on the wavelength.8-10 BrC

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particles have a broad range of chemical compositions and morphologies ranging from low

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viscosity secondary organic material to glassy tar balls and airborne soil organic particles.11-14

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BrC is composed of various types of organic components that absorb light at variable

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wavelengths but are generally strong absorbers in the UV and short wavelength of the visible

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range.15-16 While BrC light absorption is enhanced at shorter wavelengths, BC absorbs strongly

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over all wavelengths.16-17 For example, tar balls are absorbing BrC species that exhibit a large

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variability in their optical properties with refractive indices (RI) with a range of real part from

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1.56-1.88 and imaginary part from 0.002i to 0.27i. 13-14 Additionally, both BC and BrC particles

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can be coated with organic components that increase the absorption due to a lensing effect.18-21

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Field measurements use the absorption Ångström exponent (AAE) to describe the wavelength

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dependence of the light-absorption by aerosols with respect to the empirical power law of ~λ-AAE,



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where λ is wavelength. The broad range of AAE values reported in the literature (1.6-11)15, 22 is

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an inherent result of the large variability in the BrC chemical composition resulting in substantial

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uncertainty in quantitative assessment of optical properties of aerosol containing BrC and BC

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components.23-24 Most models assume that the majority of absorbance by atmospheric

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carbonaceous material is due to highly absorbing BC and do not account for BrC present in the

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atmosphere, which could be considerable.25

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Airborne soil organic particles (ASOP) were recently recognized as a distinct particle

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type with a composition similar to soil organic matter (SOM). Their substantial contribution (up

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to 60%, by number), detected at the measurement site located in the Southern Great Plains

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(Oklahoma, USA), was attributed to atmosphere–land interactions during rainfall.12 It is

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suggested that these particles may be prevalent in certain geographic areas where open soils are

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exposed to precipitation events such as agricultural fields and grasslands.12, 26 SOM is a large

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source of terrestrial carbon and produced from plant litter being decomposed in biochemical

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processes.27 The suggested mechanism of ASOP formation is through incorporation of small air

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bubbles in the aqueous layers of wet soils followed by bursting of the bubbles at the air-water

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interface releasing a fine mist of SOM-containing microdroplets that upon later dehydration form

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solid ASOP.28 Since the composition of ASOP and SOM are similar, the optical properties of

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ASOP could potentially be approximated by SOM proxies such as Suwanee River Fulvic Acid

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(SRFA) standard material.29-32

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Understanding the broad-band optical properties of individual atmospheric particles is

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essential for estimating direct effects of aerosols on climate. Methods used to measure the

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extensive optical properties of airborne ensembles of particles include: nephelometers, Fourier

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transform infrared spectrometer, cavity ring-down (CRDS) and photoacoustic spectrometer


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(PAS).33 Nephelometers and mid-IR spectrometers have a high detection limit and are limited to

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high concentrations of polydisperse samples.33-34 CRDS and PAS have lower detection limits

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(approximately 0.4 Mm-1 35 and 0.8 Mm-1 36 respectively), and thus can only measure the optical

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properties of ensemble of particles at limited wavelengths. 29-30, 34 Additionally, filter based

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methods, such as particle soot absorption photometers and aethalometers can measure light

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absorption by bulk particle samples deposited on the filters but can have significant scattering

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artifacts that require the use of correction factors.34, 37 However, all of these methods require

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either large ensembles of airborne particles or bulk samples.

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Extensive optical properties of particles depend on their size, morphology, and

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composition of each of the particle components. The RI derived using an effective medium

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approximation of a particle is a function of the chemical composition, phase, and mixing state

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that can be uniquely assessed using electron microscopy and chemical imaging methods.38-39 In

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certain cases, they can probe optical properties of individual particles of interest, identifying

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them from a complex mixture of particles collected on substrates. For example, quantitative

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analysis of the particle-specific low-loss energy spectra from electron energy-loss spectroscopy

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(EELS) coupled to the scanning transmission electron microscope (STEM) was previously used

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to quantify intensive optical properties of individual tar balls from biomass burning on a single

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particle basis.17, 40 By using the low-loss region of EELS, RI can be retrieved in the 200-1200 nm

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wavelength range for individual particles, provided that particles were stable under the electron

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beam and their morphology would allow estimation of thickness along the direction of the

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incident electron beam. With this method of examining individual particles, field samples

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containing a mixture of particle types that satisfy these conditions can be analyzed and optical

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constants can be retrieved for each particle type present in the mixture. Because of the refractory



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behavior of ASOP and their stability under the electron beam,12 EELS provides an optimal

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method to probe the optical properties of ASOP.

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In this paper, we present the optical properties of individual ASOP retrieved from low-

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loss EELS data and compare those to the laboratory generated particles composed of Suwanee

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River Fulvic Acid (SRFA) reference material. SRFA is a well-studied material with RI values

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reported in literature.29-32, 41 We analyze the chemical composition of ASOP and SRFA particles

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using scanning transmission x-ray microscopy near edge x-ray absorption fine structure

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(STXM/NEXAFS), and TEM based methods of energy dispersive x-ray spectroscopy (EDX),

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and EELS. Both ASOP and SRFA are primarily carbonaceous, however EDX detects some S

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and Na in SRFA. NEXAFS shows that ASOP have higher levels of carbonization (more

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intensive sp2 (C=C) peak) compared to the SRFA particles, while SRFA contains more

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prominent oxygenated functional groups. Consistent with their compositional characteristics,

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ASOP and SRFA have similar imaginary parts (absorption) of the RI, while the real part

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(scattering) of the RI is larger for SRFA. In addition, we show that heating the ASOP particles to

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350 °C leads to charring of their organic content, leading to substantially larger absorption.

