Pyrogenic PAHs

Unlike natural PAHs, pyrogenic PAHs are produced in an extremely short period of time through high temperature reactions, particularly the incomplete combustion of organic matter (in the broadest terms: biomass and fossil fuels). The products are unsubstituted compounds ranging from low molecular weight (100 – 200 Dalton, mostly 2-3 ring groups) to high molecular weight (>200 Dalton, or mostly 4-ring groups) compounds, e.g. those composing the 16-PAH list.

The formation mechanisms of PAHs have been the subject of intensive research over the years (e.g. Neff, 1979; Glarborg, 2007; Appel et al., 2000; Rockne et al., 2000; Frenklach, 2002;

Ladesma et al., 2000; Dobbins et al., 1998). Two mechanisms have generally been acknowledged as the best explanation of how PAHs are thermally generated: pyrolysis and pyrosynthesis. Pyrolysis involves the cracking of complex and high molecular mass organic molecules into lower molecular weight free radicals. This is immediately followed by pyrosynthesis in which the newly-created free radicals are reassembled. Benzene and further non-alkylated PAHs are produced by joining simple, individual benzene rings into double, triple and larger, multi-ringed, high molecular mass ring structures (see Fig. 2.2). Most recently, Richter and Howard (2000) reviewed and discussed the potential reaction pathways of PAH formation which follow pyrolysis. Several kinetically-governed processes have been added to the list, namely 1) oxidation, resulting in the formation of the first aromatic ring (benzene) and

A B

23 heavy molecular weight PAHs (500 – 1000 Da), 2) nucleation or inception of nascent soot particles (ca. 2000 Da, effective 1.5 nm), 3) particle mass growth due to the addition of gas phase molecules including PAH radicals and 4) coagulation involving reactive particle-particle collisions.

Fig. 2.2. Proposed mechanism of pyrosynthesis starting with ethane (adopted from Ravindra et al., 2008).

The production of PAHs is closely related to soot formation. Bockholn (1994) previously proposed a schematic concept for the production of PAHs in which they were actually precursors for soot and black carbon formation (Fig. 2.3). In principle, Bockholn's scheme includes the aforementioned pyrolysis and pyrosynthesis processes of an organic fuel. This results in small hydrocarbon radicals being created, from which acetylene (C₂H₂) is formed under fuel-rich conditions. These radicals grow and form aromatic rings. Subsequently, the formation of larger aromatic rings occurs when a surplus of acetylene molecules is present.

Formation of black carbon results from the coagulation of larger aromatic structures which form primary soot particles. The growth of black carbon in size and its increase in concentration are determined both by coagulation (which switches the molecular-length scale into the macroscopic, particle dimension) and surface growth (which snatches molecules out of the gas phase), respectively. In this respect, coagulation is responsible for the highly irregular, disordered structure of soot particles.

At the onset of black carbon formation, the concentrations of PAHs in the flame would therefore be reduced (Lima et al., 2005). But not all PAHs are transformed into soot particles, as is evidenced by considerable amounts of PAH residues remaining in both the gas and particulate phases, and/or adsorbed directly onto the black carbon itself (e.g. Butler & Crossley, 1981;

Koelmans et al., 2006).

24 Fig. 2.3. A conceptual picture proposed for soot formation in homogeneous mixture (adopted from Bockhorn, 1994).

The magnitude and relative compositions of PAH16 compounds released by combustion processes vary, resulting from a combination of factors including fuel sources (biomass vs.

fossil fuel, or substrate structures) and combustion/burning conditions (temperature, oxidants).

Literature data taken from various combustion experiments and field measurements is reviewed here to calculate out the amount (expressed in terms of the mass of PAH emitted per unit mass of fuel burned) and composition of PAHs. The products are then grouped together as petroleum, coal or biomass (wood, grasses, agricultural mixtures, and paper) combustions (Fig. 2.4. and supporting information in Appendix 2.1) Among these groups, coal combustion produces the highest PAH₁₆ emissions, ranging from ~100 – 1000 mg/Kg. Biomass burnings emit a wide range of PAH16 concentrations, varying from ~10 – 500 mg/Kg, while petroleum combustions (mainly diesel fuel) release PAHs in the range of 1 – 10 mg/Kg. Combustion of gasoline fuel emits PAH16 up to two orders of magnitude less than that of diesel fuel (e.g. Marr et al., 1999;

Miguel et al., 1998).

