Detection of electrically charged soot particles in laminar premixed flames

Combustion and Flame 159 (2012) 1082–1089

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Combustion and Flame j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m b u s t fl a m e

Detection of electrically charged soot particles in laminar premixed flames Hartmut Mätzing a,⇑, Werner Baumann a, Henning Bockhorn b, Hanns-Rudolf Paur a, Helmut Seifert a a b

Institut für Technische Chemie (ITC), Karlsruhe Institute of Technology (KIT), Postfach 3640, D-76022 Karlsruhe, Germany Engler-Bunte-Institut (EBI-VBT), Karlsruhe Institute of Technology (KIT), Postfach 3640, D-76022 Karlsruhe, Germany

a r t i c l e

i n f o

Article history: Received 4 July 2011 Received in revised form 21 September 2011 Accepted 23 September 2011 Available online 20 October 2011 Keywords: Soot formation Particle mass spectrum Particle size distribution Charged soot particles Particle inception Premixed flames

a b s t r a c t A particle mass spectrometer has been used to investigate the formation of electrically charged species and soot particles in laminar premixed flames. The mass range was from 600–6  105 amu and extends from high molecular hydrocarbons to soot particles of 10 nm diameter. The flames were stabilized on cooled porous plate burners. Acetylene/oxygen flames were investigated at low pressure (30 mbar), and ethylene/air flames were investigated at atmospheric pressure. Soot particles could only be detected in flames showing yellow luminosity, i.e. above the critical C/O-ratio for soot formation. Both positively and negatively charged particles were found, the positive charge dominating in the low pressure acetylene/oxygen flames, the negative charge dominating in the atmospheric ethylene/air flames. With the assumption of spherical shape and constant density, the mass spectra were converted to size spectra. Usually, they show a multiple peak structure which is somewhat difficult to interpret. There are indications that particles may carry multiple (1–2) charges, and also that particles of different types may coexist beside polyaromatic and polyhedral species in the early stage of particle inception. Ó 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction Soot formation in fuel rich hydrocarbon flames is of interest both from the mechanistic point of view to understand the phase transition from the gas phase to the solid phase as well as from the practical point of view to control the energy efficiency and heat transfer in combustion engines and the environmental impacts of soot emissions. In general terms, soot formation involves a variety of physico-chemical processes like particle inception, mass growth and eventual oxidation beside particle coagulation and aggregation which have been under investigation for many years. An examination of the early literature [1,2] gives some insight how these mechanisms interact and contribute to the formation of overall soot loadings as well as to the evolution of soot particle size and number density. In flat laminar premixed flames, burning conditions can be adjusted in a way to decouple interfering mechanisms to some extent [3]. One key question still is the formation of nonvolatile species and particles during the build-up of aromatics of high molecular weight (PAH). The recent research approaches this problem both with sophisticated modeling tools as well as by advanced particle sizing techniques. A very promising modeling approach is the so-called HACA (hydrogen abstraction, C2H2 addition) mechanism which assumes comparatively stable polyaromatics like acenaphthylene and/or pyrene as key intermediate species on the way to particle inception which is not driven by

⇑ Corresponding author. Fax: +49 721 608 24373. E-mail address: [email protected] (H. Mätzing).

nucleation from some supersaturated vapor but merely by reaction kinetics [e.g., 4,5]. A more recent analysis of this concept has shown its capability to reproduce measured particle size distributions and gives indications that incipient particle size distributions may be bimodal [6–8]. Bimodal and even trimodal soot particle size distributions in laminar premixed flames have also been reported in several experimental studies [e.g., 9–16]. In these investigations, the particle size was determined by mobility analyzers [9–14] or by photoionization mass spectrometry [15,16] which employ a controlled electrical charging of the particle samples. Bimodality of soot particle size distributions can result from persisting particle inception at low soot loadings or from coexistence of ‘‘primary’’ particles and their aggregates under high soot conditions [14,16]. Soot particle precursor species may be expected in the mass range below about 3000 amu (which corresponds to ‘‘particle diameters’’ below 1.7 nm numerically, if q = 2 g/cm3 is assumed as material density). In this region, the mass spectrum is densely populated with high molecular species, but the mass distribution is strongly dependent on ionization conditions: If only flame ions are investigated without additional ionization, polyynes, polyaromatic and polyhedral species are observed in sooting flames and their mass distribution appears unimodal [17]. If photoionization techniques are applied, the mass distribution shifts from unimodal to multimodal, when the ionization wavelength is decreased below k = 248 nm [18]. Reliable gas and particle sampling from high temperature, reacting flows is not trivial, and requires to minimize errors related to disturbance of the temperature and flow fields at the sampling point as well as errors resulting from gas and particle interactions

0010-2180/$ - see front matter Ó 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2011.09.014

