Reactivity of polycyclic aromatic hydrocarbons adsorbed on soot particles

C0l4-698l/8l/OlOl-iXNl %02.00/O

Armospheric Enwonmenr, Vol. IS, pp, 91 94. Q Pergamon PressLtd 1981 Printedin Great Britam

REACTIVITY HYDROCARBONS

OF POLYCYCLIC AROMATIC ADSORBED ON SOOT PARTICLES J. D.

BUTLER

and P. CROSSLEY

Department of Chemistry, University of Aston, Birmingham B4 7ET, England (First reeeiued 1 t January 1980 and jn.~nal form 28 March 1980) Abstract - Polycyclic aromatic hydrocarbons (PAH) adsorbed on soot particles do not react significantly when exposed to ambient laboratory atmospheres for periods of up to 230 days or to air containing 5 ppm SO1for 99 days. Exposure to air containing 10 ppm NO, gave decreasing recoveries of PAH with time. It has been established that the reactivity of PAH under these conditions is anthanthrene > benzo(a)pyrene > benzo(ghi)perylene > benz(a)anthracene z pyrene z benzo(e)pyrene > chrysene > fluoranthene > phenanthrene N coronene. Evidence is presented which indicates that even in the absence of photolysis or ozone that PAH in ambient NO, polluted air can be converted to nitro derivatives. These nitration reactions will reduce the carcinogenic content of polluted air, as the 6-nitro-~nzo(a)pyrene, 7nitro~nz~a)anthracene and nitrochrvsene derivatives that are formed, in contrast to the parent hydrocarbons: are non-carcinogenic. _

INTRODUCTION Particie size distribution studies carried out in the U.K., Canada and the U.S.A. (Butler and Crossley, 1979 ; Pierce and Katz, 1975 ; Demaio and Corn, 1966) show that polycyclic aromatic hydr~arbons (PAH) present in urban and suburban atmospheres are primarily associated with aerosol soot particles in the sub-micron size range. Particles of this size will have atmospheric life-times of weeks rather than hours or days and if, in addition, strong sunlight is absent, then photochemical degradation of these compounds will not occur. Under these conditions long range transport of aerosols becomes feasible without adsorbed material undergoing significant chemical charge. Such situations have recently been reported by Bjorseth et al. (1979) in which the total PAH concentration monitored during February in Norway and Sweden of an air mass that had passed over the U.K. were comparable to those found in U.K. atmospheres. In this instance poor dispersion and low U.V. light intensity giving minimal photochemi~a~ activity have preserved the original aerosoI characteristics during its passage over the North Sea. In contrast, in the U.S.A. during summer 60% degradation of benzo(a)pyrene within 40 min has been reported (Thomas et ni., 1968). On an annual basis, however, climatic conditions in Northern Europe will not be so conducive for the degradation of PAH by photolysis or interaction with ozone. The absence of these removal processes for PAH poses questions about their atmospheric life-times and focuses attention on alternative chemical reactions likely to lead to their destruction, especially during winter. Other aerosol components of polluted atmospheres that could conceivably react with PAH are the oxides of nitrogen and sulphur together with their acid derivatives. 91

Since little information appears in the literature on the stability of PAH adsorbed on soot in the presence of low concentrations of SO, and NO, we considered that the preliminary investigation outlined in this paper should be undertaken. EXPERIMENTAL (a) Production of polycyclic aromatic hydrocarbons adsorbed on soot Soot/PAH was generated in a flame from an ethene-airoxygen-nitrogen gas mixture. Gas compositions employed are shown in Table 1. The soot/PAH produced was collected in a conical nexus 0.65 m high tapering from a base dia. of 0.064 m to a throat dia. of 0.013 m ~sjtioned over the burner (Page and Ates, 1978). (b) Method of exposure Between (6 and 7) x lo-“ kg soot deposit from the nexus having a surface area of 8.2 x 10’ mz kg-’ were transferred to a Pyrex glass saucer shaped boat that was placed inside a reaction chamber of volume 0.34dm3. This reaction chamber was constructed from a 55/44 Pyrex Quickfit ground-glass joint so that soot samples could readily be removed from the chamber at the required time intervals. The chamber was connected to an air pump (C. Austin, Dymax Mk Ii) operating at 2.83 x 10-l dm’ s-’ and a thermostatted permeation tube. This system was capable of giving a calibrated and controlled flow of air containing either SO2 or NO, through the reaction chamber during the experiment.

