Internally mixed multicomponent soot: Impact of different salts on soot structure and thermo-chemical properties

Journal of Aerosol Science 70 (2014) 26–35

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Internally mixed multicomponent soot: Impact of different salts on soot structure and thermo-chemical properties Henrike Bladt, Natalia P. Ivleva, Reinhard Niessner n Analytical Chemistry, Institute of Hydrochemistry, Technische Universität München, Marchioninistr. 17, D-81377 Munich, Germany

a r t i c l e in f o

abstract

Article history: Received 12 July 2013 Received in revised form 28 November 2013 Accepted 29 November 2013 Available online 16 December 2013

An often referred feature of ambient and combustion aerosol samples is their ratio of elemental carbon (EC) to organic carbon (OC). Thermo-optical methods are commonly used to determine this ratio. As those methods generally consist of a heating step under inert atmosphere for OC quantification and a consecutive combustion step under oxidative atmosphere for EC quantification, combustion catalysts may drastically impact the measured EC/OC value. Such catalysts may be minerals, e.g. inorganic salts or oxides, that are mixed with the soot and may stem from various sources. In this study, the impact of varying content of different salts internally mixed with soot on soot structure and oxidation reactivity was studied. For this purpose, a novel method for preparation of model soot aerosols internally mixed with different inorganic salts (CaSO4, Ce(SO4)2/CeO2, Na2SO4, or NaCl) at different contents was applied by spraying aqueous salt solutions into a propane diffusion flame. Proof of production of internal mixtures of soot with the different salts was given by scanning electron microscopy (SEM). Raman microspectroscopy (RM) was utilized to characterize the soot structure. Soot oxidation reactivity was analyzed by temperature-programmed oxidation (TPO). It could be proven that doping of the soot with inorganic salts does not impact the soot structure. However, soot oxidation reactivity is strongly enhanced with increasing salt content resulting in a TPO emission maximum shifted by up to 200 K towards lower temperatures. Our results pose questions on the feasibility of thermo-optical methods for the determination of EC/OC values of carbonaceous aerosols in the presence of internally mixed minerals. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Internally mixed soot aerosol Salt Mineral Structure Reactivity Thermo-optical analysis

1. Introduction Ambient aerosols and specifically combustion-derived aerosols are often characterized by their content of elemental carbon (EC) and organic carbon (OC), which are determined by thermo-optical methods (Ram & Sarin, 2011; Watson et al., 2005). Besides organic and elemental carbonaceous materials, ambient aerosols may also contain inorganic materials. The inorganic materials may be among others oxides, sulfates, nitrates, or chlorides of alkali and earth alkali elements (mostly Na, Mg, Ca), metals (mostly Al, V, Fe, Ni, Cu, and Zn), and Si. They may stem from various sources such as primary emissions like wood or oil combustion, industry, sea spray, or mineral (soil) dust as well as secondary aerosols (Artaxo et al., 1999; Viana et al., 2008).

n

Corresponding author. Tel.: þ49 89 2180 78231; fax: þ 49 89 2180 78255. E-mail addresses: [email protected] (H. Bladt), [email protected] (R. Niessner).

0021-8502/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jaerosci.2013.11.007

