ARTICLE IN PRESS Atmospheric Environment 39 (2005) 307–314 www.elsevier.com/locate/atmosenv Long-term PAH accumulation after bud break in Quercus ilex L. leaves in a polluted environment Anna Alfania,, Flavia De Nicolaa, Giulia Maistoa, Maria Vittoria Pratib a Dipartimento di Biologia Vegetale, Università degli Studi di Napoli Federico II, via Foria 223, 80139 Napoli, Italy b Istituto Motori del CNR, via Marconi 8, 80125 Napoli, Italy Received 1 October 2003; accepted 8 September 2004 Abstract The dynamics of polycyclic aromatic hydrocarbon (PAH) concentrations in the leaves of Quercus ilex L. for 16 months after bud break (May 2001–September 2002) were studied at a polluted site in the urban area of Naples by comparison to the dynamics at a control site in the Vesuvius National Park. Twenty-seven PAHs were extracted by sonication and quantified by GC-MS. Total PAH concentrations in the leaves sampled at the urban site showed a considerable increase from bud break, with the highest values during the winter (about 3-fold greater than the initial value) and a subsequent decrease, unlike the control site. The control site exhibited PAH concentrations one order of magnitude lower than the urban site. At the urban site, the medium molecular weight PAHs, amounting to 72% of the total, appear responsible for the temporal trend, while the low and high molecular weight PAHs (respectively, 10% and 18%) exhibited only narrow variations over time. At the control site, the low, medium and high molecular weight PAHs contributed similarly to the total concentrations (32%, 31% and 37%, respectively); the low molecular weight PAHs showed the widest temporal variations. Carcinogenic PAHs showed a dynamic at the urban site comparable to that shown by the total PAHs. At the control site dibenzo(a,h)anthracene exhibited concentrations higher than at the urban site. r 2004 Elsevier Ltd. All rights reserved. Keywords: Polycyclic aromatic hydrocarbons; Air contamination; PAH leaf accumulation; PAH dynamics; Carcinogenic PAHs 1. Introduction Vegetation is exposed to dry and wet deposition of pollutants. It thus represents a sink of inorganic and organic compounds. Various plants have been found to accumulate polycyclic aromatic hydrocarbons (PAHs) both in experimental research and in situ (Alfani et al., 2001; Howsam et al., 2000, 2001; Jouraeva et al., 2002; Müller et al., 2001; Nakajima et al., 1995; Wagrowski and Hites, 1997). PAH root uptake from contaminated Corresponding author. Fax: +39 081 2538523. E-mail address: alfani@unina.it (A. Alfani). soils was found to be negligible (Blum and Swarbrick, 1977; Edwards, 1986), so the main PAH accumulation pathway is from the air to the leaf surface. Atmospheric PAHs, both in gas phase and particle phase, may enter leaves by absorption into the waxy cuticle or by stomata uptake (Simonich and Hites, 1994). Leaf concentrations can provide a time-integrated value of atmospheric PAH concentrations. Accordingly, plant leaves may represent a convenient passive sampler to monitor atmospheric PAHs. In recent years, PAHs have received particular attention because of their mutagenic and carcinogenic properties (EPA, 1992; IARC, 1987) and their monitoring is 1352-2310/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.09.001 ARTICLE IN PRESS 308 A. Alfani et al. / Atmospheric Environment 39 (2005) 307–314 regarded as necessary (European Communities, 1996). The biomonitoring of PAH air concentrations through leaf analyses is an economic and efficient system, but to evaluate atmospheric PAH contamination using plant leaves, it is necessary to determine the pattern of PAH accumulation during leaf growth and the behaviour of PAHs in the leaves. In previous studies on air PAH biomonitoring (Alfani et al., 2001), high differences of PAH accumulation were found in leaves of Quercus ilex L. in the urban area of Naples, showing clear gradients among the selected sites. In this paper, by sampling Q. ilex leaves at regular intervals after bud break over a 16-month period, we aimed to highlight the dynamics of PAH accumulation taking into account a series of variables, such as leaf ageing, the influence of meteorological conditions (temperature, rainfall) and the temporal trend of air PAH concentrations. The contribution of different molecular weight PAHs to total leaf concentrations as well as the contribution of leaf carcinogenic PAHs and the variation in the PAH profile during the observation period were also considered. 