The identification and quantification of homologous compound sequences (n-alkanes, n-alkanols, and n-alkanoic acids) present in chromatographically elutable fractions, in conjunction with the application of diagnostic parameters, provide helpful data relevant to the characterization of emission sources. The carbon preference index (CPI) has been proposed as an empirical utensil to evaluate qualitatively the weight of biogenic and anthropogenic inputs. For n-alkanes, this parameter is a ratio of the sum of odd-numbered hydrocarbons to the sum of even-numbered hydrocarbons. For n-alkanols and n-alkanoic acids, CPI is determined by inverting the ratio in order to have even-to-odd homologues. In general, the homologue compounds from epicuticular waxes of terrestrial plants exhibit high CPI values (CPI ≫ 1). CPI values near unity indicate the presence of carbon homologues derived from petroleum products or from partial thermal alteration (i.e., incomplete combustion products) of petroleum [Abas and Simoneit, 1996; Simoneit, 1989, 1999; Pio et al., 2001a, 2001b]. Cmax, the carbon number with maximum concentration in the homologous series, can also be used as an indication of relative source inputs, especially to assess the contribution of waxy components of vegetation at a regional level [Simoneit, 1989]. The concentrations of wax n-alkanes, n-alkanols and n-alkanoic acids (WNA, WNAL and WNAC) are calculated by subtraction of the average of the next higher and lower even carbon numbered homologues, taking as zero the negative values [Simoneit, 1999]. Another diagnostic parameter that can be used to assess the magnitude of petroleum contributions to atmospheric aerosols is U:R (unresolved to resolved mass ratio) [Tang et al., 2006].
3.2.1. Aliphatic Hydrocarbons
 The n-alkanes varied from C13 to C34 homologues (Table 3), with an odd carbon number predominance, and maximizing for the homologues ≥C23, which are attributable to plant waxes [Abas and Simoneit, 1996]. The average concentration of the total homologous series ranged between 1.0 ng m−3 (AZO) and 40.3 ng m−3 (KPZ), presenting higher levels in winter, as opposed to the other sites. The homologues with less than 20 carbon atoms presented, throughout the campaign, low average concentrations (<1.1 ng m−3). Simoneit and Mazurek  explain this finding by sampling and extraction deficiencies resulting from the high volatility of these compounds. Therefore concentrations for n-alkanes < C20 must be interpreted with caution. The dominance of the n-C27, n-C29 and n-C31 homologues during summer shifted toward lower carbon numbers in the range n-C21-n-C25 during the colder season. Changes in the modal chain length of n-alkane distributions have been attributed to differences in growing season temperatures of the source regions [Simoneit et al., 1991]. However, Schefuß et al.  suggest a large influence of the regional precipitation regime on the chain length distributions of leaf wax lipids, in agreement with their biologic functionality as regulators of the plant moisture balance. It is also probable that the n-C21-n-C25 modal chain length in winter has resulted from further petrogenic inputs. According to Simoneit , diesel exhaust presents a relative maximum at n-C23.
 The monthly averages of CPI and WNA are presented in Figure 2. CPI calculations indicate a significant anthropogenic influence throughout the entire sampling period in AVE (CPI < 3), and during winter for all other locations. The WNA reveal a more important contribution from vascular plants to the total n-alkanes during the warm season at all locations, probably because of the higher emissions from both deciduous and coniferous trees in summer. A lower annual average fractional contribution, approximately of 30%, is observed at AVE and SBO by comparison with values higher than 47% at the four other sites (Table 4). The stronger contribution of anthropogenic sources to the global amounts of n-alkanes in AVE and SBO is also indicated by the U:R ratio, which presents seasonal average values above 2.9 and below 2.4 for these two sites and for the remaining locations, respectively. Wood and coal combustion exhibit U:R values from 2.3 to 3.9, whereas vehicular emissions result in values higher than 4.0 [Tang et al., 2006]. Pure hydrocarbon mixtures from plant waxes have U:R < 0.1 [Azevedo et al., 2002]. U:R values for rural, mixed and urban western United States samples, for example, are 0.2–4, 1.4–3.4 and 0.9–25, respectively [Simoneit, 1984]. Seasonally, while at SBO higher average U:R ratios were found during summer, at all other sites, maxima occurred during the winter period. It seems therefore that at SBO contamination from anthropogenic emissions occurred as a result of long-range transport from polluted areas through injection of boundary layer contaminated air at high tropospheric levels. Another hypothesis is the contamination of the local atmosphere by construction works during the warmer months.
