Analysis of Polycyclic Aromatic Hydrocarbons (PAH) Adsorbed on

Combustion-related soot particles in the state of formation were sampled from the stoker system of a 0.5 MW incineration pilot plant. The plant was op...
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Environ. Sci. Technol. 2000, 34, 4780-4788

Analysis of Polycyclic Aromatic Hydrocarbons (PAH) Adsorbed on Soot Particles by Fourier Transform Laser Microprobe Mass Spectrometry (FT LMMS): Variation of the PAH Patterns at Different Positions in the Combustion Chamber of an Incineration Plant R . Z I M M E R M A N N , * ,† L . V A N V A E C K , † M. DAVIDOVIC,‡ M. BECKMANN,‡ AND F. ADAMS† Department of Chemistry, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Wilrijk, Belgium, and CUTEC-Instituts GmbH, D-38678 Clausthal-Zellerfeld, Germany

Combustion-related soot particles in the state of formation were sampled from the stoker system of a 0.5 MW incineration pilot plant. The plant was operated with wood as feeding material. Samples were taken in different air supply zones at different heights over the feed bed. Polycyclic aromatic hydrocarbons (PAH) which are adsorbed on the soot particles were analyzed by Fourier transform laser microprobe mass spectrometry (FT LMMS). The structure of the observed PAH patterns was analyzed using a multivariate statistical method (principle component analysis, PCA). The samples obtained closest to the feed bed near the first three air supply zones exhibit the most significant differences in the PAH patterns. This result is due to different contribution of the elementary processes (drying, pyrolysis, gasification, and combustion) in the different zones. Possible applications for monitoring the combustion efficiency of incinerators are discussed.

Introduction Combustion processes are responsible for most of the toxic components in anthropogenic emissions. In addition to the emission of gaseous pollutants such as CO, NOx, SOx, or volatile organic compounds, particulate matter is also emitted. The significance of particle-related effects on human health is well-documented (1). Recent investigation focused on the health impact of the fine and ultrafine particle fractions (L < 2.5 µm) (2, 3). These fractions are dominant in combustion emissions (4, 5). However, up to now, the suggested mechanisms responsible for the health impacts have been controversial. For example, the total number of the fine particles, their surface properties, or the chemical * Corresponding author phone: +49 89 3187 4544; fax: +49 89 3187 3371; e-mail: [email protected]. On leave from GSFForschungszentrum GmbH, Institut fu ¨r O ¨ kologische Chemie, Ingolsta¨dter Landstr. 1, D-85764-Oberschleissheim, Germany. † University of Antwerp (UIA). ‡ CUTEC-Instituts GmbH. 4780

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composition of the particle cores and the adsorbate layers have been considered as factors responsible for the observed effects. Soot particles from combustion processes are known to be respiratory carriers of highly carcinogenic and mutagenic compounds such as polycyclic aromatic hydrocarbons (PAH) (6). To develop innovative and efficient primary measures for process-integrated emission reduction, it is necessary to investigate the processes of particle formation in industrial combustion processes. Furthermore the formation of the adsorbate layer, which contains chemicals such as PAH, need to be studied. This work focuses on the formation of semivolatile PAH adsorbed on soot particles originating directly within and closely above the flame zone.

