The relation between polycyclic aromatic compounds in diesel fuels and exhaust particulates

The relation compounds particulates Paul T. Williams*,

between polycyclic aromatic in diesel fuels and exhaust

Keith

D. BartIe+ and Gordon

E. Andrews*

‘Department of Physical Chemistry, University of Leeds, Leeds LS2 9J7, UK * Department of Fuel and Energy, University of Leeds, Leeds LS2 9JT, UK (Received 2 1 November 1985; revised 17 March 1986)

The polynuclear aromatic compound fractions (PAC) separated by column chromatography from five diesel fuels, a gas oil and sample of kerosene were analysed by capillary column gas chromatography with simultaneous parallel triple detection. The principal polynuclear aromatic hydrocarbons of the fuels are naphthalene, fluorene and phenanthrene and their alkyl derivatives; mutagenic compounds are present in significant concentrations. The principal polynuclear aromatic nitrogen and sulphur compounds of diesel fuel are carbazole and dibenzothiophene and their alkyl derivatives. The PAC of diesel exhaust particulates are similar to those of the fuel, and follow the overall trend of particulate emission with engine load. The 2 to 4ring PAC in the exhaust are primarily unburnt fuel components. Between 0.2 and 1.0 wt% of these fuel PAC survive the combustion process and comprise a significant concentration of mutagenic compounds in the particulate. (Keywords: PAH; diesel; chromatography)

Diesel-powered vehicles now achieve 2&40”/, more distance per unit of fuel than those powered by gasoline,

partly because of their higher fuel efficiency’. The main drawback, however, is the very much higher emissions of particulate matter from diesel engines’. Associated with the solvent organic fraction of the particulate matter are polycyclic aromatic compounds (PAC) which constitute a known health hazard2-4. The identity and concentrations of numerous polycyclic aromatic hydrocarbons (PAH) in diesel particulates have been reported5,6, but there is less information’ concerning the contribution to these of PAH present in the fuel. Moreover, recent interest has centred on the nitrogen containing PAH (PANH) and sulphurcontaining PAH (PASH) in fuels and particulates. Some PANH are themselves mutagenic and/or carcinogenic’, while they may also be precursors for the nitro-PAH (NPAH) which are potent direct acting mutagens* giving rise’ to a large portion of the biological activity of diesel exhaust. Many NPAH thought” to originate by reaction of nitrogen oxides (NO,) with PAH in the hot exhaust gases have been detected’ l-l2 in diesel particulates; a contribution from unburnt PAH in the fuel to NPAH by this mechanism has recently been established13. Some PASH are also toxic14 and/or of PANH and PASH is mutagenic 15-18 . The combustion also of direct interest since these compounds give rise to the toxic and corrosive exhaust gases NO, and SO,. There are few data in the literature concerning the concentrations of PAC in diesel fuel, particularly the major PAC components. Individual PAH have been identified; for example, Lipkea et al. reported” the presence of benzo[a)pyrene and benz[a]anthracene at < 1 ppm. The low levels of concentrations of benzo[a]pyrene in diesel fuel were also confirmed by 0016-2361/86/08 1150-09$3.00 ii;: 1986 Butterworth & Co. (Publishers)

1150

Ltd.

FUEL, 1986, Vol 65, August

Williams and Swarin”. Bricklemayer and Spindt” report concentrations of several PAH: for example chrysene, pyrene, fluoranthene and benzo[a pyrene for a composite of ten diesel fuels. Leary et al.’ 7 analysed a domestic fuel oil and identified, but did not quantify, several PAH, for example, naphthalene, phenanthrene and fluorene and the alkylated homologues of naphthalene, biphenyl, dibenzofuran and phenanthrene. Henderson et ~1.~~ concluded from mass spectrometric ion intensities that the major PAH in diesel fuel are naphthalene, phenanthrene and fluorene and their alkylated homologues, with the methyl, dimethyl and trimethyl naphthalenes making up the bulk of the total PAH ; they also identified acenapht hene, ant hracene, biphenyl and alkylated biphenyls, and at low levels pyrene, fluoranthene and benzopyrenes. Mills and Howard24 reported quantitative data for PAH in gas oil and similarly found naphthalene, phenanthrene and fluorene to make up the bulk of the PAH present; they also found fluoranthene, pyrene, benz[u]anthracene and chrysene at trace concentrations. There is very little published information on the presence of PANH or PASH in diesel fuels. Leary et ~1.~~ analysed domestic fuel oil and identified, but did not quantify, the major PANH as carbazole and alkylated carbazoles. Mills et ~1.~~ identified some dibenzothiophenes in diesel fuel, with total concentrations at z 200 ppm. This paper reports the analysis25, by capillary column gas chromatography (g.c.) using simultaneous parallel triple detection, of a range of commercially available diesel fuels, gas oil and kerosene for PAH, PANH and PASH. The particulate emissions from the combustion of one of these fuels in a singlecylinder direct injection diesel engine coupled to a dynamometer were monitored, and

