Aerosol Science and Technology Investigations of SP

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Investigations of SP-AMS Carbon Ion Distributions as a Function of Refractory Black Carbon Particle Types ab

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Timothy B. Onasch , Edward C. Fortner , Achim M. Trimborn , Andrew T. Lambe , Andrea d

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J. Tiwari , Linsey C. Marr , J. C. Corbin , A. A. Mensah , Leah R. Williams , Paul Davidovits & Douglas R. Worsnop

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Aerodyne Research, Inc., Billerica, Massachusetts, USA

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Boston College, Chestnut Hill, Massachusetts, USA

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Aeromegt GmbH, Hilden, Germany

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Civil and Environmental Engineering, Virginia Tech, Blacksburg, Virginia, USA

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ETH Zurich, Institute for Atmospheric and Climate Science, Zurich, Switzerland

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Laboratory for Atmospheric Chemistry, Paul Scherrer Institut, Villigen, Switzerland Accepted author version posted online: 15 Apr 2015.

To cite this article: Timothy B. Onasch, Edward C. Fortner, Achim M. Trimborn, Andrew T. Lambe, Andrea J. Tiwari, Linsey C. Marr, J. C. Corbin, A. A. Mensah, Leah R. Williams, Paul Davidovits & Douglas R. Worsnop (2015): Investigations of SPAMS Carbon Ion Distributions as a Function of Refractory Black Carbon Particle Types, Aerosol Science and Technology, DOI: 10.1080/02786826.2015.1039959 To link to this article: http://dx.doi.org/10.1080/02786826.2015.1039959

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ACCEPTED MANUSCRIPT Investigations of SP-AMS Carbon Ion Distributions as a Function of Refractory Black Carbon Particle Types Timothy B. Onasch1,2, Edward C. Fortner1, Achim M. Trimborn3, Andrew T. Lambe1,2, Andrea J. Tiwari4, Linsey C. Marr4, J. C. Corbin5,*, A. A. Mensah5, Leah R. Williams1, Paul Davidovits2, and Douglas R. Worsnop1


1. Aerodyne Research, Inc., Billerica, Massachusetts, USA 2. Boston College, Chestnut Hill, Massachusetts, USA 3. Aeromegt GmbH, Hilden, Germany 4. Civil and Environmental Engineering, Virginia Tech, Blacksburg, Virginia, USA 5. ETH Zurich, Institute for Atmospheric and Climate Science, Zurich, Switzerland Address correspondence to Timothy B. Onasch, Aerodyne Research, Inc., 45 Manning Rd., Billerica, MA, USA. E-mail: [email protected] *Current affiliation: Laboratory for Atmospheric Chemistry, Paul Scherrer Institut, Villigen, Switzerland Short title: SP-AMS CARBON ION DISTRIBUTIONS

ABSTRACT: The Soot Particle Aerosol Mass Spectrometer (SP-AMS) instrument combines continuous wave laser vaporization with electron ionization aerosol mass spectrometry to characterize airborne, refractory black carbon (rBC) particles. The laser selectively vaporizes 1

ACCEPTED MANUSCRIPT absorbing rBC-containing particles, allowing the SP-AMS to provide direct chemical information on the refractory and nonrefractory chemical components, providing the potential to fingerprint various rBC particle types. In this study, SP-AMS mass spectra were measured for 12 types of rBC particles produced by industrial and combustion processes to explore differences in the carbon cluster (Cn+) mass spectra. The Cn+ mass spectra were classified into three categories based on their ion distributions which varied with rBC particle type. The carbon ion distributions were investigated as a function of laser power, electron ionization (on/off), and ion


charge (positive or negative).

Results indicate that the dominant positive ion-formation

mechanism is likely the vaporization of small, neutral carbon clusters followed by electron ionization (C1+ to C5+). Significant ion signal from larger carbon cluster ions (and their fragment ions in the small carbon cluster range), including mid carbon (C6+ to C29+) and fullerene (greater than C30+) ions, were observed in soot produced under incomplete combustion conditions, including biomass burning, as well as in fullerene enriched materials. Fullerene ions were also observed at high laser power with electron ionization turned off, formed via an additional ionization mechanism.

We expect this SP-AMS technique to find application in the

identification of the source and atmospheric history of airborne ambient rBC particles.

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ACCEPTED MANUSCRIPT INTRODUCTION Black carbon (BC) containing particles are generated as industrial products and as a byproduct of incomplete combustion processes. Industrial applications include reinforcing material in rubber compounds, ink pigments, as well as fullerenes and carbon nanotubes for a variety of technical and medical products (Baughman et al., 2002; Scida et al., 2011).

Combustion

generated soot particles, which contain black carbon, are an important airborne pollutant (Bond et al., 2007). Black carbon, also termed refractory black carbon (rBC) or sometimes elemental


carbon (EC) (Andreae and Gelencsér, 2006; Petzold et al., 2013), is the dominant light-absorbing particulate material that heats the atmosphere (Bond et al., 2013).