Experimental Section Soot samples were collected by a special sampling probe system from the primary combustion chamber (grating system) of the 0.5 MW incineration pilot plant at the CUTECInstitut GmbH, Clausthal-Zellerfeld, Germany. The plant consists of a 5-zone reverse acting grate (MARTIN GmbH, Mu ¨ nchen, Germany), a postcombustion chamber system as well as a heat exchanger, and a flue-gas purification unit. The plant can be operated either in a conventional one-step combustion process (i.e. the total oxygen is supplied in the stoker system; the postcombustion chamber is not active) or in a two-step process, allowing gasification on the grate and postcombustion of the gasification gas (7). During sampling of the soot particles, waste wood was burned at a feeding rate of about 88 kg/h. The plant was operated in the conventional combustion mode with an inactive postcombustion chamber. Combustion conditions were held constant during the sampling periods. The air supply rate to the four first zones of the grate was 420 kg/h. The highest supply rate was adjusted in the third zone, followed by the second, first, and fourth zone. The temperatures measured at the different sampling positions as well as the oxygen concentrations (measured with OXOR 610, MAIHAK GmbH, Germany) are given in Table 1. Further details on the plant technology and test results are available in the literature (7). Figure 1 depicts a schematic drawing of the grate system of the plant. During the sampling campaign, soot samples were collected using a special high-temperature sampling system (MAIHAK GmbH, Germany). The probe is made of heat resistant 1.4841 stainless steel with an ID of 18 mm. The probe tip was positioned in the different air supply zones (zones 1, 2, and 3) at different heights above the feed bed. Flue gas was drawn at a rate of about 0.2 m3/h by a membrane pump (Typ N 0135 ST.18, KNF, Germany), and the gas concentrations were continuously analyzed after a cleaning procedure (e.g. O2, see Table 1). Sampling times were in the 10 min range. A few milligrams of soot particles were precipitated in a heated filter head (filter head GA 54 made of 1.430 stainless steel with heating HEI 56, MAIHAK GmbH, Germany) on a ceramic filter material (siliciumcarbide, medium pore diameter 30 µm, 160 cm2 filter surface). The filter head was connected directly to the stainless steel probe. The temperature of the drawn combustion aerosol in the probe was not lower than 200 °C. The filter head temperature also was held at 200 °C. At this temperature on one hand formation of new organic substances can be neglected (8), and on the other hand no condensation of tar was observed. The impact velocity at the filter was approximately 1 cm/ s. Altogether, 10 positionally resolved samples were collected: two from zone 1 (probe position A), four from zone 2 (probe position B), and four from zone 3 (probe position 10.1021/es0000596 CCC: $19.00

 2000 American Chemical Society Published on Web 09/29/2000

TABLE 1. Characterization of the Collected Soot Samplesa case probe position grating zone sampling height temp [°C] oxygen [%]

#1 A first 70 cm 745 0

#2 A first 40 cm 678 0

#3 B second 80 cm 814 0

#4 B second 60 cm 838 0

#5 B second 40 cm 876 0

#6 B second 25 cm 886 2.5

#7 C third 40 cm 840 10

#8 C third 50 cm 933 4

#9 C third 60 cm 945 3

#10 C third 80 cm 930 2

a Position of the probe (A, B, or C, see Figure 1), zone of the grating (first, second, or third, see Figure 1), sampling height over fuel bed, temperature at the probe tip, oxygen content of the flue gas.

FIGURE 1. Schematic drawing of the combustion chamber of the 0.7 MW pilot scale incinerator with sampling probe positions A (zone 1), B (zone 2), and C (zone 3) and sampling heights (#1 to #10). C). The numbering and the respective sampling heights are indicated in Figure 1 and Table 1. The collected soot is completely black and has a light, flaky texture in all cases. Analysis of the samples was performed by laser microprobe mass spectrometry (LMMS). LMMS was initially introduced based on time-of-flight mass analyzers (9, 10). Later, Fourier Transform mass spectrometry (FTMS) based LMMS instruments became available (11-13). This study was performed with the Fourier Transform laser microprobe mass spectrometry (FT LMMS) at the University of Antwerp, Department of Chemistry, Belgium. The Fourier transform laser microprobe mass spectrometer is based on a Spectrospin CMS47X FTMS instrument (Spectrospin Fa¨llanden, Switzerland, now Bruker Instruments, Billerica, U.S.A.) and is equipped with an external ion source (13, 14). The instrument is depicted schematically in Figure 2. A sample exchange probe allows for the introduction of sample holders to a sample micropositioning system without interruption of the vacuum in the source or the cell. The attenuated quadrupled output (266 nm) of a Q-switched Nd: YAG laser system (Quanta-Ray DCR-2A, Spectra Physics, U.S.A.) is focused to a spot 5 µm in diameter on the sample surface. A collinear He-Ne laser beam indicates the target spot on the sample and can be used for sample alignment via a sample-viewing system consisting of a microscope (magnification 700×) coupled to a CCD camera. Ions formed by interaction with the 266 nm laser pulse (5 ns, ∼100 µJ) are accelerated by means of static electrical fields, pass a fieldfree drift region, and are transferred into the cell of the FTMS instrument (Spectrospin CMX 47X MicroFocus). This causes a time dispersion between the low m/z ions, which travel faster, and the higher m/z ions. Therefore the low m/z ions arrive at the cell first. To inject ions from the external