Relation

between

polycyclic

aromatics

and exhaust

the constituent PAC were correlated both with the PAC present in the fuel and with the engine operating conditions. EXPERIMENTAL Fuels

The diesel fuels were obtained from different parts of the UK to ensure that they originated from different refineries and different crude oil feedstocks. The fuels were collected from commercial fuel stations in Edinburgh, Leeds, Birmingham and London. In addition 500 gallons of diesel fuel were obtained from the Peterborough locality from the same suppliers to the Perkins Engine Company. This fuel was used as the reference fuel for the engine exhaust particulate tests. The Peterborough fuel was of A2 class as distinct from the commercially obtained diesel fuels which were Al class, as specified by British Standard 2869 which defines the minimum properties of the fuel. The major difference between the Al and A2 classes is the specified minimum cetane number which reflects different alkane contents: 50 for Al, and 45 for A2. The hydrogen content of the diesel fuels was measured by n.m.r. using an Oxford Instruments Newport fuel analyser. The results are shown in Table I together with the fuel density and the cetane index; the range of 13.1 to 13.4 wt% hydrogen is typical of UK diesel fuels. Gas oil and kerosene were obtained locally for comparison with diesel fuel. Engine

The engine used was a Petter Type AVl direct injection diesel engine, the specifications of which are shown in Tab/e 2. The engine was coupled directly to a hydraulic dynamometer. An engine governor kept the speed constant at 1500 r.p.m. as the power output was varied and consequently the air/fuel ratio. The air and fuel flows were metered directly. Sampling

system

The exhaust particulate sampling system was based on that of Begeman 26. The exhaust gases were cooled below

Table 1

Fuels and their properties

Fuel type

Location

Cetane index

Hydrogen content (wt%)

Density (kg rne3)

51.3 50.6 51.8 52.6 48.0 51.8

13.2 13.2 13.3 13.4 13.1 13.2 13.7

853 849 850 839 859 846 811

_

particulates

in diesel

Leeds Edinburgh London Birmingham Peterborough Leeds Leeds

Table 2

Engine specifications

Model : Number of cylinders: Displacement: Compression ratio: Power output: Chamber type: Injection timing: R.P.M.: Brake:

Petter type AVl 1 553 cc 19:l 3.7 kW at 1500 r.p.m. Direct injection 30” B.T.D.C. Governed at 1500 Heenan and Froude dynamometer

P. 1. Williams

et al.

52°C by means of a water cooled condenser to ensure that the vapour phase components of either unburnt fuel or combustion products are condensed out27J8. Examination of the condensate confirms Begeman’?’ conclusion that condensation of the organic fractions takes place largely onto the particulate soot. Whether the present thermal quenching process achieves the same condensation mechanism as in the air dilution process will be determined in future work using a dilution tunnel. The exhaust gases were sampled at 5 dm3 min - l via a 0.6cm diameter probe inserted into the exhaust pipe 1Ocm from the exhaust manifold; the exhaust was passed to the condenser and then to a commercial built Richard Oliver filter paper particulate sampling system. This allowed a predetermined total volume of exhaust gas to be passed through a glass fibre filter paper at a constant temperature and a fixed flow rate irrespective of filter blockage by the particulates. The filter papers were kept in the dark at constant humidity and weighed to pg accuracy before and after the test. The filter papers used were 5.5 cm diameter Whatman glass libre GF/F grade filters. It has been established that the sizes of the particulates formed in diesel exhausts are very sma11z9, of the order of 0.054.1 pm. However agglomeration occurs immediately, producing aggregates of several microns in size 29. Earlier work’ showed mean aggregate sizes of 2.3 pm, and shape, for the present sampling system, similar to those illustrated by Vuk et a1.29 The GF/F filters chosen have a specified retention of 98% of 0.7pm diameter particles and should ensure efficient collection of even the small aggregates. A comparison was made with other grades of filter having different specified retentions, ranging from 0.3 pm to 1.6pm for 98% of particles. It was found there was no difference in collection efficiency between the filter grades for diesel particulates. The engine was preconditioned for 1 hat the start of the test and then held for 30min at each different engine condition to establish equilibrium before particulate and gas analysis. The filter papers were exposed to a known volume of exhaust gas at a fixed flow rate and the total masses of particulates produced were determined by weighing. The exhaust gases were transferred to an on-line gas analysis system via a PTFE tube. The vapour phase hydrocarbons were analysed as total hydrocarbons using a flame ionization detector. Fuel and particulate