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Experimental Methods:

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Particle Samples

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Field samples of ASOP were collected from 4/28/2016 (18:30) to 4/29/2016 (04:30),

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5/5/2016 (08:00) to 5/5/2016 (21:00), and 5/15/2016 (12:00) to 5/15/2016 (16:45) local time at

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the Southern Great Plains Site (36° 36’ 18” N, 97° 29’ 6” W) in Lamont, Oklahoma. This site is

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operated by the Atmospheric Radiation Monitoring (ARM) Program of the U. S. Department of


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Energy (DOE). Particles used for EELS analysis were mainly from the 4/28/2016 sample.

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Particles were collected onto the 7th stage (D50 cut-off size of 0.32 µm) of a MOUDI impactor

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preloaded with two types of microscopy substrates: Si3N4 membrane windows (Silson, Ltd) and

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400 mesh Copper TEM grids coated with either lacey carbon or Type-B carbon film (Ted Pella,

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Inc). The meteorological conditions and air parcel backward trajectories calculated using the

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Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model42 during the

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sampling periods are given in the supplemental file (Figure S1). The backward trajectories

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indicate the air mass arrived from the north part of the Great Plains and was associated with

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relatively calm and humid weather with low wind speeds.

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Aerosolized particles generated from aqueous solutions of Suwanee River Fulvic Acid

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(1S101F, International Humic Substances Society) were used as a laboratory standard for the

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retrieval of optical properties using EELS. SRFA particles were generated by nebulization of a 1

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wt.% solution of SRFA into a nitrogen gas stream at 4 lpm using a medical nebulizer (8900-7-50,

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Salter Labs, Arvin, CA). The particle-laden stream was dried using a diffusion dryer (TSI

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306200, Shoreview, MN). The subsequent dry particles were then collected on the microscopy

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substrates in the same manner as the field collected ASOP.

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Selected field samples were heated to 350 °C inside the TEM using a furnace-type

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heating holder (Gatan, Pleasanton CA) to remove volatile and semi-volatile materials. After

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heating, STEM/EELS analysis was performed over the thermally modified ASOP. To simulate

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the conditions of the TEM heating holder, for the STXM/NEXAFS analysis, the corresponding

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field samples were heated under nitrogen in a tube furnace (MTI GSL 1300X) at 20 °C/min until

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reaching 350 °C where it was held for 5 minutes before cooling at 20°C/min to room

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temperature.



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Imaging and elemental analysis: Particle samples were first imaged by scanning electron microscopy (SEM; Quanta 3D,

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FEI, Hillsboro, OR) operated at 20 kV. These particles were imaged both orthogonally to the

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incident electron beam and at a 75° tilt angle to image the contact area between particle and

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substrate.12 Elemental composition of single particles was measured using a 10 mm2 Si(Li)

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energy dispersive x-ray (EDX) detector (EDAX Genesis, Mahwah, NJ) interfaced with the SEM

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and operated at 20 kV.43-44 Samples with a high abundance of ASOP were selected for further

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analysis by STEM and STXM.

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An aberration corrected STEM (FEI Titan 80-300 STEM, Hillsboro, OR) operated at 80

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kV was used to image single particles and the electron detection was performed using the high-

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angle annular dark-field (HAADF) detector to probe the internal composition of the particles

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based on the z-contrast. High-loss dual dispersion range EELS (Gatan, Pleasanton, CA) was used

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to collect maps in STEM mode with 0.25 eV energy resolution in the ranges of: -50 to 500 eV

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for the zero-loss peak, and 200 to 750 eV to collect data for carbon, oxygen, and nitrogen.

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Signals were integrated from 280 to 360 eV for the carbon maps and from 525 to 568 eV for the

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oxygen maps. Elemental maps were collected with a lateral resolution of approximately 3 nm.

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STXM was used to acquire NEXAFS spectra of individual particles at the carbon k-edge

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energy range (278-320 eV).12, 45-47 Monochromatic incident photons from the synchrotron source

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were focused on the sample using a Fresnel zone plate. Sets of raster scan images were acquired

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over selected fields of view of the particle samples at selected x-ray energies by recording the

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transmitted light intensity. Spectra of individual particles were then reconstructed based on the

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Beer Lambert law from the stack images using the signal collected from the areas of individual

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particles referenced to the background signal from regions without particles. STXM/NEXAFS


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was performed at the Advanced Light Source synchrotron facility at Lawrence Berkeley National

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Laboratory on beamlines 11.0.2.2 and 5.3.2.2.12, 45-47

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EELS acquisition and quantification of optical properties:

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EELS measurements for optical properties were performed using an aberration corrected

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STEM operated at 80 kV. The use of an electron beam at 80 kV reduced the Cherenkov radiation

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effects and also minimized the electron beam induced knock-on damage.17, 48 EELS

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measurements were performed using a Gatan EELS spectrometer controlled by Digital

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Micrograph software to collect high-energy resolution, single range dispersion EELS data.