Figure 2. Monthly average concentrations of anthropogenic n-alkanes (dark grey), wax n-alkanes (light grey), and global CPI (black dotted line) in aerosols collected at the six CARBOSOL sites reflecting a more important contribution from vascular plants in the warm season.
Download figure to PowerPoint
Table 4. Summer and Winter Average Values for Geochemical Parameters Applied to Organic Classes in Aerosols From the Six CARBOSOL Sitesa
|n-alkanes|| || || || || || |
| Global CPI||5.1–2.5||2.5–1.5||5.5–2.2||8.2–2.1||3.2–1.5||7.6–1.6|
| WNA, ng m−3||0.9–0.4||6.8–4.2||6.5–1.1||9.1–1.2||2.0–0.6||23.8–9.8|
| WNA, %||57.8–42.8||44.8–22.5||66.2–36.9||71.5–31.9||46.8–18.8||71.4–24.3|
|n-alkanols|| || || || || || |
| Global CPI||14.1–18.4||13.0–5.5||32.0–11.8||23.7–8.9||11.6–3.1||14.4–11.7|
| WNAL, ng m−3||3.6–3.3||17.0–16.0||23.4–2.4||29.2–2.9||7.9–0.9||20.6–31.4|
| WNAL, %||83.1–89.6||84.2–63.1||93.5–79.6||90.3–75.3||76.8–46.2||85.2–73.4|
|n-carboxylic acids|| || || || || || |
| Global CPI||5.2–4.9||4.2–6.2||8.4–5.3||5.5–4.0||5.9–6.4||5.0–4.3|
| WNAC, ng m−3||5.7–0.9||16.1–27.5||8.5–2.8||10.3–3.9||5.6–4.6||44.1–93.3|
| WNAC, %||67.3–66.7||59.2–69.9||75.8–69.0||66.4–58.9||71.6–72.6||64.0–60.4|
|Aromatics|| || || || || || |
| BeP/(BeP + BaP)||1.00–0.89||0.66–0.63||0.91–0.75||0.93–0.93||n.d.||0.83–0.63|
| Fl/(Fl + Py)||1.00–1.00||0.45–0.49||0.61–0.60||0.59–0.56||n.d.–0.83||0.54–0.53|
| IP/(IP + BgP)||0.48–0.49||0.41–0.41||0.46–0.49||0.35–0.43||n.d.–0.45||0.42–0.46|
| BA/(BA + CT)||n.d.||0.36–0.46||n.d.–0.62||0.53–0.36||n.d.–0.61||0.57–0.39|
 Petroleum molecular markers are specific indicator compounds mainly present in the hydrocarbon fractions. These kind of tracers may include the 17α(H), 21β(H)-hopane series, the 5α(H), 14α(H), 17α(H) and 5α(H), 14β(H), 17β(H)-sterane series, as well as the isoprenoids pristane and phytane [Gogou et al., 1996; Simoneit, 1984, 1999; Simoneit et al., 1991; Azevedo et al., 2002]. Pristane and phytane result from the diagenesis of phytol and are not primary components of the majority of terrestrial living organisms [Simoneit, 1984]. The presence of hopanes, steranes and isoprenoids in aerosols confirm an input source from fossil fuel utilization, especially by vehicular traffic [Gogou et al., 1996; Simoneit, 1984, 1999; Azevedo et al., 2002]. It should be noted, however, that the identification of compounds such as pristane and phytane in atmospheric samples are dependent on the sampling period. Simoneit et al.  referred to the volatile compound blow-off from the filters over a 1–2 day acquisition time and the consequent depletion of aliphatics <C21, and therefore of the petroleum tracers. In this study, the resultant weekly samples presented petroleum molecular markers only in samples from the Portuguese sampling sites. The presence of ramified hydrocarbons like pristane (Pr) and phytane (Ph) is consistent with fossil fuel sources of carbon in the interval C16–C20, which is approximately the distillation range of diesel fuels [Abas and Simoneit, 1996; Zheng et al., 1997]. The total average concentrations of Pr and Ph ranged from 10 to 40 pg m−3 and from 7 to 38 pg m−3, respectively. Much higher concentrations of these hydrocarbons (68.3 and 64.5 ng m−3) were found in southern California during a severe photochemical smog episode [Fraser et al., 1997], but also over a Portuguese rural area (<15 and <19 ng m−3) [Pio et al., 2001b]. Biogenic inputs are often dominated by a predominance of the odd carbon alkanes and the C17 isoprenoid (pristane). Since