Diesel Diesel Diesel Diesel Diesel Gas oil Kerosene

fuels:

Development

analysis

of methodology.

H.p.1.c. grade solvents were employed throughout the extraction and separation procedures but it was nonetheless found necessary to distil these before use since impurity levels were magnified during concentration procedures. Cellulose Soxhlet thimbles used for extraction of particulate filters were also found to contain aliphatic and aromatic compounds, including some containing nitrogen, and were preextracted before use with 4:l by volume, benzene/methanol for 8 h. Further extraction revealed that the resulting impurity levels were insignificant. The most commonly used solvents for the extraction of PAC from diesel particulates are dichloromethane, cyclohexane and mixtures of benzene with an alcohol. In this work, extraction efficiencies were compared for dichloromethane, cyclohexane and 4:1, by volume, benzene/methanol by Soxhlet extraction’ of portions of

FUEL, 1986,

Vol 65, August

1151

Relation

between

polycyclic

aromatics

and exhaust

particulates

the same particulate filter for 8 h with 200cm3 of the solvent. The extracts were fractionated and analysed by g.c. as described below. Similar efficiencies were observed for PAC for all three solvents but 4:l benzene/methanol (recommended by Petersen et ~1.~‘) extracted more alkanes. No further detectable quantities of PAC were removed in a second 8 h extraction. Three procedures were investigated for the isolation of a PAC fraction: (a) chromatography on silica gel with elution of aromatics with benzene; (b) silica gel chromatography with elution by 40 ~01% dichloromethane in n-pentane; and (c) partition of aromatics into dimethyl sulphoxide followed by precipitation with water. The first of these was preferred since it gave a greater yield of PAC with lower concentrations of aliphatics. Isolation of the PAC fraction. A PAC fraction was isolated from each fuel as follows. A 10 cm3 aliquot of the fuel was adsorbed on to a 45 cm x 2.2 cm column of 0.1250.250mm silica gel previously deactivated by heating at 200°C for 48 h, and wetted with n-pentane. Aliphatics were eluted with 1 dm3 n-pentane or until the fluorescence under U.Y. light extended throughout the column, and then aromatics were eluted with 1 dm3 benzene. Tests with smaller volumes of benzene showed that this volume was necessary to completely remove PAC from the column. The n-pentane and benzene eluates were rotary evaporated and then blown down with nitrogen to 10 cm3 and 5 cm3 respectively before gas chromatography. The filter papers exposed to diesel exhaust were Soxhlet extracted with 200cm3 4:1, by volume, benzene/methanol for 8 h and the extract evaporated to dryness on a rotary evaporator. The dry extract was taken up in n-pentane and the aliphatic and PAC fractions separated as above but on a 18 cm x 2.2 cm column with elution volumes of 450cm3 n-pentane and 350cm3 benzene which were then reduced to 1 cm3.

Gas chromatography After appropriate dilution the samples were spiked internally with PAH standards (acenaphthene, anthracene, triphenylene, perylene, and 1,3,5 triphenylbenzene); PANH standards (indole, 2,2’-bipyridyl, phenazine, phenanthridine, carbazole, 9-nitroanthracene, 2,2’-biquinoline and dibenzo [a,i]carbazole); and PASH standards (benzothiophene and dibenzothiophene), for quantification purposes. The PAC solutions were analysed on a modified Carlo Erba Mega Series HRGC 5300. The system, which is employed a novel 3-way effluent described elsewhere”, splitter which enabled simultaneous parallel detection of PAH, PANH and PASH by means of a FID, and nitrogen selective (NPD) and sulphur selective (SSD) detectors. The response factors for various parent PAH and PAC for the three detectors are shown in Table 3. The instrument was equipped with a cold on-column injector and a 25 m x 0.3 mm i.d. fused silica capillary column, coated with cross-linked” SE-54. The temperature programme was: 50°C for 5 min, 5&7O”C at 50°C per min, 70-280°C at 5°C per min, followed by 10min at 280°C. The carrier and make-up gas was helium, with a carrier flow rate of 2cm3 per min at 280°C. Compounds were identified with the aid of the linear retention index system of Lee et aL31 Identifications were confirmed from mass spectra recorded by means of capillary column g.c. with ion-trap detection; here the

