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Evaluation of DMA size selection of dry dispersed mineral dust particles a

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K. Ardon-Dryer , S. Garimella , Y.-W. Huang , C. Christopoulos & D. J. Cziczo

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Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA Accepted author version posted online: 31 Jul 2015.

Click for updates To cite this article: K. Ardon-Dryer, S. Garimella, Y.-W. Huang, C. Christopoulos & D. J. Cziczo (2015): Evaluation of DMA size selection of dry dispersed mineral dust particles, Aerosol Science and Technology, DOI: 10.1080/02786826.2015.1077927 To link to this article: http://dx.doi.org/10.1080/02786826.2015.1077927

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ACCEPTED MANUSCRIPT Evaluation of DMA size selection of dry dispersed mineral dust particles

K. Ardon-Dryer, S. Garimella, Y.-W. Huang, C. Christopoulos, and D. J. Cziczo Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA

Address correspondence to D. J. Cziczo, Atmospheric Science and Global Change, Pacific

Downloaded by [] at 07:56 04 August 2015

Northwest National Laboratory, P.O. Box 999, MSIN: K9-24, Richland, Washington 99352, USA. E-mail: [email protected] Short title: Evaluation of DMA size selection of DD MD particles

Abstract Mineral dust particles play a significant role in the Earth‟s radiative balance via direct interaction with solar radiation and indirectly through their ability to initiate cloud formation. Many field and laboratory studies utilize a Differential Mobility Analyzer (DMA) for particle size selection. Here we evaluate the use of a DMA to size-segregate dry dispersed mineral dust particles. We examine the post-DMA size distribution using four different techniques: a Scanning Mobility Particle Sizer (SMPS) for mobility sizing, an Optical Particle Sizer (OPS) for optical sizing, the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument for vacuum aerodynamic sizing, and Electron Microscopy (EM) for geometric sizing. While the SMPS measured a narrow mobility size distribution at the DMA-selected diameter, the OPS, PALMS, and EM in most cases showed broader distributions and a smaller mode size than that selected by the DMA.

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ACCEPTED MANUSCRIPT These techniques also observed super-micrometer particles, often extending beyond the upper size limit of a typical SMPS scan. Complicating analysis, particle shape factor (χ) was observed to be a function of mobility size, ranging from 1.3 at 500 nm to 3.1 at 1000 nm. We conclude that mobility size selection of mineral dust particles using a DMA most often does not yield particles of the desired physical size or surface area. We suggest that attempts to size-select from a broad distribution of non-spherical particles require an independent measurement downstream


of the DMA to verify the actual selected size.

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ACCEPTED MANUSCRIPT 1. Introduction Mineral dust particles, liberated from windblown soil through the saltation process, play an important role in the physics and chemistry of the Earth‟s atmosphere. These particles can directly affect the global radiation balance through scattering and absorption of solar rays and, to a lesser extent, interaction with terrestrial radiation (Hudson et al. 2008). Mineral dust particles can also indirectly affect radiative balance by influencing cloud development, cloud lifetime (Lohmann and Diehl 2006; Wallace and Hobbs 2006), and the properties of precipitation


(Rosenfeld et al. 2001) by acting as Cloud Condensation Nuclei (CCN) or Ice Nuclei (IN) (Twohy et al. 2009; Cziczo et al. 2013; Sullivan et al. 2010; Ardon-Dryer and Levin 2014).

Previous laboratory studies have investigated the effect of mineral dust particles on radiation (Curtis et al. 2008; Hudson et al. 2008) and the conditions at which different mineral dust particles serve as CCN (Herich et al. 2009; Kumar et al. 2011; Garimella et al. 2014) and IN (Hoose and Möhler 2012). Heterogeneous chemical reactions on mineral dust particles are of interest because they can change particle surface properties, which can then affect the properties of mineral dust as CCN (Vlasenko et al. 2006) or IN (Cziczo et al. 2009; Kanji et al. 2011; Jones et al. 2011).

Many types of mineral dust particles examined in the laboratory are derived from naturally dusty areas. One dust type commonly used in laboratory experiments is Arizona Test Dust (ATD), which is collected from the Arizona desert and ground to a specific size range (Marcolli et al. 2007; Kanji et al. 2008; Bundke et al. 2008; Koehler et al. 2009). Standards, typically chosen to

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ACCEPTED MANUSCRIPT represent different types of natural mineral dust, are also commonly used. Examples include montmorillonite, kaolinite and illite which are frequently used as surrogates of atmospheric mineral dust particles (Kanji and Abbatt 2006; Zimmermann et al. 2007; Salam et al. 2008; Wheeler and Bertram 2012).

The size of mineral dust particles is an important factor in their ability to interact with the environment. In general, higher CCN and IN efficiency is observed for larger particles due to the


higher average surface area (Hung et al. 2003; Archuleta et al. 2005; Sullivan et al. 2009; Kumar et al. 2011). Consideration of a specific mineral dust particle size in laboratory experiments is most often accomplished using a Differential Mobility Analyzer (DMA), which is used to select a specific mobility diameter of particles (BMI 2012a).