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and losses in the sampling line. Many investigations on soot particle formation in flames have employed optical techniques and molecular beam mass spectrometry (MBMS) which are non-invasive (optical) or, which have been optimized to a minimum of invasive errors [1,2,17]. Since molecular beam sampling is a somewhat elaborate task, more simple sampling nozzles have been developed recently which mainly focus errors in the sampling line and use optimized dilution techniques [19–21]. In addition to the sample dilution, the geometry of the nozzle and the residence time in the sampling line may affect the measured particle size distribution [10–13] or the quality of the mass spectrum [18]. Systematic errors due to intrusion of large nozzles into flame gases probably have not been discussed in the literature, yet, even under low sooting conditions, very broad soot particle size distributions have been reported which cover a diameter range of an order of magnitude [10–14,20]. Up to now, it has not yet been discussed, how these results compare to previous reports on soot particle size distributions and to relevant mean particle size measurements by optical methods [1,2]. In this study, a particle mass spectrometer (PMS) was used to detect electrically charged soot particles in flat laminar premixed acetylene/oxygen flames at low pressure (30 mbar) and in ethylene/air flames at atmospheric pressure. Two similar particle mass spectrometers were used, designed for low and atmospheric pressure applications, resp. They have two stage molecular beam sampling systems which rapidly quench all gas-particle interactions. The working principle is TOF as described earlier [22], but the newly developed design was aimed to make the instrument substantially smaller and easy to handle [23–27]. The deflection voltages were chosen to permit particle masses in the range 1021–1018 g to be detected which corresponds to soot particle diameters from 1–10 nm, if the density of soot is taken to be 2 g/cm3. The size measurements were performed as function of height above the burner plate at several mixture strengths (C/O-ratios) and flow velocities. The smallest masses detected were about 2.3  1021 g or 1400 amu and are in the size range of polyaromatic or polyhedral species. A numerical particle diameter around 1.3 nm is obtained for these species, and it should be remembered that these species probably are not yet particulate. As a rule of thumb, masses above 5000 amu can be termed to be particulate, corresponding to diameters above 2 nm. Substantial progress has been achieved concerning the quantitative evaluation of the PMS signal intensity and, in this paper, the derivation of absolute number densities is described in detail for the first time.

molecular beam probing

deflection unit

2. Experimental The laminar premixed flames were generated on a commercial Mc Kenna burner with water cooled sintered bronze plates of 60 and 75 mm diameter in a way similar to previous reports [9–14,17,18,22–27]. The low pressure burner was housed in a steel vessel and the flame was not shielded against the surrounding burnt gases. The acetylene and oxygen flow rates were adjusted with mass flow controllers to give a cold gas inlet velocity of 38 cm/s. The C/O-ratios were 1.0, 1.1 and 1.2. Flame temperatures were not measured, but can be estimated to be in the range 1900–2000 K from Table I in Ref. [22]. The PMS was attached to the vessel top and the burner was movable in vertical direction to change the distance between burner plate and sampling nozzle, i.e. height above burner (HaB). Up to HaB  50 mm, the flame shape was close to cylindrical; it changed into a conical shape, when the burner was moved further away from the sampling nozzle. At HaB  100 mm, the flame diameter decreased significantly to about 20 mm. The atmospheric pressure flames were shielded against surrounding air by an annular nitrogen stream. The ethylene and air flow rates were adjusted with mass flow controllers to give cold gas inlet velocities between 9 and 16 cm/s. The C/O-ratios were varied from 0.61 to 0.68. A stabilizing steel mesh was placed approx. 30 mm above the burner port to give the flame a cylindrical shape. The temperature profiles in the postflame gases were investigated with thermocouples and were found rather uniform with a linear dependence on HaB between 2 and 4 K/mm. The basic set-up and the measurement principle of the PMS are shown schematically in Fig. 1. The particles are sampled from the hot flame gases by a quartz nozzle coated with platinum which was kept at ground potential like other relevant parts of the system, including the burner. They are transferred into the expansion chamber where the pressure is around 103 mbar. From the expanding free jet, a molecular beam is extracted by use of a skimmer and transferred into the high vacuum detection chamber (105 mbar). Of course, some systematic shortcomings associated with probe sampling can never be avoided completely. They include, for instance, some averaging over space and temperature due to the finite sampling volume and they are known as flame perturbation effects [e.g., 28–30]. These are of special interest for axisymmetric positioning of the probe, as in case of the low pressure experiments reported here. Perturbation effects can be investigated by optical methods as has been shown earlier for

charge separated particle beams

+

faraday cups / amplifier

neutral

p = 20 ... 1000 mbar

p = 10-3 mbar

turbomolecular pump

-

+ p = 10-5 mbar

turbomolecular pump

Fig. 1. Schematic representation of the particle mass spectrometer.