2. Materials and methods Q. ilex L., an evergreen Mediterranean oak, with xeric, pubescent, lanceolate leaves, was selected for this study. It is widely distributed throughout the Mediterranean region in natural and anthropic areas, showing an ability to accumulate and retain airborne particulates and thereby becoming a helpful biomonitor. Samplings of Q. ilex leaves were carried out at two sites: a square in Naples with a high vehicular traffic flow, and a woodland in the Vesuvius National Park, 12 km from Naples, considered as a control. At the urban and control sites, ten Q. ilex trees were selected and, for every tree, four–six branches were numbered and labelled; in order to collect leaves of the same generation and from the same branches at each sampling. The samples were collected at about 2 m above the ground surface. From May 2001 to September 2002, every 2 months, three samples of leaves were collected both at urban and control sites. Each sample consisted of several leaves, each leaf being picked from a labelled branch on the selected trees. The leaves were removed by hand from the branches, and care was taken to minimise contact with the leaf surface. The leaf samples were placed in polyethylene bags and transported to the laboratory at ca. 4 1C and stored at 20 1C until analysed. The first samples, collected in May 2001, consisted of 2–3-week-old leaves, the last samples of 18-month-old leaves collected in September 2002. During sampling, the leaves from the urban site showed a conspicuous black deposit on the surface. The leaves were not washed before PAH extraction, so the data presented in this study are an expression of the PAH concentrations of the leaves plus any particles impacted and retained on the leaf surface. The extraction procedure was carried out on defrosted leaves. The leaf samples (ca. 5 g wet weight) was mixed with equal quantities of sodium sulphate anhydrous. A mix of deuterated PAHs (naphthalene D8, acenaphthene D10, phenanthrene D10, chrysene D10 and perylene D12) were added before extraction for quantification. The samples were extracted in a mixture of dichloromethane:acetone (1:1=v:v) by three consecutive sonications (Misonix, XL2020). After the first sonication (3 min) in 100 ml of mixture, the samples were recovered by vacuum filtration and the extracts were stored in a flask. The recovered samples were sonicated (3 min) again, adding 100 ml of mixture, and consecutively a third sonication (3 min) was carried out adding another 100 ml of the mixture. After the third sonication, the samples were vacuum filtered and the extracts were added to those from the first sonication. The extracts were rotary evaporated to ca. 5 ml, filtered (0.2 mm), and completely dried under a gentle nitrogen stream. Subsequently, the extracts were diluted with 4 ml of dichloromethane to be analysed. The analyses were carried out by GC-MS (HewlettPackard 5890 GC with on-column manual injection, coupled to Hewlett-Packard 5971 mass-selective detector, and equipped with an HP-5MS capillary column 30 m  0.25 mm i.d. with a 0.25 mm film thickness and a phase ratio of 250). Oven temperature program was held at 33 1C for 1 min, then ramped at 20 1C min 1 to 280 1C and held for 15 min. The on-column inlet temperature was set in oven track, with an injector temperature 3 1C higher than the oven temperature at all times. The carrier gas was helium at a constant flow rate of 1 cm3 min 1 during the analysis. The SIM (selected ion monitoring) modality of acquisition was used. The following 27 compounds were quantified after a calibration curve had been performed for each PAH: naphthalene, acenaphthylene, acenaphthene, fluorene (low molecular weight PAHs), phenanthrene, anthracene, fluoranthene, pyrene, 2,3-benzofluorene, benzo(c)phenanthrene, benzo(g,h,i)fluoranthene, benzo(b)nafto(1,2-d)tiophene, ciclopenta(c,d)pyrene, benzo(a)anthracene, chrysene+triphenylene (medium molecular weight PAHs), benzo(b+k+j)fluoranthene, benzo(e)pyrene, benzo(a)pyrene, 1,3,5-triphenylbenzene, perylene, indeno(1,2,3-c,d)pyrene, benzo(b)chrysene+ picene, dibenzo(a,c)anthracene, dibenzo(a,h)anthracene, benzo(g,h,i)perylene, antantrene, coronene (high molecular weight PAHs). The total PAHs reported in this study represent