A DMA operates by balancing the electrical force on a charged particle in an applied field with the drag force on the particle as it moves through the air in which it is contained. A DMA normally consists of a cylindrical central electrode and a coaxially aligned cylindrical housing. Particle-free air is supplied in the annular region around the central electrode. Before input to the DMA, the aerosol is typically passed through an ionized radiation source (or „neutralizer‟), such as Polonium-210 or Krypton-85, which imparts particles with a known, approximately Boltzmann, charge distribution. The „neutralized‟ particles then enter the DMA in a thin annular ring adjacent to the outer cylinder. Particles are drawn by the electric field according to their charge: they are either attracted or repelled by the potential on the DMA‟s center. Particles with a specific electrical mobility pass through an outlet slit at the end of the central rod. Particles of

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ACCEPTED MANUSCRIPT higher electric mobility (particles that are smaller and/or carry more charges) impact the center rod upstream of the sample slit whereas particles of lower mobility (larger and/or less charged) strike below the exit slit or pass out of the DMA through the output sheath flow (Hewitt 1957; Liu and Pui 1974; Knutson and Whitby 1975). The particle electrical mobility, Zp, is defined as: ZP 

neCc [1] 3 Dm


where n is the number of elementary electrical charges carried by the particle, e is the charge of an electron, Cc is the Cunningham slip correction factor, μ is the dynamic viscosity of air, χ is the particle dynamic shape factor, and Dm is the electrical mobility diameter of the particles (Hinds 1982). Electrical mobility is therefore directly proportional to particle charge and inversely proportional to particle size, so the smaller the particle and/or the higher the charge the greater the mobility (Knutson and Whitby 1975; Intra and Tippayawong 2008). Since particles entering the DMA are imparted with a Boltzmann charge distribution, the output size distribution can contain multiply charged particles of larger sizes. The most common multiply charged particles are those with two and three elementary charges, often referred to as doubly and triply charged particles. Gunn (1956) and Wiedensohler (1988) provided methodologies to estimate the fraction of multiply charged particles in the output distribution. In addition to charge, mobility also depends on the particle shape; e.g. a non-spherical particle experiences a higher drag force than that experienced by a spherical particle (DeCarlo et al. 2004). Since mineral dust particles are non-spherical (Hudson et al. 2008; Niedermeier et al. 2011; Veghte and Freedman 2012) and can

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ACCEPTED MANUSCRIPT carry significant electrical charge (Forsyth et al. 1998), special care is required to ensure the desired results from DMA size selection.

Although size selection of mineral dust particles with a DMA has been the focus of many previous laboratory studies, only a few have verified the output of the DMA after sizing (Lüönd et al. 2010; Welti et al. 2009; Koehler et al. 2010; Ladino and Abbatt 2013). Some studies theoretically calculated the fraction of doubly and triply charged particles expected downstream


of the DMA (Welti et al. 2009; Koehler et al. 2010), while others (Lüönd et al. 2010; Ladino and Abbatt 2013) used a Scanning Mobility Particle Sizer (SMPS) to examine the size distribution. Veghte and Freedman (2012) verified the size distribution downstream of the DMA by collecting and analyzing particles on microscope grids. Most of these studies report post-DMA size distributions that are (1) broader than can be explained by simple theory and/or (2) have a different mode size than selected. Based on these previous studies it is unclear if a DMA can correctly size select from a dry-generated mineral dust particle source. The purpose of this work is to evaluate DMA size selection of dry dispersed mineral dust particles by downstream measurement of size distribution using multiple techniques.

2. Methodology 2.1 Samples The mineral dust particles investigated in this study were “Nominal 0–3 μm” ATD from Powder Technology Inc., Illite (rock chips) from Clay Mineral Society, and sodium-rich montmorillonite (NaMon, unspecified size powder) from Clay Mineral Society. Grinding mineral samples prior to

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ACCEPTED MANUSCRIPT aerosolization is a common procedure to increase the number concentration of sub-micrometer particles in each sample (Veghte and Freedman 2012; Garimella et al. 2014; Ladino and Abbatt 2013). In this study, grinding was performed on the rock chips and a portion of the unspecified size powder (the unground NaMon is termed “UgNaMon”). The methodology was described in Garimella et al. (2014): ~10g of sample was wet-ground in ethanol in a Fisher Model 8-323V2 Mortar Grinder with agate pestle and mortar for three hours and then dried overnight under a flow of filtered air. This grinding process is known to result in thinner crystalline sheets and is


less likely to degrade the clay crystallinity than other types of grinding (Cicel and Kranz 1981). Reference experiments were also conducted on Ammonium Sulfate ((NH4)2SO4; hereafter AS). Polydisperse droplets were generated with a constant output atomizer. An aqueous solution of Milli-Q water (18.2 MΩ-cm) and 11 weight % (wt%) reagent grade ammonium sulfate (99%, Sigma Aldrich, St. Louis, MO) was used to produce the AS aerosols.

2.2 Experimental setup The setup for the sizing experiments consists of three parts: particle generation, particle size selection, and measurement of the output size distributions (see Figure 1). Conductive silicone tubing carried particles between all components to minimize particle loss to tubing by electrostatic forces. Prior to the experiment, all instruments were calibrated with different PSL particles in order to verify their performance. All the experiments were performed at a temperature of 25±1.5 °C, pressure of 1010±6.6 mb and relative humidity of ~50%.

2.2.1. Particle generation and impaction

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ACCEPTED MANUSCRIPT AS particles were atomized using a Brechtel Manufacturing, Inc. (BMI) 9203 Aerosol Generator. The atomized aerosol entered integrated in-line dryers where they evaporated, leaving anhydrous crystalline particles (BMI 2012b). ~2 lpm of aerosol flow exited the atomizer and entered the sizing component of the setup. No impaction was used with AS since the particles were predominantly sub-micrometer in size.