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non-sooting flames [e.g., 31]. If the results of that study can be extrapolated to sooting conditions, the soot mass growth in the sampled volume can be expected to be quenched a little already before the sample reaches the MBMS sampling nozzle, which would be favorable for the measurements presented here. In the atmospheric flames, the sampling probe was positioned perpendicular to the gas flow which reduces potential errors due to flame perturbation. No flame disturbance was visible by eye. The temperature profile in the vicinity of the sampling probe was measured using thermocouples and no temperature effects were measured at distances larger than 2 mm from the sampling probe. The nozzle diameters were 0.5 mm in the low pressure application and 0.3 mm in the atmospheric pressure application. Occasionally, some measurements were repeated with different orifice size and, no inconsistencies could be observed. This gives further confidence in the measurement method. Downstream of the skimmer, the particle sample passes a deflection unit which is equipped with two capacitors perpendicular to each other. The first capacitor has a variable DC voltage and serves to deflect particles with different mass to charge ratios from the central pathline to the Faraday cups where the resulting intensity is measured. This is shown in Fig. 1. The appropriate range of deflection voltage depends on the ion mass to be detected, on the velocity of the ions and, it is also subject to instrument dimensions. In the low pressure application, the deflection voltages were in the range 0–100 V, in the atmospheric pressure application, they were in the range 0–1000 V to cover the same m/z range. The second capacitor (not shown in Fig. 1) provides an AC voltage with an amplitude of approx. one tenth of the deflection voltage and ensures that the deflected particle beam hits the Faraday cups periodically. The flight time difference between the triggering AC voltage and the detected particle signal results in a phase shift. After preamplification, signal intensity and phase shift can be measured simultaneously with a lock-in amplifier. The correlation from phase shift to flight time and particle velocity is straight forward, since the path length is known. In the low pressure application, particle velocities were around 450 m/s, while the particles sampled from atmospheric flames had velocities around 1200 m/s. The measured signal intensity varied over several orders of magnitude depending on flame conditions and was in the range 1014– 1011 A. For particles with one single charge, this corresponds to a detection limit of approx. 107 . . . 108 per cm3. The mass resolution of the instrument can be varied by vertical slits placed in front of the Faraday cups. In the low pressure application reported here, the mass resolution was ±10% and in the atmospheric pressure application it was ±5%. For particle measurements, this is sufficient and corresponds to size resolutions Dlog dp = 0.026 and 0.013 (dp = particle diameter), resp., but for the precise detection of particle precursors (e.g., PAH), the mass resolution is somewhat poor. Increase of the flight path would also support a better mass resolution. However, the intention was to build very compact instruments with comparatively small pumps. The spectrometers are only about 0.5 m long and 0.25 m in diameter with total pumping rates of only 800 and 1600 l/s. The validity of the measurement procedure has been demonstrated earlier for the case of inorganic nanoparticles [32]. 2.1. Data evaluation The relationship between deflection voltage U[V] and particle properties has been described and illustrated elsewhere [22,33]. Briefly, the particle (ion) mass mp (kg) is proportional to U/v 2p where vp (m/s) is the particle (ion) velocity. The proportionality constant involves several instrument parameters and is given by 7.53  1019 A s in the present set-up. The particle velocity in the skimmed beam was found to vary only little with the particle mass,

but it was found strongly dependent on pressure. In the low pressure (30 mbar) application, the terminal particle velocity was close to 20% of the terminal gas velocity (vt,gas  2100 m/s) and in case of the atmospheric pressure flames (vt,gas  4200 m/s) it was a factor of three higher. The angular divergence of the skimmed particle beam was found to be less than 2°, hence no ambiguity on the size determination arises from beam divergence. The flux J of charged soot particles which are transferred to the detection chamber per time is given by the integral of the probability (size) distribution function dN/d log dp multiplied with the volume flow (dV/dt)SK transferred to the skimmer and the transfer factor atr:

J¼

  Z dV dN  atr  d log dp dt SK d log dp

ð1Þ

For small size intervals, the integral can be replaced by (dN/d log dp)  Dlog dp. The volume flow transferred to the skimmer can be calculated from the mass flux through the sampling nozzle in a way similar to that used in ICP-MS measurements [e.g., 34,35]. Since in the particle mass spectrometer the gas sample can be expanded far beyond continuum conditions with very high nozzle pressure ratios, the relationships given in Ref. [34,35] need to be modified to meet conditions which are characterized by both low and high formal Mach numbers [e.g., 36]. The volume flow transferred to the skimmer is then found to depend (i) on gas properties (the ratio of heat capacities, c = cp/cv  1.3 for postflame gas, the molar mass of the gas sample, M  25 g/mol for postflame gas, and the gas (stagnation) temperature immediately in front of the nozzle, T0  1600 K [37]), (ii) on flow properties (Mach number MaSK at the skimmer location – the use of a formal Mach number under molecular flow conditions is due to convenience [38]), (iii) on instrument geometry (diameters of the sampling nozzle, Dn, and the skimmer, DSK, distance x between the nozzle inlet and the skimmer which must be located upstream of the shock front in the zone of silence) in the following way:

rffiffiffiffiffiffiffiffiffiffiffi 1    c1  2 dV 2 1 Dn cRT 0 p 2 ¼   fM  MaSK  D dt SK 4 x cþ1 M 4 SK