The relative predominance of aromatic or aliphatic fractions in the fuel structure controls the amount of PAHs emitted. The higher the aromatic fraction of the fuel structure, the greater the possible emission level of PAHs becomes. Mastral et al (1998) explained that incomplete combustion of coal fuel results in the emission of unburned fragments consisting mainly of aromatics from the coal structure. Following the PAH formation scheme mentioned above, these unburned aromatics can readily undergo pyrosynthesis with other radicals or other aromatic rings, thereby building higher molecular weight substances at low combustion temperatures. Coal contains more aromatic structures than similar fuels. For instance, bituminous coal consists of aromatic (roughly 45% of the total mass) and aliphatic (ca. 30%)

25 fractions (Yan et al., 2004). Likewise, biomass fuels - in particular wood - also contain high percentages of aromatic ring components (as polyphenolic compounds in lignin). Phenol compounds are potential precursors for PAHs (Sharma & Hajaligol, 2003). In addition, aromatic fuels (represented by benzene) have been observed to emit PAH compounds up to 100 times larger than those produced by aliphatic ones (acetylene) under the same burning conditions as reviewed by Richter and Howard (2000). On the other hand, petroleum - which is largely made up of aliphatic structures – will produce low weight PAH16 compounds upon combustion as compared to the other groups.

High temperature (plus oxidant level) decreases the overall emissions of PAHs (e.g.

Jenkins et al., 1996). Pyrogenic PAHs can be formed in a wide range of temperatures, stretching from relatively low (ca. 300^oC) to high (~1000^oC), depending on how condensed the structure of the precursors is. The more solid the precursors, the higher the temperature needed to crack the precursors. Several studies have attempted to calculate a scale showing the optimal conditions under which PAHs are produced. For example, a pyrolysis of cellulose (vegetation) to produce PAHs occurs optimally between 300 – 650^oC (McGrath et al., 2003). Combustion of paper emits maximal PAHs at ~300^oC (Yang et al., 2005). Neff's review (1973) showed that the PAHs produced in the series of compounds from naphthalene to coronene are optimally generated at 780^oC. Bituminous coal combustion was shown to release an optimal emission of PAHs at 800^oC (Liu et al., 2000). McGrath et al. (2001) observed that increasing the burn temperature from 800 to 850 ^oC lead to significant increases in PAHs resulting from burning chlorogenic acid and cellulose; beyond this temperature the emission decreased. Khalfi et al.

(2000) observed that PAHs are optimally emitted from wood waste incinerators at 900 - 954 ^oC.

Jenkins et al. (1996) observed from the experimental burnings of biomass (wood and cereals) that the magnitude of PAH emission depends also on the flame type. They found that fewer PAHs are emitted in a vigorous flame, whereas the levels produced were much higher in both smoldering stages and less robust flames.

26 Fig. 2.4. Emission factors for the 16 PAHs on the EPA's priority list. Results taken from the combustion of various organic matter classified as coal, biomass or petroleum. Datawere evaluated from various literature sources (see Appendix 2.1. for details to the references).

The composition of PAH emissions serves to characterize the sources. Combustion processes favor production of unsubstituted compounds as compared to their alkyl homologues (Lima et al., 2005). The PAH16 represent the most common unsubstituted PAHs which are produced by such processes. Therefore, they have been quite often examined by and employed in environmental studies and assessments. Combustion of diesel fuel in modern vehicles generates high levels of lighter PAHs (~300 Da). It emits no heavier PAHs (>300 Da, e.g.

coronene), unlike gasoline (Riddle et al., 2007a). It is due to this that modern diesel vehicles have been equipped with more advanced technology, enabling high combustion temperatures during engine operation which hinder the formation and aid in the breakdown of heavier PAHs.

In contrast, older vehicle technologies produced higher amounts of PAHs (Riddle et al., 2007b).

Increasing engine temperatures during the combustion cycle fosters the production of lower molecular weight compounds. Liu et al (2000) observed an increase in the relative composition of 2- and 3-ring structures as operating temperatures increased from 783^oC to 843^oC, even though the total overall magnitude of PAHs was significantly reduced.

In order to better understand the source characteristics of PAHs with regards to their PAH16 composition, various literature data is presented which evaluates their relative individual and ring-group compositions. The relative composition of a particular compound from a given source is evaluated by normalizing the individual concentration from the corresponding PAH16. Fig. 2.5 and Fig. 2.6 show the relative composition (median values) of the common sources of pyrogenic PAH₁₆. The dashed line represents the composition pattern which

1 10 100 1000 10000

¦PAH16 (mg/Kg)

Coal Combustions

Biomass Burnings

Petroleum Combustions Emission Factor of PAHs

27 characterizes different sources. In addition, the literature data from Sumatra's peatland burning episode in 2005 (mean values, after See et al., 2007) is also presented for a comparison. These figures are particularly important, since they provide information on the conditions in the area where this study took place.