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Working at 80 kV and using the monochromator decreased the full width at half maximum of the

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zero-loss peak (ZLP) from 0.8 eV to 0.15-0.2 eV with an energy dispersion of 0.025 eV/channel.

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Energy-loss spectra were acquired at a collection angle of 45.5 mrad. For each particle, the

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energy-loss spectrum was obtained by centering the incident electron beam on the middle of the

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particle allowing the thickness to be estimated as the particle diameter. The electron dose rate to

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each particle was approximately 0.03 nA, and each spectrum took between 1-2 minutes to

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collect. After five consecutive EELS spectra were recorded there was no difference observed in

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the spectra or damage to the particle (Figure S2). Collection of multiple spectra demonstrates

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that the particles are not beam sensitive as would be observed through changes in subsequent

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spectra or visible damage to the particles. The reported retrieved RI values were averaged over

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12 measurements for SRFA particles (RISRFA), 14 measurements for ASOP (RIASOP), and 10

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measurements for ASOP heated to 350 °C (RIASOPh).

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The low-loss region (0-10 eV) in the EELS spectrum was used to derive the optical

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constants of 50-200 nm diameter particles of SRFA, ASOP, and heated ASOP following

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procedures established by Crozier and co-workers.17, 40 Figure 1 shows an example of the low-



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loss EELS spectrum of an individual ASOP, the corresponding fitted ZLP and the single

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scattering distribution, S(E), used to calculate the particle-specific RI values using the Kramers-

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Kronig relationship. In the low-loss EELS spectrum, the first peak centered around 0 eV is the

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ZLP, the second peak around 4-6 eV is the plasmon resonance of the π electrons, while the broad

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peak around 22 eV is due to the π + σ plasmon peaks.49 Retrieval of the S(E) from experimental

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EELS data requires removal of the ZLP and correction for the multiple scattering. The S(E)

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distributions for all analyzed particles are included in the supplemental information (Figure S3).

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Further discussion for obtaining the S(E) is also in the supplemental information.

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Monochromatization of the electron beam is essential to produce an electron beam with a narrow

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energy distribution, which facilitates more accurate removal of the ZLP at lower energies.

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241


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Figure 1. Example raw EELS spectra (black) with the extracted single scattering distribution,

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S(E), (blue) after removal of the zero-loss peak (red) and correction for plural scattering. The

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shaded energy region corresponds to the retrieval of the RI at 200-1200 nm.

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To retrieve the RI values, the Kramers-Kronig analysis was performed on the extracted

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S(E) using a modified version of the MATLAB KraKro code (http://tem-eels.com) employing

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Ritchie’s derivation for thin film approximation.50,51 The thin film approximation has been

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shown to be effective for particles with diameters of 40-200 nm.17 The low-loss region of the

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EELS spectrum can be described by the single scattering distribution function S(E) according to

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equation 151 =

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1 + +

(1)

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where I0 is the ZLP intensity, v is speed of light, t is the particle diameter, a0 is the Bohr radius,

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m0 is electron mass, β is the collection angle, θE is the characteristic angle of energy loss, and

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ε(E)=ε1(E)+iε2(E) is the complex dielectric function for the chemical composition of the probed

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particle. β is calculated by measuring the diffraction pattern of sapphire in the 110 and 104

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planes and measuring the diameter of the collection area to obtain the collection angle while θE is

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calculated based on the energy of the incident electron beam. The SS(E) term represents the

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energy loss due to surface contributions described by equation 2 =

260

!

"#$ /

− ' (

)

$

'

− * +

(2)

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An iterative method was used to solve eq. (1) for Im[-1/ε(E)]. Initially, the SS(E) term was set to

262

zero and then allowed to vary in subsequent iterations until the solution for Im[-1/ε(E)]

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converged.50-51 11 ACS Paragon Plus Environment


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The Kramers-Kronig relationship was then used to retrieve the Re[1/ε(E)] using the causational

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relationship between Re[1/ε(E)] and Im[-1/ε(E)] where the Cauchy principle part of the integral

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is denoted by P(eq. 3)

4

.

,- = 1 − / 05

267

1 23

1 3

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The dielectric function (ε) was then derived from Re[1/ε(E)] and Im[-1/ε(E)] according to

269

equation 4.

270

89:/;'

'= :/;>

(3)

(4)

271

Computation of the dielectric function using the Kramers-Kronig formulation converged within

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three iterations. Calculations using up to 20 iterations did not significantly deviate from the

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results with 3 iterations. The real and imaginary parts of the broadband RI(E) = n(E)+ik(E) were

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then calculated from the dielectric function using equations 5.

275 276

@$ ' ' $ ?@$ ' $ =? A = . .

(5)

Absorption and scattering cross sections (σabs and σsca) were calculated based on Mie

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theory and employing retrieved RI. The calculations were performed using BHCOAT code

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adapted for MATLAB by Mätzler 2002.52 The optical properties of ASOP are presented in terms

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of the scattering efficiency (Qabs= σabs/πr2), the absorption efficiency (Qsca= σsca/πr2), and the

280

single scattering albedo (SSA= σsca/(σsca+σabs).