ð2Þ

The first factor in this equation accounts for the reduction of the gas density to the critical value when the Mach number reaches 1 in the nozzle, the second factor is the beam attenuation due to the radial expansion, the third factor, fM, accounts for beam focussing to the centerline at elevated Mach number (see below), the fourth term is the molecular flow velocity as obtained from the terminal Mach number at the skimmer location, MaSK, and the velocity of sound at the inlet. The last term is the skimmer cross-section. Provided that the expansion is into the regime of molecular flow, the terminal Mach number at the skimmer location is defined via the Knudsen number Kn0 (i.e. gas mean free pathlength1 divided by nozzle diameter) at the nozzle inlet [36]: ð1cÞ=c

MaSK ¼ 1:2  Kn0

ð3Þ

The Mach focussing factor fM is 1 for Mach numbers up to 4 and is given by

fM ¼

3 þ cMa2 2p

Ma > 4

ð4Þ

beyond that range [38]. The product fM  MaSK is a generalized expression to obtain the centerline beam velocity and intensity. 1

600 nm in the atmospheric flames, 20 lm in the low pressure flames.

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Diameter d p [nm] using z = 1 signal intensity [10 A]

(a)

600

-15

-15

signal intensity [10 A]

2

positive charge negative charge

400

200

0 0

10

20

30

3

4

5

6

(b)

600

positive charge negative charge

400

200

0 1000

10000

deflection voltage [V]

100000

m/z [amu]

Fig. 2. Measured signal intensity as function of deflection voltage (a) and as function of mass to charge number m/z (b) in a C2H2/O2 flame, C/O = 1.1, P = 30 mbar, v = 38 cm/s, HaB = 50 mm.

ð5Þ

DI has the same sign as z. The transfer factor atr in Eqs. (1) and (5) is below 1, if ion loss occurs, and it is larger than 1, if the particle beam is expanded less than the beam of light molecules [37]. Preliminary measurements suggest atr close to 1 [41] which is used in the present data evaluation. 3. Results 3.1. Results in low pressure flames

3.0x10

9

positive charge negative charge

-3

dN DN DI  ¼ d log dp D log dp z  e  ðdV=dtÞSK  atr  D log dp

of the positively charged particles, these peaks are observed also and the overall intensity is significantly higher. In addition, a third peak becomes observable at a deflection voltage of 5 V. Hence deflection voltages change by a factor of two from peak to peak. A very similar peak pattern was observed earlier under comparable flame conditions [22]. Figure 2b shows the signal intensity as function of the mass to charge number, m/z. The number of charges z per particle is not known, and the most simple, tentative assumption is z = 1. With this assumption, all particles masses are in the range 5000–200,000 amu which corresponds to particle diameters between 2 and 7 nm, if the soot particle density is taken to be 2 g/cm3. This is indicated in the upper scale of Fig. 2b. Figure 3 shows the particle size distribution obtained from the data in Fig. 2. The total number density of charged soot particles of the main peaks in Fig. 3 is 1.7  108 cm3 and 5.6  107 cm3 for positively and negatively charged particles, resp., and the peak widths are ln rg = 0.058 when fitted to log-normal distributions. The size and the number densities of the charged particles are in good agreement with values reported previously for comparable conditions, when a particle detection at a location upstream of the skimmer was used [37]. However, the size of the neutral particles in these flames has been reported to be twice as high [33]. Similar measurements were performed for different C/O ratios and different heights above the burner (HaB). As general tendency it was observed that under low sooting conditions, the intensity spectra are shifted to lower m/z, the small side peaks vanish and the signal intensity is reduced. With increasing soot loading, the size distribution shifts to larger particle size, the signal intensity increases and the intensity of the side peaks grows. As example, Fig. 4 shows results obtained at HaB = 20 and 70 mm for the same flame as in Fig. 3. In order to summarize the various measurements, only the main (central) peak of the size distributions was used and the

d N/d logdp [cm ]