In addition to directly

absorbing sunlight, BC particles can become incorporated into cloud droplets and deposit on snow and ice surfaces, changing cloud properties and surface reflectivity, respectively (Jacobson, 2006; Hadley and Kirchstetter, 2012). These processes significantly modify the earth’s radiation balance (Ramanathan and Carmichael, 2008; Bond et al., 2013). Furthermore, atmospheric BC containing particles have been demonstrated to be deleterious to human health (Dockery et al., 1993; Brugge et al., 2007; Lee et al., 2010). The chemical composition and structure of BC particles, and thus their physical and optical properties (Bond and Bergstrom, 2006), depend on their methods of production (Vander Wal and Tomasek, 2004). For example, BC-containing soot produced rapidly by spark discharge is highly disordered, and therefore highly reactive (Schmid et al., 2011). At the other extreme, thermally-treated and laser-heated BC contains more-ordered graphitic structures (Oberlin, 1984; Vander Wal and Jensen, 1998), which are correspondingly less reactive (Schmid et al., 2011). Environmentally-relevant BC samples produced by diesel engines, aircraft turbines, or biomass 3

ACCEPTED MANUSCRIPT burning exhibit a wide range of chemical structures, which may aid in the identification of ambient soot sources (Vander Wal et al., 2010, 2014). This range of chemical and physical properties, which depends upon the efficiency of combustion, affects the BC optical properties and has been described as soot “maturity” (López-Yglesias et al., 2014).

Traditional

measurements of the chemical and physical properties of BC particulate materials include a variety of offline, filter-based techniques, such as Raman and X-ray-based spectroscopies, TEM and SEM microscopies, and EC/OC thermal optical techniques (Vander Wal, 1997; Bond and


Bergstrom, 2006; Lack et al., 2014). Laser vaporization and mass spectrometric measurements are another important approach with the potential for rapid, on-line analysis, including soot particle aerosol mass spectrometry (Onasch et al., 2012; Corbin et al., 2014) and single particle laser ablation mass spectrometry (Sinha, 1984; Ferge et al., 2006; Maricq, 2009). Mass spectrometric studies of the carbon vapors formed from heated ( 300 m/z) are a strong function of the ion impact energy at the MCP detector. While the current system has not been tested for detecting high mass ions in this regard, all of the data in the current study were Downloaded by [Lib4RI] at 01:14 28 April 2015

obtained with SP-AMS instruments operated with similar MS tuning voltages, implying similar detection efficiencies as a function of m/z. This study focuses on qualitative, rather than quantitative, changes to the measured carbon cluster ion (Cn+) mass spectra with the intent of explaining the origins for the observed carbon ion signals in the SP-AMS, and our observations and conclusions should not be significantly affected by this issue. Future work to quantitatively assess SP-AMS carbon cluster ion signals will need to explicitly account for this issue. The HR-TOF-MS was operated with a mass resolving power, m/Dm, of ~2000-3000 (V-mode) measured at m/z 200 (m is the mass of a specific ion; Dm is a measure of the peak width, e.g., FWHM) (DeCarlo et al., 2006). This resolution enables the identification, separation, and quantification of carbon ions from typical organic and inorganic ions at a given m/z, such as SO2+ (47.96698), C4+ (48.00000), and CH4O2+ (48.02113) at m/z 48. Polycyclic aromatic hydrocarbons (PAH’s), which can be co-generated by rBC particle sources, produce ions in the midrange region ~300 m/z (Dzepina et al., 2007), which may also be resolved from carboncluster ions.

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ACCEPTED MANUSCRIPT The ion optics in the SP-AMS instrument were equipped with a bipolar power supply, enabling the sequential study of both positive and negative ions. Standard impact energies of 70 eV typically generate positive ions from neutral molecules; however, secondary electrons may also attach to neutral molecules, generating negative ions. Positive ions may also be generated via other ionization mechanisms, such as thermal ionization, which may occur during particle vaporization for materials with low ionization potentials (e.g., alkali metals). In this study, we focus exclusively on the carbon cluster ion signals (Cn+ or Cn-). The sum of


these Cn+ (Cn-) ions is a measure of sampled particulate rBC mass (Onasch et al., 2012). In addition to the Cn+ (Cn-) ion signals, the SP-AMS mass spectra contained ion signals due to organic and inorganic compounds associated with the rBC materials. For laboratory generated rBC particles with little or no associated organic components (RBC < 0.5), there is negligible (10%), depending upon the chemical composition and fragmentation of the associated organic material. However, the Cn+ fragmentation patterns from organic material and from rBC material differ, with C3+ the dominate fragment (~ 0.45) of the low carbon ion signal from rBC material and C1+ the dominant fragment (> 0.6) of the low carbon ion signal from organic material. Note that one could use the correction to C1+, based on the measured C3+ ion signal from Onasch et al. (2012) to reduce the potential impact of Cn+ ion signal interference from associated organic material, though this correction was not used in the current study. Typical measurements using the SP-AMS are done with the laser vaporizer set at maximum power (~105 W/cm2), 70 eV electron ionization, and positive ion detection. Here, we varied the 9

ACCEPTED MANUSCRIPT following instrument settings to investigate the parameters affecting Cn + ion signal distributions: (i) positive and negative ion detection, (ii) ionizing electron filament on (70 eV) and off (i.e., thermal ionization only), and (iii) laser vaporizer power. Materials Studied. For this study, we selected 12 rBC particle types representing a variety of refractory carbon nanostructures, ranging from amorphous to graphitic to fullerenic (Vander Wal et al., 2010). A complete list and description is given in Table 1. The materials include: graphitic carbon nanoparticles (Aquadag), carbon black (Regal black), amorphous carbon