Table 1. Gas composition fed to the burner to generate soot in the nexus _Burner gas Percentage Percentage feed dm3 s-* by volume by weight Oxygen Nitrogen Ethene Air

7.5 x 5.0 x 1.0 x 1.0 x

1o-3 10-S 1o-Z 1o-2

29 40 31

33 38 29

92

J. D. BI!~IXKand P. CROSSLI~Y

(c) Preparation

ofpermeation tubes

(i) Stojch~ometric quantities ofnitric oxide (BOC) and air were blended and condensed at liquid nitrogen temperature to give a pale blue solid of dinitrogen trioxide: 2N0 + $0, + N,O,. The solid was allowed to melt to a deep blue liquid which was used to fill a permeation tube sealed at both ends by steel ballbearings. (ii) Sulphur dioxide (BDH) supplied in a cylinder was used to fill a permeation tube sealed in the same manner. The tubes were thermostatted and weighed periodically to determine their leakage rates. (d)

method o~a~alysjs a~po~yc~~c~ie aromatic hydrocarbon

PAHs were analysed by accurately weighing between (2.0 and 4.0) x lo-’ kg soot. These soot samples were removed from the reaction chamber at the desired time intervals and placed in a flask containing 1.5 x 10m2dm3 dimethyl sulphoxide. After heating in a boiling water bath for 6 h they were cooled and centrifuged to separate the soot from the dimethyl sulphoxide extract. This extract solution was treated with an equal volume of water and then extracted ( x 4) with n-pentane. The n-pentane extracts were combined and evaporated to dryness at room temperature. The residue was dissolved in 1.0 x low3 dm3 cyclohexane and chromatographed on a 0.12 m activated alumina column using cyclohexane as eluant. The fractions collected from the column were analysed by scanning the U.V. spectrum in the range 200-450 nm with a Unicam SP 800. RESULTS

Soot samples taken from the nexus had varying PAH content. Although the soot was well mixed before

being placed in the reaction chamber, sample inhomogeneity caused some variation in the concentration of individual PAHs measured. To assist in the interpretation of the results, standard deviation and limits expressed at the 10% confidence level have been quoted in the tables of results. Table 2 summarizes results of the analysis of PAH on soot after exposure to laboratory air and air containing 5 ppm SOz for periods up to 231 days and 99 days, respectively. In the case of the SO2 experiment Table 2. Concentrations “__ _. _.~__.___--

PAH

the gas stream entering the reaction chamber passed over a thimble of dia. 1 x 10-’ m fused to the wall of the chamber which contained 2.5 x 10W3dm” water. This was done because Novakov et al. (1974) have shown that SO, can be converted into sulphuric acid in the presence of water vapour, air and soot. The soot acts as a catalyst and in attempting to reproduce natural conditions as far as possible interaction of PAH with sulphuric acid must be represented. Table 3 shows the effect of air containing 10 ppm NO,. In this experiment, decreasing concentrations of PAH were recovered after 5, 12. 2429 and 51 days of exposure. The results are reported in terms of firstorder rate constants for the disappearance of PAH along with an estimate of half-life in days under these conditions. DISCUSSION

The concentration of 5 ppm SO* used in this work corresponds to the threshold limit value (TLV) for occupational exposure and is certainly in excess ofthat normally encountered in air pollution episodes. The point, however, is that by analogy with our results the PAH/soot system appears relatively stable towards ambient air and air containing 5 ppm SOz. It seems unlikely on this basis that sulphonation of PAH occurs or that a derivative is obtained which initiates molecular degradation, The TLVs of 25 ppm NO and 5 ppm NOz, on the other hand, are much higher that the highest NO, values found in urban atmospheres of around 1 ppm in close proximity to motor traffic (Butler, 1979). In these conditions 80-90% will comprise NO and the remainder NOz. The air flow rate over the permeation tube used in this work gave NO, concentrations between the TLV and the maximum values found in heavily polluted atmospheres. Such concentrations will exceed typical ambient values and, furthermore, the NO to NOz ratio derived from the dinitrogen

of polycyclic aromatic hydrocarbons adsorbed on soot particles after exposure to air and air containing 5 ppm SO2