phytane is rarely found in biological material (except some bacteria), most biological hydrocarbons have a Pr/Ph ratio ≫ 1.0. The Pr/Ph ratios ranged from 0.21 to 5.60 in AZO and from 0.45 to 1.74 in AVE. Values approaching the unity for AVE indicate a hydrocarbon signature derived from petrochemical use, analogous to others described for urban aerosols. The 17α(H), 21β(H)-hopanes were only detected in AVE, and mainly during the winter period (Table 5). The series presented the typical 22S:R pairs and maximize at C29. Concentrations of the 22S hopanes were always higher than those for the corresponding 22R pairs. This type of distribution is commonly found in aerosols emitted by gasoline and diesel engines. Biogenic precursors contain only the 22R 17β(H), 21β(H)-hopane configuration [Peters and Moldowan, 1993; Peters et al., 2005].
Table 5. Seasonal Average Concentrations of Molecular Markers in Aerosols From the CARBOSOL Sampling Sitesa
|6, 10, 14-trimethylpentadecan-2-one||summer||263.37||4.17||82.75||5.71||244.24||459.04|
|6, 10, 14-trimethylpentadecan-2-one||winter||200.22||8.27||19.48||10.03||71.11||289.08|
|Octadecanoic acid (C18:0)||summer||636.09||3293.67||1554.91||2117.89||1250.53||6511.33|
|Octadecanoic acid (C18:0)||winter||208.41||3956.49||583.64||860.57||1914.91||13518.29|
|Oleic acid (C18:1)||summer||259.00||462.27||1371.20||1795.67||1445.31||1782.31|
|Oleic acid (C18:1)||winter||75.01||568.78||207.52||231.12||266.39||5660.45|
|Linoleic acid (C18:2)||summer||136.49||240.54||1826.82||1367.13||833.69||2132.48|
|Linoleic acid (C18:2)||winter||25.38||367.41||51.12||49.58||66.22||955.09|
|Palmitoleic acid (C16:1)||summer||51.84||⋯||35.44||323.06||47.55||⋯|
|Palmitoleic acid (C16:1)||winter||27.19||⋯||9.43||1.92||15.04||⋯|
3.2.2. Aromatic Hydrocarbons
 PAHs detected in samples from the six nonurban European sites ranged from phenanthrene to coronene, with individual levels never exceeding 5.8 ng m−3. This maximum concentration was obtained for fluoranthene in KPZ during February 2002. This concentration is two to six times higher than the maximum levels measured in Greek (2.0 ng m−3) [Gogou et al., 1996], Swedish (3.0 ng m−3) and Finnish (0.7 ng m−3) rural areas [Prevedouros et al., 2004]. The average concentrations of total PAHs are higher at the continental low-level sites of AVE and KPZ, which present levels comparable to those from Algiers City (14.7 ng m−3 [Yassaa et al., 2001]) and from an urban area of Birmingham during winter (18.1 ng m−3 [Smith and Harrison, 1996]). The average concentrations measured for KPZ are more than 20 times higher than the results obtained for AZO, PDD, SIL and SBO (Table 3). The results obtained for these four locations are in the range of those reported for rural and urban U.S. west coast areas (0.01–2.2 ng m−3 [Simoneit, 1984]), and for a rural area in Birmingham during summer (1.02 ng m−3 [Smith and Harrison, 1996]). Higher average concentrations are found during winter for AVE, PDD, SIL and KPZ, and during summer for the site under upslope weather conditions (SBO). At AZO, with mild climate all year, no marked seasonality is observed (Table 3). Higher PAH levels are generally observed in winter as a consequence of an increase in wood burning and in consumption of fossil fuel combustibles, lower losses due to photochemical degradation and the fact that meteorological conditions during this period (i.e., more frequent and lower temperature inversions) are less favorable for pollutant dispersion [Kalaitzoglou et al., 2004].