At moderate nozzle pressure pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi ratios and low terminal Mach numbers, fM  MaSK  2=ðc  1Þ, the latter term being applied in Eq. (2) usually [34,37]. In case of the low pressure flames, MaSK was found to be 2.5. In case of the flame burning at atmospheric pressure, MaSK = 5. Therefore, fM = 1 is used in the low pressure case and, in case of atmospheric pressure, fM = 5.7 is obtained. The high Mach number would thus correspond to a reduced apparent beam divergence at the skimmer entrance. This was investigated experimentally by placing a flat target plate at the skimmer position and measuring the diameter of the impinging particle beam. Therefrom, a beam divergence of 6° (half angle) was obtained. This result confirms the considerations above and, it is in agreement with comparable measurements obtained with supersonic gas injectors which are investigated elsewhere [39]. The important observation in the present context is that Mach focussing can increase the detection limit of the particle mass spectrometer remarkably. (Note, in addition, that the PMS is operated outside the range of significant cluster formation [40].) From Eqs. (2)–(4), the volume flows transferred to the skimmer are calculated to be nearly equal, (dV/dt)SK = 0.05 cm3/s for the low pressure flame and (dV/dt)SK = 0.045 cm3/s for the atmospheric pressure flame. This is in agreement with the experimental finding that the pumping rates required at the detection chamber are equal in both cases. The ion current DI in the size interval Dlog dp is now given by DI = z  e  (dV/dt)SK  atr  (DN/Dlog dp)  Dlog dp where e is the elementary charge and z is the charge number which is set to ±1 here, as will be discussed below. Hence the size distribution function is obtained from

2.0x10

1.0x10

9

9

0.0

Figures 2a and b show typical results obtained in low pressure flames. In Fig. 2a, the measured intensity spectrum is plotted as function of deflection voltage without further data evaluation. In case of the negatively charged particles, the intensity spectrum shows two peaks at deflection voltages of 11 V and 22 V. In case

1

2

4

6

8

Diameter d p [nm] Fig. 3. Size distribution of charged soot particles in a C2H2/O2 flame, C/O = 1.1, P = 30 mbar, v = 38 cm/s, HaB = 50 mm.

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9

8x10

8x10

-3

8

d N/d logdp [cm ]

-3

d N/d logdp [cm ]

(a) positive charge negative charge

6x10

8

4x10

8

2x10

(b)

9

6x10

positive charge negative charge

9

4x10

9

2x10

0

0

1

2

4

1

6

2

Fig. 4. Size distribution of charged soot particles in a C2H2/O2 flame, C/O = 1.1, P = 30 mbar,

6

8

10

v = 38 cm/s, (a) HaB = 20 mm and (b) HaB = 70 mm.

10 positive charge (main peak)

8

C/O 1.2

6

1.1 1.0

4 2

Diameter dp [nm]

10

Diameter dp [nm]

4

Diameter dp [nm]

Diameter d p [nm]

0

negative charge (main peak)

8

C/O 1.2 1.1 1.0

6 4 2 0

0

20

40

60

80

0

100

20

40

60

80

100

Height above burner, HaB [mm]

Height above burner, HaB [mm]

Fig. 5. Size of positively and negatively charged soot particles vs. HaB in C2H2/O2 flames, C/O = 1.0, 1.1 and 1.2, P = 30 mbar,

m/z [amu]

10

(a)

6.0x10

10

10000

v = 10 cm/sec

4.0x10

positive charge negative charge

10

2.0x10

0.0 1

1000

1.2x1010

100000

2

4

6

8

10000

9.0x10 9

v = 12 cm/sec

6.0x10 9

positive charge negative charge

3.0x10 9 0.0 1

10

2

100000 2x10 v = 14 cm/sec

1000

9

positive charge negative charge

2x109

0

10000

(d)

-3

dN /d logd p [cm ]

-3

dN /d logd p [cm ]

4x109

(c)

6

8

10

8

10

m/z [amu]

m/z [amu] 10000

4

Diameter d p [nm]

Diameter d p [nm]

1000

100000

(b)

-3

1000

dN /d logd p [cm ]

10

-3

dN /d logd p [cm ]

m/z [amu] 8.0x10

v = 38 cm/s.

100000

v = 16 cm/sec positive charge negative charge

1x10

9

0 1

2

4

6

8

10

Diameter d p [nm]

1

2

4

6

Diameter d p [nm]

Fig. 6. Mass and size distribution of positively and negatively charged species and particles in atmospheric C2H4/air flames, C/O = 0.68, HaB = 14 mm, with variable gas inlet velocities v. In Fig. 6a, the solid lines are log-normal fits.

corresponding particle diameter was plotted as function of the height above burner. As shown in Fig. 5, the particle size increases with increasing height above burner up to approx. 50 mm and reaches some final constant value thereafter. The mass and size of the particulate ions agree well with the previous report by Roth and Hospital [22].