ion source, the potential wall at the entrance front plate must be opened by an appropriate pulse. Once inside the cell, the ions reflect against the second trapping wall and escape unless the first potential wall is restored (preventing heavier ions to enter). In practice, the cell now contains the lightest m/z ions (that have traveled the length of the cell twice) together with all the higher m/z ions arrived at the cell before the low m/z ions escaped. The experimental Tgate parameter shifts the observable mass window along the m/z scale. For all measurements presented in this paper the same Tgate ) 500 µs parameter was used to optimally display the PAH in a mass range of about 300 to 400 m/z. The soot samples were used as collected without sample pre-preparation for generation of microprobe samples. A few milligrams of soot were pressed on the target region of the stainless steel sample holder. In this procedure, the soot particles are pressed to a smooth, shining black layer which adheres on the sample holder. By this procedure the sample texture is unified, which is important for the comparison of the LM FTMS data obtained from the different samples. During the acquisition of LMMS signals the effect of the laser radiation on the sample surface can be observed via the sample viewing system. Impacted spots appear darker due to a roughening of the smoothly prepared surface. The real ablation of material is hardly visible; as a rule of thumb, the single laser shot ablation depth for this kind of densified material is in the range of a few hundred nanometers. However, this also depends on the sample preparation technique. For example, the observed signals for PAH are considerably lower or even absent, when the particles are loosely adhered on double sided adhesive tape. Under these conditions large amounts of material are ablated, making the correct focusing of the microprobe laser beam difficult. VOL. 34, NO. 22, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Schematic representation of the Fourier transform laser microprobe mass spectrometer with external ion source.

TABLE 2. Signal-to-Noise Ratios (S/N) and Sum of S/N Ratios of the Observed PAH Species for the Presented Spectra #1 to #10a nominal mass [m/z]

case #1 [S/N]

case #2 [S/N]

case #3 [S/N]

case #4 [S/N]

case #5 [S/N]

case #6 [S/N]

case #7 [S/N]

case #8 [S/N]

case #9 [S/N]

case #10 [S/N]

250 252 274 276 278 298 300 302 324 326 328 348 350 352 372 374 376 398 400 422 424 446 448 ΣS/N

1 1 3 10 1 1 13 9 12 24 5 8 26 5 6 15 8 5 6 3 4 1 1 168

2 3 4 17 3 3 20 16 22 41 7 12 38 10 8 23 12 7 8 4 5 1 2 268

2 6 6 32 5 8 32 23 28 60 11 16 56 16 9 31 19 10 16 5 10 3 5 409

1 2 2 10 2 2 9 8 6 16 3 4 13 3 2 7 3 2 3 2 1 1 2 104

3 7 6 36 5 5 35 25 15 43 12 5 28 9 3 15 6 3 5 1 1 1 1 270

3 9 5 24 3 4 19 13 9 21 4 3 15 4 2 7 3 1 1 1 1 1 1 154

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 23

1 1 1 1 1 1 5 2 5 10 1 3 15 4 3 9 5 2 2 1 1 1 1 76

1 1 1 1 1 1 1 1 1 3 1 1 4 1 1 4 1 1 2 1 1 1 1 32

3 8 9 41 7 10 39 30 33 72 13 20 64 18 11 37 22 12 17 5 9 2 5 487

a

PAH mass peaks with an intensity lower than S/N ) 2 are considered as not detectable and are set to 1.