(carbon nanoparticles and glassy carbon spheres), surface oxidized carbon (NoritSX activated charcoal), combustion generated soots (premixed ethylene flame and methane diffusion flame soots generated at several fuel equivalence ratios, f), and fullerene enriched carbon (Alfa Aesar fullerene soot and Nano-C fullerene black). In addition, we aerosolized and sampled particles of 99.9% pure C60 (buckminsterfullerene), specified by the manufacturer (MER Corporation). All of these rBC particle types were measured by the SP-AMS to have rBC mass fractions greater than 0.65 (RBC < 0.5), with the majority having rBC mass fractions greater than 0.9. All the manufactured materials, except for C60, were dispersed in distilled water via sonication, aerosolized using a TSI constant output atomizer (model 3076), and dried in a diffusion drier to less than 20% RH prior to sampling by the SP-AMS instrument. The materials were sampled as either polydisperse or monodisperse aerosol, with peak and selected mobility diameters in the range of 100 to 400 nm. The C60 material was milled in a nitrogen atmosphere and aerosolized via a dry powder dispersion process (Tiwari et al., 2013). The laboratory-based combustion soot particles, generated using either premixed ethylene (Cross et al., 2010) or methane diffusion

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ACCEPTED MANUSCRIPT flame (Stipe et al., 2005) sources, were subsampled from the flame effluent, diluted with particle free air, and mobility size selected prior to measurement. In addition to the laboratory generated rBC particle types, we sampled rBC particles in situ using extractive sampling methods from open flame burning of biomass materials obtained during the FLAME3 study at the Montana Fire Science Laboratory (McMeeking et al., 2009) and from the exhaust of diesel trucks in traffic at the Caldecott Tunnel in Berkeley, California (Dallmann et al., 2014). The diesel truck exhaust was measured to have an average rBC mass


fraction of 0.38±0.12 (RBC ~ 1.6) (Dallmann et al., 2012). The biomass burning soots exhibited measured average rBC mass fractions that ranged from 0.07 (lodgepole pine and turkey oak; RBC ~ 13) to 0.13 (gallberry; RBC ~ 7) to 0.47 (manzanita; RBC ~ 1.1).

RESULTS AND DISCUSSION Positive carbon ion mass spectra. We obtained SP-AMS Cn+ mass spectra for all of the rBC materials listed in Table 1. Figure 1 shows the positive ion mass spectra for four laboratorygenerated particle types selected for their distinct spectral profiles: (a) Regal black, (b) premixed ethylene flame soot generated with a fuel equivalence ratio (f) of 2, (c) Nano-C fullerene black, and (d) C60 (buckminsterfullerene). The number of carbon atoms comprising the Cn+ in each mass spectrum are separated into three basic categories (low carbon: C1+ to C5+, mid carbon: C6+ to C29+, and fullerenes: C30+/ C602+ to C166+), which may be related to the stable structures of the positive carbon cluster ions (i.e., linear, rings, and fullerenes) as indicated at the top of Figure 1 (Bowers, 2014).

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ACCEPTED MANUSCRIPT A common feature of all the mass spectra shown in Figure 1 is the presence of significant signals at low carbon numbers (C1+ to C5+). The higher signals of cations with odd numbers of carbons is consistent with their greater stability (Drowart et al., 1959). In the mass spectra shown in Figure 1, the mass spacing for the dominant Cn+ ion signals below C30+ is m/z = 12 (C1), whereas the spacing for dominant carbon ion signals above C30+ is m/z = 24 (C2). This spacing change may represent a transition from carbon clusters with open 1-D and 2-D structures (e.g., linear chains, monocyclic and polycyclic rings) to closed or hollow 3-D, geodesic


structures (Rohlfing et al., 1984; Bloomfield et al., 1985; von Helden et al., 1993; Handschuh et al., 1995; Bowers, 2014). The mass spectrum of Regal black particles (Figure 1a) consists primarily of the carbon cluster ions C1+ to C5+ (~99% of total Cn+ signal), with a minor contribution from clusters greater than C5+. Similar carbon ion distributions were measured for Aquadag (graphitic carbon), carbon nanoparticles and glassy carbon spheres (amorphous carbon), methane diffusion flame soot (fnet = 0.7), and NoritSX (activated charcoal). Mass spectra of ethylene flame soot (f = 2) contained significant fractions of ions greater than C5+ (Figure 1b), including measureable, though small, ion signals for carbon clusters beyond m/z 2000 (C166+) (not shown). In particular, ethylene flame soot exhibited significant ion signals in the mid carbon range between m/z 60 (C5+) and m/z 360 (C30+ and C602+) (~0.25 of total Cn+ signal). The rBC materials with significant mid carbon ion fractions (ranging from >0.5 to ~0.03) in decreasing order included: (i) laboratory-based ethylene soot generated in a pre-mixed, flat-burner flame source (f = 5, 4, 3, 2), (ii) biomass burning soot (gallberry, turkey oak, lodgepole pine, and manzanita), and (iii) Alfa Aesar fullerene soot. All other samples generated 12

ACCEPTED MANUSCRIPT negligible (95% of total Cn+ signal), though it also exhibited signals at low carbon numbers. While the large carbon cluster ion


distribution peaks around m/z 1200, there are large signals at m/z 720 (C60+) and m/z 840 (C70+) which stand out in the sequence (i.e., as “magic numbers”), indicating the presence of especially stable fullerenes (Fowler, 1986). This positive carbon ion series has been identified as being dominated by fullerene ions, especially for carbon numbers greater than 45 (Bowers, 2014). In this work, we define total positive fullerene ion signals as being the sum of Cn+ signal from C30+/ C602+ to C166+. The carbon ion mass spectrum for C60 (Figure 1d) was dominated by signals at m/z 720 (C60+) and m/z 360 (C602+). The carbon-13 isotope abundances at m/z 720 to 726 match those of C60+; in addition, the carbon-13 isotope abundances at m/z 360 also match those of C60+, but at every half mass (i.e., doubly charged). The formation of doubly charged fullerenes, C602+ and C702+ in particular, is also observed in the spectra of Nano-C fullerene black in Figure 1c. The C60 mass spectrum (Figure 1d) contained ions reflecting the loss of C2 from both C60+ (m/z spacing of 24) and C602+ (m/z spacing of 12) during fragmentation of the parent ions (O’Brien et al., 1988; Scheier et al., 1994). The C60 mass spectrum also shows ion signals at low carbon numbers, which may represent Cn+ ion signals due to potential break-up of C60 during preparatory ball 13