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In the specific case of traffic-generated soot aerosols, fuel-borne metal-based catalysts may be a source of metal oxides such as iron or cerium oxide (Naschke et al., 2008; Song et al., 2006; Vouitsis et al., 2008). Also the lubricant in marine or automotive engines may contain additives in order to improve its performance. Such additives may be surfactants such as calcium sulfonate (Hudson et al., 2006), which decompose to form calcium sulfate CaSO4 during engine combustion (McGeehan et al., 2009). Not only additives but also impurities in fuel and lubricant are a source of minerals mixed in engine exhaust aerosols. Particularly, oxides and sulfates of iron, vanadium, nickel, zinc, and calcium can be found at significant concentrations in the exhaust of ship engines using low grade heavy fuel oil (Lyyränen et al., 1999; Moldanová et al., 2013). Such additives and impurities in the fuel and/or lubricant are directly introduced into the soot formation process. Thus, the generated soot particles consist partly of soot and minerals and the soot possesses a very intimate contact to the minerals. We therefore term this type of soot as soot internally mixed with minerals. This definition is analogous to Adachi & Buseck (2008) who examined internally mixed soot, sulfates, and organic matter in ambient aerosol particles. Adachi & Buseck (2008) define soot particles coated with organic matter or sulfate as well as internally mixed soot. There exist several studies on the effect of various catalysts on the oxidation of carbonaceous material, specifically soot (Stanmore et al., 2001). Some of these studies used doped fuels or flames for the production of internally mixed soot aerosols. In such studies mainly Ce-, Fe-, Pb-, Cu-, V-, Mn-, and/or K-based catalysts were used to form (mixed) oxides of these elements internally mixed with soot (Bladt et al., 2012; Bonnefoy et al., 1994; Kim et al., 2010; Laheye et al., 1996). However, to our knowledge there are no publications on the systematic laboratory production and characterization of soot internally mixed with non-oxide minerals such as CaSO4, Na2SO4, or NaCl. In contrast, most studies cover physically mixed soot with potential catalysts yielding in externally mixed multicomponent soot. To produce this externally mixed soot, the mineral species were mixed with the mineral-free soot after the actual soot formation process generating a loose contact between the soot and the catalyst that is not as intimate as for internally mixed soot. Thus, externally mixed soot consists of separate soot particles and mineral particles, whereas internally mixed soot contains particles, which are composed of both soot and minerals. In the mentioned studies of externally mixed soot, soot was either impregnated with aqueous catalyst solutions (Castoldi et al., 2009; Matarrese et al., 2008) or mechanically mixed with the catalyst (Ciambelli et al., 1996; Neeft et al., 1998). Neeft and co-workers additionally differentiated between “loose” and “tight” contact for externally mixed soot. The former was produced by loose mixing of soot with the catalyst, for the latter both components were milled. Several metal catalysts (MoO3, Fe2O3, V2O3, etc.) were tested and ranked by Neeft et al. (1996). In general, tight contact yielded in higher oxidation reactivity than loose contact. Furthermore, for both soot externally mixed with metal oxides at tight contact (Ciambelli et al., 1996; Neeft et al., 1998) as well as internally mixed iron oxide-containing soot (Bladt et al., 2012; Kim et al., 2010), a significant fall in combustion temperatures with increasing catalyst content was observed. Reaction rates decreased significantly with increasing mass ratio of catalyst to soot (Ciambelli et al., 1996; Kim et al., 2010). For graphite oxidation, alkali salts are also known as catalysts besides metal oxides (Patai et al., 1952). However, we are not aware of any study systematically demonstrating the catalytic effect of diverse salts like CaSO4, Na2SO4, or NaCl internally mixed with soot on its oxidation as we have done with this study. Despite the knowledge of the catalytic effect of certain minerals on soot oxidation, the feasibility of thermo-optical methods for the quantification of EC and OC in aerosol samples is rarely questioned. Depending on the applied protocol of such methods, e.g. IMPROVE (Chow et al., 1993), NIOSH 5040 (National Institute of Occupational Safety and Health, 1999), etc., the aerosol filter samples are firstly heated under inert atmosphere (usually He) up to a certain temperature to quantify OC. Secondly, the samples are reheated under oxidative atmosphere (1–100% O2 in He depending on protocol) for EC quantification. In some protocols, the effect of OC charring to form EC during the first heating step under inert atmosphere is accounted for by a so-called pyrolysis correction. Thereby, the thermo-optical reflectance (TOR) and/or the thermo-optical transmittance (TOT) of laser light illuminating the filter sample are monitored. This pyrolysis correction shifts the split point between OC and EC evolution to the point where the reflectance or transmittance reaches its initial level (Watson et al., 2005). Thermo-optical analyses of ambient aerosols have been numerously published and according to Watson et al. (2005), it is doubtful that future carbon comparisons would lead to more information unless they include components that systematically study the effects of sample properties and the analysis variables. For this reason, we present a detailed and systematic characterization of internally mixed soot containing different salts concerning soot structure and thermo-chemical properties. Thereby, useful conclusions on the applicability of thermooptical methods for mineral-containing soot can be drawn. Particularly, it is possible that the EC/OC ratio can be severely reduced in presence of salts due to a reduction in oxidation temperature of EC and an enhancement in OC charring as found by Wang et al. (2010) using the NIOSH 5040 protocol for externally mixed multicomponent soot. For this purpose Wang et al. (2010) deposited metal particles on top of filters loaded with diesel exhaust particles producing external mixtures of both components. According to Wang et al. (2010) the split point correction is more dependent on changes in EC oxidation temperature than it is on OC charring. In further studies Wang et al. (2012) quantified black carbon in ice and snow following the IMPROVE TOR protocol. It could be demonstrated that inorganic material can change its optical properties during thermal treatment and thus can interfere with pyrolysis correction. The aim of this study is to systematically demonstrate the catalytic effect of diverse salts on soot oxidation. For this purpose, we developed a new method of producing internally mixed model soot aerosols by spraying aqueous solutions of