the sum of the concentrations of all compounds. The concentrations of all the investigated PAHs are calculated considering the recovery of deuterated PAHs. The recoveries ranged from 40% to 80%, with the highest losses for naphthalene and acenaphthene. ARTICLE IN PRESS 309 A. Alfani et al. / Atmospheric Environment 39 (2005) 307–314 240 The dynamics of the total PAH concentrations in Q. ilex leaves over a period (May 2001–September 2002) of 16 months, after bud break, shows a conspicuous PAH increase up to January 2002 at the urban site (Fig. 1). Subsequently the concentrations decrease until July 2002, and do not show noticeable variations from July to September 2002. The total PAH leaf concentrations expressed as ng g 1 dry weight show a range between 1134 in July 2001 and 3482 in January 2002. The decrease in PAH concentrations from May to July 2001 (Fig. 1) is misleading: in this period, the xeromorphism of the young Q. ilex leaves starts to become accentuated and the leaf dry weight per fresh weight increases (Fig. 1, box). At the control site, leaf PAH concentrations appear lower than at the urban site and the temporal variations are narrow. The concentrations range from 240 to 647 ng g 1 dry weight, with the lowest concentration in March 2002 and the highest in November 2001 (Fig. 1). Rainfall and temperature patterns in Naples during the sampling period (Fig. 2) show a warm, dry summer and cold, rainy winter in 2001; the summer in 2002 appears particularly rainy. The contribution of different molecular weight PAHs (low, medium and high), as reported by Harner and Bidleman (1998), to the total concentrations at the urban site differs over the study period (Fig. 3). At the urban site, the medium molecular weight PAHs increase 3-fold from May 2001 to January 2002 and then decrease until September 2002, reaching roughly the values of May 2001. The trend of the medium molecular 200 160 120 80 40 40 20 T (°C) Precipitations (mm) 3. Results 0 0 M J J A S O N D J F M A M J J A S Fig. 2. Rainfall (bars) and temperatures (black triangles) applicable to the urban and control sites from May 2001 to September 2002. 4000 400 control site urban site 200 PAH (ng g-1 d. w.) 3000 0 MJ S N J MMJ S 2000 1000 Sept 02 July 02 May 02 Mar 02 Jan 02 Nov 01 Sept 01 July 01 May 01 0 Fig. 3. Dynamics of low (white triangles), medium (white circles) and high (black diamonds) weight PAH concentrations (7s.e.) in Q. ilex leaves at the urban and control (in the box) sites. 4000 0.8 0.6 0.4 PAHtot (ng g-1 d.w.) 3000 0.2 0.0 leaf d.w./leaf w.w. MJ S NJ M MJ S 2000 1000 Sept 02 July 02 May 02 Mar 02 Jan 02 Nov 01 Sept 01 July 01 May 01 0 Fig. 1. Dynamics of total PAH concentrations (7s.e.) in Q. ilex leaves at the urban (black circles) and control (white triangles) sites. In the box, the dry/wet weight ratios in Q. ilex leaves at the urban and control sites are indicated. weight PAHs was the same as the total PAH concentrations. Low and high molecular weight PAHs show only small variations at this site and differ from the trend of total PAHs (Figs. 1 and 3). At the control site, the low molecular weight PAHs exceed the values of the other two groups from May to November 2001, unlike that at the urban site (Fig. 3, box). Table 1 shows the percentage of low, medium and high molecular weight PAHs in the leaves for each sampling, both at the control and urban sites. The mean percentages over the study period are also reported. At the urban site the medium molecular weight PAHs constitute the highest fraction (72%), while the low and high molecular weight PAHs represent, respectively, 10% and 18% of the total concentrations (Table 1). At the control site the low, medium and high molecular ARTICLE IN PRESS 310 A. Alfani et al. / Atmospheric Environment 39 (2005) 307–314 Table 1 Q. ilex leaf concentrations of low, medium and high molecular weight PAHs (LMW, MMW and HMW, respectively) and carcinogenic PAHs as percentages of totals during the studied period at urban and control sites; mean values over time are also reported 2001 2002 Mean May July September November January March May July September Urban site LMW MMW HMW Carcinogenic 14.30 73.60 12.10 9.24 9.80 70.59 19.61 13.88 19.70 62.10 18.20 10.42 8.71 82.85 8.44 9.94 4.75 82.44 12.81 7.71 4.36 79.17 16.47 10.74 8.97 66.56 24.47 13.82 11.28 63.28 25.45 12.73 9.36 69.70 20.94 11.35 10.1 72.3 17.6 