1152

FUEL, 1986, Vol 65, August

in diesel

fuels:

P. T. Williams

Table 3 Relative molar response with the three detectors

et al.

factors for PAH, PANH

and PASH

FID

NPD

SSD

PAH Anthracene Triphenylene

100 100

5 5

0 0

PANH Carbazole Phenazine

100 100

100 100

0 0

PASH Benzothiophene Dibenzothiophene

100 100

5 5

100 100

Compound __ .~

cross-linked SE-54 coated column was 10m long and coupled to an ion trap detector (ITD) via a heated transfer line. The ITD has a mass range from 20 to 650 a.m.p. with scan times of between 0.125 and 2 s. Quantitative analyses were made with the aid of an integrator. RESULTS

AND DISCUSSION

Polycyclic aromatic compounds in the fuels Table 4 shows the results of the PAH analyses along with a carcinogenicity index for the five diesel fuels, together with gas oil and kerosene. Figure 1 shows a typical gas chromatogram profile for diesel fuel. The profiles for all the diesel samples and gas oil were similar whilst kerosene showed only the lower boiling range fraction, reflecting its lower distillation temperature. Figure 1 shows good resolution between compounds and it was possible for individual isomers to be identified; however some of these have been grouped together in Table 4 for clarity. The PAH making up the greatest proportion of the total are: naphthalene, fluorene and phenanthrene and their alkyl substituted homologues in agreement with previous work22-24. The results also show the considerable variations in the PAH contents of the fuels obtained from different parts of the UK, reflecting their different sources of crude feedstock, and refinery operation. The major components of the PAH fraction show a wide variation in concentration; for example the methylnaphthalenes range from 1400-4600 ppm, the dimethylnaphthalenes from 4900-9400 ppm , the trimethylnaphthalenes from 220&4000 ppm, the methyl fluorenes from 90&2300 ppm, and the methylphenanthrenes from 1600-2500ppm, i.e. a 20&3000/, variation between the fuels. However, the concentrations of individual PAH relative to each other remain fairly constant. Diesel fuel E from Peterborough was the A2 grade fuel with lower cetane number than the other Al grade diesel fuels, but the PAH content of this fuel is mid-range and thus the grade of the fuel appears to have little effect on the PAH content. Biologically active PAH are present in the fuels in significant concentrations. Longwell has shown that phenanthrene, the methylphenanthrenes and fluoranthene are mutagenic in both bacterial and human cell tests, while methylfluorenes are active in mutagenicity bioassays33. All these compounds were in high concentrations in the diesel samples. The higher boiling point PAH of known carcinogenicity’ are also present in lower but still significant concentrations; for example benz[a]anthracene, chrysene and methylchrysenes.

Relation Table 4

between

PAH concentrations

polycyclic

aromatics

and exhaust particulates

in diesel fuels: P. T. Williams

et al.

(ppm) in the fuels -___

PAH

A

B

C

D

E

Gas oil

Kerosene

Carcinogenicity

Naphthalene Methylnaphthalenes Dimethylnaphthalenes Acenaphthene Trimethylnaphthalenes Fluorene Methylfluorenes Phenanthrene Anthracene Methylphenanthrenes Dimethylphenanthrenes Fluoranthene Pyrene Methylfluoranthenes Benz[a]anthracene Chrysene 3-Methylchrysene 6-Methylchrysene Benzo[e]pyrene Benzo[u]pyrene Perylene

100 3800 9400 800 4000 700 2300 800 200 2ocQ 300 200 150 130 <5
400 4600 8600 1000 2600 500 1500 900 150 2500 300 200 150 130 <5 < 10 < 10 <5
100 1400 5OQO 700 2700 600 1100 1000 100 2000 200 100 50 <30 <5
400 3200 6600 600 2200 400 900 600 150 1700 300 200 150 <30 <5
100 1500 4900 700 2900 400 2100 700 150 1600 200 100 100 <30 <5 < 10
300 3500 7300 500 3600 700 1700 900 200 2000 200 200 100 <30 <5 < 10
1500 14800 16200 600 700 100 _