 Retene, a completely dehydrogenated resin diterpenoid also detected, is a pyrolysis end product from diterpenoids that have the abietane or pimarane skeletons. It has been proposed as molecular tracer for coniferous wood [Simoneit and Mazurek, 1982; Simoneit, 1999, 2002]. It is known that the burning conditions have an impact on this biomarker: in an oxygen-sufficient and high-temperature environment, less retene is expected because of thermal cracking, while the opposite conditions promote its formation [Fang et al., 1999]. Retene was only detected in AVE and PDD with average concentrations between 0.01 ng m−3 and 0.33 ng m−3, and presenting higher values during the winter period. The contribution of retene to the total measured organic concentration did not surpass, on average, 0.30%. Although AVE and PDD showed average concentrations of retene below the low end of the range (6–8 ng m−3) measured for cities where wood is used for heating in 50% or more of all households [Ramdahl, 1983], similar levels were observed in some rural and urban areas of Mississippi, Georgia and Florida [Zheng et al., 2002].
 Further assessment of processes affecting the composition of hydrocarbons can be obtained from concentration ratios between different PAH species [Pio et al., 2001b]. These parameters must be used with caution because they assume only minor modifications following emission and no sampling artifacts (especially in the case of the reactive benzo[a]pyrene) [Gogou and Stephanou, 2000]. During the CARBOSOL campaign, weekly sampling did not include special devices, such as diffusion denuders and foam plugs. Therefore volatilization losses or adsorption artifacts may have occurred on the filter for semivolatile organic constituents, especially for the low molecular weight compounds because of their high volatility [Kavouras et al., 1999]. The mean values of the sum of nine combustion nonalkylated compounds to the total concentrations of PAHs (CPAHs/TPAHs) were generally higher than 0.50 (Table 4), excepting for the background oceanic site (AZO). The higher ratio values observed, suggest that a large fraction of PAHs originated from pyrogenic sources, rather than from fossil fuel volatilization [Alves, 2001]. Methylphenanthrenes and phenanthrene were absent from samples of the AZO and SBO sites throughout the campaign, while at SIL they were detected only in summer. The mean MP/P ratios in AVE, PDD and KPZ are higher than 1, indicating a predominance of petrogenic emissions (e.g., vehicular exhausts), rather than stationary combustion sources [Gogou et al., 1996; Tsapakis et al., 2002]. The BeP/(BeP + BaP) is affected by the strong reactivity in the atmosphere, since benzo[a]pyrene is easily decomposed by light and oxidants. Most of the fresh exhausts have similar contents of benzo[e]pyrene and benzo[a]pyrene, thus the increase of the ratio can be regarded as an index of the aging of particles [Tsapakis et al., 2002]. The mean values obtained for this ratio suggest fresher emissions during the entire sampling period in AVE and during winter at KPZ (values close to 0.50). The summer periods present, in general, higher ratios, indicating more intense photochemical or thermal reactions with ozone, nitric oxides and hydroxyl radicals. The mean Fl/(Fl + Py) ratios in CARBOSOL sites are in the range 0.5–0.8. These values are similar to those previously published for vehicular emissions [Rogge et al., 1993a]. The IP/(IP + BgP) ratio is normally used to distinguish between vehicular emissions and stationary sources (including wood and coal domestic heating). Similar IP/(IP + BgP) ratios were obtained in all CARBOSOL sites (0.35–0.49), with higher values usually during winter. A comparison of these values to those previously published (0.18 for cars, 0.37 for diesel, 0.56 for coal [Grimmer et al., 1983], and 0.62 for wood smoke impacted aerosols [Gogou et al., 1996]) indicates that the importance of mobile source contributions may be surpassed by the stationary inputs during the wintry months, because of the probable increase in domestic wood burning. The mean Ba/(Ba + CT) ratio presents values between 0.36 and 0.61, which are similar to those calculated for oil combustion sources, such as vehicles [Tsapakis et al., 2002]. Overall, the PAH concentration diagnostic ratios estimated for the CARBOSOL sites suggest that aromatic compounds are associated with a variety of stationary and mobile (e.g., vehicular) sources, hinting a predominance of wood burning emissions in winter. The low-level continental sites are the most affected by petrogenic and pyrogenic emission sources.