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these salts or correspondent precursors directly into a propane diffusion flame. Following, different analytical techniques were applied for the comprehensive characterization of the laboratory produced soot aerosols. Ion chromatography (IC) of the aqueous extracts of the soots was used for quantifying the salt content in soot. Scanning electron microscopy (SEM) proved the existence of an internal mixture of soot with mineral species by showing the crystallite needles of the minerals deeply embedded in the soot matrix and thereby demonstrating the intimate contact between soot and minerals. The generated aerosol samples were subjected to Raman microspectroscopy (RM). RM has been shown to be an effective tool for soot structural analysis (Ivleva et al., 2007; Sadezky et al., 2005). In addition, RM is able to identify mineral species (www.ens-lyon.fr) that are internally mixed in the prepared model soot. Finally, the thermo-chemical properties of our internally mixed soots were studied by temperature-programmed oxidation (TPO), which has been demonstrated to be an expressive method for this purpose (Knauer et al., 2009; Neri et al., 1997; Schmid et al., 2011) (Handbook of Minerals Raman Spectra, 2013). Thereby, soot-loaded filter samples are heated under oxidative atmosphere and the combustion products carbon monoxide CO and carbon dioxide CO2 are monitored by FTIR spectroscopy depending on applied temperature. Furthermore, several studies describe correlations between the soot structural parameters analyzed by RM and the soot oxidation reactivity derived from TPO or TGA (thermo-gravimetric analysis) experiments (Al-Qurashi & Boehman, 2008; Knauer et al., 2009; Schmid et al., 2011). For all internally mixed salt-doped soots of this study we found a strong decrease in temperature of maximum COþCO2 emission Tmax with increasing salt content in soot. As RM showed the soot structure not to be affected by the salt and its content in soot, the catalytic effect of the tested salts is the only reason for the enhancement in soot oxidation. We observed similar results for internally mixed Fe-containing soot in earlier studies (Bladt et al., 2012). The results question the feasibility of common thermo-optical techniques for the quantification of elemental carbon (EC) and organic carbon (OC) in ambient aerosols and specifically in combustion-derived aerosols in the presence of internally mixed minerals. 2. Materials and methods 2.1. Production of salt-containing model soot aerosol A custom-made diffusion burner was used for the laboratory preparation of salt-containing propane soot aerosols (schematic setup see Supplementary data Fig. S1). The burner nozzle was delivered with a flow of propane (0.14 L min  1), which was surrounded by a concentric flow of air (4.15 L min  1). The flame was additionally seeded with calcium sulfate (CaSO4), cerium(IV) sulfate (Ce(SO4)2), sodium sulfate (Na2SO4), or sodium chloride (NaCl). For this purpose the air flow was led through a nebulizing aerosol generator to produce an aqueous salt spray that was then conducted into the flame. The salt solution in the nebulizer contained the aforementioned salts in pure water (Milli-Q, MerckMillipore, MA, USA), except for the production of CaSO4-containing soot. As mentioned, calcium sulfonates are common additives to lubricants and decompose to form calcium sulfate in the engine. For the production of CaSO4-containing soot, an aqueous solution of calcium methanesulfonate (Ca(MeSO3)2; MeQCH3) was used due to its high solubility in water compared to CaSO4. By varying the salt concentrations (from 0 g L  1 to 52 g L  1) in the spray solutions, soots internally mixed with different salts at different contents were obtained. Undoped (non-mineral-containing) soot was generated for reference by spraying pure water (Milli-Q) into the flame under same conditions. The soot exhaust from the flame was diluted and cooled by an additional nitrogen flow (3.00 L min  1) streaming orthogonally to the top of the flame. All gas flows were controlled and adjusted using gas flow meters (Rota, Germany). Emitted particles were collected simultaneously on a set of three quartz fiber filters, which were preheated at 773 K for 24 h before sampling. One filter was used for ion chromatography, one for Raman microspectroscopy, and one for temperatureprogrammed oxidation. 2.2. Ion chromatography (IC) To determine the salt content in the soot samples, ion chromatography was applied. For this purpose, filter samples were heated at 423 K for 30 min to remove volatile organic compounds that may disturb the IC analysis. Afterwards, the samples were extracted in 20 mL of pure water (Milli-Q, MerckMillipore, MA, USA) at room temperature for 3 days. At the beginning and the end of the extraction procedure the samples were ultrasonified for 30 min. Afterwards the solutions were filtered through a disposable syringe filter (Cellulose, pore size 0.20 μm, Carl Roth GmbH, Germany), which was used as guard column to avoid clogging of the anion exchange column in the ion chromatograph by dispersed, micrometer-sized soot agglomerates of the soot filter sample. The anions in the solutions were qualified and quantified by an ion chromatograph Dionex LC25 (Thermo Fisher, MA, USA) with an anion-exchange column (AS9 Thermo Fisher, MA, USA) using conductivity detection after chemical suppression. In case of soot prepared with Ca(MeSO3)2, Ce(SO4)2, or Na2SO4 the content of sulfate 2 (SO4 ) was analyzed and in case of NaCl-doped soot the chloride (Cl  ) content was determined. From the anion content the total content of CaSO4, Ce(SO4)2, Na2SO4, or NaCl was calculated, verifying that these species are the only present minerals in