In addition, it has previously been observed, that the ion formation rate decreases when large amounts of material are ablated (14). For this reason it is important to perform the sample preparation procedure in a very reproducible manner. For the PAH signals, signal-to-noise ratios (S/N) were determined as a semiquantitative parameter for pattern comparison (Table 2). The noise level was measured in the mass range 480-500 m/z. For each sample, the sum of the S/N values for all observed PAH related masses is given as a parameter for the total PAH concentration (Table 2). For S/N values below 2 (detection limit) the S/N level is set to 1. For many PAH masses multiple PAH isomers are possible. The detected PAH masses and some possible PAH molecular species are listed in Table 3. 4782

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The structure of the observed data was investigated using a multivariate statistical approach using principle component analysis (PCA). The theory of the PCA technique is described elsewhere (15, 16). The PCA loading plot and the corresponding score plot (Figure 8) reflect the variance of the PAH patterns of the 10 different samples (cases) and of the investigated S/N values of the PAH mass peaks (variables), respectively. Sample inhomogenities are the cause of nonrepresentative results in microprobe analysis. Therefore the soot samples were carefully mixed before preparation of the LMMS target. The representativity and significance of the observed patterns was tested by multiple FT LMMS measurements on

TABLE 3. Typical Observed PAH Masses and Selected PAH Species mass [m/z]

composition

PAH species (exemplary)

250.080 252.096 276.096 278.112 300.096 302.112 326.112 328.128 350.112 372.096 374.112 376.128 398.112 400.128 424.128 448.128 450.144 472.128

C20H10 C20H12 C22H12 C22H14 C24H12 C24H14 C26H14 C26H16 C28H14 C30H12 C30H14 C30H16 C32H14 C32H16 C34H16 C36H16 C36H18 C38H16

corannulene benz[a],[b]pyrenes, benz[a],[b],[j],[k]fluoranthenes, perylene 1,2-benzoperylene, anthanthrene, benzo[ghi]perylene dibenzo[a,c],[a,h],[a,j]anthracenes coronene benzo[b]perylene, dibenzo[a,e],[a,f],[a,k],[b,k],[j,l]fluoranthenes, dibenzo[a,ghi],[b,ghi],[e,ghi]perylenes, peropyrene, naphtho[1,2,3,4-ghi]perylene anthraceno[2′.1′,1.2]anthracene benzo[a]coronene circobiphenyl dibenzo[a,bc]coronene 1.2,7.8-dibenzantanthrene ovalene dibenzo[a,g],[a,j]coronenes, naphtho[2,3-a]coronene, periflanthene benzonaphthocoronene benzo[a]ovalene decacyclene, 1.2,3.4,5.6,10.11-tetrabenzanthanthrene, anthraceno[2.3-a]coronene dibenzo[a,bc]ovalene

FIGURE 4. Online real time recorded resonance-enhanced multiphoton ionization (REMPI) mass spectrum of wood combustion flue gas. The spectrum was recorded at the 0.5 MW incinerator during an online measurement campaign in 1996. and 0.415 for #2. The standard deviation obtained for the ratio (276 m/z)/(326 m/z) was below 20%. This shows that the PAH patterns of the samples #2 and #6 are significantly different.

Results and Discussion

FIGURE 3. FT LMMS spectrum of PAH from soot particles sampled in the combustion chamber of the 0.5 MW incinerator during wood combustion (mixed sample from different probe positions). the same soot sample. Four FT LMMS spectra, each accumulated over 100 laser shots, were recorded from sample #6. The standard deviation obtained for a PAH signal with a S/N ratio of ca. 8 (276 m/z) was below 10%. The ratio (276 m/z)/(326 m/z) shows a large difference for the different patterns and can therefore be considered as ”indicator” for the different patterns. The average value (n ) 4) of the ratio (276 m/z)/(326 m/z) is 1.2 for #6