281 282 283 284

Results and Discussion: ASOP collected for this study at the SGP site are similar in morphology and composition to those collected previously at SGP.12 For the sample collected on 4/29/2016 and used for 12 ACS Paragon Plus Environment

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further analysis, the HYSPLIT back trajectories indicate that the air parcels spent most of their

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time over the Great Plains regions where the region is dominated by agriculture and grasslands

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where ASOP formation is expected (Figure S1).12 For the 5/5/2016 and 5/15/2016 samples, the

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air mass originated primarily from the North and the previous 72 hours was spent mostly over

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the Great Plains region. All samples contained an externally mixed particle population and

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ASOP are observed in all samples. Representative SEM images acquired at 75° tilt angle are

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shown in Figure 2 with ASOP marked by arrows. Number fractions of ASOP are around 20% for

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the 4/29/2016 sample, 45% for the 5/5/2016 sample, and 40% for the 5/15/2016 sample. Their

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sizes are between 100-800 nm with a mean diameter of 500 nm. These particles exhibited a

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spherical shape with an aspect ratio (width/height) near one, indicative of glassy organic aerosol

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particles.53 Other collected particles with higher aspect ratios (oblate shapes) were presumably

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deformed upon impaction on the substrate. These deformed particles with high aspect ratios are

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often observed in the atmosphere and are usually described as liquid, low viscosity organic

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products.53 Because ASOP are spherical and resistant to electron beam exposure, they are ideal

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for the optical constant retrieval based on STEM/EELS measurements.

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Figure 2. Tilted SEM images at 75° of particles collected at the ARM SGP site on a) 4/29/2016,

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b) 5/5/2016, c) 5/15/2017, and d) laboratory generated SRFA particles. The arrows indicate

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glassy ASOP. Other particles have relatively low viscosities and presumably flattened during

305

impaction onto the substrate.

306 307

HAADF STEM images in Figure 3 illustrate representative particles of SRFA, ASOP,

308

and heated ASOP. All of these particles are near spherical with an aspect ratio close to one and

309

exhibit no visible signs of damage during electron beam exposure. In addition, as demonstrated

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by the uniform signal over the particle areas, they do not contain any internal structures. A

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fraction of ASOP appears to be coated with a thin layer of less viscous (semi-liquid)

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carbonaceous material adhering to the lacey carbon support film (Fig 3b). After heating to 350

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°C, ASOP retain their spherical shape with the less viscous material removed (Figure 3c

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compared to Figure 3b). Heating particles leads to removal of the coating so that the core particle 14 ACS Paragon Plus Environment

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can be analyzed. Literature reports54-55 suggest that heating carbonaceous particles past ~400 °C

316

leads to charring, as is confirmed by the optical data presented below. ASOP and SRFA particles

317

presented here meet the requirements (stability under the electron beam and spherical

318

morphology) for analysis with EELS to retrieve RI values.

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321 322

Figure 3. Representative HAADF STEM images of particles used for RI retrieval: a) SRFA, b)

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ASOP with a thin coating of semi-liquid carbonaceous material, and c) ASOP heated to 350 °C.

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Note that the lacey carbon substrate is on the right for all three particles (indicated by the

325

arrows).

326 327

STEM images acquired at the element-specific EELS energies were used to map internal

328

distributions of C and O in ASOP and SRFA particles. Representative images, with a lateral

329

resolution of 3 nm, are shown in Figure 4. The particles also contained nitrogen, but because of

330

its low intensity in the EELS spectra its lateral distribution could not be reliably mapped. STEM

331

imaging of the SRFA particles indicated that the particles are nearly spherical with uniform

332

distributions of C and O. STEM images of ASOP (Figs. 3b and 4d) display visible particle 15 ACS Paragon Plus Environment


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Page 16 of 38

333

coatings adhered to the carbon support substrate. Comparison of carbon and oxygen maps (Figs

334

4b, e, h and 4c, f, i, respectively) indicates that particles are mainly carbonaceous. The map of

335

the ASOP oxygen signal (Figure 4f) is enhanced in a thin outer layer (3-6 nm) of the particle.

336

The oxygen-rich coating is indicative of atmospheric processing of the particles through a

337

diffusion limited process.56 Oxygen-rich coatings have been previously observed on tar balls, but

338

much thicker covering approximately the outer 40 nm of the particles.13, 57 In contrast, the maps

339

of heated ASOP show a uniform distribution of carbon and oxygen indicating removal of the

340

oxygen-rich coating.

341

342


Page 17 of 38

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343

Figure 4. STEM and EELS (carbon and oxygen) maps of: a-c) SRFA particle, d-f) ASOP, and g-

344

i) heated ASOP. Note that the lacey carbon substrate is on the bottom for the SRFA and on the

345

right for the ASOP and heated ASOP.