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the respective soot samples by the ion chromatogram. Results are given as percentage of mineral mass on total soot mass (% (m/m)).

2.3. Scanning electron microscopy (SEM) Scanning electron microscopy (SEM) was applied to investigate the composition of the internally mixed soot containing 42% (m/m) CaSO4. For this purpose, the soot filter sample was analyzed by a field emission scanning electron microscope (Sigma HD, Carl Zeiss Microscopy GmbH, Germany) with an energy selective backscattered electron (ESB) detector (Carl Zeiss Microscopy GmbH, Germany) and an in-lens secondary electron detector (InLens, Carl Zeiss Microscopy GmbH, Germany). The sample was measured under vacuum (2.53  10  4 mbar) at an acceleration voltage of 30 kV, a probe current of 50 pA, and a working distance of 2.5 mm.

2.4. Raman microspectroscopy (RM) Raman microspectroscopy was conducted with a Horiba LabRAM HR Raman microscope system (Horiba Jobin Yvon, Japan) at an excitation wavelength λ0 of 633 nm (He–Ne laser, 40 mW initial intensity, 14 mW at the sample at 100% laser intensity). The instrument was calibrated using the characteristic first-order phonon band of pure Si of a silicon wafer at 520 cm  1 and zero order correction of a blazed grating with a groove density of 600 lines mm  1. Spectra were recorded with a 50  magnification objective and in the Stokes shift range from 50 to 2000 cm  1. For sample analysis, the focused laser beam was scanned over an area of 20 μm  20 μm using DuoScan™. For untreated soot samples, the laser intensity was reduced to 1% of initial laser power (i.e. 0.14 mW at the sample) to avoid sample degradation. For soot samples after TPO exposure, 10% or 100% of laser power (i.e. 1.4 mW or 14 mW at the sample) were focused on the sample. Each soot sample was analyzed at 5–8 randomly selected positions throughout the filter surface. For each position 20 spectra of an integration time of 20 s were accumulated for a good signal-to-noise ratio. Multipoint baseline correction was performed for all spectra using LabSpec 5.58.25 software (Horiba Jobin Yvon, Japan) and the spectral intensities were normalized with the G peak at around 1600 cm  1. One mean Raman spectrum was calculated from the spectra of all observed positions of one filter sample, as spectra at different positions of the same filter sample were not distinguishable.

