- Email: [email protected]
Distribution of n-alkanes and PAHs in atmospheric aerosols
Atmospheric Research 46 Ž1998. 223–231
Distribution of n-alkanes and PAHs in atmospheric aerosols A. Gelencser ´ a b
a,)
´ Molnar , T. Barcza b, Gy. Kiss a , A. ´ a, J. Hlavay b, E. Meszaros ´ ´ b
Air Chemistry Group of the Hungarian Academy of Sciences, P.O. Box 158, H-8201 Veszprem, ´ Hungary Department of Analytical Chemistry, UniÕersity of Veszprem, ´ P.O. Box 158, H-8201 Veszprem, ´ Hungary
Abstract The concentrations of n-alkanes and polycyclic aromatic hydrocarbons ŽPAHs. are determined in atmospheric aerosol samples collected at a rural sampling site in Hungary. For the n-alkanes the chromatographic profiles are established and the average carbon number and carbon preference index ŽCPI. are calculated. An attempt is made to obtain the origin of n-alkanes found in atmospheric aerosol samples. Based on the results of the measurements the probable importance of a round-the-year biogenic source for the n-alkanes with CPIs close to unity is emphasized. q 1998 Elsevier Science B.V. Keywords: n-Alkanes; PAH; Particle origin; Carbon preference index; Biogenic source
1. Introduction Although organic compounds constitute a significant fraction of atmospheric aerosol particles, their inventory is far from being complete. In contrast, semi-volatile organic compounds in atmospheric aerosol particles have been thoroughly characterized by chromatographic techniques ŽAceves and Grimal, 1993; Leuenberger, 1988; Radzi Bin Abas and Simoneit, 1996; Sicre et al., 1987, etc... The majority of studies on these topics focus on the determination of potentially hazardous compounds, mostly from urban samples. The substances most frequently analyzed are alcohols, n-alkanes, aldehydes, carboxylic acids, pesticides, phenols, polychlorinated biphenyls and polycyclic aromatic hydrocarbons. Aliphatic and aromatic hydrocarbons together are particularly preferred compounds of such studies. n-Alkanes are especially good candidates for studies directed more to the origin and )
Corresponding author.
0169-8095r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 8 0 9 5 Ž 9 7 . 0 0 0 6 4 - 1
224
A. Gelencser ´ et al.r Atmospheric Research 46 (1998) 223–231
fate of atmospheric aerosol than in the compounds themselves. They are present in all aerosol samples irrespective whether urban, continental or maritime aerosols are observed. Moreover, they can be derived from both biogenic and anthropogenic sources, which can be differentiated from their distribution. Although their mass is undoubtedly a minor fraction of the total organic matter of atmospheric aerosols, their features referred to above and their relatively low reactivity and low volatility entitle them to be worth monitoring as tracers of both atmospheric transport and particle origin. A key parameter associated with n-alkanes is the well-established carbon preference index ŽCPI. which considers the relative abundance of compounds containing odd and even number of carbon atoms ŽTissot and Welte, 1984.. This parameter is known to indicate the source of n-alkanes in the atmosphere. According to a simplified model ŽSchneider et al., 1983. in the range of C 19 –C 35 , CPI ) 5 indicates natural sources Žprimarily cuticular waxes of higher plants., whereas CPI f 1 implies anthropogenic influences Žcombustion of fossil fuels.. Chromatograms of the latter also show a characteristic feature: an unresolved complex mixture ŽUCM. in the range of C 16 –C 36 , which may represent a major fraction of the hydrocarbons present in the sample ŽRadzi Bin Abas and Simoneit, 1996.. It was shown, however, that the lack of odd-to-even carbon number preference may not necessarily be the sign of anthropogenic origin: n-alkanes from the marine biota also exhibit CPIs close to unity, though at a much lower concentration level ŽWarneck, 1988.. By mapping the size distribution of n-alkanes in a background sampling site it was found that the n-alkanes originating from higher plant sources preferentially turn up in the larger size fraction ŽSicre et al., 1987.. On the contrary, n-alkanes of anthropogenic origin favour the fine particles. These observations are reinforced by the mass median diameter ŽMMD. calculated from normalized cumulative distributions. Terrestrial n-alkanes show high values of MMD Ž1.19–2.63 m m. confirming that the origin of these compounds is related to the emission of larger particles from cuticular waxes of higher plants or soil dust. On the other hand, MMD values for anthropogenic n-alkanes are less than 1 m m. This implies the initial emission of these compounds into the vapour phase followed by condensation on sub-m m particles. The other group of compounds, the polycyclic aromatic hydrocarbons ŽPAHs. originate almost exclusively from anthropogenic sources, mostly from combustion processes. Their importance is their toxicity and persistence in the environment. It is their absolute concentration and, to a lesser extent, the relative ratio of some selected compounds which may be indicative of their atmospheric transport and origin. The objective of the present paper is to trace the origin of n-alkanes associated with atmospheric aerosol particles on the basis of some key parameters and to reveal the relative share of a biogenic source for these compounds of no odd-to-even carbon number preference in atmospheric aerosol particles. 