3.2. Results in atmospheric flames In atmospheric ethylene/air flames, unimodal and bimodal size distributions have been observed under low sooting conditions. As example, Fig. 6 shows a series of measurements which were performed at a constant C/O-ratio of 0.68 and at a constant height

H. Mätzing et al. / Combustion and Flame 159 (2012) 1082–1089

above the burner, HaB = 14 mm, but with variable fresh gas inlet velocity v. Increase of the inlet velocity corresponds to an increase of the flame temperature. In contrast to the low pressure acetylene/oxygen flames, the intensity of the negatively charged species predominates in these flames. At the lowest inlet velocity investigated here, v = 10 cm/s, the size distribution appears to be unimodal with a peak particle diameter around 7 nm (assuming z = 1). At the lower (left) end of the mass (size) distribution, the signal intensity and the size distribution function decrease continuously into the range of the detection limit. At v = 12 and 14 cm/s, the mass distribution is shifted to smaller m/z – values and a second peak appears in the low mass range, while the number density of the former particle peak is decreased. At v = 16 cm/s, only one peak remains and is located around m/z = 3000. If z = 1, this peak would be in the mass range of high molecular hydrocarbons (PAH) which may not yet be particulate. Very similar observations were also made when decreasing the C/O-ratio while keeping the inlet velocity and the height above burner constant. Since a decreasing C/O-ratio is also associated with decreasing soot loading and increasing flame temperature, all these observations are consistent and may reflect a temperature effect close to the threshold of particle inception. Due to some spatial limitations of the experimental set-up, the smallest accessible height above the burner was 11 mm in these experiments. Starting from there, the signal intensity of the low mass peak decreased continuously down to the detection limit with increasing HaB, and, in parallel, the signal intensity of the high mass mode increased continuously until HaB = 20 mm [26,48]. There are indications that the signal intensity of the high mass mode does not increase further [41]. Obviously, the low mass mode is best observed close to the flame front (i.e. the hottest part of the flame) and does not persist generally in the postflame region.

4. Discussion In the low (30 mbar) pressure flames, charged species were detected with m/z well above 5000. Species with positive charge were dominating and showed a trimodal mass distribution, while the negative species showed two modes only. If the peaks are fitted to log-normal shape, standard deviations ln rg of the order of 0.06 are obtained which means that the peaks are very sharp (monodispersity is usually attributed to size distributions with ln rg < 0.1). The peaks are separated by a factor of two in deflection voltage, and in the absence of further information, it is not clear, whether this is due to multiple charge or due to multiple mass. For low pressure flames with temperatures around 2000 K, Balthasar and Mauss [42] have calculated the thermal ionization as function of soot particle size. According to these calculations, thermal ionization is unlikely to produce substantial particle fractions with multiple charges, as long as the diameter is below 10 nm. This supports the assumption made here, z = 1. Following this picture, particles with half and twice the mass of the main peak would then be observable as result of difference in formation and consumption (coagulation) rates. However, it remains difficult to understand, why one of the peaks should be missing for one polarity. Roth and Hospital [22] found the charge distribution to vary significantly with the height above burner. Homann and Wolf [33] attributed the concentration ratio of positive to negative soot ions to temperature and to mechanistic effects. In the flames burning at a pressure of 30 mbar, the size of the particulate ions appears to be significantly smaller than the size of the neutral particles, as mentioned above. This is in agreement with previous findings [33,43] that particulate ions appear to grow more slowly and remain much smaller than neutral particles.

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Usual explanations for this observation are in terms of thermal ionization and chemi-ionization as probable ion formation mechanisms. However, there are more mechanisms which contribute to and affect the concentration of particulate ions, all of which have been reviewed by Franke [44]. A brief summary is given here: – Thermal ionization according to the Boltzmann (or Saha) equation [29,42]. – Chemi-ionization at the particle surface [29,45]. – Attachment/detachment of gas phase ions to/from particles (diffusion charging and its reverse) which increases/decreases the total amount of particulate ions, but leaves their size almost unchanged [46]. – Ion-neutral coagulation which changes the size, but not the total amount of ions, and which increases the fraction of charged particles in flame regions where the inception of neutral particles is negligible. – Ion recombination (particle–particle and particle-gas) which reduces the charge number (i.e. the total amount of ions under present conditions) and which can be expected to reduce the fraction of charged particles in many cases. Although there is no extensive knowledge available on all these processes, the following general remarks are possible: (i) There is no need to postulate a charge balance in the particulate phase, because charge exchange between the particles and the gas phase may occur. It has not been possible yet to measure the concentrations of all charged species in flames under sooting conditions completely and, in particular, the charge balance in the particulate phase has hardly ever been reported to be complete in the literature. Rather, there appears to be clear evidence that the charge distribution among the particles changes with the experimental conditions [11,17,22,33,45], as is observed in the present work, too. (ii) Since high molecular ions are not observed below the soot threshold, they may be formed preferentially by decomposition of large ionic particles above the soot threshold in addition to other formation pathways which start from small units [17,29]. Decomposition of large particles would make the occurrence of multiple size modes somewhat plausible. In some of the atmospheric ethylene/air flames, two distinct and partially overlapping size modes were detected. The peak signal intensities vary over two orders of magnitude and the concentration of the negatively charged species exceeds that of the positively charged species. So the charge distribution is found opposite compared to the low pressure flames as consequence of all the mechanisms outlined above. The peaks are significantly broader than in the low pressure flames and, when fitted to lognormal distributions, their standard deviations are around ln rg  0.2–0.3. Hence, the size distribution width is close to and below the limit of self-preserving size distributions with ln rg  0.3. The size distributions obtained with conventional dilution probe sampling are usually broader (ln rg  0.5) or deviate from log-normal shape substantially. This has given rise to detailed discussions of the errors associated with conventional dilution probe sampling [10–14,20,21]. The residence times in conventional sampling trains range from 10 ms up to 250 ms and were claimed acceptable with respect to particle coagulation. In the early stages of particle inception, however, particle number densities of the order of 1011–1012 cm3 must be accounted for [1,2], which are associated with coagulation half-life times in the range of 1 ms and less. Moreover, condensation effects (10–100 ls time scale) due