The mineral dust particles were dry-generated by placing ~5 g of the desired sample in a 1000


mL Erlenmeyer flask containing 3 Teflon-coated stir bars. The mineral dust particles were agitated using a Lab Line 3589 Shaker, and 2 lpm of dry, filtered nitrogen flowed through the flask to suspend the particles. The suspended particles then passed through a 500 mL unshaken Erlenmeyer flask used as a buffer volume to maintain constant particle concentrations and size distributions during an experiment. This generation methodology, based on that used by Kumar et al. (2011), is described in Garimella et al. (2014). This technique was developed to simulate the natural saltation process and generates size distributions similar to those found in naturally dusty source regions (Lafon et al. 2006).

After mineral dust particles were generated, they either (1) directly entered the DMA to be size selected or (2) were passed through two URG Corporation cyclone impactors with 50% cut sizes of 2.5μm (URG-2000-30EHS) and 1.0μm (URG-2000-30EHB) at 16.7 lpm. The later were used to eliminate particles larger than 1μm. In this case, a flow of 14.7 lpm of dry nitrogen was added to the 2 lpm polydisperse flow upstream of the impactors. Excess flow not drawn into the DMA was filtered and vented (see Figure 1).

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2.2.2. Size selection A Brechtel Manufacturing, Inc. (BMI) 2002 DMA (BMI 2012a) was used to select particles at mobility sizes of 500, 750, and 1000 nm. This DMA (#1 in Figure 1, hereafter DMA #1) was outfitted with a standard BMI impactor with a 50% cutoff at 650 nm at 1 lpm and a Polonium210 radioactive source (Kim et al. 2005). A 5 lpm sheath flow and 0.5 lpm sample flow were used in all experiments to maintain a sheath to sample flow ratio of 10:1. It should be noted that


the use of the 650 nm impactor prior to the DMA was to reduce the presence of large particles. The use of this impactor did not completely eliminate particles above the cut point but did reduce their concentrations. This is discussed further in subsequent sections.

2.2.3. Characterization of post-DMA size distributions The output of DMA #1 was characterized in real time using a Scanning Mobility Particle Sizer (SMPS), Optical Particle Sizer (OPS), and the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument. In addition, ATD size-selected by DMA #1 was impacted on grids for offline analysis using Electron Microscopy (EM).

2.2.3.1. Scanning Mobility Particle Sizer (SMPS) A second BMI 2002 DMA (#2 in Figure 1, hereafter DMA #2) and a BMI 1700 Mixing Condensation Particle Counter (MCPC, hereafter r CPC) were used as a SMPS for measuring the mobility size distributions downstream of DMA #1. Unlike in DMA #1, this SMPS has neither an impactor nor a neutralizer in order to avoid altering the measured size distributions. The

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ACCEPTED MANUSCRIPT sample flow through DMA #2 was set to 0.5 lpm, 0.36 lpm of which was drawn by the CPC; the remaining 0.14 lpm was pumped through a filter and exhausted (see Figure 1). The sheath flow in DMA #2 was set to 5 lpm to maintain a fixed 10:1 ratio of sheath to sample flow. For each mobility size selected by DMA #1, the SMPS performs three 5-minute scans and measures mobility size distributions between 10 and 103 nm.

2.2.3.2. Optical Particle Sizer (OPS)


A TSI Model 3300 OPS (TSI 2012) was used to measure the size distributions downstream of DMA #1 over the size range of 350 to 104 nm. Since the OPS draws 1 lpm, 0.5 lpm of dry nitrogen was mixed with the 0.5 lpm aerosol flow that exits DMA #1. Size spectra measured by the OPS were 5-minute averages. Estimates of the density, complex refractive index, and the shape factor for each particle type were specified in the collection software prior to an experiment in order to provide accurate optical size distributions. These data were collected from the literature and can be found in Table 1.

2.2.3.3. Particle Analysis by Laser Mass Spectrometry (PALMS) The laboratory PALMS instrument, described previously (Murphy and Thomson 1995; Cziczo et al. 2006), was used to measure the vacuum aerodynamic diameter of particles exiting DMA #1. Vacuum aerodynamic diameter is frequently measured in instruments that use low-pressure aerodynamic lens systems as inlets. Sizing with an aerodynamic lens is accomplished by measuring the size-dependent velocity that the particles acquire during the mild supersonic expansion into vacuum that occurs at the end of the aerodynamic lens (DeCarlo et al. 2004). In

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ACCEPTED MANUSCRIPT this work, the particles were introduced to PALMS via an aerodynamic lens inlet. A continuous 532 nm Nd:YAG laser beam is split into two beams to detect the particles via scattering, and the time difference between the signals provides information for determining vacuum aerodynamic size (Cziczo et al. 2006). The sizing range for detection by PALMS is 150 to 3x103 nm. In this study, >500 particles were measured for each sample at each selected size. In this configuration, PALMS drew ~0.5 lpm so no additional flow was required.


2.2.3.4. Electron microscopy (EM) In addition to analysis using the aforementioned on-line methods, EM was used to investigate size-selected ATD particles. Particles were collected on EM grids using a Micro-Orifice Uniform-Deposit M135-10 conventional impactor. A 0.5 lpm flow was drawn through the impactor after exiting DMA #1. Electron Microscopy Sciences FCF200 gold microscope grids with carbon support film were used as the substrate. The grids were analyzed using a FEI Tecnai Multipurpose (G2 Spirit TWIN) Transmission Electron Microscopy at the MIT Center for Materials Science and Engineering to determine the size distributions. A total of six grids were sampled, one for each selected size of ATD particles, both with and without the upstream cyclone impactors. As suggested by Kulkarni et al. (2011), sampling was randomized by choosing a subsection of the microscope grid at low magnification and then proceeding to measure the particles in that section. Furthermore, sample areas were chosen from the interior of the grid to minimize shatter and other artifacts along the outer edge. ~500 particles were analyzed per grid sample.