-.--._- ~~~~

___.___

Concentration of PAH analysed, mg kgg ‘, after 0, 3, 14, 20, 34, 52, 143 and 231 days exposure to laboratory air Mean concentration

d

II

timitf ‘,, Mean ”

Concentration of PAH analysed, mg kg- r+ after 0, 12, 19, 27, 49, 66, 87 and 99 days exposure to air containing 5 ppm SOZ Mean concentration

Phenanthrene 32k 18 26 8 25 123 & 29 Fluoranthene 2441 * 222 310 7 9 4530 + 354 Pyrene 4717 ?; 439 625 8 9 7794 + 658 Chrysene 212 & 34 41 6 16 438 + 71 Benz(a)anthracene 671 _t 145 206 8 22 1172+424 Benzo~a)pyrene 1272 + 117 166 8 9 1246 & 249 Benzo(e)pyrene 812 rf: 93 141 8 11 956 k 226 Anthanthrene 1130 Ifr99 129 7 9 1209 k 245 Benzo(ghi)perylene 2681 & 184 237 I 7 2907 k 540 Coronene 688 + 149 212 8 22 696 & 137 ._ .__~ g denotes standard deviation and N the number of determinations performed.

0

n

L’mlts 0 Mean ”

41 496 921 79 536 314 3t7 342 682 192

8 8 8 6 1 7 8 R 7 8

24 8 8 16 36 20 24 70 19 20

._.~~... .~_

.---~-~-

93

Reactivity of polycyclic aromatic hydrocarbons

Table 3. Concentration of PAH in mg kg -* recovered from soot samples after exposure to air containing 10 ppm NO, as a function of time with estimates of half-life time in days Time, days PAW

0

_.^_ Phenanthrene Coronene Fluoranthene Chrysene Benzo(e)pyrene Pyrene Benz(a)anthracene Benzo(ghi)perylene Benzo(a)pyrene Anthanthrene

244 620 7443 443 1108 7743 1355 2931 1557 1209

5

12

21

25

39

29

51

71 122 195 183 198 137 154 339 108 584 417 360 322 322 6327 4987 3573 3498 5210 2754 3573 151 360 195 359 268 251 242 188 931 886 609 432 565 355 6503 3692 2693 2286 1323 1084 144 163 78 1003 409 434 203 312 88 1688 993 351 342 513 347 52 31 683 436 187 98 247 0 0 435 145 24 24 24 302 161

trioxide in the permeation tube will be richer in NOz than that which normally obtains in air. Nevertheless, the use of N,OJ is very convenient experimentally and this advantage we consider to out-weigh the disadvantages, at least for these preliminary experiments, to justify its use. On this basis the results given in Table 3 should be analogous to the behaviour of a heavily polluted ambient aerosol containing PAH adsorbed on soot particles. The information given in Table 3 shows that individual PAH exhibit different half-lives which range from about 3.7 days in the case of anthanthrene to about a month for phenanthrene and coronene. These half-life times when arranged systematically from the shortest to the longest correlate with the localization energy or reactivity number for the position of substitution of the PAH ring system by electrophilic reagents (Dewar, 1952; Dewar et al., 1956a, b) and partial rate factors for nitration (Dewar et al., 1956~). Thus, the reactivity order found for PAH exposed to air/NO, atmospheres in this study and the correlation with Dewar’s work strongly suggests that nitro derivatives have been formed at the ring positions of the parent PAH denoted in Table 4. As these positions are specifically susceptible to nitrations under the mild room temperature conditions that have been employed, it is reasonable to believe that these nitration reactions can take place in

Rate constant, k,day-’

Standard deviation

0.023 f 0.009 0.024 + 0.006 0.026 i_ 0.007 0.027 * 0.018 0.029 i 0.005 0.050 + 0,006 0.064 + 0.013 0.082 + 0.015 0.101 f 0.023 0.187 f 0.033

0.011 0.008 0.009 0.014 0.006 0.008 0.016 0.019 0.030 0.042

Phenanthrene Coronene Fluoranthene Chrysene Benzo(e)pyrene Pyrene Benz(a)anthracene Benzo(ghi)peryIene Benzo(a)pyrene Anthanthrene