2.5. Temperature-programmed oxidation (TPO) TPO was used for simulation of soot oxidation in diesel exhaust aftertreatment systems (Bladt et al., 2012; Messerer et al., 2006; Schmid et al., 2011). Thereby, soot on quartz fiber filters was oxidized in the temperature range from 373 to 973 K and in a total gas flow (3.0 L min  1) of nitrogen (Z99.9990%, Air Liquide, France) including 5% of oxygen (99.9990%, Westfalen AG, Germany). The temperature was raised at a rate of 5 K min  1. Gas temperature control and adjustment were carried out using a type K thermocouple (HKMTSS-150, Newport Omega, Germany) placed in immediate proximity of the soot surface. Combustion products were analyzed by an FTIR spectrometer (IFS 66/s, Bruker, USA) equipped with a 2-L gas flow cell of an optical path length of 6.4 m. Before TPO experiments the soot-loaded quartz fiber filters were pretreated at 423 K under air for 30 min to remove residual water disturbing FTIR analysis. Results of the TPO experiments are TPO profiles showing the emission of CO2 þCO in dependence of applied gas temperature at the soot layer of the filter. The temperature of maximum CO2 þ CO emission Tmax is used as criterion for soot oxidation reactivity (Schmid et al., 2011). The methodic standard deviation of Tmax was determined by a set of seven filters of the same soot sample to be 10 K.

3. Results and discussion 3.1. Salt content The salt content of the soot samples analyzed by anion chromatography in dependence of applied salt concentration in the aqueous spray solution is shown in Fig. 1. It can be observed that the salt content in soot increases with applied salt concentration in spray solution for every tested salt. It proves that the method of spraying aqueous salt solution into the flame was successful for generating mineralcontaining soot. Furthermore, the high content of sulfate found in soot produced by spraying aqueous Ca(MeSO3)2 solution into the flame confirms that this approach for the generation of CaSO4-containing soot was successful. In case of Ce(SO4)2containing soot, the sulfate content was low compared to soot types containing different salts generated at similar salt concentration in the spray solution. We assume, that Ce(SO4)2 decomposes to a great extent to CeO2, which was not covered by ion chromatography. This would lead to an underestimation of actual total mineral content (composed of Ce(SO4)2 and CeO2) for Ce(SO4)2-doped soot. In the following, we will refer to this soot containing mostly CeO2 and only little Ce(SO4)2 as Ce(IV)-containing soot.

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Fig. 1. Concentration of CaSO4, Ce(SO4)2, Na2SO4, and NaCl in soot by anion chromatography versus initial concentration of Ca(MeSO3)2, Ce(SO4)2, Na2SO4, and NaCl in spray solution. Graph for Ce(IV)-containing soot is enlarged.

Fig. 2. SEM image of secondary electrons (left) and backscattered electrons (right) of internally mixed soot sample containing 42% (m/m) CaSO4.

3.2. Internal mixture of soot with minerals The soot sample containing 42% (m/m) CaSO4 was subjected to scanning electron microscopy (SEM) to visualize the internal mixture of soot with the minerals. The images of the secondary electrons and the backscattered electrons are shown in Fig. 2 (left and right, respectively). The image of the secondary electrons illustrates the porous morphology of the soot matrix. In contrary, the image of the backscattered electrons demonstrates the material contrast of the CaSO4 crystallites in white and the soot shown in dark gray. The crystallites are deeply embedded in the soot matrix. Thus, the existence of an internal mixture characterized by a very intimate contact between soot and minerals is proven. If the soot was externally (physically) mixed with CaSO4, separated particles of soot and minerals would have been observed. 3.3. Structural analysis All mineral-containing soot types were analyzed by Raman microspectroscopy (RM) to study the impact of the different salts on the soot structure. Spectra of CaSO4-containing soots are shown in Fig. 3. Spectra of soots containing Ce(IV), Na2SO4, or NaCl are shown in the Supplementary data (Fig. S2). All Raman spectra reveal the soot-characteristic first-order Graphitic (G) peak and Disordered (D) peak at  1600 cm  1 and 1335 cm  1, respectively (Sadezky et al., 2005). The spectra of undoped soot and soot mixed with the different salts at different concentrations are not distinguishable. Thus, neither the salt species, nor the salt content in soot impact the structure of the graphitic soot layers. Same observations were made for iron-containing soot in earlier studies (Bladt et al., 2012). According to Al-Qurashi and Boehman (2008), Knauer et al. (2009), and Schmid et al. (2011), soot structure correlates with soot oxidation reactivity. As in our case, the soot structure does not change with varying salt species and salt content we can exclude a change in soot oxidation reactivity due to soot structural changes in our internally mixed multicomponent soot aerosol. In some of our spectra of soot containing Na2SO4 or Ce(IV) at high contents, very small peaks at 992 or 465 cm  1 can be observed, respectively. According to the literature, these peaks are typical for Na2SO4 and CeO2, respectively (Murugan et al., 2000; Twu et al., 1997). Moreover, this proves that Ce(SO4)2 mostly decomposes in the flame yielding soot containing mostly CeO2 and little Ce(SO4)2. For untreated CaSO4-containing soot such typical mineral Raman signals were not observed.