ACCEPTED MANUSCRIPT milling or carbon vapor (e.g., C2) lost due to fullerene fragmentation during laser vaporization and electron ionization. In addition to the laboratory rBC material, we also examined rBC containing particles from two common ambient sources. Figure 2 shows measured SP-AMS Cn+ ion mass spectra for (a) on-road diesel truck exhaust and (b) open air biomass burning soot. The diesel truck exhaust was sampled from a single on-road vehicle in the Caldecott Tunnel (Dallmann et al., 2012). The Cn+ mass spectrum for the single diesel truck matches the average rBC mass spectrum obtained


over 2 hours (12:00 – 14:00 local time) of continuous sampling during which ~5% of the vehicles sampled were diesel trucks, suggesting that diesel soot was the dominant source of rBC (Dallmann et al., 2014). The diesel soot Cn+ mass spectra were dominated by low carbon ion signals (>0.97). The biomass burning soot (Figure 2b) was sampled during the open-air combustion of lodgepole pine during the FLAME 3 laboratory experiments (McMeeking et al., 2009). Three other biomass fuel types (gallberry, turkey oak, and manzanita) were included in this study to show a range of measured Cn+ ion distributions (mass spectra not shown). All of these biomass fuel types exhibited SP-AMS Cn + ion distributions with significant ion signals at low carbon numbers (0.68 to 0.92), though lower than diesel soot. It is apparent from Figure 2 that two distinct combustion sources (diesel fuel and lodgepole pine) generated different SP-AMS Cn+ ion mass spectra. These differences illustrate the potential of the SP-AMS Cn + measurements to provide identification of ambient particulate rBC sources. In the following sections, we explore some of the complexities in the observed mass spectra that are inherent in the SP-AMS technique. 14

ACCEPTED MANUSCRIPT Multiple Sources of Positive Fullerene Ions. Positive ion mass spectra were obtained with 70 eV electron ionization as well as with the electron source (heated tungsten wire filament) turned off. With the filament off, the only ions observed from the sampled rBC particles were alkali metal ions, such as sodium and potassium, and fullerene ions (no multiply charged ions or smaller carbon fragment ions were observed). For rBC particles with no measureable fullerene ion signal using electron ionization (e.g., Regal black and other graphitic and amorphous rBC particle types), no Cn+ ions were observed with the filament off.


Figure 3a and 3b compare the positive ion mass spectra obtained with the filament on and off for (a) Nano-C fullerene black and (b) Alfa Aesar fullerene soot, respectively. Figure 3c shows the ratio of the positive ion signal with the filament off-to-on for Nano-C fullerene black, Alfa Aesar fullerene soot, and C60 as a function of m/z. Note that the C60 filament off-to-on ratios were only included if the absolute signals had sufficient signal-to-noise; the C60 ratios at m/z 720 were lower than the ratios observed for the fragment ions at C58+, C56+, C54+, and C52+. As is evident in Figure 3c, the ratio of fullerene ions during filament off-to-on (i) increased with increasing carbon number, with a few exceptions, and (ii) varied significantly between the fullerene-containing materials. These observations indicate that in addition to electron ionization there is, at least, one other mechanism capable of producing positive fullerene ions during CW-laser vaporization. It is likely that the additional ion formation mechanism is via thermionic emission (i.e., thermal energy driven ionization). Two possible mechanisms for thermionic emission include (1) incoherent, sequential photoexcitation of fullerene molecules in the gas phase after vaporization from the laser-heated rBC particles (Ding et al., 1994) and (2) direct release of fullerene ions 15

ACCEPTED MANUSCRIPT from laser-heated rBC surfaces during the vaporization process. Mechanism (1) would depend on the laser power density (W/cm2) within the ion formation chamber of the instrument and the transit time of the vaporizing fullerene molecules within the laser beam. Mechanism (2) would depend on fullerenes remaining on the laser-heated rBC particle surfaces until heated to the high temperatures of incandescence/sublimation, at that temperature thermal ionization may become efficient. We are unable to positively identify the ionization mechanism that generates positive fullerene ions with the electron filament turned off.

However, it is apparent that positive


fullerene ion signal levels (i.e., quantification) and ion distributions in the SP-AMS may be affected by other ionization mechanisms in addition to electron ionization and, therefore, may change as a function of the laser vaporizer power. Laser Vaporizer Power Dependence. Laser vaporizer power is an important instrument parameter, which influences the total amount of energy available per rBC particle and its corresponding heating rate, and must be controlled to ensure the laser light intensity is high enough to achieve complete vaporization of the rBC particles passing through the laser (Onasch et al., 2012). Increasing the laser power increases both the total light intensity and the laser beam cross section, which is important given the potentially incomplete overlap between the laser beam and sampled particle beams (Willis et al., 2014). The results of laser power experiments represent ensemble averages over all of the different trajectories of rBC particles through a given laser beam profile, as governed by the laser beam – particle beam overlap. Figure 4 shows selected carbon ion signals (C1+ to C5+, C602+, C58+, C60+, and the total fullerene ion signals) as a function of laser vaporizer power for three rBC particle types: (a) Regal black, (b) Nano-C fullerene black, and (c) C60. All of the carbon ion signals show apparent saturation 16