11.1 Control site LMW MMW HMW Carcinogenic 56.59 28.94 14.46 10.96 39.55 19.77 40.69 34.02 63.03 15.34 21.63 15.54 54.11 26.28 19.61 15.46 12.05 53.71 34.24 20.29 11.44 44.15 44.41 23.57 12.73 29.27 58.00 32.69 25.78 29.57 44.65 14.50 15.02 33.96 51.02 10.25 32.3 31.2 36.5 19.7 1000 150 350 urban site control site 280 control site 140 PAH (ng g-1 d.w.) 600 M J S N J MM J S 400 PAHcarc (ng g-1 d. w.) 50 70 0 urban site 100 210 800 150 100 0 M J S N J MM J S 50 200 0 Sept 02 July 02 May 02 Mar 02 Jan 02 Nov 01 Sept 01 July 01 May 01 Sept 02 July 02 May 02 Mar 02 Jan 02 Nov 01 Sept 01 July 01 May 01 0 Fig. 4. Dynamics of phenanthrene (black circles), fluoranthene (black triangles), pyrene (inverted black triangles), benzo(g,h,i)fluoranthene (white circles), chrysene+tryphenylene (white diamonds) and naphthalene (white squares) concentrations (7s.e.) in Q. ilex leaves at the urban and control (in the box) sites. Fig. 5. Dynamics of carcinogenic PAHs: benzo(a)anthracene (black circles), benzo(b+k+j)fluoranthene (black triangles), benzo(a)pyrene (inverted black triangles), indeno(1,2,3-c,d)pyrene (white circles), dibenzo(a,h)anthracene (white diamonds) concentrations (7s.e.) in Q. ilex leaves at the urban and control (in the box) sites. weight PAHs constitute, respectively, 32%, 31% and 37% of the total (Table 1). At the urban site, over the studied period, the 3–4 ring PAHs (pyrene, fluoranthene and phenantrene) exhibit the highest concentrations among the medium molecular weight PAHs (Fig. 4), making up from 30% to 60% of the total PAHs. At the control site, naphthalene shows the highest concentrations among the low molecular weight PAHs, with values higher than at the urban site, particularly from May to November 2001 (Fig. 4, box). Among the studied PAHs the benzo(a)pyrene, benzo(b+k+j)fluoranthene, indeno(1,2,3-c,d)pyrene, benzo(a)anthracene and dibenzo(a,h)anthracene represent the carcinogenic PAHs. The dynamics of these are shown in Fig. 5. At the urban site, the highest concentrations were found in November 2001 and January 2002 with elevated values of benzo(b+k+j)fluoranthene, benzo(a)anthracene and dibenzo(a,h)anthracene, with the same trend shown by the total PAH dynamics. The benzo(b+k+j)fluoranthene concentrations increase from May 2001 to January 2002 and remain high until September 2002. Benzo(a)anthracene and dibenzo(a,h)anthracene show a peak in November 2001 and then decrease with different trends. Indeno(1,2,3-c,d)pyrene and benzo(a)pyrene appear to have ARTICLE IN PRESS A. Alfani et al. / Atmospheric Environment 39 (2005) 307–314 the lowest and most constant concentrations over time (Fig. 5). At the control site dibenzo(a,h)anthracene shows even higher concentrations than at the urban site. The concentrations of the other carcinogenic PAHs appear more constant and lower than at the urban site (Fig. 5, box). 4. Discussion PAH concentrations of Q. ilex leaves at the site in Naples are high, showing in January 2002 values about 7-fold higher than at the control site, highlighting a conspicuous accumulation of airborne PAHs in leaves in the urban area. These high PAH accumulations in leaves are in broad agreement with the dense covering of hairs on both adaxial and abaxial Q. ilex leaf surfaces that have been shown to be effective at trapping particles (Alfani et al., 1997; Rauret et al., 1994). Indeed leaves with hairs present a larger surface and boundary layer than hairless leaves, enhancing the potential for capture and retention of PAHs from the air (Howsam et al., 2000). PAH leaf accumulation provides a temporal integration of PAH air pollution (Alfani et al., 2001; Howsam et al., 2001; Simonich and Hites, 1995; Tremolada et al., 1996; Wagrowski and Hites, 1997). In particular, leaf PAH concentrations of Laurus nobilis L., another evergreen Mediterranean species like Q. ilex, from 13 urban sites in Tuscany (Italy) proved to be correlated to air PAH concentrations collected at the same sites (Lodovici et al., 1998). At the urban site Q. ilex leaves showed PAH concentrations 3-fold higher than at the control site already in May 2001, 2–3 weeks after bud break. Similarly, trace metal concentrations accumulated rapidly in new leaves of Q. ilex at a polluted site in Naples (Alfani et al., 1996). The active metabolism of Q. ilex leaves, after bud break, probably enhanced foliar uptake of PAHs whose high