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Fig. 3. Raman spectra of soot with different CaSO4 content in % (m/m). Soot-characteristic G and D peaks are marked in gray.

Fig. 4. Raman spectra of CaSO4-containing soot (a) before and (b) after TPO; (c) Raman spectrum of pure crystalline Ca(MeSO3)2 as used in spray solution.

No Raman signals of NaCl were observed for the correspondent soot as NaCl does not possess first-order Raman bands, but only weak second-order Raman-bands (Burstein et al., 1965; Rasetti, 1931). After the TPO experiments soot samples were again subjected to Raman analysis. In all cases, neither residual soot could be observed in the microscopic images nor soot characteristic Raman bands in the spectra. Only the remaining salt crystals were found in the microscopic images and identified by their Raman spectra (Supplementary data, Fig. S3). In case of Na2SO4-doping, the characteristic peaks for Na2SO4 at 1131, 992, 647, and 450 cm  1 (Murugan et al., 2000) were observed. For Ca(MeSO3)2-doping, only the characteristic peaks for CaSO4 at 1127, 1016, 626, and 416 cm  1 (www.ens-lyon.fr) (Handbook of Minerals Raman Spectra, 2013) were detected and no additional signals for Ca(MeSO3)2 were observed (Fig. 4). For Ce(SO4)2-doping, the characteristic bands of cerium(IV) oxide (CeO2) at 465 cm  1 (Twu et al., 1997) could be observed at all salt concentrations after TPO. NaCl does not show Raman bands.

3.4. Thermo-chemical reactivity Thermo-chemical reactivity of the soots containing different minerals at different contents and thus the catalytic activity of these mineral species were analyzed by temperature-programmed oxidation (TPO). The TPO profile of undoped soot exhibits an emission peak at 878 K, which is comparable to the Tmax of automotive diesel soots (Schmid et al., 2011). There were no further emission peaks or shoulders observed for undoped soot.