ACCEPTED MANUSCRIPT at the highest laser power attained for each rBC particle type except the fullerene signals in the Nano-C fullerene black sample. Regal black. The carbon ion distribution (C1+ - C5+) for Regal black was invariant with laser power. That is, the relative ratios between the individual carbon ions (C1+ - C5+) were constant with laser power, even as the total ion signal for all ions increase with increasing laser power. The plateau of the carbon ion signals with increasing laser power suggests that all Regal black particles passing through the effective region of the laser vaporizer beam were fully vaporized


and detected at the higher laser powers. Therefore, the measured C1+ - C5+ ion distributions may be a fundamental signature of this carbon type (i.e., carbon black), as there was no evidence for interferences from other ionization mechanisms or fragmentation from larger carbon cluster ions. Nano-C fullerene black. The laser power dependence of the selected ion signals for the fullerene black sample shown in Figure 4b is more complex. As shown in Figure 1, Nano-C fullerene black samples exhibited ion signals at low (C1+ - C5+) and high (fullerene ions > C30+, with some doubly charged fullerene ions, such as C602+) carbon numbers. The low carbon ion signals for Nano-C fullerene black, for clarity shown in Figure 4b using C3+ only, followed a similar trend to the low carbon ion signals of Regal black (Figure 4a), saturating at high laser power with invariant relative ratios (latter not shown on figure). On the other hand, the total fullerene ion signals for Nano-C fullerene black increased with increasing laser power. Several distinct trends with laser power can be identified for select fullerene ions in the NanoC fullerene black case. Specifically, the C60+, which is zero at zero laser power, rapidly reaches a relatively constant value at intermediate laser vaporizer powers and subsequently increases significantly with increasing laser power. The C60+ power dependence at higher laser power 17

ACCEPTED MANUSCRIPT matches the total fullerene ion signal trend. In contrast, C602+ appears to rapidly saturate with increasing laser power, with only a slight decrease at high laser power. These observations suggest that C60+ from Nano-C fullerene black are produced by two different mechanisms as a function of laser power, one that dominates at low laser power that also generates C602+ and one at high laser power that does not generate C602+ ions.

All these observations appear to be

consistent with the conclusions of the previous section of an additional laser-power-dependent ionization mechanism for fullerene ions that does not generate multiply charged fullerene ions or


fragment ions. C60. The selected C60 carbon ions shown in Figure 4c did not change significantly with laser power. The parent ion signals, C60+ and C602+, and the small carbon cluster ion signals, C3+ and C58+ (generated via fragmentation of the parent ions during ionization or potentially due to nonC60 carbon material generated by the break-up of C60 during preparatory ball milling), all increased with increasing laser power and appeared to level off at higher laser powers, similar to the behavior of the low carbon ion signals for Regal black and Nano-C fullerene black, but different from the fullerene ion signals (C60+, C602+ and total fullerene ions) of the Nano-C fullerene black. In contrast to Nano-C fullerene black, the fullerene ions from the C60 sample appear to be generated by only one dominant ionization mechanism over the studied range in laser power, which generates both C60+ and C602+ in a constant ratio. This observation agrees with the ratios in Figure 3c, which suggests that the C60 sample generates fewer C60+ ions (approximately a factor of ~7 lower) with the filament off than Nano-C fullerene black sample under high laser power conditions. We do not know why the C602+ ions, and associated C60+ ions, saturate at higher laser power conditions for the C60 sample, compared with the Nano-C fullerene 18

ACCEPTED MANUSCRIPT black. We included the C60 sample in this study as a known quantity, in contrast to the other fullerene enriched carbon samples whose compositions are not well characterized; however, pure C60 may have a different absorption cross section than other rBC samples which may affect its laser power dependence in the SP-AMS. Negative ion mass spectra. In addition to positive ion mass spectrometry, we also obtained mass spectra of negatively charged carbon ions for several of the sampled rBC materials. This mode of operation provides additional information relevant to understanding the SP-AMS


technique. Figure 5 shows normalized negative carbon ion mass spectra for three of the four rBC materials shown in Figure 1 (Regal black, Nano-C fullerene black, and C60). Five features are notable in Figure 5. First, at low carbon numbers, the Cn- ion distributions extend to larger cluster sizes (C2 - - C10 - compared to C1+ - C5+) and exhibit an even carbon number preference. This trend is opposite to that observed with the low carbon Cn+ distributions and is consistent with previous observations (Bloomfield et al., 1985). Second, virtually no negative mid carbon ions (here defined as between C10- and C30 -) were observed. Third, negative fullerene ions were only observed for rBC samples which produced positive fullerene ions (e.g., Nano-C fullerene black and C60, but not for Regal black, Aquadag, carbon nanoparticles, glassy carbon spheres, or methane diffusion flame soot). Fourth, the apparent fragmentation of C60 in negative ion mode (Figure 5c) was significantly reduced compared to fragmentation observed in the positive ion mode (Figure 1d); low carbon negative ion signals were very low (note the scale changes for Figure 5c), and the negative C2-loss ions (C(60-2n)-) were absent. There were also no multiply charged negative fullerene ions. Fifth, the negative fullerene ion distribution for Nano-C fullerene black (Figure 5b) was significantly different than the positive fullerene ion distribution 19

ACCEPTED MANUSCRIPT (Figure 1c); the negative ion distribution exhibited a narrower range of fullerene ions with the exceptionally-stable C60 - and C70- ions more prominent than their positive ion counterparts (Figure 1c). Origins of carbon cluster ions in SP-AMS Low carbon: C1+-C5 +. All rBC materials sampled using the SP-AMS exhibit measureable positive ion signals in the low carbon category (C1+ - C5+). There are two potential sources for these low carbon ion signals: (i) electron ionization of small carbon clusters in the vapor phase


and (ii) ions generated by vapor phase fragmentation of larger carbon clusters during laser vaporization and/or electron impact.