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ACCEPTED MANUSCRIPT Surface area-equivalent (Dse) and volume-equivalent diameters (Dve) of the mineral dust particles were determined using the EM (Garimella et al. 2014). Mineral dust particle geometries are approximated as cylinders with average diameter (DEM) and thickness (hEM). For a population of aerosol particles on a grid, the distribution of DEM is constructed using the visible face-on particles, and the distribution of hEM is constructed using the edge-on particles (an example of EM image with particles DEM and hEM can be found in Figure 2). Each particle was measured several times (from one side to the other), the average of those measurements represented the


particle average DEM or hEM values. These two orientations are distinguishable based on particle aspect ratios and surface morphologies (Figure 2), since the edge-on particles have large aspect ratio and display the layered structure of the clays. These observations provide estimates for Dse and Dve where: 1/2

1 2  Dse   DEM  DEM hEM  2 

[2]

1/3

3 2  Dve   DEM hEM  2 

[3]

Furthermore, observed values for Dve can be combined with specified values of the particle‟s electrical mobility diameter Dm to provide estimates for particle shape factors using Equation (4) (see section 2.4).

2.4. Conversion to volume-equivalent diameter (Dve) To facilitate the comparison of results from different instruments, electrical mobility and vacuum aerodynamic diameters were converted to Dve, the value reported by the OPS. These conversions

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ACCEPTED MANUSCRIPT were based on the calculations of DeCarlo et al. (2004) where the relationship of a particle‟s Dve to its Dm is expressed by rearrangement of Equation (25) in DeCarlo et al. (2004):

Dve 

1 Cc  Dve  D  Cc  Dm  m

[4]

Similarly, the relationship between a particle‟s Dve and its vacuum aerodynamic diameter can be expressed by rearrangement of Equation (28) in DeCarlo et al. (2004):


 1  p Cc  Dve   Dve      0 Cc  Da  

1/2

[5]

Da

where ρp is the particle density, ρo is the reference density of the calibration particles (in this case Polystyrene Latex Sphere (PSL) spheres particles with a density of 1000 kg m-3), and Dα is the particle‟s vacuum aerodynamic diameter. Both of the above equations can be solved numerically to determine Dve. In general, a size distribution shifts to smaller size after conversion to Dve. Particle densities and shape factors are provided in Table 1.

3. Results Here we present results from experiments where particles were size-selected using DMA #1 at three different mobility diameter sizes: 500, 750 and 1000 nm. The output of this size selection was examined using three instruments: the SMPS for mobility sizing, the OPS for geometric sizing, and PALMS for vacuum aerodynamic sizing. In addition, ATD particles were also sampled on EM grids for geometric sizing. Results from AS and ATD size selection are detailed.

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ACCEPTED MANUSCRIPT It is noteworthy that each of the post DMA instruments used in this work has its advantages and limitations. While the SMPS can detect small particles, with sizes as low as 10 nm, it cannot detect particles larger than 103 nm. On the other hand, PALMS and the OPS can detect very large particles (up to 3x103 and 104 nm, respectively). They are limited in the detection of small particles, however: PALMS cannot detect particles smaller than 150 nm and the OPS cannot detect particles smaller than 350 nm. EM, on the other hand, can detect a very broad range of particle sizes, but is a labor-intensive off-line technique. The lack of SMPS data above 103 nm


(where the OPS and PALMS show significant particle concentrations) or the larger relative abundance of particles from the SPMS at small sizes (where there are no OPS and PALMS counts) is thus related to the different sensitivities and not a disagreement in the results. These different instrument sensitivities are inherent in the following figures. For clarity, the comparison of data in the figures has been normalized but the different instrument sensitivities means that the normalization, to the peak in each distribution, is not the same for each instrument. Furthermore, because of these different sensitivities, no instrument should be considered „truth‟ and data should be considered as a comparison. 3.1. AS Size distributions of AS particles measured by the SMPS, OPS and PALMS downstream of DMA #1 are presented in Figure 3. The concentration of each figure was normalized based on its maximum concentration. The upper row of Figure 3 (a-c) shows size distributions measured by the SMPS, the middle row (d-f) shows size distributions measured by the OPS, and the third row (g-i) shows size distributions measured by PALMS. The middle column (size selection of 750 nm particles) also shows fitted normal (for the SMPS) and lognormal (for the OPS and PALMS)

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ACCEPTED MANUSCRIPT distributions. Normal and lognormal fits were chosen due to their similarity to the measured size distributions. These fits, and those described hereafter, are computed using only data from the first (singly charged, based on the minima between peaks) mode to avoid artificial broadening due to the presence of doubly and triply charged particles. The three instruments measure similar size distributions. Note that the SMPS has an upper size limit of Dm = 1000 nm at this flow ratio, so information for larger sizes is absent. The SMPS distribution was processed by assuming a Boltzmann distribution based on the default procedure from the BMI software (BMI 2012a).