2. Experimental details 2.1. Sampling Aerosol samples were collected weekly with a Whatman GFrF filter of 0.45 m m on top of a hill in Tihany, at Lake Balaton, Hungary, starting from June 1995. The
A. Gelencser ´ et al.r Atmospheric Research 46 (1998) 223–231
225
sampling period was 48 h, at a flow rate of 20 lrmin. For most part of the year the sampling site can be considered rural, but during the summer local transport sources may intervene. In winter, samples were also collected with a Berner-type 9-stage cascade impactor on top of a five-storey building in Veszprem, ´ Hungary. It is considered to be an urban sampling location. The cutoff diameters Žat 50% collection efficiency. were Žfrom the first stage.: 0.0625, 0.125, 0.25, 0.5, 1, 2, 4, 8 and 16 m m. 2.2. Analysis of n-alkanes The filters were extracted in an ultrasonic bath 3 times with 10.0 ml of n-hexane for 15 min. The extracts were cleaned up on a 500-mg silica cartridge and then evaporated to 1.0 ml. After the addition of 20 m l of internal standard solution Ž n-undecane in n-hexane., 1.0 m l was injected into the gas chromatograph connected to a mass spectrometer in splitless mode. The column was a 30 m SPB-1 fused silica capillary of 0.32 mm i.d. and 0.25 m m film thickness. The temperature program applied was 408C from 1 min, then 88Crmin to 3008C, held for 10 min. The mass spectrometer was operated in selected ion recording ŽSIR. mode, monitoring ions of 57 and 85 amu. 2.3. Analysis of PAHs The samples were extracted successively with 8 ml of methanol and 8 ml of dichloromethane repeated 3 times. The extract was purified on a C18 solid phase extraction ŽSPE. cartridge and evaporated to 500 m l, then transferred to methanol. A total of 20 m l of the purified extract was injected into a Waters HPLC system equipped with a 470 scanning fluorescence detector. The column was a polymeric type LiChrospher PAH, 250 mm = 4 mm, 5m m. Gradient elution was used from 40% acetonitrile in water to 100% acetonitrile.
3. Results and discussion 3.1. Summer samples Samples taken in Tihany during spring and summer have been analyzed for n-alkanes. From the concentrations of individual compounds the total concentration of n-alkanes, the carbon preference index ŽCPI. and the mass median average carbon number Ž Nmax . have been calculated. These parameters are shown in Table 1, with reference to the date of sampling. As it can be seen from the table, the parameters varied considerably during the sampling period. The most significant changes were observed in the CPI which fluctuated within the range of 1.09–6.79. As for the Nmax parameter, the variations are less pronounced, though, considering that it is an average quantity, they represent a significant shift in the distribution of individual n-alkanes Ž Nmax 24.1–28.8.. The total
A. Gelencser ´ et al.r Atmospheric Research 46 (1998) 223–231
226
Table 1 The calculated total concentration of n-alkanes and their carbon preference index ŽCPI. and mass median carbon number Ž Nma x . in the summer samples No.
Date
Ctotal wngrm3 x
CPI
Nmax
1 2 3 4 5 6 7 8 9 10 11 12
12–14 06 95 19–21 06 95 26–28 06 95 21–23 06 95 13–15 07 95 27–29 07 95 07–09 08 95 21–23 08 95 04–06 09 95 18–20 09 95 02–04 10 95 16–18 10 95
29.2 41.6 14.6 18.9 24.7 34.6 19.3 31.0 23.9 34.4 26.2 18.7
1.09 1.15 4.97 6.79 1.54 1.79 2.01 1.47 1.98 2.33 1.67 2.12
24.1 24.3 28.1 28.8 26.1 26.3 27.3 25.6 27.2 27.5 24.8 26.9
concentrations varied somewhat with the parameters: the lowest concentrations were found in samples with the highest CPI and Nmax and vice versa. It must be added that the variations remained within a factor of 3 and the total mass of aerosol particles was not measured. From the series of observations, three distinct types of distributions can be separated on the basis of the parameters. These types correspond to samples for which meteorological conditions remained constant over the period of sampling. The chromatographic traces of these distributions are shown in Fig. 1 where the carbon numbers of the homologues are also indicated.
Fig. 1. Selected ion chromatograms of summer samples reflecting different types of n-alkane distribution.
