Research Communications Loss of Unburned-Fuel Hydrocarbons from Combustion Aerosols during Atmospheric Transport RAFEL SIMO ´ ,* JOAN O. GRIMALT, AND JOAN ALBAIGE ´S Department of Environmental Chemistry, CID-CSIC, 08034 Barcelona, Catalonia, Spain
Urban, coal-fired power plant, coastal, and marine aerosols were compared in terms of pyrogenic (PHC) and unburned-fuel (FHC) hydrocarbon contributions. It was observed that the relative content of FHC decreased from near combustion source to remote samples. Artificially irradiated and aerated combustion aerosols also exhibited selective loss of FHC along with photoreactive compounds. Furthermore, different correlation to soot content has been obtained for the two fractions in marine aerosols. We suggest that PHC and FHC present a decoupled association to soot. Whereas soot-compound interactions protect PHC from major transformations, FHC occur more exposed to vapor/particle partitioning. This is consistent with the decoupled size distribution of the two fractions in aged aerosols: PHC mostly occur in small (soot-rich) particles whereas FHC distribute onto all sizes. Our findings are relevant to source reconciliation modeling and aerosol chemistry.
Introduction A significant fraction of aerosols generated from combustion processes is constituted by carbonaceous particles (soot) (1, 2), mostly submicron-sized, which can easily undergo longrange aeolian transport (3). Soot contains organic pollutants and long-lived black carbon, thus representing a significant vector for carbon dispersion and burial on a global scale (4). Pyrogenic and uncombusted-fuel hydrocarbons (PHC and FHC, respectively) occur admixed in the organic fraction of combustion aerosols with distinguishable compound distribution patterns: PHC are represented by polycyclic aromatic hydrocarbon (PAH) mixtures with an abundance of pericondensed, alicyclic, and non-substituted (parent) species (5-8), and FHC are characterized by unresolved complex mixtures (UCM) and the predominance of alkylated-PAH over parent homologues (5-8). This fingerprinting has been extensively used to assess the relative inputs of pyrolysates and fossil fuel residues to the atmosphere and throughout the environment (5-10). However, the usefulness of hydrocarbons as combustion tracers relies on the knowledge of their atmospheric transformations and fate. On the basis of field measurements and laboratory aging experiments with near-source and remote aerosols, we present evidence that FHC are preferentially lost over PHC from combustion * Corresponding author e-mail:
[email protected]; fax: 34-32045904.
S0013-936X(96)00994-7 CCC: $14.00
1997 American Chemical Society
particles during atmospheric transport. We suggest that this is mainly accounted for by differences in hydrocarbon-particle association. Our findings are relevant to source reconciliation modeling and aerosol chemistry.
Methods Sampling. We collected aerosols characteristic of different environments by means of high-volume air filtration through precleaned glass fiber filters (GF/A). Commonly, air volumes between 400 and 1400 m3 were withdrawn at a flow rate of 60 m3 h-1. Caution was applied to prevent the filters from contacting surfaces other than those of cleaned Teflon and metal. Sampled filters were wrapped with aluminum foil and stored frozen (-20 °C) until analysis. Analyses. HC analysis was performed as described in ref 11. In short, samples were Soxhlet-extracted with dichloromethane:methanol 2:1 (v:v) and fractionated through silica + alumina by column chromatography to separate the aliphatic, the aromatic hydrocarbon, and the polar lipid fractions. The aliphatic and the polar fractions were analyzed by gas chromatography (GC) in a Carlo Erba 5300 HRGC equipped with a flame ionization detector and a SE-54 capillary column. Up to 31 PAH were determined in the aromatic fraction by GC coupled to mass spectrometry in the selected ion monitoring (SIM) mode using a Hewlett Packard 5995 with an HP 300 data acquisition system. In all fractions, internal and external standards were used for quantification of individual compounds. Non-mineral particles were characterized and quantified by microscopy after dissolution of the filter in HF acid. Pollen grains, fungal spores, plant debris, and soot were identified among the most abundant particles. Soot content was estimated by measuring areas covered by black carbon in the microscope samples and applying statistical methods as for standard pollen quantification. Irradiation and Aeration Experiments. Portions of sampled filters were located in a side-open quartz container and irradiated for 4-12 h in a Suntest apparatus (Heraeus, Hanau, Germany) equipped with a xenon arc lamp (light spectrum in the range of 300-800 nm, i.e., very close to natural light) under continuous air flushing. Temperature was kept below 40 °C.