Figure 3 shows FT LMMS data of a soot sample sampled from the grating during a wood combustion series. Sampled fractions obtained from probing at the positions A, B, and C at different sampling heights were combined. Five hundred laser shot transients were accumulated prior to the Fourier transformation. The upper spectrum shows the mass range from 20 to 500 m/z. The two highest peaks are due to the potassium isotopes at 39 (100%, truncated) and 41 m/z (∼6%). Potassium is present in large amounts in the soot or fly ash resulting from biomass combustion and is ionized easily in the laser microprobe process. The peak at 113 m/z is due to K+(KCl) cluster ions. Due to the applied Tgate value of 500 µs, potassium ions which are formed within the 5 ns laser pulse are largely suppressed. However, the ion formation process, in the case of alkaline metals in particular, lasts for several 100 µs after laser impact (14). Thus a fraction of the potassium ions formed during the delay manages to reach the FT LMMS cell. In the mass range of 30 to about 150 m/z, the typical fragmentation pattern of aliphatic organic compounds is observed (intensity decreases exponentially with mass). Typical are the masses 55 and 57 (C4H7/9+). The peak at 213 is due to (K2SO4)K+. In the range of 252 to 472 m/z, several VOL. 34, NO. 22, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Two FT LMMS spectra of soot particles sampled at different heights above the fuel bed in the first air supplying zone (probe position A) at the 0.7 MW incinerator. For sample details see Table 1.

FIGURE 6. Four FT LMMS spectra of soot particles sampled at different heights above the fuel bed in the second air supplying zone (probe position B) at the 0.5 MW incinerator. For sample details see Table 1. PAH species with nominal mass numbers of e.g. 250, 252, 274, 276, 300, 326, 350, 374, 398, 424, 448, and 472 m/z can be detected. The element composition of some of the major species was determined by examination of the C/H ratio, utilizing the high mass resolution of the FT LMMS instrument. For example, several carbon clusters have the same nominal mass as the PAH. The lower part of Figure 3 shows an enlarged view of the observed PAH signals. 4784

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The ionization process of the organic species depends strongly from the sample matrix. In previous experiments we tried to detect PAH from fly ash particles composed of mainly minerals. No PAH signals were obtained even from spiked fly ash particle samples containing up to 1% of a single PAH species (pyrene). On the other hand organic molecules adsorbed on soot particles can be ionized efficiently in a one-step laser ionization procedure (17, 18). Black carbon efficiently absorbs light radiation. This ensures a high

FIGURE 7. Four LM-FTMS spectra of soot particles sampled at different heights above the fuel bed in the third air supplying zone (probe position C) at the 0.5 MW incinerator. For sample details see Table 1. incoupling rate of the laser energy and thus a high ionization efficiency in the laser microprobe process. This property of black carbon has been employed, for example, in a technique named “graphite-assisted laser desorption/ionization” in which graphite particles embedded in a liquid matrix were used for efficient MALDI detection of proteins or polymers (19, 20). Additionally, we performed FT LMMS experiments on pure carbon particles as well as on a compact graphite surface. These experiments clearly revealed that no new PAH were generated from the carbonaceous matrix upon laser impact under the chosen experimental conditions. The LMMS technique has the advantage that semi- or nonvolatile high molecular weight PAH can be analyzed and only minimal sample amounts and preparation are required. This is not possible with the standard techniques such as gas chromatography-mass spectrometry (GC-MS) or high performance liquid chromatography (HPLC) with e.g. fluorescence detection. However, isomeric PAH cannot be distinguished in the LMMS process. Due to the high temperatures in the furnace, PAH of relative high molecular weight are mostly present on the soot particle surfaces. On the other hand, the occurrence of very high molecular weight PAH species may be limited by the trend of decreasing thermal stability of PAH with increasing size. Soot particles that are transferred to colder parts of the plant may adsorb low molecular weight species such as naphthalene or phenanthrene. The latter compounds have been detected in LMMS studies on soot particles collected