Our observations suggest that both sources may be

important, though the latter may be secondary. Previous work has shown that neutral C1-C5 carbon clusters represent the main vapor components from heated graphitic carbon materials (Pflieger et al., 2008) and the most stable geometries of these clusters are linear (von Helden et al., 1993). The positive and negative ion mass spectra for Regal black show that the measured carbon ion fraction in the low carbon category (C1+ - C5+ or C2- - C10-) is greater than 99%. As the distribution of low carbon ions did not change as a function of laser power (Figure 4a) and low carbon ions were absent in the filament-off mass spectra (Figure 3), we conclude that the low carbon ion signals from Regal black particles, a high temperature carbon black, are generated by electron ionization of neutral carbon clusters vaporized directly from the rBC particulate material at ~4000 K (i.e., incandescence) (Schwarz et al., 2006; Moteki and Kondo, 2010). Other rBC materials that also exhibited only low carbon ion signals in the SP-AMS included graphitic nanoparticles

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ACCEPTED MANUSCRIPT (Aquadag), amorphous carbon (carbon nanoparticles and glassy carbon spheres), and ‘mature’ soot (methane diffusion flame soot). It is also possible that a fraction of low carbon ion signals may be generated from fragmentation of larger carbon clusters. Previous work has shown that fullerene ions, such as C60+, can fragment by expelling small carbon clusters, such as C2, and reclosing into smaller fullerene ions, such as C58+ (Kroto et al., 1991; Foltin et al., 1993); smaller carbon cluster ions ( 60% of total Cn+ ion signal) by ion signals in the low carbon numbers (C1+ - C5+), with the source-specific differences in the low, mid, and high carbon ion signals being secondary features. We have not observed SP-AMS Cn+ ion distributions from ambient measurements that resemble the industry-generated fullerene-rich


rBC samples included in this study. Based on our current observations, we recommend using low carbon number ions (C1+ - C5+) for ambient rBC mass calibrations and quantitative rBC mass and size measurements for the following reasons: (1) ambient Cn+ ion measurements are dominated by low carbon ion signals, and (2) low carbon ion signals appear to be generated solely via electron ionization in the SP-AMS, unlike fullerene ions, and therefore appear to be most directly related to the vaporized components of rBC particulate material. Mid carbon: C6+-C29+. The mid carbon category includes Cn+ ions in the mass spectral region from m/z 72 to 348 (C6+ to C29+), excluding multiply charged larger carbon cluster ions. Carbon clusters in this category have been reported to be typically most stable as ring structures (von Helden et al., 1993) and likely fragment via C3 expulsion (Geusic et al., 1987). The rBC materials during this study that exhibited significant mid carbon ion signals were generated by fuel-rich, incomplete combustion processes, suggesting that mid carbon ion signals may originate from incompletely graphitized rBC or not mature soot. The mid carbon ions were always associated with rBC materials that also exhibited fullerene ions, suggesting that mid carbon ions may form, at least in part, via fragmentation of fullerenes. 22

ACCEPTED MANUSCRIPT As previously noted, the mid carbon ion signals were sensitive to either source conditions for the rBC particles or instrument parameters. This variability is illustrated by three Cn + mass spectra for nominally the same Alfa Aesar fullerene soot (Product 40791, different Lots) sampled by different SP-AMS instruments (not shown). In all spectra, low carbon and fullerene ions are present, but the mid carbon ion fractions varied from significant (0.2) to essentially zero (108 W/cm2) have been shown to produce fullerene ions as artifacts of the vaporization process (Buseck, 2002). In contrast, low laser power densities (103105 W/cm2) have been shown to be less prone to generate species not originally present and yield 23

ACCEPTED MANUSCRIPT carbon clusters distributions similar to thermal desorption results, indicating minimal influence of the laser power density on the resulting carbon clusters, including fullerenes (Buseck et al., 1992; Wilson et al., 1993; Pflieger et al., 2008). In our SP-AMS experiments, the laser power density was low (~105 W/cm2), minimizing the potential for producing fullerene structures by the laser vaporization process (Greenwood et al., 1991). This is supported by the fact that the SP-AMS measured fullerene ion signals where expected in the sampled rBC material (i.e., fullerene black/soot and C60) and none where


fullerenes were not expected (e.g., graphitic and amorphous carbon), for both positive and negative ion detection. However, laser power dependence was observed for the fullerene ion signals (Figure 4b), indicating that the laser power density plays a substantial role in the fullerene ion production. Our studies suggest that increasing the laser power density may increase the measured fullerene ion signal due to an additional ionization mechanism (i.e., thermionic emission).

In addition to this observation, HRTEM studies in the literature highlight the

potential for rapid annealing of rBC particles in laser-based systems such as the SP-AMS (Ugarte, 1994; Vander Wal and Choi, 1999). We discuss both issues below. The formation of positive fullerene ions with the filament off (i.e., ion formation via thermionic emission) indicates that there is more than one mechanism for generating fullerene positive ions.