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Analogous to undoped soot, all mineral-doped soots show a single emission peak with Tmax and no emission shoulders. However, compared to undoped soot, the emission profiles of all mineral-doped soots are significantly shifted towards lower temperatures with increasing salt content. This illustrates that not only metal oxides such as iron or cerium oxide, as shown by our Ce(IV)-doped soot and by the studies of Bladt et al. (2012), Neri et al. (1997) and Stratakis & Stamatelos (2003), but also minerals such as CaSO4, Na2SO4, or even NaCl promote soot oxidation. The emission profiles of all Na2SO4-containing soots with the correspondent Tmax are shown exemplarily in Fig. 5. The emission profiles of all other analyzed soots are given in Supplementary data, Fig. S4. Soot containing Na2SO4 at the highest tested level of 78.5% (m/m) exhibits its Tmax at 678 K, which is 200 K lower than for undoped soot. But the Tmax of soot containing the lowest tested Na2SO4 content of only 1.0% (m/m) is already decreased by 140 K to 738 K compared to undoped soot with a Tmax of 878 K. For NaCl-doped soot, a salt content of only 0.3% (m/m) is sufficient to shift the emission peak by 95 K to a Tmax of 783 K. Hence, already little amounts of the different salts significantly shift Tmax and thus increase soot oxidation reactivity drastically. Further increase in content of the different tested salts yields only in little further increase in soot oxidation reactivity. The Tmax of all prepared soots in dependence of salt content are shown in Fig. 6. Furthermore, the emission profiles of Na2SO4-containing soot and to a lower extent those of NaCl-containing soot show an increasing steepness in emission slope with increasing mineral content in soot. Thus, the emission profiles narrow with rising mineral content. This indicates an increase in reaction rate in addition to the decrease in activation energy for soot oxidation with increasing salt content in soot as described in Ciambelli et al. (1996) and Kim et al. (2010). Nevertheless, we desist from calculating kinetic parameters based on our TPO measurements, as such non-isothermal experiments can be prone to diverse errors (Vyazovkin & Wight, 1998). Isothermal studies for the evaluation of kinetic parameters on the oxidation of soot internally mixed with minerals will be subject of future studies. Moreover, no SO2 or SO3 emissions were detected in the infrared spectra of the TPO experiments with soot prepared with Ca(MeSO3)2, Ce(SO4)2, or Na2SO4. Firstly, it implies that CaSO4 and Na2SO4 do not decompose during thermal treatment. Secondly, it proves that CeO2 is not only the main Ce species after TPO, as shown by Raman microspectroscopy, but is also initially the main Ce species present in Ce(IV)-doped soot before TPO. Figure 7 demonstrates the relation between mineral content in soot and the CO/CO2-ratio at Tmax for all prepared soots. It can clearly be seen, that this ratio decreases when soot is internally mixed with salts. For as little as the lowest tested salt contents in soot, it reaches a lower limit of approximately 0.1. Same observations with a similar lower limit of CO/CO2-ratio were made for Fe-containing soot in our earlier studies (Bladt et al., 2012). According to Neeft et al. (1997), this ratio decreases with decreasing temperature. Nevertheless, the extent to which the CO/CO2-ratio decreases for our soots appears not to be completely explained by Neeft's observation. It rather implies that complete soot oxidation is promoted by internally mixed salts. In difference to our tested salts CaSO4, Na2SO4, and NaCl, metals oxides such as iron oxides or our tested cerium oxide are possibly able to donate oxygen for soot oxidation, be then reoxidized by external gaseous oxygen and thus close the catalytic cycle as proposed by Neri et al. (1997). It is assumed that carbon adsorbs metal-bound oxygen atoms faster than molecularbound oxygen (Stanmore et al., 2001). However, CaSO4, Na2SO4, and NaCl do not offer the possibility of such a redox-like oxidation mechanism. Nevertheless, the extent of decrease in Tmax and CO/CO2-ratio with increasing mineral content is in the same range for CaSO4-, Na2SO4-, and NaCl-doped soot as for Ce(IV)-doped soot (containing mainly CeO2 as shown by Raman microspectroscopy) and Fe oxide-containing soots as shown in Bladt et al. (2012). Since similar results were obtained

Fig. 5. TPO profiles of soot containing different content of Na2SO4.

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Fig. 6. Temperature of maximum CO2 þ CO emission Tmax in dependence of salt content in soot (n ¼5, m¼ 1 for CaSO4-, Ce(IV)-, and Na2SO4-containing soot; n ¼4, m¼ 1 for NaCl-containing soot).

Fig. 7. CO/CO2-ratio at temperature of maximum CO2 þCO emission Tmax in dependence of salt content in soot (n¼ 5, m¼ 1 for CaSO4-, Ce(IV)-, and Na2SO4containing soot; n¼ 4, m¼ 1 for NaCl-containing soot).

for metal salts and oxides, it appears to be a rather physical than chemical feature of minerals to promote soot oxidation. Neeft et al. (1997) and Castoldi et al. (2009) propose that the catalytic activity is furthermore affected by the mobility of the catalyst that positively influences the carbon/catalyst contact. In contrary to such studies concerning externally mixed multicomponent soots, it is further plausible for our internally mixed soots, that the salt aerosols directly introduced into the flame already impact soot formation. We believe that the salts decrease the size of primary soot particles and/or increase the distance between primary soot particles in agglomerates. This would facilitate the interaction of external oxygen with carbon. This effect will be more pronounced by increasing mineral content in soot. In that case, our tested salts could not be called catalysts anymore in the strict sense of a catalyst being a substance taking part in reaction by the formation of an intermediate species. We will devote future studies to this effect. Finally, the results of our TPO experiments raise questions on the applicability of thermo-optical methods for EC/ OC quantification if the carbonaceous material is internally mixed with salts. As demonstrated, various salts are able to shift the emission during thermal treatment towards lower temperatures. It is highly possible that not only the tested salts possess this ability, but also various other minerals frequently found in ambient or specifically trafficemitted aerosols. Consequently, the emission of EC may be shifted towards lower temperatures or even into protocol regions before the pyrolysis split point, e.g. regions where usually OC is expected. This would drastically decrease the EC/OC ratio as it was indicated by Wang et al. (2010) for externally mixed soots. In addition, it leads to the false assignment of different OC or EC fractions to certain temperature steps applied under inert or oxidative atmosphere,