in the stack gas region (17, 18). The gas phase, however, contains several, more volatile PAH. The pattern of low molecular weight aromatic compounds is a measure of the actual status of the combustion process. Recently, a laser based technique for online monitoring of PAH patterns from combustion flue gas was developed (21, 22). The so-called resonance-enhanced multiphoton ionization-time-of-flight mass spectrometry technique (REMPI-TOFMS) uses a highly selective and efficient two-step photoionization method (resonance photoexcitation step and photoionization step). Figure 4 shows a REMPI-TOFMS spectrum of wood combustion flue gas recorded online at the same incineration pilot plant during a measurement series in 1996. A comparison of the Figures 3 and 4 shows the complementary character of the information on combustion related PAH obtainable by LMMS technique on soot particles and the REMPI-TOFMS method on the (particle free) flue gas. The flue gas REMPI spectrum in Figure 4 shows smaller PAH like naphthalene (128 m/z), acenaphthene (154 m/z), phenanthrene (178 m/z), and pyrene (202 m/z). Furthermore, benzene (78 m/z), dibenzofuran (168 m/z), and some methylated species such as methylnaphthalene (142 m/z) or methylphenanthrene (192 m/z) are prominent. The larger PAH species are present on the particle phase and were, therefore, precipitated in the dust filtering unit of the online sampling system (22). The LMMS technique allows for the detection of this low volatile PAH, as shown in Figure 3. Note that the laser microprobe approach with TOFMS detection for characterization of particulate matter can be VOL. 34, NO. 22, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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indicated as follows: measurements from the first zone: b, measurements from the second zone: 9, and measurements from the third zone: [. In Figure 8b the score plot is depicted, which describes the variance of the variables (here: the intensity of PAH masses). In the score plot the variables are marked as follows: PAH masses g 350 m/z: O, and PAH masses < 350 m/z: b. The first principal component (PC1) describes 83% of the variance of the data set, the second (PC2) 12%. The PAH patterns from the first zone (#1 and #2) are very similar, and the loading values are therefore grouped. Interestingly, the PAH pattern of samples taken at the highest sampling position in the second and third zone (#3 and #10, respectively) also exhibits large similarity to the PAH patterns from the first zone and are therefore are grouped together in a cluster with cases #1 and #2. Considering the PAH patterns measured at the lower sampling position at the second zone, the PCA reveals a logical sequence: with increasing sampling height (from sample #6 to #3), the loading values get closer step by step to the abovementioned cluster (#1, #2, #3, #10). This process is indicated by arrows in Figure 8a. A somewhat similar trend with respect to PC1 can be seen for the cases from the third zone (also marked by arrows in Figure 8a).

FIGURE 8. Principal component analysis (score plot and loading plot) of the observed PAH patterns. (a) The PCA score plot shows the correlation between the cases (measurements #1-#10) according to the two most important principal component axes. (b) The PCA loading plot shows the distribution of the variables (PAH masses) according to the two most important principal component axes. used in an “online” measurement setup, the Aerosol-TOFMS technique, which makes the analysis of single particles in an airborne state possible (23-25). Figures 5-7 show the observed FT LMMS spectra from the soot samples obtained at different heights over the first three air supply zones. In Figure 5, the FT LMMS spectra of the soot samples which were collected above the fuel bed in first zone (sampling position A) of the grating at heights of 40 cm (#2) and 70 cm (#1) are depicted. The observed FT LMMS PAH patterns in both samples are very similar, i.e., the PAH at 276 and 300 m/z are half as abundant as those at 326 and 350 m/z, and the intensity of the higher PAH decreases exponentially with mass. Figure 6 shows four FT LMMS mass spectra of the soot samples collected in the second zone of the grating (sampling position B). The respective sampling heights were 25 (#6), 40 (#5), 60 (#4), and 80 (#3) cm. The PAH of the soot sample collected in the position closest to the fuel bed (#6) shows a significantly different pattern with respect to the samples from the first zone (#1 and #2). Here the peaks at 252 and 276 m/z are larger, while those at 300 and 326 m/z are slightly less intense. However, the PAH peaks at 350 and 374 m/z are significantly smaller in comparison with the patterns observed in the first zone; higher PAH masses cannot be detected. The spectra obtained from the third zone of the grating are shown in Figure 7. With the exception of the sample taken at the highest sampling point (80 cm, #10), the spectra show a PAH pattern of low intensity (#9 and #8). At the lowest sampling point (40 cm, #7) no PAH signals were obtained. To reveal the structure of the obtained data set, principal component analysis (PCA) was preformed. Figure 8a depicts the PCA loadings plot, describing the variance of the cases (measurement #1 to #10). The cases in the PCA plot are 4786