A. Gelencser ´ et al.r Atmospheric Research 46 (1998) 223–231
227
The upper chromatogram of sample 4 clearly shows a strong odd-to-even carbon number predominance as reflected in CPI s 6.79. The Nmax is the highest for this type of distribution, 28.8. All of these parameters are known to indicate biogenic sources, the emission of n-alkanes from cuticular waxes. It corresponds to what can be expected at a rural sampling site in the growth season. Much doubt arise, however, about the origin of the n-alkane distribution of sample 1 shown on the middle chromatographic trace. As compared to the top chromatogram, its mass centre is shifted towards lower carbon number compounds Ž Nmax s 24.1., even below C 25 which have rarely been observed in summer aerosol samples elsewhere ŽSicre et al., 1987.. Its CPI is close to unity, the slight predominance of odd carbon number compounds are largely due to the biogenic contribution among the higher homologues which can be observed at the end of the chromatogram. The bottom chromatogram shows a distribution Žsample 5., the origin of which is fairly obvious. The main characteristics of this profile is the presence of an unresolved complex mixture of aliphatic hydrocarbons ŽUCM. among which the straight-chain homologues are sometimes difficult to identify. By considering only the n-alkanes the mass centre of the chromatogram Ž Nmax . is at carbon number 26.1, contrasting with those of the other two types. This finding corresponds to the results obtained in urban air ŽAceves and Grimal, 1993.. The total concentration of n-alkanes may be misleading since there are many other aliphatic hydrocarbons in the UCM at much higher concentrations which have not been taken into account. Overall, this chromatogram is a typical example of hydrocarbons of petrogenic origin, closely resembling the chromatographic traces of vehicular exhaust. This is understandable because the sample in question was collected in the summer when local traffic is significant. As far as the frequency of the occurrence of these distributions are concerned, they were rarely encountered in pure form. In most cases a sort of a mixture of the upper two profiles were obtained, as reflected in the parameters CPI and Nmax in Table 1. This is presumably due to the relatively long period of sampling during which the different effects were averaged. Two contributions were clearly identified but the question about the third remains to be answered. 3.2. Winter samples The uncertainty about the origin of one n-alkane distribution calls for a more thorough study outside the growth season, involving the simultaneous determination of n-alkanes and polycyclic aromatic hydrocarbons ŽPAHs. as well as of the particle size distribution of n-alkanes, though the latter at a different sampling location. The reasons for these experiments are the following: outside the growth season, when the majority of the higher plants are dormant, the contribution of the known biogenic sources of n-alkanes should be negligible. On the other hand, anthropogenic n-alkane production increases since domestic heating becomes important. Altogether, one would expect n-alkane distributions with CPIs close to unity and positive correlation between the total concentrations of n-alkanes and PAHs. The particle size distribution can also refer to the origin of the compounds studied: the compounds of anthropogenic origin are accumulated on finer particles whereas those from biogenic sources, if they are present, favour
A. Gelencser ´ et al.r Atmospheric Research 46 (1998) 223–231
228
Table 2 The calculated total concentration of PAHs and n-alkanes and the carbon preference index ŽCPI. and mass median carbon number Ž Nma x . of n-alkanes in the winter samples No.
Date
Ctotal PAHs wngrm3 x
Ctotal n-alkanes wngrm3 x
CPI
Nmax
1 2 3 4 5 6 7 8 9 10 11 12
20–22 11 95 04–06 12 95 11–13 12 95 28–20 12 95 08–10 01 96 15–17 01 96 22–24 01 96 29–31 01 96 12–14 02 96 19–21 02 96 26–28 02 96 04–06 03 96
33.3 55.4 14.2 18.1 8.9 71.1 26.7 10.6 67.5 39.9 20.2 14.7
19.1 22.4 35.7 42.8 41.0 20.3 36.2 48.9 17.1 24.6 37.8 33.9
1.37 1.28 1.19 1.16 1.28 1.43 1.25 1.35 1.11 1.17 1.34 1.12
24.5 24.5 24.5 24.3 24.8 24.7 24.4 24.9 24.8 24.5 25.0 24.3
the coarser particles. The total concentrations of n-alkanes and PAHs and the corresponding indices are summarized in Table 2. In accordance with expectation, the distribution of n-alkanes in all samples is remarkably uniform, the Nmax values vary little, between 24.3 and 25.0 and the CPI values are in all cases below 1.50, presumably reflecting some emission of coniferous plants. The characteristic chromatographic trace is shown in Fig. 2. It is worth noting that the peaks showing slight odd-to-even carbon number predominance are shifted to lower homologues either due to the different emission sources or the
Fig. 2. The typical selected ion chromatogram of winter samples.
A. Gelencser ´ et al.r Atmospheric Research 46 (1998) 223–231
229
Fig. 3. The total concentrations of PAHs plotted as a function of the corresponding total concentrations of n-alkanes in the case of winter samples.