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to dilution and cooling could not be ruled out completely in conventional dilution probe sampling [10] and severe disturbances of the flow field (and hence of the temperature field) were admitted [20]. In the present experimental set-up, care was taken to stabilize the flow field of the flame gases and no significant temperature effects were measured upon intrusion of the sampling nozzle (see Section 2). Further, the molecular beam sampling system used in the present investigation employs sampling times less than about 10 ls and quenches both particle–particle and gas–particle interactions fast enough to minimize the corresponding errors. At an inlet flow velocity of 10 cm/s and HaB = 14 mm, only one strong peak is detected which corresponds to a modal particle diameter close to 7 nm (2  105 amu). This result is in good agreement with previous optical size measurements which were performed with similar flames [47]. Up to now, no evidence for multiply charged particles was found in this size range which is in agreement with previous reports and model calculations [10,12,42]. These findings imply that the charged soot particles can have the same size as the total ensemble of particles under certain conditions. The measured number density of all charged particles in Fig. 6a is 3.8  1010 cm3. Previous measurements [1,2,14,33,47] suggest a total particle number density around 1011 cm3 under these conditions, hence the result obtained here indicates a fraction of charged particles close to 40% which appears acceptable. When increasing the inlet flow velocity from 10 to 16 cm/s, the large particle peak shifts into the lower size range and finally, it vanishes completely. In parallel, another peak starts to develop in the low mass range. This peak is observed at a modal mass of 7000 amu at v = 12 cm/s. It shifts to modal masses of 5000 and 3000 amu, when increasing the inlet velocities to v = 14 and 16 cm/s, resp. At even higher inlet velocity, this peak also vanishes completely. Due to its broadness, this peak covers the mass range from large molecules (1000 amu) to incipient particles (>3000 amu) and, from the measurements made here, it is not obvious, if there is a change of phase from gaseous to particulate across the peak width or, if there is a change of material density. The observation of this peak confirms previous reports about multimodal size distributions in these flames [9–15], but a general persistence of this peak under various flame conditions has not been observed, as mentioned above. Roth and Hospital [22] indeed have reported some low mass mode of negatively charged species (m  3400 amu) in low pressure acetylene/oxygen flames which survives far into the postflame gases and they contributed it to fullerene species. In the atmospheric ethylene/air flames studied here, bimodality appears to be related to the inlet flow velocity and to the height above burner (i.e. reaction time) and hence to flame temperature in a way similar to other findings [21]. The measured appearance and behavior of the low mass mode compares well to the inception mode predicted in some of the previous model calculations [6,8]. Moreover, under sufficiently low sooting conditions, the low mass mode can be observed isolated, i.e. in the absence of mature soot particles (Fig. 6d). The nature of this low mass mode, however, is still unknown. Fullerene structures have been ruled out as possible candidates in atmospheric ethylene/air flames [49,50]. Also, the low mass mode may be expected to cover those high molecular species which have been termed nanoparticles of organic carbon (NOC) previously [9]. From Fig. 6, it is obvious that the peak in the low mass range is clearly separated from the peak of the large particulate ions. Neither surface growth nor coagulation seem to fill the mass gap between the two peaks. This may mean that the low mass mode rather is an indication of soot particle decomposition than an indication of soot particle inception or that it originates from a mixture of both particle formation and particle decomposition. Further investigations concerning the detectability of this peak and species characterizations are under way.