3.2.3. Aldehydes and Ketones
 Atmospheric particulate n-alkanals and n-alkanones with less than 20 carbons result predominantly from oxidative processes, anthropogenic activities, or microbial action over alkanes, alkenes or other constituents [Simoneit et al., 1988; Stephanou and Stratigakis, 1993]. Compounds with higher molecular weights may occur in the epicuticular waxes as a result of the reaction of O3 on unsaturated hydrocarbons [Alves, 2001]. In CARBOSOL samples, the homologous series of n-alkanals ranged from C11 to C30, presenting average total concentrations between 1.0 ng m−3 (AZO and SBO) and 19.0 ng m−3 (KPZ). Excepting for AVE, levels are higher in summer than in winter (Table 3). In general, all CARBOSOL sites present n-alkanals with maximum concentrations for compounds with chain lengths above C22 during the entire sampling period, indicating an origin in particulate abrasion products from leaf surfaces and biomass burning. The only exception occurred during winter in SBO, where C14 and C15 became prevalent, in accordance with a local significant microbial source throughout this season [Simoneit et al., 1988; Stephanou and Stratigakis, 1993]. The homologous distributions of n-alkan-2-ones varied between C14 and C31 for almost all locations. The single exception occurred in AVE, where only the C29 ketone was detected. The average concentrations of total n-ketones ranged between 0.2 ng m−3 (AZO and PDD) and 3.4 ng m−3 (KPZ), presenting higher values during summer (Table 3). Geochemical parameters for the carbonyl families are not presented because of the discontinuity of the homologous series during the sampling period, at all locations.
 One of the predominant ketones found in the atmospheric aerosol samples was the phytone (6, 10, 14-trimethylpentadecan-2-one). It is produced by thermal alteration and oxidation of phytol, which is part of the chlorophyll molecule, and has been proposed as a marker for secondary biogenic aerosol [Brown et al., 2002; Gogou et al., 1996; Pio et al., 2001b; Simoneit et al., 1988]. This isoprenoid compound has been previously detected in rural aerosols from Crete [Gogou et al., 1996], Portugal [Pio et al., 2001b], Big Bend National Park, Texas [Brown et al., 2002] and rural Nigeria [Simoneit et al., 1988] at levels ranging between values close to 1 and dozens of ng m−3. The small coastal city of AVE and the forested mountain site of SIL received rather low concentrations of this isoprenoid ketone throughout the year and, inclusively, it was absent from some samples. The Atlantic background site of AZO and the Hungarian plains of KPZ presented the highest levels. Aside from the two sites where the ketone is not noteworthy, the seasonal variation of phytone showed higher concentrations during the summer months. The wintry decrease suggests that secondary aerosol formation from biomass degradation may be less important in these sample composites [Brown et al., 2002].
3.2.4. Alcohols and Sterols
 In aerosols from the CARBOSOL sites, the typical distributions of n-alkanols ranged from C12 to C30 with a strong even-to-odd carbon number predominance, reflecting a prevailing biogenic origin (Tables 3 and 4 and Figure 3). In opposition to the higher molecular weight compounds, the homologues <C20 are not in fresh vascular plant waxes and may have a microbial origin [Alves et al., 2001]. The average concentrations varied from 3.4 ng m−3 (AZO) to 35.4 ng m−3 (KPZ). By contrast with the other locations, AVE and KPZ present higher concentrations during winter. This may reflect an enhanced microbial and/or anthropogenic component during the colder season, since the wax constituents have a smaller percentage contribution to the aerosol content in this period of the year compared to the summer time. The WNAL comprise the majority of the mass of the homologous series, representing up to 94%. The highest concentrations of WNAL are observed in summer (Table 4), reflecting some seasonality of wax production.