An additional ionization mechanism complicates the quantification and

interpretation of the fullerene content distribution within sampled particles. The additional ionization may be the reason for the large increase observed in the total fullerene ion signals with increasing laser power. Figure 4b shows that the mechanism responsible for the increase in C60+ (and total fullerene ions) in Nano-C fullerene black at high laser power does not generate C602+ 24

ACCEPTED MANUSCRIPT ions, consistent with the lack of C602+ ion formation with the filament off in Figure 3. As C602+ appears to be generated via electron ionization only, the C602+ ion signal may be useful in separating the measured C60+ ion signal into fractions generated by the dominant ionization mechanisms: electron ionization and thermionic emission. The C602+ ion signal (and presumably the corresponding C60+ ion signal) appears to be laser power independent (Figure 4b), suggesting that C60 structures are not being produced by the laser and that the carbon cluster ion signals generated by electron ionization may represent the preexisting C60 content. Therefore, SP-AMS


fullerene ion signals may reflect pre-existing fullerene content in the sampled rBC particulate material, although the amount and distribution are not yet quantitatively determined. Another laser power density-dependent mechanism that may be playing a substantial role in governing the Cn+ mass spectra obtained with the SP-AMS may be the annealing of the sampled rBC particles at high temperatures in the laser vaporizer. While the SP-AMS laser is considered to have a relatively low laser power density, rBC particles transit time through the laser on the order of microseconds and heat to incandescence (~4000 K), generate heating rates on the order of 1e7 K/s. Heating rBC higher than 2000 K allows disordered rBC materials to anneal, meaning that the disordered carbon structures of the initial rBC undergoes solid-state rearrangement to a more thermodynamically-stable configuration. Previous HRTEM studies have shown partial annealing occurring in rBC particles due to pulsed laser light (i.e., laser induced incandescence techniques) at power densities ~107 W/cm2, where the heating (cooling) rates were in excess of 1e11 K/s (1e9 K/s) and the total time at elevated temperature conditions is on the order microseconds (Vander Wal and Jensen, 1998; Vander Wal and Choi, 1999). Therefore, some level of annealing may be expected to occur within rBC particles in the SP-AMS. 25

ACCEPTED MANUSCRIPT The chemical processes that govern the annealing of carbon materials at high temperatures (3000-4000 K) are not well understood, though some form crystalline graphite (i.e., graphitizing carbon) and some not (i.e., non-graphitizing carbon) (Harris, 2004, 2005). HRTEM and Raman results indicate that some non-graphitizing carbons generate fullerene-like structures during heattreatment (Ugarte, 1994; Burian and Dore, 2000; Harris, 2005). Thus, it is possible that the laser power dependence of the fullerene ion signals from the Nano-C fullerene black sample (Figure 4b) may be due, in part, to heat-related annealing processes prior to vaporization. In this case,


strong fullerene ion signals in the SP-AMS may result from rBC materials with preexisting carbon structures that facilitate fullerene formation during heating. This potential mechanism is consistent with both our observations of fullerene ions only being generated from a subset of rBC materials, not including graphitic (e.g., mature soot) or amorphous carbon, and our observations of an additional ionization mechanism for generating positive fullerene ions. Furthermore, variations in the initial chemical composition of the rBC material and particle heating rates in the SP-AMS may influence the extent of annealing prior to complete vaporization and may explain the variations observed in the fullerene and mid carbon ions distributions. CONCLUSIONS The Soot Particle Aerosol Mass Spectrometer (SP-AMS) utilizes a new technique designed to detect and identify, in real time, airborne refractory black carbon (rBC) containing particles using a CW laser vaporizer. The instrument aims to measure the size, mass, and chemical composition of rBC particles, including both refractory and nonrefractory materials. The present study focused on the origin and nature of refractory carbon cluster ions (Cn+) produced in the SP26

ACCEPTED MANUSCRIPT AMS. Cn+ mass spectra were measured for 12 types of rBC particles produced by industrial and combustion processes, including diesel truck particulate exhaust and biomass combustion soot. The measured carbon cluster ion distributions varied as a function of rBC particle type and may serve as a fingerprint for the nature of the particles. The Cn+ ion distributions were classified into three categories (low, mid, and fullerene carbons), based on the carbon ion cluster size and the measured mass spectra. The low carbon number (C1+ - C5+) ion signals are consistent with a direct measure of small, neutral carbon


clusters in the vapor phase, with potential contributions from ions generated by the fragmentation of larger carbon clusters. The mid carbon ions may be related to the fragmentation of fullerenelike structures, but have not been consistently reproducible and warrant further study. The fullerene ions were likely produced by the electron ionization of pre-existing fullerenes or fullerene-like structures vaporized from the sampled rBC materials and modified by an additional, thermionic ionization mechanism under high laser power densities. The SP-AMS carbon ion distributions are likely governed, in part, by the annealing of the rBC particulate material at high temperatures in the laser vaporizer prior to complete vaporization, a process that is dependent upon the initial chemical composition of the rBC material and the rate of particle heating. To date, most observed ambient and ambient-related rBC particle sources exhibit SP-AMS measured carbon ion distributions that are dominated by ion signals in the low carbon numbers (C1+ - C5+) (Fortner et al., 2012; Massoli et al., 2012; Corbin et al., 2014; Dallmann et al., 2014), with the source-specific differences presented here being a secondary perturbation. Therefore, we recommend using low carbon number ions (C1 + - C5+) for ambient rBC mass calibrations and 27

ACCEPTED MANUSCRIPT quantitative rBC mass and size measurements. We expect our observations to provide a basis for the interpretation of future SP-AMS studies aiming to identify the source and atmospheric


history of airborne ambient rBC particles.

28

ACCEPTED MANUSCRIPT ACKNOWLEDGMENT The authors acknowledge Jason Olfert and Rouzbeh Ghazi for help with the inverted diffusion flame source and helpful discussions with Joakim Pagels and Axel Eriksson from Lund University. This research was supported by DOE ASR grants DE-FG02-05ER63995, DESC0006980 and DE-SC0011935, DOE SBIR DE-FG02-07ER84890, NASA SBIR NNX10CA32C, NSF Atmospheric Science Program grants 1244918 and 1244999, NSF Center for the Environmental Implications of Nanotechnology grant EF-0830093, an EPA STAR


Graduate Fellowship to AJT, and EPA STAR grants R833747 and 83503301. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not reflect the views of the funding agencies.