5. Conclusions Electrically charged soot particles have been detected in flat premixed flames burning at low pressure and at atmospheric pressure using TOF particle mass spectrometers of particularly compact design. In the low pressure flames, positive ions were prevailing over negative ions and the size distributions were bi- and trimodal with one clearly dominating peak. Under the experimental conditions, m/z was well above 5000 amu and with the plausible assumption of single charge (z = 1), all particle sizes were in the range 2–10 nm diameter. The peaks were very sharp with geometric standard deviations around ln rg  0.06. In the atmospheric flames, negative ions were prevailing over positive ions and the observed mass distributions were monoand bimodal, depending on inlet gas flow rate. The size distribution width was close to the self-preserving limit. Keeping the height above burner constant, monomodal distributions were observed at the low and high flow rates, bimodal distributions at intermediate flow rates. With increasing flow rate, i.e. with increasing temperature and decreasing reaction time, the observed mass peaks (m/z) may fall below 3000 amu and include the mass range of large molecules. The nature and composition of these small species should be investigated further. The puzzling question arises which part of the observed species is due to soot particle inception and, which part is due to soot particle decomposition. Acknowledgments The technical assistance by Mrs. M. Hauser, Mr. S. Fozin Foyet and Mr. M. Gensch is gratefully acknowledged. References [1] H.Gg. Wagner, Proc. Combust. Inst. 17 (1978) 3–19. [2] B.S. Haynes, H.Gg. Wagner, Prog. Energy Combust. Sci. 7 (1981) 229–273. [3] H. Böhm, D. Hesse, H. Jander, B. Lüers, J. Pietscher, H.Gg. Wagner, M. Weiss, Proc. Combust. Inst. 22 (1988) 403–411. [4] F. Mauss, T. Schäfer, H. Bockhorn, Combust. Flame 99 (1994) 697–705. [5] J. Appel, H. Bockhorn, M. Frenklach, Combust. Flame 121 (2000) 122–136. [6] M. Balthasar, M. Kraft, Combust. Flame 133 (2003) 289–298. [7] J. Singh, R.I.A. Patterson, M. Kraft, H. Wang, Combust. Flame 145 (2006) 117– 127. [8] J. Appel, H. Bockhorn, M. Wulkow, Chemosphere 42 (2001) 635–645. [9] A. D’Alessio, A. D’Anna, P. Minutulo, L.A. Sgro, in: H. Bockhorn, A. D’Anna, A.F. Sarofim, H. Wang (Eds.), Combustion Generated Fine Carbonaceous Particles, KIT Scientific Publishing, Karlsruhe, Germany, 2009, pp. 205–230. [10] M.M. Maricq, S.J. Harris, J.J. Szente, Combust. Flame 132 (2003) 328–342. [11] M.M. Maricq, Combust. Flame 137 (2004) 340–350. [12] M.M. Maricq, in: H. Bockhorn, A. D’Anna, A.F. Sarofim, H. Wang (Eds.), Combustion Generated Fine Carbonaceous Particles, KIT Scientific Publishing, Karlsruhe, Germany, 2009, pp. 347–366. [13] M.M. Maricq, J. Aerosol, Science 40 (2009) 844–857. [14] L.A. Sgro, A. D’Anna, P. Minutolo, Combust. Flame 158 (2011) 1418–1425. [15] H.-H. Grotheer, K. Hoffmann, K. Wolf, S. Kanjarkar, C. Wahl, M. Aigner, Combust. Flame 156 (2009) 791–800. [16] H. Wang, A. Abid, in: H. Bockhorn, A. D’Anna, A.F. Sarofim, H. Wang (Eds.), Combustion Generated Fine Carbonaceous Particles, KIT Scientific Publishing, Karlsruhe, Germany, 2009, pp. 367–384. [17] Ph. Gerhardt, S. Löffler, K.H. Homann, Proc. Combust. Inst. 22 (1988) 395–401. [18] J. Happold, H.-H. Grotheer, M. Aigner, in: H. Bockhorn, A. D’Anna, A.F. Sarofim, H. Wang (Eds.), Combustion Generated Fine Carbonaceous Particles, KIT Scientific Publishing, Karlsruhe, Germany, 2009, pp. 277–288. [19] M. Kasper, K. Siegmann, K. Sattler, J. Aerosol Sci. 28 (1997) 1569–1578. [20] B. Zhao, Z. Yang, J. Wang, M.V. Johnston, H. Wang, Aerosol Sci. Technol. 37 (2003) 611–620. [21] B. Zhao, Z. Yang, Z. Li, M.V. Johnston, H. Wang, Proc. Combust. Inst. 30 (2005) 1441–1448. [22] P. Roth, A. Hospital, Proc. Combust. Inst. 24 (1992) 981–989. [23] H. Mätzing, W. Baumann, M. Hauser, H.-R. Paur, H. Seifert, A. van Raaij, P. Roth, in: Nanofair 2003, VDI–Berichte 1803, VDI Verlag, Düsseldorf, Germany, 2003, pp. 327–330. [24] H.-R. Paur, W. Baumann, H. Mätzing, H. Seifert, Nanotechnology 16 (2005) S354–S361. . [25] W. Baumann, H. Mätzing, H.-R. Paur, M. Hauser, A. van Raaij, Partikel-MassenSpektrometer zur Detektion von Nanopartikeln. Patent, DE-PS 10 344 462, 2007.

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