Figure 3. Monthly average concentrations of anthropogenic (dark grey) and wax n-alkanols (light grey) and global CPI (black dotted line) evaluation in aerosols from CARBOSOL sites, showing a major contribution of leaf epicuticular constituents of the vegetation.
Download figure to PowerPoint
 Phytosterols consist of sterols of higher photosynthetic plants originated by a biosynthetic pathway of cyclization, where squalene is the precursor [Simoneit et al., 1991]. Whereas β-sitosterol (C29) and stigmasterol (C29) are indicators for vegetation in general, campesterol (C28) has been pointed out as a specific molecular tracer for gramineae [Simoneit, 2002; Oros and Simoneit, 1999]. In this study, campesterol was not detected in AZO, and stigmasterol was present only in the Portuguese sampling sites. Pio et al. [2001b] report that the absence of campesterol might be due to its degradation toward other compounds, or as a result of its absence from the vegetation waxes affecting the aerosol composition. Cholesterol (C27) is an important constituent of cell membranes found in animal tissues. It has been proposed as a molecular marker of meat frying and grilling operations [Nolte et al., 1999]. However, this C27 sterol should not be viewed as an unique tracer for emissions during meat cooking, since it was also detected in pine, oak and eucalyptus wood smoke [Nolte et al., 2001]. Simoneit and Elias  report that the presence of cholesterol in oceanic aerosols probably indicates an input from marine sources such as algae, which also can contribute to continental cholesterol sources in environments downwind from lacustrine areas.
 Different carbon number distributions have been obtained in distinct experiments, suggesting that the phytosterol patterns may be related to geographical characteristics, particularly the specific plant communities and the climatic conditions [Pio et al., 2001b; Simoneit, 1989; Simoneit et al., 1988, 1990]. The sterol carbon number distributions, considering the total average concentrations, reveal similar patterns for AVE and KPZ with C29 > C28 > C27, while at the continental higher mountain sites (PDD, SIL and SBO) a distribution of C29 > C27 > C28 was evident. AZO reveals a sterol distribution with C29 > C27. The pattern found in PDD, SIL and SBO is similar to that reported by Simoneit et al.  for aerosol samples from Nigeria and for the Amazon region [Simoneit et al., 1990]. This distribution correlates with the predominance of wax components from vegetation and also from grass [Simoneit et al., 1988]. Like in AZO, in a Portuguese semirural area, the C28 phytosterol was not present at detectable levels, but an opposite pattern was reported with C27 > C29 [Pio et al., 2001b]. In general, while AVE and KPZ present higher average concentrations for all the identified sterols during the cold season, all other locations show higher levels during summer (Table 3). The distinct seasonality between elevated and surface continental sites is possibly a consequence of emission and transport. The seasonal pattern might indicate the predominance of vegetation waxes exudations in more remote sites, biomass burning and probably meat cooking operations as predominant sources in AVE and KPZ. Concentrations at the elevated sites are likely related to the effective decoupling of the lower layers from the midtroposphere during the cold season and to a more efficient upward transport of air masses from the boundary layer in summer than in winter.
 Fatty acids represent another major group of solvent-extractable compounds present in the aerosol phase. The distribution patterns ranged from C7 to C32, comprising saturated and unsaturated homologues. Depending on the site, a modal or bimodal distribution was observed, maximizing at n-C16 and/or n-C22-n-C24. Alkanoic acids homologues > n-C20 are constituents of vegetation waxes. Compounds with lower chain lengths can be derived from microbial sources [Alves et al., 2001]. Moreover, n-alkanoic acids < C18 also originate from biomass burning, food preparation, vehicle exhaust and tire wear debris [Rogge et al., 1991, 1993c]. The annual average concentrations for the whole members of the homologous series ranged from a minimum of 4.3 ng m−3 in the Atlantic site (AZO) to an upper limit of 117.2 ng m−3 in KPZ. As verified for other compounds, at the two continental low-elevation sites concentrations maximize during winter, whereas an opposite tendency is observed at other locations (Table 3). CPI values >4 and a contribution from wax components higher than 60% are indicative that n-carboxylic acids are mainly derived from biogenic sources (Table 4).