concentrations in the urban area can be attributed to vehicular traffic. Absorption and/or adsorption of PAHs by leaves has been found to be very quick in young leaves exposed to PAH contaminated air (Simonich and Hites, 1994). On the other hand, the increase in leaf lipid and cuticular waxes facilitates accumulation in the leaves of PAHs, lipophilic compounds, as shown by the increase in PAH concentrations during leaf growth at the Naples urban site. Moreover, Brabec et al. (1981) and Little and Wiffen (1977) showed that senescent leaves of a species were more effective at retaining airborne metals and particles than non-senescent ones. Leaf PAH concentration continues to increase during the winter, and in January at the urban site the value is about 4-fold higher than in July and one order of magnitude greater than at the control site. This trend is 311 in broad agreement with that of PAH concentrations found in mature Q. ilex leaves from six sites of the Naples area in the same period (De Nicola et al., unpublished data). These concentration data show that PAH leaf accumulation does not change appreciably depending on the age in mature leaves of Q. ilex, and therefore reflect the degree of contamination of the atmospheric PAHs in the studied area. Total PAHs as well as their profile do not change with respect to leaf ageing. Only during the first 2 months after bud break, when the leaves do not exhibit xeromorphism, is pollutant accumulation likely to be faster. Mature xeromorphic leaves of Q. ilex appear good monitors at each time during ageing. Moreover, both the increase in total PAH concentrations in the leaves of Q. ilex in winter and the decrease in the summer appear consistent with the seasonal pattern found by Caricchia et al. (1999) in the air particulate collected in two sampling campaigns at several sites in the urban area of Naples. Also in other cities, higher PAH concentrations directly in the particulate (Lodovici et al, 2003; Menichini et al., 1999; Wada et al., 2001) or in plant leaves (Lodovici et al., 1998; Nakajima et al., 1995) have been found in winter than in summer. During the winter, the higher air concentrations of PAHs are due to increases in sources, both in number and magnitude, compared with the summer (Howsam and Jones, 1998; Park et al., 2002). The degradation reaction rate of PAHs in air, lower in winter than in summer (Halsall et al., 2001), contribute to produce a particularly high PAH burden in wintertime. The decrease in PAH concentrations, both in suspended particulate matter and in leaf tissues, reported by many investigators (Caricchia et al., 1999; Lodovici et al., 2003; Howsam et al., 2001; Nakajima et al., 1995; Wada et al., 2001), has been attributed to elevated PAH photodegradation in summer, caused by strong sunlight, high temperature and high ozone concentrations (Lodovici et al., 2003; Papageorgopoulou et al., 1999), especially in the Mediterranean region. McCrady and Maggard (1993) have shown that photodegradation is a significant removal mechanism at the leaf surface. Lower PAH concentrations in the leaves of Q. ilex sampled in Naples during the warmest months support the theory of photo- and thermodegradation of PAHs. It has been proposed that PAHs adsorbed on plant surfaces are volatilised as temperatures rise (Halsall et al., 2001) and are degraded, particularly after reactions with hydroxyl radicals, to a greater extent in summer than in winter (Niu et al., 2003). The plentiful and frequent rains recorded during the investigated period are unlikely to affect leaf PAH concentrations. Indeed, the highest leaf PAH concentrations were measured in winter 2001 when more plentiful rains but lower temperatures than the next summer were recorded. Moreover, leaves washed in distilled water did ARTICLE IN PRESS 312 A. Alfani et al. / Atmospheric Environment 39 (2005) 307–314 not show variations in PAH concentrations with respect to unwashed leaves (unpublished data; Lodovici et al., 1998). In the leaves of Q. ilex from the urban site, the medium molecular weight PAHs, representing the highest fraction (mean 72%) of the total, appear responsible for the increase and the subsequent decrease of total PAH concentrations. The condensate of vehicle exhausts is relatively enriched in fluoranthene, pyrene, benzo(g,h,i)perylene and coronene; of these the first two, medium molecular