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ACCEPTED MANUSCRIPT Vander Wal, R.L., Bryg, V.M., and Hays, M.D. (2010) Fingerprinting soot (towards source identification): Physical structure and chemical composition. J. Aerosol Sci. 41: 108–117. Vander Wal, R.L., Bryg, V.M., and Huang, C.-H. (2014) Aircraft engine particulate matter: Macro- micro- and nanostructure by HRTEM and chemistry by XPS. Combust. Flame 161: 602–611. Vander Wal, R.L. and Choi, M.Y. (1999) Pulsed laser heating of soot: morphological changes.


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Table 1. Refractory black carbon (rBC) materials sampled in this work.

#

Particle Type

1

Aquadag

Carbon Type (CAS) and Usage Graphite (7782-42-5). Used for coatings for light absorption and electrostatics.

2

Biomass burning

Open air biomass burning emissions.

3

Carbon nanoparticles

4

Diesel exhaust

Amorphous carbon (7440-44-0), made by laser decomposition, average particle size 30nm. On road diesel truck exhaust.

5

Ethylene premixed flame soot C60

6

7

Alfa Aesar fullerene soot

8

Nano-C fullerene black

Source of Particles (Lot #) A cheson Colloids Company, Port Huron, Michigan (Produce 5307562, Lot 8867) FLAME3 study (McMeeking et al., 2009) Sigma-Aldrich Inc. (Produce 633100)

Caldecott Tunnel (Callmann et al., 2014) Fuel equilivance Premixed flat ratios from 2-5. burner flame source. Buckminsterfullerene, Materials and Buckyballs, CarbonElecrochemical 60 (99685-96-8), Research (MER) sublimed. Corporation Fullerene enriched Alfra Aesar soot generated by (Produce 40971, KräetschmerLots F12S011 Huffman arc process and L20W054) (Kräetschmer et al., 1990). Fullerene enriched Nano-C, carbon black Westwood, MA generated by Nano(Lot DRC’s proprietary II-G 070224) low-pressure, combustion-based 40

Generation Technique Atomized

Open air flame

Atomized

Internal diesel engine Lame (Cross et al., 2010) Dry powder aerosolization (Tiwari et al., 2013) Atomized

Atomized

ACCEPTED MANUSCRIPT 9

Glassy carbon spheres

10

Methane diffusion flame soot Norit SX activated charcoal

11


12

Regal blank

technology. Amorphous carbon (7440-44-0).

Net equivalence ratios from 0.6-0.9. Acid washed, steam activated carbon with high S/V ratio and adsorptive capacity. Carbon black or furnace black (133386-4). Used as an additive for plastic and rubber, pigment, chemical reagent, batteries, and refractories.

41

Yokai Carbon Co. and Alfa Aesar (Product 398004, Type 1, Lot A 06Q23) Diffusion flame source

Atomized

Norit

Atomized

Cabot Corporation, Billerica, MA (Produce 400R, Lot GP-3901)

Atomized

Flame (Stipe et al., 2005)


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Figure 1. Normalized positive ion mass spectra for (a) Regal black, (b) Ethylene soot, (c) NanoC Fullerene black, and (d) C60. Mass spectra represent the measured refractory carbon ion count rate (Hz) normalized to the total carbon ion signal as a function of the mass-to-charge ratio (m/z) up to m/z 2000. The displayed m/z scale corresponds to carbon cluster ions from C1+ (m/z 12) to C166+ (m/z 1992) as shown on the top axis. The C1+-C5+ region has been expanded for clarity. The top of the figure shows the SP-AMS mass spectrometric categories and the related structures for stable carbon clusters. 42


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Figure 2. Positive carbon ion mass spectra measured in the (a) exhaust emissions from a single on-road diesel truck and (b) smoke from the biomass burning of lodgepole pine. The C1+-C5+ region (1-60 m/z) has been expanded for clarity and the intensity scales for the C6+ to C83+ region (60-1000 m/z) has been expanded to 1:10 (right axes).

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Figure 3. (a) Normalized carbon ion mass spectra for Nano-C fullerene black with the electron beam on (black) and off (red). (b) Normalized mass spectra for Alfa Aesar fullerene soot with electron beam on (black) and off (red). The laser vaporizer is on in both cases. (c) The ratio of carbon ions with the electron beam OFF-to-ON for both materials as a function of m/z. The fractional carbon ion signals for electron beam OFF-to-ON from pure C60 is also included in (c) for the five m/z ion peaks with reasonable signal-to-noise; C60+ is the lowest point at the highest m/z in this case.

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Figure 4. Laser power studies for (a) Regal black, showing the invariance of C1+-C5+ with laser power, (b) Nano-C fullerene black, showing the different laser power dependences for C3+, C602+, C60+ and the total fullerene ion signal (C30+/ C602+ to C166+), and (c) C60, showing the similarities in laser power dependences for the parent C60+ and C602+ ions and the fragment C3+ and C58+ ions. The laser power axis is a relative measure of the intracavity laser vaporizer power obtained by measuring the light leaking through the mirror end of the laser cavity (see text for details).

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Figure 5. Normalized negative carbon ion mass spectra for (a) Regal black, (b) Nano-C fullerene black, and (c) C60. The scales for the right axes of (a) and (b) are 1:70; the scale for the left axis of (c) is 1:200. The C1--C10- region has been expanded for clarity.

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