reduced volatilization effect at lower temperatures. The higher share of homologues below C 25 , as reflected in the Nmax values, can also be explained by this effect, though the continued activity of unidentified sources cannot be excluded, either. The lack of UCM in the chromatogram may also support the latter hypothesis. To prove that the predominant source is of anthropogenic origin, the total concentrations of n-alkanes are plotted against those of the PAHs, as shown in Fig. 3. As it is fairly obvious from the figure, no positive correlation was found between the two quantities, even some inverse tendencies can be observed. Should the n-alkanes derive overwhelmingly from anthropogenic sources, one would expect positive correlation between their total concentration and the total concentration of PAHs which are solely of anthropogenic origin. Instead, the largest n-alkane concentrations were found in the samples in which the concentrations of PAHs were at their minimum. On the other hand, for samples in which high PAH concentrations were detected, the concentration of n-alkanes were found to be quite low. This observation may imply that there may be a major biogenic source for n-alkanes in winter in the Northern Hemisphere. In order to support this hypothesis, samples were taken in Veszprem ´ with a Berner-type cascade impactor. As an example, the results of the analysis of 02–05 February 1996 are presented in Table 3. In harmony with the filter samples, no significant increase of CPIs can be observed with particle size. Similarly, the Nmax values do not show any particular tendency with
230
A. Gelencser ´ et al.r Atmospheric Research 46 (1998) 223–231
Table 3 The calculated total concentration of n-alkanes and their carbon preference index ŽCPI. and mass median carbon number Ž Nma x . in the fractions of a cascade impactor sample taken between 02–05 February 1996 Fraction
Cut-offw m mx
Ctotal n-alkanes wngrm3 x
CPI
Nmax
1 2 3 4 5 6 7 8
0.0625 0.125 0.25 0.5 1 2 4 8
12.4 1.5 2.3 3.4 2.6 6.1 9.9 6.6
1.52 0.93 0.95 1.10 1.05 1.17 1.21 1.27
24.4 24.8 23.5 24.2 24.5 24.7 25.4 24.8
particle size. The total concentrations of n-alkanes in the fractions, though, varied with particle size. The majority of the n-alkanes were found in the smallest and the three largest particle size fractions, exhibiting a bimodal distribution. It is worthy of note that similar distributions of n-alkanes were found over the Mediterranean—though at a lower concentration level ŽSicre et al., 1987.. In the smallest size fraction compounds of anthropogenic origin are expected to be abundant whereas larger fractions contain, to a larger extent, biogenic n-alkanes. These results of urban aerosol may also reinforce the previous assumption that biogenic sources play an important role in n-alkane production even outside the growth season. One might expect that if biogenic n-alkanes dominate over their anthropogenic counterparts in urban aerosol close to the points of emission, then the overall terrestrial biogenic production of n-alkanes would dominate their anthropogenic emission by orders of magnitude. It may also prove to be a viable explanation for the lack of positive correlation between the total concentrations of n-alkanes and PAHs as was found on the filter samples at the rural sampling site. Moreover, the same may account for the unidentified source of n-alkanes during the growth season. Yet at least one question remains to be answered: what can be this significant biogenic source of n-alkanes which is active throughout the year? Obviously, it cannot be the well-known mechanism involving higher plant waxes since it produces n-alkanes with odd-to-even carbon number preference. But are there microorganisms which are active throughout the year and are capable of producing n-alkanes without odd carbon number predominance during the decay process? The answer may lie in a recent discovery in organic geochemistry, namely that the odd carbon number predominance of n-alkanes diminishes during the decay of organic matter. It has been found that many types of bacteria and fungi producing even carbon number n-alkanes below C 22 also biosynthesize n-alkanes with no carbon number preference in the C 23 –C 34 range ŽGrimalt et al., 1986; Blazso´ and Schulten, 1990.. Although these findings refer to the study of low rank coals and aerobic sediments, it may also apply to other decomposition processes. Nevertheless the observations with atmospheric aerosol seem to support such a hypothesis. Further studies are required both on emission and aerosol formation to prove that decay processes contribute significantly to the overall production of n-alkanes. In addition, such studies may also prove to be important contributions to aerosol science.
A. Gelencser ´ et al.r Atmospheric Research 46 (1998) 223–231
231
Acknowledgements The authors are indebted to the OTKA F 0116272, F016262 and the Hungarian Academy of Sciences for their financial support.
References Aceves, M., Grimal, J.O., 1993. Atmos. Environ. 2, 51. Blazso, ´ M., Schulten, H.R., 1990. Pyrolysis-field ionization mass spectrometry of low rank coals. Org. Geochem. 1, 87–99. Grimalt, J., Albaiges, ´ J., Alexander, G., Hazai, I., 1986. Predominance of even carbon-numbered n-alkanes in coal seam samples of Nograd Basin ŽHungary.. Naturwissenschaften 73, 729. Leuenberger, C., 1988. Aliphatic and polycyclic aromatic hydrocarbons in urban rain, snow and fog. Atmos. Environ. 4, 695. Radzi Bin Abas, M., Simoneit, B.R.T., 1996. Composition of extractable organic matter of air particles from Malaysia: initial study. Atmospheric Environ. 15, 2779–2793. Schneider, J.K., Gagosian, R.B., Cochran, J.K., Trull, T.W., 1983. Particle size distribution of n-alkanes and 210 Pb off the coast of Peru. Nature 304, 429–432. Sicre, M.A., Marty, J.C., Saliot, A., Aparicio, X., Grimalt, J., Albaiges, J., 1987. Aliphatic and aromatic hydrocarbons in different sized aerosol over the Mediterranean sea: occurrence and origin. Atmos. Environ. 10, 2247–2259. Tissot, B.P., Welte, D.H., 1984. Petroleum Formation and Occurrence, Springer, Berlin, p. 101. Warneck, H., 1988. Chemistry of the Natural Atmosphere, Academic Press, New York.