The results shows that the compositions of PAH16 stemming from pyrogenic sources is relatively similar with respect to high levels (>30%) of naphthalene (NAPH) and low levels (<5%) of high molecular weight compounds (Fig. 2.5a). This suggests that all biomass, coal and petroleum combustions emit a significant amount of naphthalene. This is particularly true in the case of petroleum combustion, in which NAPH comprises roughly 65% of the emitted PAH16. However, the ring-group composition shows a different pattern for petroleum and biomass-coal combustion, particularly in their relative compositions of 2 and 3 ring compounds (Fig. 2.6.a).

The decreased levels of 3-ring groups from petroleum combustion is due to a lack of acenapthylene (ACYN) and acenaphthene (ACEN) produced. In contrast, the high levels of 3-ring PAHs for biomass and coal combustion are mainly caused by phenanthrene (PHEN) and acenaphthylene (ACYN). Within the biomass group, wood burnings emit more ACYN than grass burnings (Fig. 2.5b). Therefore, the relative compositions of the 2- and 3-ring groups can probably be used to distinguish between petroleum and biomass-coal combustion simply by comparing the mass ratios of these ring groups. As far as ring-groups are concerned (Fig. 2.6a), the patterns emitted by biomass and coal combustion are quite similar, thus making it quite difficult to differentiate between them. Therefore, the composition of individual compounds such as fluorene (FLU), fluoranthene (FLA) and pyrene (PYR) and most of the other high molecular weight compounds can be used as additional clues during separation, due to their unique signatures and patterns. For example, the ratio of the relative composition of FLA to PYR for biomass burnings is three-fold higher than that of coal combustion. Therefore, any mass ratio between those compounds can be developed to help provide clues for source differentiation.

In comparison to biomass burnings, Fig. 2.5b shows the relative compositions of individual PAH compounds between two distinct locations. Dumai and Pekanbaru (the capital city of Riau province) were affected by the tremendous volumes of smoke stemming from peatland burnings in 2005. It shows that Sumatran peatland burnings emit less NAPH and ACYN. The relative composition of the PAH16 between Dumai and Pekanbaru is also significantly different. This is particularly true for the composition of PHEN, ANTH, PYR, as well as benzo(b)fluoranthene (BbFLA), benzo(k)fluoranthene (BkFLA), benzo(a)pyrene (BaP), dibenzo(a,h)anthracene (DANTH), and indeno(1,2,3-c,d)pyrene (IPYR). Although there is only a brief discussion on the composition of PAH16 provided here by the authors, the evidence suggests that PAHs derived from the peatland smoke experienced alterations due to

28 decomposition or additions from local sources. This data can therefore be a useful aid in comparison for PAH assessments in aquatic systems of those particular studied areas.

Fig. 2.5. Relative composition of 16 individual PAHs of the EPA priority list from (A) common pyrogenic sources including coal, biomass and petroleum combustion (median values) (see Appendix 2 for references); (B) a subset of biomass burning sources comprising wood and grass burnings; and (C) Sumatra peatland burnings (mean values, after *See et al., 2007). Note: the y-axis for graph A and B is different.

Fig. 2.6. Relative composition of the ring groups of the 16 PAHs on the EPA priority list from (A) common pyrogenic sources including coal, biomass and petroleum combustion (median values) (see Appendix 2 for references); (B) Sumatra peatland burnings (mean values, after *See et al., 2007).

0 5 10 15 20 25 30 35 40

Relative Composition (%) Biomass Burnings

Wood Burnings Grass Burnings A

C 0

5 10 15 20 25 30 35

Relative Composition (%) Pyrogenic PAHs Biomass Burnings Coal Combustions Petroleum Combustions

0 5 10 15 20 25 30 35

Relative Composition (%)

Sumatra Peatland Burnings in 2005 Dumai Pekanbaru 65,1%

B

0 10 20 30 40 50 60 70 80

2 rings 3 rings 4 rings 5 rings 6 rings

Relative Composition (%)

Sumatra Peatland Burnings in 2005*

Dumai Pekanbaru

0 10 20 30 40 50 60 70 80

2 rings 3 rings 4 rings 5 rings 6 rings Relative Composition (%) Pyrogenic PAHs

Biomass Burnings Coal Combustions Petroleum Combustions

A B

29