346 347

The elemental composition of each particle type was analyzed using SEM/EDS,

348

STEM/EELS, and STXM/NEXAFS (Figure 5). ASOP particles were identified in the tilted SEM

349

images at 75° for further EDS and STXM/NEXAFS experiments. The EDS analysis of each

350

particle type is shown in Figure 5a, where the spectra are scaled to the same height of the oxygen

351

peak to facilitate visual comparison. SRFA is primarily composed of C, N, and O as expected for

352

biodegradation material. SFRA also contains traces of S and Na. ASOP are mainly carbonaceous

353

containing C, N, and O with no additional peaks present from other elements. Similarly, heated

354

ASOP contained only C, N, and O. Figure 5b confirms, by EELS, the presence of C, N, and O in

355

all three samples. The oxygen peak for ASOP was more intense than in the heated ASOP and

356

SRFA particles. ASOP has a relatively higher π* to σ* peak intensity compared to SRFA,

357

consistent with the NEXAFS data shown in Figure 5c. The carbon bonding showed both

358

similarities and differences between particle types. For SRFA particles the most intense peak is

359

RCOOH (288.5 eV) followed by R(C=O)R/C-OH (286.5 eV) and then the C=C sp2 (285.4. eV)

360

peak. The SRFA spectrum shown here is similar to literature reports of other humic acid

361

substances58 which are water-soluble and highly oxidized fractions of the soil organics

362

susceptible to aerosolization by the ‘rain drop mechanism’.28 The NEXAFS spectrum of ASOP is

363

similar to our previously reported spectrum12 and shows a prominent sp2 peak at 285.4 eV and

364

minor peaks of oxygenated functional groups at 286.5 and 288.5 eV. Overall, the spectral

365

features are similar to the ‘free-light’ lower density SOM NEXAFS spectra reported.12 Spectrum



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366

of the heated ASOP shows the sp2 peak is the most prominent peak with smaller oxygen peaks

367

present. This enhancement of the sp2 peak is indicative of charring of the particles which would

368

cause them to be more absorbing and similar to black carbon.54-55, 59 With the heating of the

369

ASOP to remove the coating, it was found that in addition to the coating being removed,

370

composition of the inner particle part has changed as indicated by the enhanced sp2 peak.

371

Relative intensities and shapes of the characteristic features in EDX, EELS, and NEXAFS

372

spectra indicate that ‘free-light’ SOM12 is more similar to ASOP than SRFA.

373


Page 19 of 38

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374 375

Figure 5. Elemental comparison of SRFA, ASOP, and ASOP heated to 350 °C particles inferred

376

from: a) EDX spectra normalized to the oxygen counts (Cu and Si are background signals

377

originated from the substrate grid and detector, respectively), b) EELS, and c) STXM/NEXAFS. 19 ACS Paragon Plus Environment


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378 379

Figure 6 shows broadband values of RISRFA retrieved over the wavelength range of 200-

380

1200 nm. Previous publications examined the optical constants of atmospheric particles using

381

EELS.17, 40 However, they did not compare measurements on standards to results from other

382

optical methods to determine any systematic errors in the EELS measurements and applications

383

of the Kramers-Kronig analysis. Here, we used SRFA as a standard to compare the single particle

384

EELS method with bulk methods previously employed to retrieve the RISRFA.29-30 The retrieved

385

RISRFA at 390 and 532 nm from this work are compared to those from optical retrieval methods

386

utilizing cavity ring-down spectroscopic methods29-30, 32 in Table 1. Other methods reported the

387

RISRFA for ranges of 315-345 nm,41 360-420 nm,31-32 and single wavelengths of 390 and 532

388

nm.29-30 At 390 nm, the imaginary part of the RISRFA reported here (k=0.08 ) is similar to those

389

reported from other optical methods (k=0.05-0.1).30-32 In contrast, the real part of the RISRFA

390

reported here (1.20) is considerably lower than that measured by optical methods (1.60-1.69).30-32

391

For SRFA we report RISRFA at 532 nm of 1.22-0.07i for the retrieval with EELS, while results

392

from CRDS are between 1.52-0.02i and 1.65-0.02i.29-30 The discrepancy in the CRDS results

393

stems from the use of pulsed CRDS30 compared to continuous wave CRDS29 and a different lot

394

of SRFA which leads to considerable differences in retrieving the optical properties. At these

395

wavelengths, the real part of the reported RISRFA is slightly lower, while the imaginary part of the

396

RISRFA is slightly higher. Overall, the imaginary part of the RISRFA retrieved from EELS is

397

consistent with those from optical methods, while at all wavelengths the real part of the RISRFA is

398

lower. This is similar to previous EELS experiments of polystyrene spheres that were modified

399

using a high electron beam dose and subsequent beam exposure showed no change to the particle

400

morphology .17 Figure S4 shows the comparison of the RI from EELS17 to reported values for


Page 21 of 38

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401

polystyrene60 indicating that the real part of the refractive index is substantially lower than the RI

402

retrieved by other methods. Some part of the discrepancy may be due to the modification of the

403

polystyrene spheres with the electron beam. The lower values of the real part of RISRFA retrieved

404

from EELS compared to those derived by the optical methods could also arise from the drier

405

conditions that particles experience under vacuum in the STEM and from using the SRFA

406

standard derived from a purified natural source.29-30 With these caveats in mind, the retrieval of

407

the RI values of atmospheric particles using EELS data, values derived for the RI of the field

408

collected carbonaceous particle presented in this work and in earlier literature17, 40 can be put into

409

a relative perspective.