0 0

Total A= London concentration;

diesel; B= Leeds diesel; C= Edinburgh diesel; D= Birmingham diesel; E= Peterborough + = carcinogenecity index after Lee et a/.‘; * = mutagenic according to Longwel13’

0 0 _ *

_ _

0 *

_ _ _ _

0 + + + + + + +$ 0

*

_

control

diesel;

< =indicates

range

of

SSD XI

NPD X2

FID X6 Time

I

I

I

I

I

I

I

(min)

0

IO

20

30

40

50

60

Temperature

(“Cl

1 50

II 50 70

I

I

100

150

I

I

I

200

250

280

Figure 1 Typical gas chromatogram of the PAC fraction of a diesel fuel. 25 m cross-linked injection. Simultaneous flame-ionization (FID), nitrogen-selective (NPD), and sulphur-selective

SE-54 fused-silica (SSD) detection

capillary

Hold

column

with on-column

FUEL, 1986, Vol 65, August

1153

Relation

between

polycyclic

aromatics

and exhaust

particulates

PANH

concentration A

B

Carbazole l-Methylcarbazole 3-Methylcarbazole 2-Methylcarbazole 4-Methylcarbazole 6-Phenylquinoline 1.4-Dimethylcarbazole 2-Phenylindole l,2-Dimethylcarbazole 1,3-Dimethylcarbazole 9_Cyanoanthracene* 9_Cyanophenanthrene* 4_Aminophenanthrene* 2-Azapyrene Benzo[de&arbazole 3_Aminophenanthrene* 2_Aminophenanthrene* 2_Aminoanthracene* 9-Phenylcarbazole Benz[e]acridine Benz[a]acridine Benzo[c]carbazole Benzo[a]carbazole

IO 20 4 5 10 II I9 5 I 4 9 9 6 2 5 3 4 5 6 <1
7 20 5 6 10 7 16 5 8 6 9 10 4 I 3 2 2 5 4
I146

130

A = London diesel; B= Leeds diesel; C = Edinburgh Lee et ~1.~; * = tentative identification

1154

FUEL, 1986,

Polycyclic

aromatic compounds

in exhaust

purticulates

Figure 2 is a typical gas chromatogram of the PAC fraction of the diesel exhaust particulate from the baseline fuel E. The concentrations of PAH in the exhaust are summarised in Table 7 for low, medium and high engine loads. The results are expressed as concentration &g m- 3, of the PAH in the exhaust and as mg PAH/kg of fuel cornbusted using the method of Andrews et al.’ Certain PAH are shown graphically in Figures 3 and 4. The PAH exhaust profiles for the different engine load conditions were similar, showing the presence of naphthalene, fluorene and phenanthrene and their alkylated homologues, which are the main PAH present in the diesel fuel; combustion of e.g. vegetable-oil based alternative fuels gives greater concentrations of parent

(ppm) in the fuels

PANH

Total

et al.

appears to have no relation to the grade of fuel. The A2 grade fuel (lower cetane number) has a similar PANH content to the other (Al grade) diesel fuels. Most of the PANH compounds identified in diesel fuel are largely mutagenically inactive; however active compounds such as benzacridines and benzocarbazoles are present although in low concentration. The presence of PANH in concentrations up to 20ppm may be important in the formation of NPAH by oxidation in the exhaust gas stream. The presence of nitrogen containing PAH may enhance the formation of these highly carcinogenically active derivatives. The PASH chromatographic profiles (e.g. Figure I) were similar for the five diesel fuels and for gas oil, but no PASH compounds were detected for kerosene. The PASH compounds present are (Table 6) dibenzothiophene and its alkylated derivatives, mainly methyl and dimethyl dibenzothiophenes. The concentrations of PASH in the diesel fuels are approximately an order of magnitude lower than the PAH content, and an order of magnitude higher than the PANH content. The PASH levels follow the same trends as those of PAH and PANH.