Since DMA #1 selects particles with a given mobility (including doubly and triply charged particles), a downstream SMPS is expected to show a highly monodisperse distribution if no additional neutralization is used. That is to say, in the absence of a change in particle charge any doubly and triply charged particles transmitted by DMA #1 would appear as part of a single mobility peak transmitted by DMA #2. Multiply charged particles output by DMA #1 will be observed at larger sizes in the OPS and PALMS distributions, however. This behavior is apparent in Figure 3 and explains the secondary peak at larger size in the OPS and PALMS spectra. In order to compare between the selected and the observed diameters, mode sizes were converted to Dve (see section 2.4). For AS particles with selected mobility diameters of 500, 750 and 1000 nm, the converted Dve is 484, 725 and 965 nm, respectively. Mode sizes and standard deviations are provided in Table 2. Figure 4 presents a graphical comparison between the selected and observed volume-equivalent diameters of size-selected AS particles. The three downstream instruments observed similar mode sizes to those which were selected. This result provides validation that DMA #1 correctly accomplishes a size selection of approximately spherical particles. In addition, SMPS, OPS and PALMS determinations of diameter, when

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ACCEPTED MANUSCRIPT converted to analogous size distributions using literature values, are equivalent. This agreement has also been shown in previous studies (Heintzenberg et al. 2002; Veghte and Freedman 2012).

3.2. Mineral dust particles Size distributions of cyclone-impacted ATD particles output by DMA #1 were measured by the SMPS, OPS, PALMS, and EM. Data are presented in Figure 5, analogous to Figure 3 for AS, where the upper row (a-c) shows size distributions measured by the SMPS, the second row (d-f)


size distributions measured by the OPS, the third row (g-i) size distributions measured by PALMS and the fourth row (j-l) size distributions measured using EM. Since the distributions of particle face areas and particle widths are independently measured, low and high estimates of the particle surface areas were used to provide a bracketing range for the EM size distributions. These estimates are represented by the white and black histograms in Figure 5 (j-l), where the white histograms are the mean particle width minus one standard deviation and the black histograms are mean particle width plus one standard deviation. For clarity, fitted normal (for the SMPS) and lognormal (for the OPS, PALMS) distributions are only shown in the middle column.

Data taken without impactors is shown in Figure 6. As mentioned previously, it is known that dust samples contain high concentration of large particles. The use of the impactors minimized the presence of these large particles but did not eliminate them. This can be seen in Figures 5 and 6. Even with the use of three impactors (cut off at 2.5 μm, 1 μm and 650 nm), large particles were still present in the distribution. Such behavior likely explains the motivation for the use of

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ACCEPTED MANUSCRIPT several impacors in previous studies (Ladino et al. 2011; Koehler et al. 2010; Ladino and Abbatt 2013). It is also worth noting that shattering of particles on the EM grid was not apparent in either ATD case (with and without the use of impaction). This observation is based on EM images (e.g., Figure 2) and the existence of a single mode in the size distribution plot (Figures 5 and 6 j-l).

Smaller mineral dust particles, size-selected at 100 and 250 nm, were also tested (Figure 7). A


comparison between different instruments for these small particle sizes could not be performed as completely as for the larger sizes because the selected mode size falls below the detection thresholds of PALMS and the OPS (150 and 350 nm, respectively). EM samples were not taken for these sizes. While the mode size cannot be considered, the transmission of larger particles – those within the PALMS and OPS range - is possible. While the SMPS exhibits a narrow distribution at the selected size, PALMS and the OPS both record a broader distribution extending to larger sizes. The mode size measured by the OPS and PALMS were larger than the selected sizes which is expected given the size limitation of these instruments. This behavior is similar to the findings by Veghte and Freedman (2012), where the mean area-equivalent diameters of particle size-selected below 300 nm were larger than the selected diameter. We note that Garimella et al. (2014) suggested that it is unlikely for dry dispersed dust particles smaller than 300 nm to be correctly selected using a DMA because these sizes are often smaller, and of significantly lower concentration, than the mode sizes of the initial distribution.

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ACCEPTED MANUSCRIPT Table 3 presents volume and surface area-equivalent mode diameters and standard deviations for ATD derived from EM measurements. Table 3 also details inferred shape factors for ATD, calculated using Equations (4) and (5) and by comparing the selected mobility diameters with the observed Dve. While Möhler et al. (2008) and Endo et al. (1998) specify a single χ value for ATD particle (χ =1.3 and 1.5, respectively), the results from this study suggest that χ changes as a function of particle mobility for ATD particles. We found that χ increases from 1.3 at 500 nm to 3.1 at 1000 nm and an agreement was found between the impacted and non-impacted methods.


The values of small particle sizes also agreed with the values found by Möhler et al. (2008). Kaaden et al. (2008) and Reid et al. (2003) also found an increase of shape factor values with increasing dust particles sizes.

Figure 8 presents a comparison between the selected and observed volume-equivalent diameters for ATD particles (analogous to that shown for AS in Figure 4), both with and without the cyclone-impactors (Panels a and b, respectively). Diameters were converted to Dve for comparison. For ATD particles with selected mobility diameters of 500, 750 and 1000 nm, the calculated Dve is 403, 598 and 791 nm, respectively. As expected, the SMPS distributions have a similar mode sizes to those selected by DMA #1. The other instruments measure smaller mode sizes, significant when compared to the agreement found for AS in Figure 4. This is not in contrast to the observation of larger than selected particles at 100 and 250 nm since, in those cases, the mode size was below the OPS and PALMS threshold. In all cases, from 100 to 1000 nm, the OPS and PALMS observe the transmission of particles above the selected size. Similar behavior is observed both with and without the cyclone impactors. The measured mode sizes

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ACCEPTED MANUSCRIPT (Dve) and standard deviations from the SMPS, OPS and PALMS are presented in Table 2. Comparisons between the selected and observed Dve for the other mineral dust types investigated (Illite, NaMon and UgNaMon), both with and without the use of impaction, exhibit the same behavior. Data for all experiments is provided in Table 2. Note that Dm of 500, 750 and 1000 nm corresponds to Dve of 484, 725 and 965 nm for illite and 462, 690 and 918 nm for NaMon and UgNaMon.