410

411 412

Figure 6. Comparison of the retrieved broadband refractive index of Suwanee River Fulvic Acid

413

(RISRFA) with data from optical methods.29-32, 41 The real part of the RISRFA(n) is shown in the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

414

upper panel, the imaginary part (k) is shown in the lower panel. Shaded region indicates one

415

standard deviation of the refractive index for all particles measured

Page 22 of 38

416 417

Table 1. Summary of RI values retrieved from STEM/EELS data compared to previous literature

418

values from EELS (*) and optical methods (†) Sample

SRFA

This Study

Literature

(wavelength)

(wavelength)

1.22-0.07i (532 nm)

1.52-0.02i (532 nm) 29, † 1.65-0.02i (532 nm) 30, † 1.602-0.1i (390 nm) 30, †

1.20-0.08i (390 nm)

1.69-0.05i (390 nm) 31, † 1.61-0.05i (390 nm) 32, †

ASOP

1.29-0.07i (532 nm)

-

1.28-0.09i (390 nm) Heated

1.90-0.38i (532 nm)

ASOP

1.81-0.44i (390 nm)

Brown

-

-

1.4-0.1i (550 nm)17,* 1.67-0.27i (550 nm)40,*

Carbon

1.79-0.15i (550 nm)14,† 1.88-0.27i (550 nm)14,† Black Carbon

-

1.8-0.5i (550 nm)17,* 1.95-0.79i (550 nm)40,*

419 420

A comparison of the retrieved RI for SRFA and ASOP is shown in Figure 7. The real part

421

of the RI is larger for ASOP than for SRFA over the entire wavelength region. However, the

422

imaginary parts of the RI for ASOP and SRFA are consistent over the range of 200-1200 nm. For


Page 23 of 38

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423

instance, the RIASOP at 532 nm is 1.29-0.07i compared to RISRFA of 1.22-0.07i. Tabulated RIASOP

424

values are included in Table S1. The uncertainty in the retrieved RI SRFA is relatively small with

425

the standard deviation from all particles measured in the real part of 0.02 and in the imaginary

426

part of 0.01 at 532 nm. The uncertainty for the RIASOP values are higher, at 0.06 for the real part

427

and 0.02 for the imaginary part at 532 nm. A larger uncertainty is expected for field-collected

428

samples due to variance in composition. No trend is observed with the variance of the RIASOP

429

with size over the size range of probed particles (50-200 nm). Comparison of the individual

430

single scattering distributions can be found in Figure S3 where the spectra are similar for all

431

particles. The differences between RI SRFA and RIASOP demonstrate that although their absorption

432

properties are comparable, ASOP scatters more efficiently than pure SRFA. Both of these

433

particle types originate from natural organic matter, so similarity in RI would be expected. The

434

major chemical difference between ASOP and SRFA particles is that ASOP has a larger

435

contribution of sp2 to total carbon and an oxygen rich surface.



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Page 24 of 38

436 437

Figure 7. Comparison of the broadband values of RI SRFA (green) and RIASOP (brown). Upper

438

panel shows the real part (n) and lower panel shows the imaginary part (k). Shaded region

439

indicates one standard deviation.

440 441

Figure 8 compares the retrieved values for RIASOP with reported EELS data for field

442

collected tar balls40, amorphous carbon, and graphitic carbon.17 Other BrC particles that have

443

been analyzed previously using EELS to determine the RI were tar balls from biomass burning,

444

collected above the Yellow Sea during the Asian Pacific Regional Aerosol Characterization

445

Experiment.40 Both ASOP and tar balls are derived from plant degradation materials, but with

446

different emission mechanisms. ASOP are emitted from wet soils during rain events.12, 28 Tar

447

balls are generated through the thermal decomposition (pyrolysis) of biomass.13, 61 The RIASOP


Page 25 of 38

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448

values (1.29-0.07i at 532 nm) are considerably lower than the RItar balls (1.67-0.27i at 550 nm),40

449

but similar to the amorphous carbon spheres (1.4-0.1i at 532 nm).17 The RItar balls values are

450

slightly lower than the RIASOPh retrieved for heated ASOP and the RIGC of graphitic carbon.17

451

Reported RItar balls values could be slightly higher due to the relativistic effects of using a higher

452

energy electron beam in that study (120 kV versus 80 kV here). Recent laboratory studies show

453

that the retrieved RI values for laboratory generated tar ball proxies are between 1.79-0.15i and

454

1.88-0.27i at 550 nm.14 Additional studies show that tar balls can also have a much lower RI

455

values (1.56-0.02i 13; 1.72-0.008i,1.81-0.006i, 1.75-0.002i).62 When ASOP were heated, the

456

particles charred and had a higher RI than the unheated particles. The retrieved RIASOPh at 532 nm

457

was 1.90-0.38i, which is similar to that reported for graphitic carbon (1.8-0.5i to 1.95-0.79i17 and

458

1.95-0.79i40 at 550 nm). This RIASOPh value is also higher than those reported previously for tar

459

balls. The NEXAFS spectra shown in Figure 5 indicate that when the ASOP were heated, their

460

sp2 peak increased while the oxygenated peaks decreased, indicating charring of particle

461

material. The higher fraction of sp2 carbon present in the heated particles will lead to higher

462

RIASOPh and absorption in the visible light range.63



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Page 26 of 38

463 464

Figure 8. Comparison of the retrieved values of RI SRFA, RIASOP and RIASOPh compared with

465

previous literature employing EELS based method. Upper panel shows the real part (n) and

466

lower panel shows the imaginary part (k) of RI.