Distribution of n-alkanes and PAHs in atmospheric aerosols A. Gelencser ´ a b
a,)
´ Molnar , T. Barcza b, Gy. Kiss a , A. ´ a, J. Hlavay b, E. Meszaros ´ ´ b
Air Chemistry Group of the Hungarian Academy of Sciences, P.O. Box 158, H-8201 Veszprem, ´ Hungary Department of Analytical Chemistry, UniÕersity of Veszprem, ´ P.O. Box 158, H-8201 Veszprem, ´ Hungary
Abstract The concentrations of n-alkanes and polycyclic aromatic hydrocarbons ŽPAHs. are determined in atmospheric aerosol samples collected at a rural sampling site in Hungary. For the n-alkanes the chromatographic profiles are established and the average carbon number and carbon preference index ŽCPI. are calculated. An attempt is made to obtain the origin of n-alkanes found in atmospheric aerosol samples. Based on the results of the measurements the probable importance of a round-the-year biogenic source for the n-alkanes with CPIs close to unity is emphasized. q 1998 Elsevier Science B.V. Keywords: n-Alkanes; PAH; Particle origin; Carbon preference index; Biogenic source
1. Introduction Although organic compounds constitute a significant fraction of atmospheric aerosol particles, their inventory is far from being complete. In contrast, semi-volatile organic compounds in atmospheric aerosol particles have been thoroughly characterized by chromatographic techniques ŽAceves and Grimal, 1993; Leuenberger, 1988; Radzi Bin Abas and Simoneit, 1996; Sicre et al., 1987, etc... The majority of studies on these topics focus on the determination of potentially hazardous compounds, mostly from urban samples. The substances most frequently analyzed are alcohols, n-alkanes, aldehydes, carboxylic acids, pesticides, phenols, polychlorinated biphenyls and polycyclic aromatic hydrocarbons. Aliphatic and aromatic hydrocarbons together are particularly preferred compounds of such studies. n-Alkanes are especially good candidates for studies directed more to the origin and )
Corresponding author.
0169-8095r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 8 0 9 5 Ž 9 7 . 0 0 0 6 4 - 1
224
A. Gelencser ´ et al.r Atmospheric Research 46 (1998) 223–231
fate of atmospheric aerosol than in the compounds themselves. They are present in all aerosol samples irrespective whether urban, continental or maritime aerosols are observed. Moreover, they can be derived from both biogenic and anthropogenic sources, which can be differentiated from their distribution. Although their mass is undoubtedly a minor fraction of the total organic matter of atmospheric aerosols, their features referred to above and their relatively low reactivity and low volatility entitle them to be worth monitoring as tracers of both atmospheric transport and particle origin. A key parameter associated with n-alkanes is the well-established carbon preference index ŽCPI. which considers the relative abundance of compounds containing odd and even number of carbon atoms ŽTissot and Welte, 1984.. This parameter is known to indicate the source of n-alkanes in the atmosphere. According to a simplified model ŽSchneider et al., 1983. in the range of C 19 –C 35 , CPI ) 5 indicates natural sources Žprimarily cuticular waxes of higher plants., whereas CPI f 1 implies anthropogenic influences Žcombustion of fossil fuels.. Chromatograms of the latter also show a characteristic feature: an unresolved complex mixture ŽUCM. in the range of C 16 –C 36 , which may represent a major fraction of the hydrocarbons present in the sample ŽRadzi Bin Abas and Simoneit, 1996.. It was shown, however, that the lack of odd-to-even carbon number preference may not necessarily be the sign of anthropogenic origin: n-alkanes from the marine biota also exhibit CPIs close to unity, though at a much lower concentration level ŽWarneck, 1988.. By mapping the size distribution of n-alkanes in a background sampling site it was found that the n-alkanes originating from higher plant sources preferentially turn up in the larger size fraction ŽSicre et al., 1987.. On the contrary, n-alkanes of anthropogenic origin favour the fine particles. These observations are reinforced by the mass median diameter ŽMMD. calculated from normalized cumulative distributions. Terrestrial n-alkanes show high values of MMD Ž1.19–2.63 m m. confirming that the origin of these compounds is related to the emission of larger particles from cuticular waxes of higher plants or soil dust. On the other hand, MMD values for anthropogenic n-alkanes are less than 1 m m. This implies the initial emission of these compounds into the vapour phase followed by condensation on sub-m m particles. The other group of compounds, the polycyclic aromatic hydrocarbons ŽPAHs. originate almost exclusively from anthropogenic sources, mostly from combustion processes. Their importance is their toxicity and persistence in the environment. It is their absolute concentration and, to a lesser extent, the relative ratio of some selected compounds which may be indicative of their atmospheric transport and origin. The objective of the present paper is to trace the origin of n-alkanes associated with atmospheric aerosol particles on the basis of some key parameters and to reveal the relative share of a biogenic source for these compounds of no odd-to-even carbon number preference in atmospheric aerosol particles. 