Results and Discussion A suite of diverse atmospheric particulate samples, encompassing urban, coal-burned power plant, coastal, and remote marine aerosols, was studied (Table 1). Resolved and unresolved alkanes, PAH, n-alkanols, fatty acids and sterols as well as some other minor molecular markers were identified in the dichloromethane:methanol extracts, indicating that vascular plant waxes and debris, microbiota, secondary aerosols and, likely, greases, and road dust contribute to the total particle loads along with combustion aerosols. In all cases, PHC and FHC were found at detectable concentrations (>10 pg m-3). PHC consisted of typically pyrogenic PAH such as benz[a]anthracene, benzofluoranthenes, benzo[e]pyrene, benzo[a]pyrene, indeno[1,2,3-c,d]pyrene, benzo[g,h,i]perylene, and coronene, which are generated during the combustion of both fresh and fossil organic matter. The FHC fraction was composed of methylated phenanthrenes, fluoranthenes, and pyrenes, highly abundant in fossil fuels, and a high molecular-weigh aliphatic UCM, typical of fossil fuels although
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TABLE 1. Description of Aerosol Samples Studied aerosol type urban high traffic low traffic coal-fired power plant near-source off-source marine continental coast shipboard island
location
location/sampling characteristics
date (d/m/y)
Barcelona Barcelona
densely populated area low population area
22/11/91 30/04/90
10 km Andorra Power Plant, NE Spain 70 km Andorra Power Plant, NE Spain
downwind under the plume downwind, top of hill under the plume
03/03/92 19/06/92
Cap Ferrat, S France CYBELE cruise, NW Mediterranean
coastal, rural, non-polluted site ship transect across the Catalan Sea, face-to-wind sampling high altitude (1000 m), near-shore, non-polluted site; sampling only when wind blowing from sea
28/09/89 27/04/90
Mallorca, NW Mediterranean
11/04/89 29/04/89 01/05/89 11/10/89 29/10/89 30/10/89 04/11/89 12/11/89 18/11/89 24/11/89 02/12/89
TABLE 2. Effect of Irradiation and Aeration Treatment onto Relative Concentrations of Fossil-Fuel and Photolabile Hydrocarbons in Aerosols Sampled near Combustion Sources sample urban aerosol high traffice low traffic f coal power plant aerosolg PAH adsorbed on soil dusth
U/Ra
MePhen/ Phenb
(MeFla + MePy)/ (Fla + Py)c
photolabile PAH/ stable homologuesd
8.5 f 7.2i 7.1 f 2.6 1.5 f 0.0
2.9 f 1.9 1.4 f 1.2 2.0 f 0.2
0.4 f 0.1 0.3 f 0.1 0.3 f 0.0
1.0 f 0.4 0.3 f 0.2 0.6 f 0.4
1.5 f 1.5
0.1 f 0.0
0.4 f 0.0
a Unresolved (UCM)/resolved aliphatic hydrocarbons. b (C1-phenanthrenes + C2-phenanthrenes)/phenanthrene. c (C1-fluoranthenes + C1-pyrenes)/ (fluoranthene + pyrene). d (Cyclopenta[c,d]pyrene + benz[a]anthracene + benzo[a]pyrene)/(chrysene + triphenylene + benzo[e]pyrene). e Treated in Suntest for 4 h. f,g Treated in Suntest for 9 h. h PAH were Soxhlet-extracted from an urban filter and adsorbed onto precleaned soil dust by solvent evaporation. Treated in Suntest for 12 h. i Numbers at the two sides of the arrows are ratio values before and after Suntest treatment.