467 468

Understanding the optical properties of ASOP is important because they may contribute a

469

significant fraction of the total aerosol present in certain environments.12 Once the RIASOP over a

470

broad wavelength range is retrieved, Mie theory can be used to estimate their contribution to the

471

radiative forcing. Figure 9 shows the scattering efficiency (Qsca), absorption efficiency (Qabs),

472

and single scattering albedo (SSA) calculated for ASOP in the size range of 50-800 nm based on 26 ACS Paragon Plus Environment

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473

the broadband (200-1200 nm) values of RIASOP. Tabulated data for Qsca, Qabs, and SSA is

474

included in Table S1. Since no substantial difference in the RI was observed for particles

475

between 50-200 nm, the retrieved RI was applied for diameters up to 800 nm to calculate Qsca,

476

Qabs, and the SSA. For particles in the size range of 400-800 nm, the maximum scattering

477

efficiency is approximately at 430 nm. For particle sizes smaller than 400 nm, the scattering

478

efficiency decreases with increasing wavelength. At all particle sizes, the maximum absorption

479

efficiency is at the shortest wavelength (200 nm) and decreases as the wavelength increases. The

480

SSA has a maximum at approximately 600 nm for particles in the size range of 400-800 nm. No

481

maximum absorption was observed for smaller particles that exhibit a decrease in the SSA with

482

increasing wavelength.

483

484 485

Figure 9. Comparison of a) the scattering efficiency (Qsca), b) absorption efficiency (Qabs), and

486

c) single scattering albedo (SSA) of ASOP calculated from the retrieved RIASOP over the

487

wavelength range of 200-1200 nm and particle diameters of 50-800 nm. Note the visible

488

wavelength region is shown by the corresponding colors, while the UV and IR are faded to

489

black.

490 491



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 38

492

The highest prevalence of ASOP was observed on stage 7 of the MOUDI impactor (size

493

range of 0.32-0.56 µm) with most particles having a diameter between 400-600 nm (Figure S5).

494

Stages with either larger or smaller cut-off sizes did not contain ASOP in sufficient quantities to

495

analyze. The size distribution is similar to the previously reported median size of approximately

496

500 nm by Wang et al. 2016 where a Sioutas cascade (SKC Inc.) impactor (size range of 0.25-

497

0.5 µm, stage D) was used.12 A comparison of the Qsca, Qabs, and SSA of 500 nm diameter

498

particles for SRFA, ASOP, and heated ASOP is shown in Figure 10. ASOP have consistently

499

higher Qsca values than SRFA, with the highest scattering efficiency between 200-400 nm

500

decreasing rapidly with increasing wavelength. ASOP and SRFA particles have similar Qabs over

501

all wavelengths, with ASOP being slightly more absorbing. In both cases, Qabs decreases as the

502

wavelength increases. The absorbance enhancement at lower wavelengths is consistent with

503

previously reported optical properties for atmospheric BrC.15-16 The ASOP have a consistently

504

higher SSA than SRFA particles, and show a similar trend with a decreasing SSA as the

505

wavelength increases. The major difference between ASOP and SRFA is in the carbon content,

506

with the ASOP particles having a higher contribution of sp2 carbon and lower contributions of C-

507

OH and –COOH compared to SRFA particles. Additionally, minor contributions of Na and S in

508

SRFA may contribute to the differences in optical properties as well. The optical properties of

509

these particles are similar to other BrC species with an AAE of 1.8 for 200 nm size particles

510

which is in the range of 1.6-11 for BrC.15, 22 For the heated ASOP, the Qsca peaks at a wavelength

511

of approximately 780 nm, and heated ASOP consistently scatter more than non-heated ASOP at

512

all wavelengths above 400 nm. The Qabs of heated ASOP is fairly consistent over the range of

513

200-400 nm with a slight increase from 600-800 nm. This relatively flat absorbance over all

514

wavelengths is consistent with BC species.16 This is in contrast to ASOP and SRFA which are


Page 29 of 38

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515

highly absorbing at lower wavelengths and decrease in absorptivity at higher wavelengths;

516

characteristic trends of BrC. Heated ASOP show invariable SSA across all wavelengths

517

consistent with the optical properties of carbonized materials. The effect of retrieving an accurate

518

RI over a wide wavelength range coupled with knowledge of the size distribution of particles

519

will help assess the effects of ASOP on climate forcing.

520



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Page 30 of 38

521

Figure 10. Comparison of the a) the scattering efficiency (Qsca), b) absorption efficiency (Qabs),

522

and c) single scattering albedo (SSA) of 500 nm sized particles of SRFA, ASOP, and heated

523

ASOP particles.