2. Experimental details 2.1. Sampling Aerosol samples were collected weekly with a Whatman GFrF filter of 0.45 m m on top of a hill in Tihany, at Lake Balaton, Hungary, starting from June 1995. The
A. Gelencser ´ et al.r Atmospheric Research 46 (1998) 223–231
225
sampling period was 48 h, at a flow rate of 20 lrmin. For most part of the year the sampling site can be considered rural, but during the summer local transport sources may intervene. In winter, samples were also collected with a Berner-type 9-stage cascade impactor on top of a five-storey building in Veszprem, ´ Hungary. It is considered to be an urban sampling location. The cutoff diameters Žat 50% collection efficiency. were Žfrom the first stage.: 0.0625, 0.125, 0.25, 0.5, 1, 2, 4, 8 and 16 m m. 2.2. Analysis of n-alkanes The filters were extracted in an ultrasonic bath 3 times with 10.0 ml of n-hexane for 15 min. The extracts were cleaned up on a 500-mg silica cartridge and then evaporated to 1.0 ml. After the addition of 20 m l of internal standard solution Ž n-undecane in n-hexane., 1.0 m l was injected into the gas chromatograph connected to a mass spectrometer in splitless mode. The column was a 30 m SPB-1 fused silica capillary of 0.32 mm i.d. and 0.25 m m film thickness. The temperature program applied was 408C from 1 min, then 88Crmin to 3008C, held for 10 min. The mass spectrometer was operated in selected ion recording ŽSIR. mode, monitoring ions of 57 and 85 amu. 2.3. Analysis of PAHs The samples were extracted successively with 8 ml of methanol and 8 ml of dichloromethane repeated 3 times. The extract was purified on a C18 solid phase extraction ŽSPE. cartridge and evaporated to 500 m l, then transferred to methanol. A total of 20 m l of the purified extract was injected into a Waters HPLC system equipped with a 470 scanning fluorescence detector. The column was a polymeric type LiChrospher PAH, 250 mm = 4 mm, 5m m. Gradient elution was used from 40% acetonitrile in water to 100% acetonitrile.
3. Results and discussion 3.1. Summer samples Samples taken in Tihany during spring and summer have been analyzed for n-alkanes. From the concentrations of individual compounds the total concentration of n-alkanes, the carbon preference index ŽCPI. and the mass median average carbon number Ž Nmax . have been calculated. These parameters are shown in Table 1, with reference to the date of sampling. As it can be seen from the table, the parameters varied considerably during the sampling period. The most significant changes were observed in the CPI which fluctuated within the range of 1.09–6.79. As for the Nmax parameter, the variations are less pronounced, though, considering that it is an average quantity, they represent a significant shift in the distribution of individual n-alkanes Ž Nmax 24.1–28.8.. The total
A. Gelencser ´ et al.r Atmospheric Research 46 (1998) 223–231
226
Table 1 The calculated total concentration of n-alkanes and their carbon preference index ŽCPI. and mass median carbon number Ž Nma x . in the summer samples No.
Date
Ctotal wngrm3 x
CPI
Nmax
1 2 3 4 5 6 7 8 9 10 11 12
12–14 06 95 19–21 06 95 26–28 06 95 21–23 06 95 13–15 07 95 27–29 07 95 07–09 08 95 21–23 08 95 04–06 09 95 18–20 09 95 02–04 10 95 16–18 10 95
29.2 41.6 14.6 18.9 24.7 34.6 19.3 31.0 23.9 34.4 26.2 18.7
1.09 1.15 4.97 6.79 1.54 1.79 2.01 1.47 1.98 2.33 1.67 2.12
24.1 24.3 28.1 28.8 26.1 26.3 27.3 25.6 27.2 27.5 24.8 26.9
concentrations varied somewhat with the parameters: the lowest concentrations were found in samples with the highest CPI and Nmax and vice versa. It must be added that the variations remained within a factor of 3 and the total mass of aerosol particles was not measured. From the series of observations, three distinct types of distributions can be separated on the basis of the parameters. These types correspond to samples for which meteorological conditions remained constant over the period of sampling. The chromatographic traces of these distributions are shown in Fig. 1 where the carbon numbers of the homologues are also indicated.
Fig. 1. Selected ion chromatograms of summer samples reflecting different types of n-alkane distribution.