30 29 27 26 24 14 11 8 7 3.7

ambient aerosols. Obviously, the higher the NO, concentration in polluted air the greater the likelihood of the conversion of PAH into nitro derivatives. We suggest, therefore, on this evidence that nitration can be an effective route for the removal of PAH from the atmosphere. In view of this suggestion it is pertinent to enquire into the carcinogenicity of these nitro derivatives, because if they are non-carcinogenic, then an aged aerosol in polluted air should not be as hazardous as one newly formed. The survey carried out by the U.S. Department of Health, Education and Welfare lists (1970-71) 6-nitrobenzo(a)pyrene, 7nitrobenz(a)anthracene, nitropyrene and nitrochrysene as non-carcinogenic. Hence, we conclude that even in the absence of photolytic degradation of atmospheric PAH a chemical route still exists in polluted atmospheres which converts benzo(a)pyrene into the non-carcinogenic 6-nitro derivative. Acknowledgements - We thank the Department of the Environment through the Transport and Road Research Laboratory, the apartment of Health and Social Security for a research studentship to P.C. and the Health and Safety Directorate of the Commission of the European Communities, Luxembourg, for supporting this work. We are also indebted to Dr. C. E. Searle of the Department of Cancer Studies, The Medical School, Birmingham, for discussion and advice on the carcinogenicity of the nitro compounds mentioned in this paper.

Table 4. Comparison of half-life times of polycyclic aromatic hydrocarbons adsorbed on soot with reactivity number at denoted ring positions and partial rate factors for nitration referred to benzene as unity PAH

Half-life time, days

Position in ring system

Reactivity number

10 1

1.79 1.80

6 3 1 7

1.67 1.63 1.51 1.35

6 1

1.15 1.03

Haff-hfe time, days 30 29 27 27 24 14 11 8 7 3.7

Partial rate factor 490 11.50 3500 17,000 108,000 156,GQO

J. D. B~II-t.tn and P. Cnoss~.r:v

94 REFERENCES

Bjorseth A., Lunde G. and Lindskog A. (1979) Long-range transport of poiycyciic aromatic hydrocarbons. Atmospheric Environment 13, 45-53. Butler J. D. (1979) Air Pollufion Chemistry. Academic Press, London, p. 34. Butler J. D. and Crossley P. (1979) An appraisal of relative airborne sub-urban concentrations of poiycyclic aromatic hydrocarbons monitored indoors and outdoors. Sci. Totul Envir. 11, 53358. Demaio L. and Corn M. (1966) Poiynuciear aromatic hydrocarbons associated with particuiates in Pittsburgh air. J. Air ~~~~uf. Control Ass. 16, 67. 71‘ Dewar M. J. S. (1952) A molecular theory of organic chemistry. VI--aromatic substitution and addition. J. Am. them. sot. 74, 3357-3363. Dewar M. J. S., Mole T., Urch D. S. and Warford E. W. T. (1956a) Eiectrophiiic substitution. Part IV. The nitration of diphenyi, chrysene, benzo(a) pyrene and anthanthrene. J. them.

Sot.,

3.5%

3576.

Dewar M. J. S., Mole T. and Warford E. W. T. I 1956b) Eiectrophiiic substitution, Part Y. Competitive nitration 3. them. SOL.., 3576 -3580. Dewar M. J. S.. Mole T. and Warford E. W. ‘I, (lU56c) Eiectrophiiic substitution. Part VI. The nitration of aromatic hydrocarbons; partial rate factors and their interpretation. J. them. Sot.. 3581 3586. Novakov T., Chang S. G. and Harker A. B. (1974) Sulfates as pollution particuiates; catalytic formation on carbon (soot) particles. Science. N.Y. 186, 259 261. Page F. M. and Ates F. (1978) Evaporation combustion of fuels, In Advuttcrs in Chemisrry Series 166 (Edited by G. T. Zong), Am. them. Sot,, Washington, DC, p. 190. F’ierce R. C. and Katz M (1975) Dependency of polynu~i~r aromatic hydrocarbon content on size distribution of atmospheric aerosols. Em+. Sci. Trchnol. 9, 347 353. Thomas J. F., Mukai M. and Tebben B. D. (1968) Fate of airborne henzo(a)pyrene. Encir. Sci. Technol. 2, 33 39. U.S. Department of Health, Education and Welfare (1972) Survey of compounds which have been tested for carcinogenic activity, 1970. 71,