The major PAH present in the diesel fuels are not carcinogenically active. However they may be precursors of more compounds in the exhaust. For example Henderson et ~1.‘~ found that certain PAH, e.g. pyrene and phenanthrene when added to aliphatic fuel cause increased emission of the same PAH, and also increased the emission of the NPAH corresponding to the parent PAH; no increase in soot production was observed however. Other PAH (1-methylnaphthalene, acenaphthene and benzo[a]pyrene) increased the overall emissions of several PAH and soot, and altered the patterns of NPAH emissions. These results may equally apply to other major PAH components which show great variation. Thus the variability of the PAC component of diesel fuels shown by this work can significantly effect the PAH concentration, soot forming capability and biological hazards of the exhaust particulate. A typical NPD gas chromatogram for diesel fuel is shown in Figure 1. As for the PAH, all the diesel fuels and the gas oil showed similar PANH chromatographic profiles; however kerosene contained no detectable PANH. Kerosene has a lower distillation range than diesel or gas oil, and it can be seen from Figure I that the PANH compounds present in the diesel fuel have a high boiling range, between that of carbazole and benzo[a]carbazole. The main PANH in diesel fuel (Table 5) are carbazole and its methyl derivatives which together make up approximately two-thirds of the total PANH. Other compounds present are, on the evidence of correspondence of retention indices31, amino and cyano derivatives of anthracene and phenanthrene. The concentrations of PANH (with a maximum of about 20ppm) are 2-3 orders of magnitude lower than those of the PAH in the diesel fuels. The levels follow the same trends as those of PAH, i.e. when the concentration of PAH is low then the concentration of PANH is also low. As for PAH, the concentration of PANH in the fuel

Table 5

in diesel fuels: P. T. Williams

Vol 65, August

C

9 2 6 2 2

4 2


D

E

5 IO 3 3 5 4 8 3 4 2 4 4 1 I 3 2 2 4 3
8 21 6 8 10 -I I7 4 8 4 7 7 4 2 3 2 3 5 4 il
71

130 diesel; E = Peterborough

Gas oil 5 9 1 1 3 5 4 1 2 1 3 4 1 1 1 I

Kerosene

Carcinogenicity 0

_

_ _ _.

1 1 2 il
+ + +

47 control

diesel; + =carcinogenicity

index after

Relation Table 6

Concentration

between

of PASH

PASH

polycyclic

t

2.8-Dimethyldibenzothiophene 3,7-Dimethyldibenzothiophene 3,8-Dimethyldibenzothiophene 1,7-Dimethyldibenzothiophene Total A = London

B

C

D

E

Gas oil

145 200

125 180

130 200

175 300

115

140

85

75

75

130

45 15 40

75 20 125

45 15 55

40 15 25

45 15 90

70 20 90

225

315

130

130

180

275

125

155

80

85

105

185

1

45

60

30

30

40

70

1010

1430

785

705

880

1315

diesel; B= Leeds diesel; C = Edinburgh

NPD%i

et al.

in diesel fuels: P. T. Williams

170 370

170 230

I-Methyldibenzothiophene 3-Ethyldibenzothiophene 4,6-Dimethyldibenzothiophene 2,GDimethyldibenzothiophene 2-Ethyldibenzothiophene

and exhaust particulates

in diesel fuel (ppm) A

Dibenzothiophene 4-Methyldibenzothiophene 2-Methyldibenzothiophene 3-Methyldibenzothiophene

aromatics

diesel; D = Birmingham

diesel; E = Peterborough

Kerosene

diesel; + = carcinogenicity

Carcinogenicity

index after Lee et ~1.~

f--

FID X?

Ll I

I

I

I

I

I

I

Tmc (mm)

0

IO

20

30

40

50

60

Tempmtwc

50

I

I I 50

70

I

I

100

150

I 200

I 250

I 260

Hold

C’C)

Figure 2

Typical

gas chromatogram

of the PAC fraction

of diesel exhaust

PAH relative to alkyl derivatives24. However the results show how the proportions of certain PAH are markedly increased relative to the others, for example phenant hrene and the methylphenanthrenes. These PAH have become more concentrated in the exhaust either by being formed on combustion or due to lower combustion efficiencies.

particulate.

Chromatographic

conditions

as in Figure

I

PAH emissions (Figure 3 and 4) follow the overall trend of particulate emission with engine load, i.e. high PAH at low load, decreasing at mid-load, and increasing at higher load. Mills et a1.24 found the highest PAH were obtained at low engine load; in fact a detailed examination of their data for 2350 r.p.m. at 0, l/4, l/2,3/4 and full loads shows

FUEL, 1986, Vol 65, August

1155

Relation

between

Table 7

Exhaust

polycyclic

aromatics

PAH concentration

and exhaust

(pg mm3) and relation

particulates

in diesel

to mass of fuel burnt

P. T. Williams

et al.