4. Discussion The major result of this work is that, while all instruments report similar size distributions for common density and quasi-spherical AS particles, only the SMPS (mobility size) agrees with the initial selection from DMA #1 for the mineral dust samples. In contrast, the OPS and EM (independent optical and geometric size measurements, respectively) and PALMS (vacuum aerodynamic size) report broader distributions that extend to sizes above that selected. Although of lesser abundance for measurements with the impactor, this observation is apparent in both the impacted and non-impacted aerosol samples. Large particles (500, 750 and 1000 nm) had a mode size smaller than the selected mobility diameter. Small particles (100 and 250 nm) had a mode size larger than the selected mobility diameter, although the mode was below the OPS and PALMS instrumentation thresholds.

Although size selection of dry-generated mineral dust particles with a DMA has been used by many previous laboratory studies (Lüönd et al. 2010; Welti et al. 2009; Ladino et al. 2011; Koehler et al. 2010; Ladino and Abbatt 2013), few have verified the size distribution

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ACCEPTED MANUSCRIPT downstream of the DMA. Of those, who did perform a verification, Lüönd et al. (2010) and Ladino and Abbatt (2013) used a SMPS, while Veghte and Freedman (2012) used EM. These studies also reported a broad mobility diameter distributions at a mode size roughly similar to that selected. In addition, Veghte and Freedman (2012), who size selected calcium carbonate particles in sizes between 300 to 900 nm using a EM, found that the resulting size distributions were more polydisperse compared to the sizing of AS particles. The mean area equivalent diameter was larger than the mobility diameter for smaller sizes and smaller for larger sizes. In


addition, the differences between the mean area equivalent diameters and mobility diameters were 5.2% for 400 nm and 7.5% for 600 nm. Based upon the data presented here and these previous literature studies of dry dispersed mineral dust particles, we suggest that the assumption that a DMA produces only the selected particle size is insufficient for verifying the actual output size distribution. Several factors affect the ability of a DMA to size-select dry-generated mineral dust particles. These include the particle charge, upstream size distribution, and shape. Dry-generated mineral dust particles can have higher charge compared to other species, such as organics and inorganic salts (Forsyth et al. 1998; Niedermeier et al. 2010). This may affect the efficacy of the neutralizer and potentially allow abundant highly charged particles (i.e., doubly, triply and greater charged particles) to pass through the DMA. The upstream polydisperse size distribution of particles can also affect the ability of the DMA to size select particles. Mineral dust samples often contain a significant number of coarse mode particles and, when coupled to the increased probability of multiple charges, this can lead to a high number of inadvertently transmitted particles. This work suggests that neither cyclone impaction nor grinding (comparing NaMon and UgNaMon)

20

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ACCEPTED MANUSCRIPT eliminates the abundance of the larger particles in the post-DMA #1 particle distributions. Particle shape can also affect the ability of the DMA to size select particles. Sizing becomes ambiguous for particles with irregular shapes, especially when compared to sizing quasispherical AS particles with known density. For a sample with significant inhomogeneity in shape, particles that are similar volume-equivalent size but different χ can experience different amounts of drag in the DMA column, especially if their orientations are also different. These effects can account for the observations of similar geometric diameter, volume-equivalent, and


surface area-equivalent diameters of particles at different selected mobility size. Since the shape factors of mineral dusts are >1, and in some cases >3, Equation (4) indicates that similar observed diameters tend to be smaller than the selected mobility diameter. Each of the factors mentioned above could have potentially affected the ability of the DMA to size-select the drygenerated mineral dust particles in this work. The use of multiple impactors and/or the grinding of the samples should reduce the upstream concentration of large particles. In all cases investigated here, transmission of particles above the selected size was observed. We specifically note the shape of the particles was irregular, as observed using EM, which resulted in high shape factor values that have not been fully considered previously. We suggest that shape, in addition to the upstream size distribution, can play a role in the DMA ability to accurately size select. We therefore suggest a combination of factors can affect the ability of a DMA to size-select drygenerated mineral dust particles. Another finding from this work is that mineral dust particle shape factor is not constant as a function of particle size. An increase in  was observed for both impacted and non-impacted samples. The relationship between shape factor and dust particle sizes has been considered in

21

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ACCEPTED MANUSCRIPT past studies. Okada et al. (2001) did not find a clear trend between the two for atmospheric dust particles whereas Reid et al. (2003) and Kaaden et al. (2008) found that shape factor had a strong dependency on particle size for mineral dust sampled in Africa. Kaaden et al. (2008) reported shape factors of 1.11, 1.19 and 1.25 for mobility diameters of 800, 1000 and 1200 nm. Reid et al. (2003) reported that smaller particles had a shape factor near unity while larger particles had a shape factor of ~3. 5. Conclusions


The purpose of this work was to evaluate the ability of a DMA to size select dry dispersed mineral dust particles, a common practice for laboratory studies of an atmospherically relevant aerosol types. A post-DMA size distribution was examined using four techniques: (1) an SMPS was used for mobility sizing, (2) an OPS for optical sizing, (3) PALMS for vacuum aerodynamic sizing, and (4) EM for geometric sizing. A “control experiment” with known density and quasispherical AS particles showed matching mobility, geometric, and volume-equivalent size distributions. Size-selection of dry dispersed mineral dust particles, on the other hand, resulted in a broader distributions and a smaller than selected mode size. EM imaging of ATD particles provided estimates of physical diameters to infer that particle shape factor was a function of particle size. We conclude that mobility size selection of mineral dust particles using a DMA most often does not yield particles of the desired physical size and surface area. Instead, inhomogeneity in particle morphology can result in particles of similar size experiencing different amounts of drag in the DMA, which then changes their (shape-dependent) electrical mobility relative to one another. This finding is important for laboratory experiments that investigate the size-dependent