A. Gelencser ´ et al.r Atmospheric Research 46 (1998) 223–231
227
The upper chromatogram of sample 4 clearly shows a strong odd-to-even carbon number predominance as reflected in CPI s 6.79. The Nmax is the highest for this type of distribution, 28.8. All of these parameters are known to indicate biogenic sources, the emission of n-alkanes from cuticular waxes. It corresponds to what can be expected at a rural sampling site in the growth season. Much doubt arise, however, about the origin of the n-alkane distribution of sample 1 shown on the middle chromatographic trace. As compared to the top chromatogram, its mass centre is shifted towards lower carbon number compounds Ž Nmax s 24.1., even below C 25 which have rarely been observed in summer aerosol samples elsewhere ŽSicre et al., 1987.. Its CPI is close to unity, the slight predominance of odd carbon number compounds are largely due to the biogenic contribution among the higher homologues which can be observed at the end of the chromatogram. The bottom chromatogram shows a distribution Žsample 5., the origin of which is fairly obvious. The main characteristics of this profile is the presence of an unresolved complex mixture of aliphatic hydrocarbons ŽUCM. among which the straight-chain homologues are sometimes difficult to identify. By considering only the n-alkanes the mass centre of the chromatogram Ž Nmax . is at carbon number 26.1, contrasting with those of the other two types. This finding corresponds to the results obtained in urban air ŽAceves and Grimal, 1993.. The total concentration of n-alkanes may be misleading since there are many other aliphatic hydrocarbons in the UCM at much higher concentrations which have not been taken into account. Overall, this chromatogram is a typical example of hydrocarbons of petrogenic origin, closely resembling the chromatographic traces of vehicular exhaust. This is understandable because the sample in question was collected in the summer when local traffic is significant. As far as the frequency of the occurrence of these distributions are concerned, they were rarely encountered in pure form. In most cases a sort of a mixture of the upper two profiles were obtained, as reflected in the parameters CPI and Nmax in Table 1. This is presumably due to the relatively long period of sampling during which the different effects were averaged. Two contributions were clearly identified but the question about the third remains to be answered. 3.2. Winter samples The uncertainty about the origin of one n-alkane distribution calls for a more thorough study outside the growth season, involving the simultaneous determination of n-alkanes and polycyclic aromatic hydrocarbons ŽPAHs. as well as of the particle size distribution of n-alkanes, though the latter at a different sampling location. The reasons for these experiments are the following: outside the growth season, when the majority of the higher plants are dormant, the contribution of the known biogenic sources of n-alkanes should be negligible. On the other hand, anthropogenic n-alkane production increases since domestic heating becomes important. Altogether, one would expect n-alkane distributions with CPIs close to unity and positive correlation between the total concentrations of n-alkanes and PAHs. The particle size distribution can also refer to the origin of the compounds studied: the compounds of anthropogenic origin are accumulated on finer particles whereas those from biogenic sources, if they are present, favour
A. Gelencser ´ et al.r Atmospheric Research 46 (1998) 223–231
228
Table 2 The calculated total concentration of PAHs and n-alkanes and the carbon preference index ŽCPI. and mass median carbon number Ž Nma x . of n-alkanes in the winter samples No.
Date
Ctotal PAHs wngrm3 x
Ctotal n-alkanes wngrm3 x
CPI
Nmax
1 2 3 4 5 6 7 8 9 10 11 12
20–22 11 95 04–06 12 95 11–13 12 95 28–20 12 95 08–10 01 96 15–17 01 96 22–24 01 96 29–31 01 96 12–14 02 96 19–21 02 96 26–28 02 96 04–06 03 96
33.3 55.4 14.2 18.1 8.9 71.1 26.7 10.6 67.5 39.9 20.2 14.7
19.1 22.4 35.7 42.8 41.0 20.3 36.2 48.9 17.1 24.6 37.8 33.9
1.37 1.28 1.19 1.16 1.28 1.43 1.25 1.35 1.11 1.17 1.34 1.12
24.5 24.5 24.5 24.3 24.8 24.7 24.4 24.9 24.8 24.5 25.0 24.3
the coarser particles. The total concentrations of n-alkanes and PAHs and the corresponding indices are summarized in Table 2. In accordance with expectation, the distribution of n-alkanes in all samples is remarkably uniform, the Nmax values vary little, between 24.3 and 25.0 and the CPI values are in all cases below 1.50, presumably reflecting some emission of coniferous plants. The characteristic chromatographic trace is shown in Fig. 2. It is worth noting that the peaks showing slight odd-to-even carbon number predominance are shifted to lower homologues either due to the different emission sources or the
Fig. 2. The typical selected ion chromatogram of winter samples.
A. Gelencser ´ et al.r Atmospheric Research 46 (1998) 223–231
229
Fig. 3. The total concentrations of PAHs plotted as a function of the corresponding total concentrations of n-alkanes in the case of winter samples.