524 525

526 527

Conclusions: The RI values for ASOP and SRFA particles were determined using STEM/EELS on a

528

single particle basis. SRFA is a common standard for retrieving optical properties and is similar

529

to ASOP since they are both from biological degradation products. The RISRFA was 1.22-0.07i at

530

532 nm, which is a slightly lower real part and higher imaginary part than retrieved through

531

optical methods. The RIASOP ranged from 1.21-0.11i to 1.29-0.08i over the 200-1200 nm

532

wavelength region. These values are similar to the retrieved values for amorphous carbon17 and

533

lower than tar balls40 by EELS. EELS retrieval of the RI of less absorbing particles (such as

534

SRFA and ASOP) overestimates the imaginary part of the RI for particles with a low absorption,

535

which in turn will lead to an underestimation of the real part of the refractive index. The optical

536

properties were related to particle compositions and the NEXAFS data indicated that ASOP had

537

higher fractions of sp2 carbon while the SRFA particles showed higher contributions from R-OH

538

and –COOH functional groups. The ASOP composition is consistent with ‘free-light’ SOM

539

indicating that the particles are likely from decomposition of natural organic materials. The

540

higher sp2 carbon fractions present in the ASOP results it an overall higher RI than that of SRFA.

541

When compared to previous literature with the EELS based retrievals of RI, the RIASOP values

542

were much lower and distinct from tar balls and were similar to values retrieved for amorphous


Page 31 of 38

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543

carbon. Heating of the ASOP led to charring and removal of the oxygen-rich exterior of the

544

particles as shown by EELS and NEXAFS. Correspondingly, this led to higher values of

545

retrieved RI that were similar to those of graphitic carbon. Since the optical properties of BrC

546

species are not commonly categorized or used in atmospheric models, incorporation of the

547

RIASOP values for modeling predictions in the regions where they might be a considerable

548

fraction of the aerosol population can lead to better modeling of the radiative forcing. Proposed

549

mechanisms for formation of ASOP are through impaction of rain droplets on soil.28 It is

550

suggested that ASOP would be abundant in areas that have exposed open soils, such as

551

agricultural areas or grassland, that experience large amounts of rainfall.12 Various factors affect

552

the emission rates of ASOP: rainfall intensity, rain drop size, relative humidity and temperature

553

after rainfall, and seasonal variation that affect chemical composition of water soluble soil

554

organic matter. Additional studies are needed in order to understand the emission mechanism of

555

ASOP, contribution to the total aerosol budget, and variability in their chemical composition.

556

Suitable standards that have a similar refractive index to the particles of interest and are robust

557

under the electron beam are needed to calibrate the retrieved RI. Since the RIASOPh was very

558

close to the RI of graphitic carbon, the EELS method to retrieve the RI of single particles could

559

be more robust for highly absorbing species. For example, highly absorbing flame soot64 and

560

slightly absorbing humic-like substances65 can be used for further validation of the technique.

561

By retrieving the optical constants based on EELS measurements of individual particles within

562

multicomponent particles samples, radiative properties of individual components can be

563

evaluated for subsequent use in atmospheric models.

564

565

Supporting Information: 31 ACS Paragon Plus Environment


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566 567

Back trajectories for field collected samples (Figure S1); Comparison of consecutive spectrum

568

for a single SRFA particle and STEM image after approximately 10 minutes of beam exposure

569

(Figure S2); Single scattering distributions for SRFA, ASOP, and heated ASOP (Figure S3);

570

Comparison of the RI of polystyrene spheres retrieved from EELS and CRD measurements

571

(Figure S4); Size distribution of ASOP (Figure S5); Tabulated data of ASOP (Table S1)

Page 32 of 38

572

573

Acknowledgements:

574

We are grateful to D. Bonanno and G. Kulkarni for assistance in sample collection at the SGP

575

site. We are grateful the ARM field site staff for assistance in sample collection at the Southern

576

Great Plains. The Pacific Northwest National Laboratory (PNNL) group acknowledges support

577

from the Science Acceleration Project of the Environmental Molecular Sciences Laboratory

578

(EMSL) at PNNL. The Lawrence Berkeley National Laboratory (LBNL) group acknowledges

579

support from the US Department of Energy's Atmospheric System Research Program, an Office

580

of Science, Office of Biological and Environmental Research (OBER). J. W. acknowledges the

581

student exchange program between the University of Würzburg and U.C. Berkeley (curator

582

Professor A. Forchel, Würzburg and NSF IGERT program at UCB, DGE-0333455, Nanoscale

583

Science and Engineering - From Building Blocks to Functional Systems.) The CCSEM and

584

STEM/EELS analyses were performed at EMSL, a National Scientific User Facility sponsored

585

by OBER at PNNL. PNNL is operated by the US Department of Energy by Battelle Memorial

586

Institute under contract DE-AC06-76RL0. STXM/NEXAFS analysis at beamlines 5.3.2.2 and

587

11.0.2.2 of the Advanced Light Source at LBNL is supported by the Director, Office of Science,

588

Office of Basic Energy Sciences of the US Department of Energy under Contract No. DE-AC0232 ACS Paragon Plus Environment

Page 33 of 38

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589

05CH11231. We acknowledge use of the NOAA Air Resources Laboratory for the provision of

590

the HYSPLIT transport and dispersion model and READY website (http://www.ready.noaa.gov)

591

used in this publication.

592 593



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