 Unsaturated n-fatty acids (alkenoic acids) are emitted to the atmosphere from microbial sources and from the processing, degradation and combustion of plant and animal constituents [Rogge et al., 1993b]. Meat cooking is an important source of n-alkenoic acids, mainly oleic (C18:1) and palmitoleic acid (C16:1) [Rogge et al., 1991]. Phytoplankton and bacteria also contain a number of unsaturated fatty acids [Rogge et al., 1993b; Simoneit et al., 2004]. Biomass burning presents, in general, as primary components palmitic (C16:0) and stearic (C18:0) acids [e.g., Oros and Simoneit, 1999]. Once emitted into the atmosphere, unsaturated fatty acids, which are indicators of recent biogenesis [Simoneit et al., 1991], are likely to be attacked by free radicals, ozone and other oxidants, producing aldehydes, lower-weight carboxylic acids and dicarboxylic acids [Rogge et al., 1993b]. In this study, oleic and linoleic acids were detected in almost all occasions in all sampling sites. Total average concentrations ranged between 0.20 ng m−3 (AZO) and 3.72 ng m−3 (KPZ) for oleic acid and between 0.08 ng m−3 (AZO) and 1.54 ng m−3 (KPZ) for linoleic acid. Palmitoleic acid was measured in almost all occasions in AZO, occasionally in PDD, SIL and SBO, and never in AVE and KPZ. The total average concentration reached 0.16 ng m−3 in SIL, 0.040 ng m−3 in AZO, 0.031 ng m−3 in SBO, and 0.022 ng m−3 in PDD. Seasonally, the average levels were higher during summer for the more remote locations (AZO, PDD, SIL and SBO) and during winter for AVE and KPZ, suggesting a strong contribution of wood burning and/or meat cooking from October to March at these two sites (Figure 4). The concentration levels measured for oleic and linoleic acids are comparable to those of remote, rural and suburban areas in the United States [Brown et al., 2002; Zheng et al., 2002]. Values obtained for palmitoleic acid are lower than those measured in remote, rural and suburban areas in the United States and in the western north Pacific [Zheng et al., 2002; Simoneit et al., 2004].
Figure 4. Monthly average concentrations of anthropogenic n-acids (dark grey), wax n-acids (light grey), and global CPI (black dotted line) values in aerosols from CARBOSOL sites showing higher concentrations during winter possibly due to biomass burning at the two continental low-elevation sites and global CPI values characteristic of a predominant biogenic origin.
Download figure to PowerPoint
 One method that has been used to gauge the age of aerosol is to take the ratio between the concentrations of the saturated C18 alkanoic acid (C18:0) and the monounsaturated C18 acid (C18:1). This ratio is used as an aerosol age indicator since the monounsaturated acid breaks down much faster by atmospheric oxidation than the saturated analogue. The abundance of the saturated acid compared to the monounsaturated homologue can, therefore, indicate a relative decomposition rate [Brown et al., 2002]. The C18:0/C18:1 ratio measured during the CARBOSOL experiment presents successively decreasing averages for AVE (7.91), KPZ (6.28), SBO (4.27), SIL (3.45), AZO (2.75) and PDD (2.27). Simoneit et al.  reported a noticeable increase of this ratio with increasing height above ground in Chinese urban areas. An opposite pattern was observed for the CARBOSOL samples during summer, showing an apparent enhancement with declining height. However, this correlation is not observable in the winter time. Similar results to AVE and KPZ were reported between 5 and 11 with an average of 6.6 for rural samples collected in remote Big Bend National Park, Texas. These high, rural-like ratios were ascribed to a combination of local rural biogenic emissions and aged aerosol advected from urban areas [Brown et al., 2002]. Seasonally, the C18:0/C18:1 ratio presented higher averages during winter than summer for all studied locations, suggesting that the aerosols were more aged in colder periods that those of summer (Table 6). Except for AVE, the significant increase of the C18:0/C18:2 ratios in the winter samples confirms the existence of more aged air masses with relatively long residence time since their formation during the colder months [Fang et al., 1999]. This suggests the superimposition of long-distance transport on local characteristics, rather that photochemical decomposition of unsaturated acids.