reduced volatilization effect at lower temperatures. The higher share of homologues below C 25 , as reflected in the Nmax values, can also be explained by this effect, though the continued activity of unidentified sources cannot be excluded, either. The lack of UCM in the chromatogram may also support the latter hypothesis. To prove that the predominant source is of anthropogenic origin, the total concentrations of n-alkanes are plotted against those of the PAHs, as shown in Fig. 3. As it is fairly obvious from the figure, no positive correlation was found between the two quantities, even some inverse tendencies can be observed. Should the n-alkanes derive overwhelmingly from anthropogenic sources, one would expect positive correlation between their total concentration and the total concentration of PAHs which are solely of anthropogenic origin. Instead, the largest n-alkane concentrations were found in the samples in which the concentrations of PAHs were at their minimum. On the other hand, for samples in which high PAH concentrations were detected, the concentration of n-alkanes were found to be quite low. This observation may imply that there may be a major biogenic source for n-alkanes in winter in the Northern Hemisphere. In order to support this hypothesis, samples were taken in Veszprem ´ with a Berner-type cascade impactor. As an example, the results of the analysis of 02–05 February 1996 are presented in Table 3. In harmony with the filter samples, no significant increase of CPIs can be observed with particle size. Similarly, the Nmax values do not show any particular tendency with
230
A. Gelencser ´ et al.r Atmospheric Research 46 (1998) 223–231
Table 3 The calculated total concentration of n-alkanes and their carbon preference index ŽCPI. and mass median carbon number Ž Nma x . in the fractions of a cascade impactor sample taken between 02–05 February 1996 Fraction
Cut-offw m mx
Ctotal n-alkanes wngrm3 x
CPI
Nmax
1 2 3 4 5 6 7 8
0.0625 0.125 0.25 0.5 1 2 4 8
12.4 1.5 2.3 3.4 2.6 6.1 9.9 6.6
1.52 0.93 0.95 1.10 1.05 1.17 1.21 1.27
24.4 24.8 23.5 24.2 24.5 24.7 25.4 24.8
particle size. The total concentrations of n-alkanes in the fractions, though, varied with particle size. The majority of the n-alkanes were found in the smallest and the three largest particle size fractions, exhibiting a bimodal distribution. It is worthy of note that similar distributions of n-alkanes were found over the Mediterranean—though at a lower concentration level ŽSicre et al., 1987.. In the smallest size fraction compounds of anthropogenic origin are expected to be abundant whereas larger fractions contain, to a larger extent, biogenic n-alkanes. These results of urban aerosol may also reinforce the previous assumption that biogenic sources play an important role in n-alkane production even outside the growth season. One might expect that if biogenic n-alkanes dominate over their anthropogenic counterparts in urban aerosol close to the points of emission, then the overall terrestrial biogenic production of n-alkanes would dominate their anthropogenic emission by orders of magnitude. It may also prove to be a viable explanation for the lack of positive correlation between the total concentrations of n-alkanes and PAHs as was found on the filter samples at the rural sampling site. Moreover, the same may account for the unidentified source of n-alkanes during the growth season. Yet at least one question remains to be answered: what can be this significant biogenic source of n-alkanes which is active throughout the year? Obviously, it cannot be the well-known mechanism involving higher plant waxes since it produces n-alkanes with odd-to-even carbon number preference. But are there microorganisms which are active throughout the year and are capable of producing n-alkanes without odd carbon number predominance during the decay process? The answer may lie in a recent discovery in organic geochemistry, namely that the odd carbon number predominance of n-alkanes diminishes during the decay of organic matter. It has been found that many types of bacteria and fungi producing even carbon number n-alkanes below C 22 also biosynthesize n-alkanes with no carbon number preference in the C 23 –C 34 range ŽGrimalt et al., 1986; Blazso´ and Schulten, 1990.. Although these findings refer to the study of low rank coals and aerobic sediments, it may also apply to other decomposition processes. Nevertheless the observations with atmospheric aerosol seem to support such a hypothesis. Further studies are required both on emission and aerosol formation to prove that decay processes contribute significantly to the overall production of n-alkanes. In addition, such studies may also prove to be important contributions to aerosol science.
A. Gelencser ´ et al.r Atmospheric Research 46 (1998) 223–231
231
Acknowledgements The authors are indebted to the OTKA F 0116272, F016262 and the Hungarian Academy of Sciences for their financial support.
References Aceves, M., Grimal, J.O., 1993. Atmos. Environ. 2, 51. Blazso, ´ M., Schulten, H.R., 1990. Pyrolysis-field ionization mass spectrometry of low rank coals. Org. Geochem. 1, 87–99. Grimalt, J., Albaiges, ´ J., Alexander, G., Hazai, I., 1986. Predominance of even carbon-numbered n-alkanes in coal seam samples of Nograd Basin ŽHungary.. Naturwissenschaften 73, 729. Leuenberger, C., 1988. Aliphatic and polycyclic aromatic hydrocarbons in urban rain, snow and fog. Atmos. Environ. 4, 695. Radzi Bin Abas, M., Simoneit, B.R.T., 1996. Composition of extractable organic matter of air particles from Malaysia: initial study. Atmospheric Environ. 15, 2779–2793. Schneider, J.K., Gagosian, R.B., Cochran, J.K., Trull, T.W., 1983. Particle size distribution of n-alkanes and 210 Pb off the coast of Peru. Nature 304, 429–432. Sicre, M.A., Marty, J.C., Saliot, A., Aparicio, X., Grimalt, J., Albaiges, J., 1987. Aliphatic and aromatic hydrocarbons in different sized aerosol over the Mediterranean sea: occurrence and origin. Atmos. Environ. 10, 2247–2259. Tissot, B.P., Welte, D.H., 1984. Petroleum Formation and Occurrence, Springer, Berlin, p. 101. Warneck, H., 1988. Chemistry of the Natural Atmosphere, Academic Press, New York.