(mg kgg’) Mid load

Low load Exhaust

fuels:

Exhaust

mp mf

Exhaust

mp

(mg kg-‘)

cont. (pg m3)

(mg kg-r)

mp mf

PAH

cont. (itg m

Naphthalene Methylnaphthalenes Dimethylnaphthalenes Acenaphthene Trimethylnaphthalenes Fluorene Methylfluorenes Dibenzothiophene Phenanthrene Methylphenanthrenes Dimethylphenanthrenes Fluoranthene Pyrene

6 13 47 13 90 I4 95 26 72 177 42 22 IS

0.3 0.7 2.5 0.7 4.7 0.7 4.9 1.3 3.7 9.2 2.2 I.1 0.8

4 13 45 II 50 5 52 9 32 60 I3 11 6

0.1 0.4 I.4 0.4 1.5 0.1 I.6 0.3 1.0 1.8 0.4 0.3 0.2

I2 24 73 14 86 10 90 I7 77 177 34 20 I1

0.2 0.5 1.4 0.3 I.7 0.2 1.8 0.3 I.5 3.4 0.6 0.4 0.2

632

32.8

311

9.5

645

12.5

Total

(mg kg-‘)

?

_. mpjmf=mass

of pollutanti’mass

cont. (pg mu ‘)

High load

mf

of fuel burnt -l

Total

-2

IC M-Ph

i

P

7 a E” -

I I(

6

: g z 4

-5: I

0

I

I

20

I

I

40 Air/fuel

(mass/mass

I

60

I

-I

basis)

20

40 Air/fuel

Engine load

High

Mid

( mass /mass

60 basis)

Law Engine load

High

Mud

Low

Figure 3 Exhaust PAH and total particulate concentration at low and high engine load. Methylphenanthrenes, M-Ph; methylfluorenes, M-F; phenanthrene, Ph; fluoranthene, Fl; pyrene, Py

Figure 4 Mass flow rate of PAH in the exhaust/mass Symbols as in Figure 3

similar reduction in certain PAH at mid-engine loads, for example fluorene, methylfluorenes, phenanthrene and methylphenanthrene. A similar trend was observed by Bricklemayer and Spindt21. Figure 5 shows some of the data from Table 7 expressed in terms of mass of PAH in the exhaust/mass of PAH in the fuel; also shown are the gaseous total unburnt hydrocarbons (UHC) and these are shown to be a similar proportion of the total fuel burnt as the PAH fractions. Consequently it may be concluded that, for the engine conditions used in this work (single engine, injection timing and fuel), the PAH in the exhaust particulate is primarily unburnt fuel component. The

combustion efficiency of the PAH in the fuel is similar to that of the total fuel; while different PAH show different behaviour at high load, between 0.03 and 0.4 wt % (rising to between 0.04 and 1.O wt% at low load) of 24 ring PAH in the fuel survive the combustion process. This adds further weight to the preliminary conclusions of Andrews et al.‘; however, in the latter work, the PAH content of the fuel was underestimated so that the combustion efficiency of the PAH fuel component was less than in the present work. The biological activity of diesel exhaust particulate is well known, but, as suggested by Yu and Hites34, the

1156

FUEL, 1986, Vol 65, August

flow rate of fuel.

Relation

between

polycyclic

aromatics

and exhaust particulates

Total I

IO

40

20 Air/fuel

Engine

High

(mass/mass Mid

60 basls) LOW

in diesel fuels: P. T. Williams

et

al.

elution of aromatics with benzene, allows ready separation ofa PAC fraction from diesel fuels and exhaust particulate extracts. Pre-extraction of Soxhlet thimbles and purification of solvents by distillation is vital before their use in extraction and clean-up. PAC fractions from fuels and particulates are conveniently analysed by capillary gas chromatography with simultaneous parallel triple detection of PAH, PANH and PASH. The principal PAH of five UK diesel fuels, a gas oil and kerosene are naphthalene, fluorene and phenanthrene and their alkyl derivatives. There are considerable variations in the PAH content of different diesel fuels, but the fuel grade has little effect. Mutagenic compounds, principally alkyl fluorenes and phenanthrenes and fluoranthene, are present in the fuefs in significant concentrations. The principal PANH and PASH of diesel fuel are respectively carbazole and dibenzothiophene and their alkyl derivatives. Their concentrations are two to three orders of magnitude (PANH) and one order of magnitude (PASH) lower than those of the PAH. The PAC of diesel exhaust particulates are similar to those of the fuel, and their concentrations follow the overall trend of particulate emission with engine load. The 2 to 4-ring PAC in the exhaust are primarily unburned fuel components. Between 0.2 and 1.0% of these fuel PAC survive the combustion process and comprise a significant concentration of mutagenic compounds in the particulate.