22

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ACCEPTED MANUSCRIPT properties of mineral dust, such as interaction with radiation, chemical reactivity and cloud nucleating potential (e.g. CCN and IN). We suggest an independent measurement downstream of the DMA is required to verify the actual size distribution after sizing dry dispersed mineral dust particles. Ideally, this measurement should verify the size of the particles matches that of interest. For example, a validation of surface area for studies of chemical reactivity or nucleation potential would be warranted since this is the influential particle property. This work also challenges the common simplification of parameters that remain constant across


mineral dust sizes, with one example being shape factor. Based on the data obtained in this study, where the shape factor changed as a function of particle mobility size, we suggest that relevant parameters be verified as a function of mineral dust particle size. Furthermore, as the instrumentation used in this work had a comparison limitation of 350 nm (the lower threshold of the OPS), we suggest a need for future experiments below this size. Such measurements could help to improve our ability to correctly size select dry-generated non-spherical particles and provide information on the relationship of shape factor and effective density to mobility diameter. Acknowledgements We acknowledge the NOAA OAR Climate Program for their support of this project via grant number NA11OAR4310159. We thank Michael Rösch, Fred Brechtel and Karl Froyd for useful discussions.

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ACCEPTED MANUSCRIPT Niedermeier, D., Hartmann, S., Shaw, R. A., Covert, D., Mentel, T. F., Schneider, J., Poulain, L., Reitz, P., Spindler, C., Clauss, T., Kiselev, A., Hallbauer, E., Wex, H., Mildenberger, K., and Stratmann, F. (2010). Heterogeneous freezing of droplets with immersed mineral dust particles – measurements and parameterization. Atmos. Chem. Phys. 10:3601-3614. Niedermeier, D., Shaw, R. A., Hartmann, S., Wex, H., Clauss, T., Voigtländer, J., and Stratmann, F. (2011). Heterogeneous ice nucleation: exploring the transition from stochastic to singular freezing behavior. Atmos. Chem. Phys. 11:8767-8775.


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ACCEPTED MANUSCRIPT Sullivan, R. C., Petters, M. D., DeMott, P. J., Kreidenweis, S. M., Wex, H., Niedermeier, D., Hartmann, S., Clauss, T., Stratmann, F., Reitz, P., Schneider, J., and Sierau, B. (2010). Irreversible loss of ice nucleation active sites in mineral dust particles caused by sulphuric acid condensation. Atmos. Chem. Phys. 10:11471-11487. TSI: Trust Science Innovation, Optical Particle Sizer Model 3330 Manual, 6 ed., Shoreview, MN, 2012. Twohy, C. H., Kreidenweis, S. M., Eidhammer, T., Browell, E. V., Heymsfield, A. J., Bansemer,


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ACCEPTED MANUSCRIPT Wheeler, M. J., and Bertram, A. K. (2012). Deposition nucleation on mineral dust particles: a case against classical nucleation theory with the assumption of a single contact angle. Atmos. Chem. Phys. 12:1189-1201. Wiedensohler, A. (1988). An approximation of the bipolar charge distribution for particles in the submicron size range. J. Aerosol Sci. 19:387-389. Zimmermann, F., Ebert, M., Worringen, A., Schütz, L., and Weinbruch S. (2007). Environmental scanning electron microscopy (ESEM) as a new technique to determine the ice nucleation


capability of individual atmospheric aerosol particles. Atmos. Environ. 41:8219-8227.

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ACCEPTED MANUSCRIPT Table 1: Density, refractive index and shape factor of the samples used in this study. Particles type

Complex

Dynamic shape

refractive index

factor (χ)

1.77

1.56 ± 0.03i

1.04

Kuwata and

Flores et al.

Kuwata and

Kondo (2009)

(2012)

Kondo (2009)

2.65

1.51 ± 0.001i

1.3

Kanji et al.

Glen and Brooks

Möhler et al.

(2013)

(2013)

(2008)

Clay Mineral

2.6

1.4 ± 0.001i

1.3

Society (rock

Hudson et al.

Egan and

Hudson et al.

chips)

(2008)

Hilgeman (1979)

(2008)

2.35

1.55 ± 0.003i

1.11

Barthelmy

Egan and

Hudson et al.

(2014)

Hilgeman (1979)

(2008)

Source

Ammonium Sulfate

Sigma-Aldrich

((NH4)2SO4) Powder Arizona Test

Technology Inc.

Dust

(“Nominal 0–3


μm particles”) Illite (custom ground)

Density (g/cm3)

Sodium-rich Montmorillonite,

Clay Mineral

(custom ground

Society

& unground:

(unspecified

(NaMon &

particle size)

UgNaMon)

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ACCEPTED MANUSCRIPT Table 2: Measurements of the DMA output by three instruments (SMPS, OPS and PALMS). Each value is mode size ± Standard Deviation (SD) measured in volume-equivalent diameter space (Dve). 500, 750 and 1000nm electrical mobility diameter AS particles (Dm) are equal to volume-equivalent diameter 484, 725 and 965nm, respectively (see Eq. 4). Corresponding ATD and illite volume-equivalent diameters are 403, 598 and 791nm and NaMon and UgNaMon volume-equivalent diameter are 462, 690 and 918nm.