Table 6. Seasonal Averages for the Ratios Between C18 and C16 Fatty Acids
|Octadecanoic acid (C18:0)/oleic acid (C18:1)||winter average||3.26||8.13||2.82||4.59||6.41||7.61|
|Octadecanoic acid (C18:0)/oleic acid (C18:1)||summer average||2.23||7.69||1.71||2.31||2.13||4.94|
|Octadecanoic acid (C18:0)/hexadecanoic acid (C16:0)||winter average||0.55||0.55||0.60||0.89||0.86||0.49|
|Octadecanoic acid (C18:0)/hexadecanoic acid (C16:0)||summer average||0.79||0.80||0.60||0.69||0.55||0.60|
|Octadecanoic acid (C18:0)/linoleic acid (C18:2)||winter average||8.21||10.77||11.40||17.36||28.92||14.15|
|Octadecanoic acid (C18:0)/linoleic acid (C18:2)||summer average||4.66||13.69||0.85||1.55||1.50||3.05|
 While stearic and palmitic acids are, as individual compounds, not source specific, the C18:0/C16:0 fatty acid ratio is unique and can be used in source apportionment studies. The major contributors for particulate matter have C18:0/C16:0 ratios ranging from 0.17 to 0.71, depending on source type. In countries where dried cattle dung is used for cooking purposes, fine particulate smoke presents the characteristically elevated ratio around 2, whereas for foliar vegetation or wood smoke and car exhaust, values below 0.5 are typical. The surface soil and dusts from feedlots and open lot dairy farms showed an average C18:0/C16:0 ratio of 3.0 [Rogge et al., 2006]. The fine particulate matter from CARBOSOL sites presented average ratios ranging from 0.55 to 0.89. Curiously, the elevated mountain sites showed higher values during the winter period, while at the low-level sites summer maxima dominated the seasonal variation. Values between 0.5 and 1 were also found in agricultural fields, dust from paved and unpaved roads and in PM2.5 of rural and urban sources, such as hamburger charbroiling [Rogge et al., 2006, and references therein].
 Resin acids, which are biosynthesized mainly by gymnosperms (e.g., pine and spruce) in temperate regions [Rogge et al., 1998; Simoneit, 2002], were found in significant concentrations. They include unaltered (pimaric, isopimaric and sandaracopimaric acids) and thermal degradation products (dehydroabietic and 7-oxodehydroabietic acids). While the thermal degradation acids were found during almost all the sampling period at the different locations, the unaltered acids were more often detected in AVE and PDD (Table 6). The seasonal concentration of these resin constituents is characterized by a winter maximum, the highest levels and interseasonal difference being observed at the Portuguese coastal site of AVE. Surprisingly, at this site, the contribution of resin acids to the identifiable organic compounds may exceed 80% during wintry weather conditions (Figure 5). At the other locations, these wood smoke tracers represent a fraction of the identifiable organic matter ranging from about 20% to 40%, showing that domestic biomass burning is the greatest contributor to the organic matter. Dehydroabietic acid has been proposed as a candidate marker compound for coniferous wood combustion [Standley and Simoneit, 1994; Rogge et al., 1998]. It was the major resin acid quantified in the aerosols, presenting average concentrations between 6 ng m−3 and 44 ng m−3 in KPZ and AVE, respectively, the two sites with the highest abundance. The other locations revealed average concentrations between 0.03 ng m−3 and 0.61 ng m−3. Dehydroabietic acid was not identified in aerosols from Nigeria and Amazonia, in accordance with the absence of conifer vegetation in those regions [Simoneit et al., 1988, 1990]. In cities where wood is used for heating, ambient dehydroabietic acid concentrations ranged from 48 to 440 ng m−3 [Standley and Simoneit, 1994]. The compound was also detected in oceanic samples at concentrations ranging from 0.0001 to 0.4 ng m−3, whereas in terrestrial aerosol particulate matter, it was present at much higher levels (0.23–440 ng m−3). The presence of this tracer in atmospheric matter over the ocean confirmed the long-range transport of smoke from biomass burning of the continents [Simoneit and Elias, 2000; Simoneit et al., 2004].
Figure 5. Monthly average concentration of some resin acids and their contribution to the identifiable organic matter illustrating the significant input of wood smoke constituents in wintertime, particularly at the low-level sites.
Download figure to PowerPoint