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This gives a similar picture for the behavior of the PAH patterns in the second and third zone. The PAH patterns of the particles sampled close to the fuel bed (#2, #6, #7) vary the most. All particles collected in these samples surely originate from the combustion fuel bed of the respective air supply zones. The score plot shows the variance of the variables within the coordinate system of the principal components (PC1 and PC2). The PAH masses which are usually the most intense, like 350, 326, 374, or 300 m/z, are displayed at high values of the PC1. The variable cluster with PC1 values below zero is due to relatively weakly appearing PAH masses, like 250, 252, 400, or 328 m/z. Thus, the first PC is described to some extent by the intensity of the PAH signals. Considering the higher PAH masses g 350 m/z, marked with O and the lower PAH masses < 350 m/z, marked with b, it is obvious that the higher mass variables are preferably grouped at positive PC2 values (i.e. are displayed at the top of the plot), while the low mass variables are grouped at negative PC2 values. The second PC therefore includes information about the dominance of higher or lower PAH masses (variables) within the 10 individual PAH patterns (objects). The results of the PCA lead to the following interpretations: In the first zone drying, pyrolysis and gasification processes at a relatively low temperature are dominant. Under these conditions, which may be described more adequately as smoldering rather than combustion, relatively large amounts of soot particles are formed. Both samples from this zone (#1, #2) exhibit an intense PAH pattern (see Figure 5). In the second zone, combustion related processes are more dominant (higher temperatures). The pattern obtained at the lowest sampling point in the second zone (#6), shows a very different pattern in comparison with the samples from the first zone (#1, #2), which is clearly reflected by the PCA analysis. A comparison of the FT LMMS spectra #2 and #6 shows that in the latter case the pattern is shifted towards lower molecular weight PAH. While the 252 m/z peak is quite prominent here, no molecular weights higher than 374 m/z were observed in soot sample #6. In the lowest sampling position at the third zone it was not possible to detect any PAH related signals by FT LMMS, although this sample was run in triplicate. In the third zone, combustion is effective, and the PAH content of the particles

is reduced below the FT LMMS detection limits due to oxidative and thermal degradation processes. The similarity of the PAH patterns and the relative high PAH loading of the soot samples from the first zone and the soot samples from highest sampling points in the zones two and three (see spectra #1, #2, #3, and #10) are most likely caused by the course of the flue gas streams in the combustion chamber. One part of the flue gas stream which originated from the first zone runs nearly unchanged under the top of the combustion chamber toward the flue gas outlet. Thus combustion aerosol from the first zone was sampled from the highest probing points in zones two and three. When considering the spectra of the samples from the second air supply zone, large but systematic pattern changes are observed. As mentioned above, the PAH composition from the probing position close to the fuel bed is characterized by a dominance of lower mass PAH (“second zone PAH pattern”), while the highest probing position revealed a pure “first zone PAH pattern”, with more pronounced higher mass region. The spectra of the two probe positions between show superimposed patterns; the characteristics of the “first zone PAH pattern” increase from sample #5 via #4 to #3. This is due to mixing effects between the flue gas flows from the first two grate zones. Further molecular growth effects should play a role, which tend to shift the PAH pattern towards higher molecular weight compounds with increasing reaction time (4). In the third zone a similar behavior is observed. No PAH signals could be detected near the fuel bed, while the highest sampling point in this zone reveals again a pure “first zone PAH pattern”. The soot particle samples collected at the medium sampling heights show weak PAH signals. Although it is difficult to characterize the PAH patterns of the latter samples (#8 and #9) due to the low S/N ratios of the signals, they seem to exhibit much “first zone PAH pattern”. The course of the oxygen concentration and the flue gas temperature over the height of the combustion chamber (see e.g. for sampling points in the third zone given in Table 1) confirm the assumption of influence of reactive PAH formation due to molecular growth processes. The relatively high O2 concentrations in the lowest sampling positions of zones 2 and 3 are associated with a low molecular mass dominated PAH pattern and low PAH concentrations, respectively. In summary, polycyclic aromatic hydrocarbons (PAH) adsorbed on soot particles collected at different regions of a stoker combustion chamber were analyzed by Fourier transform laser microprobe mass spectrometry (FT LMMS). Near to the fuel bed, specific PAH patterns were detected for particles collected from the different air supply zones. The drying, pyrolysis, and gasification processes at relatively low temperatures in the first air supply zone cause formation of relative large amounts of soot particles, heavily loaded with PAH. In the second and third zone of the grating, considerably fewer particles were collected. In the fourth zone, the soot particle mass was even lower making it impossible to collect sufficient material for the LMMS analysis. With increasing distance from the fuel bed, the flues gas streams originated from the different air supply zone at the grating undergo mixing effects. A part of the flue gas stream from the first air supply zone runs beneath the top of the combustion chamber toward the flue gas outlet and can contribute to the samples drawn in the second and third air supply zones at higher sampling points. Further a modification of the PAH patterns (towards higher PAH masses) due to molecular growth is likely (4, 26). The positionally resolved analysis of soot particle based PAH fingerprints makes it possible to follow the local flue gas streams in the combustion chamber and identify the regions of high soot and PAH formation. Such positional resolved information is of particular importance for under-