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respectively (Chow et al., 2001). Pyrolysis correction is furthermore impacted by minerals changing their chemical and/or physical properties during thermal treatment. Such changes may include phase transitions or decomposition as shown by Bladt et al. (2012) and Wang et al. (2012). Moreover, the RM experiments of this study showed that Raman signals of minerals are barely observable in internally mixed soots before thermal treatment. After oxidation, the samples exhibit strong Raman signals if the minerals are Raman-active. Hence, internally mixed minerals may change their optical properties during the course of thermo-optical experiments and thereby impact pyrolysis correction in an unpredictable manner. Thus, apart from the general problem of high discrepancies between EC/OC ratios of same samples analyzed by different thermo-optical protocols (Chow et al., 2001; Schmid et al., 2001), a comparison of EC/OC values of different aerosol samples containing minerals of different quality and quantity would not even be possible if exactly the same protocol for thermo-optical analysis was applied. 4. Conclusions In this study, we present a systematic and detailed characterization of internally mixed multicomponent soot aerosols. A novel preparation method for such soot aerosol is applied by spraying aqueous solutions of various concentration of calcium methanesulfonate (Ca(MeSO3)2), cerium(IV) sulfate (Ce(SO4)2), sodium sulfate (Na2SO4), or sodium chloride (NaCl) into a propane diffusion flame. The method was successful for generating internally mixed soot containing CaSO4, Ce(IV), Na2SO4, or NaCl at different contents from 0.3% to 78.5% (m/m), respectively. Scanning electron microscopy (SEM) proved the existence of an internal mixture of soot with minerals by showing the mineral crystallites embedded in the soot matrix. Raman microspectroscopy (RM) demonstrated that the soot structure is not affected by different contents of the salts. Besides, the Raman signals of the tested salts were only barely observable in some soot samples before TPO. After soot oxidation by TPO, the residual salts exhibited strong Raman signals. However, soot oxidation reactivity drastically enhanced with increasing salt content in soot for every tested salt. But already little salt contents, e.g. 0.3% (m/m) NaCl, shifted the temperature of maximum emission Tmax by 95 K compared to undoped soot. Furthermore, complete soot oxidation is promoted by the salts as the CO/CO2 ratio at Tmax decreased with increasing salt content in soot. Thus, it could be shown that TPO is a powerful tool to analyze the thermo-chemical properties of internally mixed multicomponent soot. Not only metal oxides but also diverse salts are able to stimulate soot oxidation. The results of this study raise questions on the feasibility of thermo-optical methods applied for EC/OC quantification of ambient aerosols. Those aerosols may be internally or externally mixed with inorganic material of various composition and quantity. In the presence of inorganic materials, it is highly possible that the EC and OC emission are shifted leading to a wrong assignment to different OC and EC fractions or even to wrong estimations in the EC/OC ratio. Consequently, EC/OC ratios of different ambient samples are not comparable even if analyzed by the same thermo-optical protocol.

Acknowledgments Financial support by the Deutsche Forschungsgemeinschaft (DFG, Grant NI 261/26-1) is gratefully acknowledged. The authors appreciate the support in ion chromatography and TPO measurements by B. Apel and I. Pfeffereder from Technische Universität München, as well as O. Popovicheva (Moscow State University) for discussions. Furthermore, the authors thank Dr. A. Schertel, Dr. G. Ganskow and Dr. W. Horn (Carl Zeiss Microscopy GmbH, Germany) for conducting the SEM measurements and providing the SEM images. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jaerosci. 2013.11.007.

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