Iood Figure5 Variation of mass of PAH in exhaust/mass of PAH in the fuel with engine load. Symbols as in Figure 3; unburned hydrocarbon, UHC

major mutagens are met hylphenant hrenes and methylfluorenes, rather than the often citedS benzo[a]pyrene. The current work shows, moreover, that these mutagens may largely originate from unburnt fuel components, rather than pyrosynthesis during combustion. PANH were not detected in the particulate samples when the present clean-up method was employed, as might be expected in view of their much reduced concentration in the fuel as compared with PAH (lower by two or three orders of magnitude). A further concentration step, possibly by high-performance liquid chromatography, is necessary. PASH were, however, detected in the exhaust particulate (Figure 2) and were identified, as in previous work34, as dibenzothiophene and its methyl- and dimethyl derivatives. These are precisely the same compounds as are found in the fuel, and their presence and concentrations afford further evidence for the origin in the fuel of particulate PAC. As for the PAH, the concentrations of dibenzothiophene (Table 7) in the exhaust reflected the profile of the total exhaust particulate of the engine (Figure 3)-high at low load, lower at mid-load, and increasing again at high load. The concentrations of alkyl dibenzothiophenes in the exhaust were in the same ratio to that of the parent PASH as was found in the fuel, and again reflected the total exhaust particulate profile.

ACKNOWLEDGEMENTS The authors thank the Science and Engineering Research Council for support of this work through grants, and Mr D. Bacon and Mr F. Brear (Perkins Engines) for help and advice. They are also grateful to Mr G. Cole, Mr B. Frere, Mr D. Mills and Mr A. Wheeler (University of Leeds) for help with analyses. REFERENCES Cuddihy,

4

5

6

7

8 9 10 11

CONCLUSIONS PAC are eficiently extracted from diesel particulates by 4:l benzene: methanol; chromatography on silica gel, with

R. G., Griffiths,

W. C. and McClellan.

R. 0. Environ.

Sci. Technol. 1984, 18, 14

12 I3

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FUEL, 1986, Vol 65, August

1157

Relation 14 15 16

17 18 19 20 21 22

23 24

1158

between

polycyclic

aromatics

and exhaust particulates

Eastwood, D. A., Booth, G. M. and Lee, M. L. Arch. Environ. Contam. Toxico[. 1984, 13, 105 Tilak, B. D. Tetrahedron 1960, 76 Karcher, W., Nelson, A., Depaus, R., van Eiik, J., Glaude, P. and Jacob, J. ‘Polynuclear Aromatic Hydrocarbons: Chemical Analvsis and Biological Fate’ (Eds. M. Cooke and A. Dennis). Bat&he Press, Columbus, Ohio, 1981, p. 317 Pelroy, R. A., Stewart, D. L.,Tominaga, Y ., Iwao, M., Castle. R. N. and Lee, M. L. Murat. Res. 1983, 117, 31 McFall, T., Booth, G. M., Lee, M. L., Tominaga, Y., Pratap. R., Tedja-Mulia, M. and Castle, R. A. Mutut. Res. 1984, 135, 97 Lipkea, W. H., Johnson, J. H. and Vuk, C. T. SAE Paper 7801 OX, 1978 Williams, R. L. and Swarin, S. J. SAE Paper 790419, 1979 Bricklemayer, B. A. and Spindt, R. S. SAE Paper 780115 Leary, J. A., Lafleur, A. L., Longwell, J. P., Peters, W. A., Krurel, E. L. and Biemann, K. Proc. 7th Int. Symp. on Polynuclear Aromatic Hydrocarbons, Columbus, Ohio, 1982 Henderson, T. R., Royer, R. E., Clark, C. R., Harvey. T. M. and Hunt, D. F. J. Appl. Toxicol. 1982, 2, 231 Mills, G. A. and Howard, A. G. J. Inst. Eneryj, Sepr. 1983. 131

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34

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