weight PAHs, appear particularly accumulated in leaves of Q. ilex at the urban site, where the effect of traffic flow is high. Phenanthrene, fluoranthene and pyrene, the highest accumulated PAHs in Q. ilex leaves, are also the three most abundant PAHs in the moss Tortula muralis at the urban area of Ferrara especially in the industrial sector (Gerdol et al., 2002). Diamond et al. (2000) found these same three compounds as the most abundant PAHs also in an organic film on an urban surface. At the control site, naphthalene, a low molecular weight PAH, shows the highest concentrations. The low molecular weight PAHs constituted a much higher fraction of total PAHs both in leaves (Alfani et al., 2001; Wagrowski and Hites, 1997) and moss (Gerdol et al., 2002) from rural areas. The sources of emissions, the distance from the source and the physico-chemical properties of the single PAHs, that control the partition of each PAH between gaseous and particulate phase and thus determine their mobility and transport, affect the air concentrations of each PAH. These factors result in lower values at the control site compared to the urban site and different contributions of single compounds to the total, as a function of site typology. In urban areas, motor vehicle exhaust is the principal source of PAHs (Alfani et al., 2001; Katz and Chan, 1980; Nielsen, 1996; Takada et al., 1990; Valerio et al., 1992). Low molecular weight compounds, in the vapour phase, are more widely dispersed (Yang et al., 1991) and may represent potential contaminants for remote areas. Indeed, at the control site they exhibit a higher percentage of the total than at the urban site. Carcinogenic PAHs show different trends between the urban and control sites. Dibenzo(a,h)anthracene unexpectedly exhibits higher concentrations at the control than at the urban site, over the period July 2001–May 2002. The high concentrations of dibenzo(a,h)anthracene at the control site are responsible for the higher percentage of carcinogenic to the total PAHs, compared to the urban site. All the other carcinogenic PAHs show higher values at the urban than the control site, with an increase during the winter and a decrease in summertime. This concentration decrease is not exhibited by benzo(b+k+j)fluoranthene either at the control or urban sites. The occurrence of some PAHs in remote areas is explained mainly by aerial transport from distant anthropogenic sources. On the basis of our findings, this appears likely for naphthalene but is less evident for dibenzo(a,h)anthracene. Indeed, the period of elevated naphthalene leaf concentrations at the control site is connected with the period of high PAH leaf concentrations also at the urban site, even if naphthalene leaf concentrations in the latter are lower. Dibenzo(a,h)anthracene leaf concentrations do not show parallel dynamics at the two sites; moreover the highest concentrations were detected at the control site. Lodovici et al. (1998) found a positive and significant correlation between carcinogenic PAH levels in air and the carcinogenic PAH concentrations in L. nobilis leaves. Plant pollutant accumulation has been particularly useful for identifying unknown point sources, as reported by Simonich and Hites (1995). The high concentrations of dibenzo(a,h)anthracene in Q. ilex leaves from the control site in the Vesuvius National Park most likely indicate a natural or anthropic source that calls for further research. 5. Conclusions The leaves of Q. ilex, a Mediterranean evergreen oak, show a considerable capacity to accumulate in situ PAHs. Young leaves accumulate PAHs rapidly, within 2–3 weeks from bud break, already showing clear differences between sampling sites at different pollutant loads. Mature leaves (at full development) exhibit a sharp seasonal trend in PAH contamination, as characterised in polluted habitats by air particulate analyses, with increases in the winter season and decreases in summertime. The differences between the control and the urban sites, not only in total and different molecular weight PAH concentrations but also in temporal dynamics, are also evident. Q. ilex mature xeromorphic leaves are useful in determining regional PAH air contamination and, if sampled over time, can indicate changes in contamination trends at every site in the monitored area. 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