standing the complex conditions of the combustion aerosols in large scale incinerators. The formation of compounds such as polychlorinated dibenzo-p-dioxins and -furans strongly depends on particulate matter and its carbon content (27, 28) and probably also from the presence of PAH species (29). Furthermore, detailed positional resolved information on the occurrence of particulate matter and its loading with e.g. PAH is a prerequisite for the development of new concepts for process integrated measures for emission reduction of PAH, dioxins, or other air toxics from industrial incinerators. Fine soot-particles (ca. 0.2-1 µm) can hardly be precipitated by secondary measures, and recent studies focus on health effects of the fine particles in particular (2, 3). The large differences in PAH concentration observed in the adsorbate layer as a function of the probe position suggest that it may be also possible to minimize the amount of carcinogenic PAH in the adsorbate layer by primary measures. The type of investigation presented here can be important for both the design and the evaluation of primary emission reduction measures. Based on a more detailed analysis of the soot formation and its PAH loading, it should be possible to design e.g. additional air supply nozzles or fossil fuel fired burners in order to reduce the amount of soot particles and/ or the loading with carcinogenic PAH. For example it may be advantageous to add secondary air directly into the flue gas stream originated from the first zone close to the fuel bed in order to reduce the amount of soot and PAH loading. Further, changes in the design of the combustion chamber, e.g., introducing mixing units for the flue gas streams, can be implemented and evaluated. Future studies on the composition of the combustion aerosols should include also online measurement techniques in order to record dynamic emission effects of particulate matter and particle based emission of toxic chemicals. Under normal, slightly hyperstoichiometric combustion conditions, a large fraction of the soot particles is considered to be reburned prior to the emission of the respective flue gas plug. However, under instationary combustion conditions and transient changes of process conditions (22, 30) PAH and soot particles are emitted in relative high amounts. Such events occur frequently in industrial combustion. Recent investigation of particle size distribution and number concentration of soot and fly ash particles in flue gas (700 °C) and stack gas of an industrial incinerator by online particle size classification showed that process steering measures can cause massive changes in the particle size distribution in the flue gas. Furthermore temporal increases of the emission concentrations of fine and ultrafine particles were observed (31). The latter observation motivates the application of new online measurement techniques for determining the PAH content of soot particles such as the photoelectric aerosol sensor (5) or the aerosol-TOFMS technique (23-25) for future studies on time-resolved emission effects of industrial-scale incinerators.

Acknowledgments L. Van Vaeck is indebted to the Flemish Science Foundation (FWO), Brussels as research director. R. Zimmermann thanks Prof. A. Kettrup for continuous support and M. Blumenstock for help with the PCA analysis. Further R.Z. is indebted to the UIA, Antwerp and GSF, Oberschleissheim for the possibility to work at the Department of Chemistry at the UIA. Funding from the BMBF, Germany (HGF-Strategiefonds) is gratefully acknowledged The authors thank Dr. J. Maguhn, GSF for critically reviewing the manuscript.

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Received for review March 17, 2000. Revised manuscript received July 11, 2000. Accepted August 10, 2000. ES0000596