AS Instru ment

SMPS

OPS

PALM S

Dm (n m) 50 0 75 0 10 00 50 0 75 0 10 00 50 0 75 0 10 00

Witho ut impact ors 513±8 5 777±9 2 933±8 1 531±2 01 740±1 26 1006± 584 447±9 0 730±8 3 920±1 10

NaMo n Witho Witho Witho With With With ut ut ut impact impact impact impact impact impact ors ors ors ors ors ors 441±6 427±8 434±7 423±1 492±1 493±8 7 6 1 04 02 0 654±7 652±6 654±7 656±7 752±9 741±1 1 7 4 0 2 17 784±2 775±3 767±4 771±3 882±4 885±6 4 5 1 9 8 3 451±1 449±1 391±1 496±1 458±4 448±2 58 15 14 77 18 68 468±1 504±1 437±2 430±2 448±1 453±1 63 73 31 50 60 90 511±1 549±5 433±2 427±2 397±1 438±2 39 07 42 43 24 57 314±2 314±2 213±1 195±1 259±2 249±3 36 15 29 52 33 03 314±2 319±2 223±1 210±1 281±2 250±3 00 11 34 53 35 19 312±2 299±2 227±1 233±1 302±2 267±3 27 19 52 58 76 18 ATD

ATD

Illite

34

Illite

NaMo n

UgNa Mon With impact ors 499±5 0 778±8 7 895±3 6 397±1 24 438±2 37 435±2 30 216±1 77 252±2 29 222±3 12

UgNa Mon Withou t impact ors 503±6 9 755±8 4 892±4 0 371±2 44 421±2 44 444±1 53 237±2 90 227±2 71 264±2 74

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Table 3: ATD shape factor and equivalent diameter derived from the EM images. Volume and


surface area, calculated using Eq. 2 and 3, respectively. Selected

Impactor

Dynamic shape

Volume-equivalent

Surface area-equivalent

mobility size

used?

factor (χ)

diameter, in nm (Dve)

diameter, in nm (Dse)

500 nm

Yes

1.3 ± 0.9

301 ± 132

360 ± 182

750 nm

Yes

2.8 ± 2.2

234± 119

287 ± 166

1000 nm

Yes

3.1 ± 1.6

308 ± 108

372 ± 153

500 nm

No

1.3 ± 0.5

348 ± 104

419 ± 144

750 nm

No

2.2 ± 0.9

337 ± 99

410 ± 141

1000 nm

No

3.1 ± 1.1

337 ± 89

403 ± 122

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FIG 1: Experimental schematic. The setup consists of three sections: (1) particle generation and impactors, (2) particle size selection using the first (#1) DMA and (3) measurement of the output with multiple instruments: a SMPS consisting of a DMA (#2) and CPC used for mobility sizing, an OPS for optical sizing, PALMS for vacuum aerodynamic sizing, and EM for geometric sizing. Additional description is provided in the text.

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FIG 2: Example image of particles with face-on (wide particles, F-O, for diameter) and edge-on (narrow particles, E-O, for thickness) positioning.

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FIG 3: Size distributions of AS particles at three selected sizes (columns) using different techniques (rows). Top row (a-c): SMPS scans, middle row (d-f): OPS scans, and bottom row (gi): PALMS distributions. Left column (a, d and g): 500 nm, center column (b, e and h): 750 nm, and right column (c, f and i): 1000 nm. Normal (SMPS) and lognormal (OPS, PALMS) fits of the singly charged mode, described in the text, are shown in the center column.

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FIG 4: Observed versus selected volume-equivalent diameter for AS particles using three instruments from the data shown in Figure 3. Error bars show the standard deviations about the mode size. The dashed line is a 1:1 correlation. SMPS and PALMS data are slightly shifted horizontally for clarity.

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FIG 5: Size distributions of ATD particles with impactors at three selected sizes (columns) using different techniques (rows). Top row (a-c): SMPS scans, second row (d-f): OPS scans, and third row (g-i): PALMS distributions. Bottom row (j-l): distributions from EM grid analysis for low (white) and high (black) estimates of surface area and mode size depending on assumed particle thickness (see text for details). Left column (a, d and g): 500 nm, center column (b, e and h): 750 nm, and right column (c, f and i): 1000 nm. Normal (SMPS) and lognormal (OPS, PALMS) fits of the singly charged mode, described in the text, are shown in the center column.

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FIG 6: Size distributions of ATD particles without impactors at three selected sizes (columns) using different techniques (rows). Top row (a-c): SMPS scans, second row (d-f): OPS scans, and third row (g-i): PALMS distributions. Bottom row (j-l): distributions from EM grid analysis for low (white) and high (black) estimates of surface area and mode size depending on assumed particle thickness (see text for details). Left column (a, d and g): 500 nm, center column (b, e and h): 750 nm, and right column (c, f and i): 1000 nm. Normal (SMPS) and lognormal (OPS, PALMS) fits of the singly charged mode, described in the text, are shown in the center column.

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FIG 7: Size distributions of 100 and 250 nm (right and left column, respectively) size-selected ATD particles with impactors using different techniques (rows). Top row (a-b): SMPS scans, middle row (c-d): OPS scans and bottom row (f): PALMS distributions. Gray areas represent sizes below the detection threshold of PALMS and the OPS (150 and 350 nm, respectively).

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FIG 8: Observed versus selected volume equivalent diameter for ATD particles using four different instruments with (Panel a) and without (Panel b) impactors corresponding to the data in Figures 5 and 6, respectively. The dashed lines are a 1:1 correlation. Different instrumental data are slightly shifted horizontally for clarity. Note the lack of correlation which was observed for the AS case shown in analogous Figure 4.

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