minor amounts contributed also by lubricant oils, asphalt dust, and grease from food cooking. While near-source aerosols exhibited significant proportions of both PHC and FHC fractions, FHC content (UCM and alkylated PAH) tended to decrease from near-source toward remote samples. This is illustrated in Figure 1, where the relative concentrations of phenanthrene and its alkylated derivatives are plotted. Urban (A) and coal-burning (C) particles show a large contribution from uncombusted fossil fuel (higher proportion of alkylated phenanthrenes) that is partially lost in analogue aerosols collected further away from the emission sites (B and D). Marine aerosols (E and F) exhibit a pyrogenic pattern (predominance of parent phenanthrene) consistent with what is found in remote surface sediments where the main PAH source is long or meso-range atmospheric transport (G and H). Equivalent trends were observed for fluoranthene, pyrene, and their C1-substituted homologues for which the relative proportion of substituted species decreased from near-source to remote aerosols and sediments by factors of 2-4. In order to ascertain whether this relative decrease of alkylated PAH is related to processes undergone by FHC during aerosol transport, we carried out an experiment where near-source aerosols were irradiated and aerated in a Suntest apparatus to simulate atmospheric aging processes such as photodegradation and vapor/particle partitioning. Significant changes in composition were observed (Figure 2 and Table 2), involving the relative removal of UCM, alkylated phenathrenes, fluoranthenes, and pyrenes as well as the more photoreactive PAH (cyclopenta[c,d]pyrene, benz[a]anthracene,
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benzo[a]pyrene) with the consequent enrichment in nalkanes, parent, and photostable PAH (chrysene, triphenylene, benzo[e]pyrene), respectively. Conversely, in a further experiment where PAH were extracted from an urban aerosol and redeposited onto cleaned soil dust particles by gentle solvent evaporation, Suntest treatment led to the quasidisappearence of the photolabile species, but the relative content of alkylated to parent PAH remained almost unchanged (Table 2, bottom). The apparent preference of FHC loss over PHC removal during aging in both field-collected and irradiated aerosols can have two possible, although not strictly exclusive, explanations. Either the two fractions differ in intrinsic lability or they differ in exposition to weathering because of their different association to combustion particles. There is no clear evidence to support or reject the first explanation. Fossil aliphatic UCMs mostly consist of branched and cyclic alkanes (12) that might be more reactive to (photo-) oxidation than the n-alkanes, although no experimental evidence for that has been reported. Also, it has been observed that PAH containing a benzylic carbon atom are susceptible to dark oxidation when associated with fly ash (3, 14). Thus, nonphotochemical oxidation could certainly account for the selective removal of alkylated PAH that we observe in aged aerosols. Conversely, preferential photooxidation of alkylated PAH is unlikely since the results of the irradiation experiment with PAH condensed on soil dust show that, in the absence of soot and soot-compound interactions, substituted and parent homologues exhibit equivalent photoreactivities.
FIGURE 1. Alkyl homologue distributions of phenanthrenes in nearsource and remote aerosols and in remote surface sediments. C1 and C2 indicate methyl- and dimethyl-substituted phenanthrenes. Concentrations have been normalized to the most abundant homologue in each plot. Aerosol sample description is in Table 1. The marine sediment distribution is representative of what is found in the deep central basin of the northwestern Mediterranean Sea (20). The lake sediment plot shows the average of eight European remote lakes with relatively shallow water columns and low diagenetic processes (27). In both cases, atmospheric deposition is the main source of the PAH distributions observed in the sediments. Altogether, our results are more consistent with the second explanation: the decoupling of FHC and PHC into two fractions with different exposition to weathering processes. The occurrence of so-called exchangeable and non-exchangeable fractions of hydrocarbons in aerosols has been suggested from considerations based on hydrophobicity and partitioning models (15, 16). Non-exchangeable hydrocarbons are those whose measured concentration in particles is higher than the model prediction. It is commonly accepted that pyrogenic PAH, generated along with soot, adsorb onto soot surfaces, which protect them from major transformations (14, 17) not only in the atmosphere but also throughout water columns and sediments (18-20). Less is known about the composition of the exchangeable fraction that contains the compounds most exposed to physicochemical losses such as photodegradation, vapor/particle partitioning, and dissolution into droplets. Figure 3 plots the results of the combined analyses of soot content and pyrogenic and alkylated PAH in marine (i.e., aged) aerosols collected at Mallorca Island (West Mediterranean). Whereas pyrogenic PAH exhibited a significant linear
FIGURE 2. Gas chromatograms of the aliphatic hydrocarbon fraction of an urban aerosol treated with irradiation and aeration. Scales have been normalized. Abbreviations: is, internal standard; UCM, unresolved complex mixture. Position of the peaks of n-alkanes with 29 and 31 carbon atoms is indicated as reference. Note the selective loss of the UCM at longer radiation time, as expressed by a decrease in the U:R (unresolved/resolved) ratio. relation with soot content (r2 ) 0.9043, n ) 11), no correlation was observed for the alkylated PAH (r2 ) 0.1844). This provides additional evidence for a decoupled association of FHC and PHC with soot. Indeed, studies of hydrocarbon distribution in size-fractionated aerosols conducted in our laboratory have shown that PHC essentially occur in the submicron (soot-rich) fractions whereas FHC, although still predominating in smaller particles, are more homogeneously distributed throughout all sizes (22-24). This trend is more pronounced in remote than in urban aerosols (22-24), thus indicating that partitioning processes do affect FHC-soot association during aerosol transport. Thus, our results overall indicate for the first time that FHC mostly belong to the exchangeable fraction of combustion aerosol hydrocarbons. The fractionation mechanism could be that FHC condense as a coating onto already-formed soot particles in combustion exhausts, as suggested for volatile PAH (21). Due to weak association to soot, FHC will undergo vapor/particle partitioning during transport, being redistributed onto all-sized particles. We also suggest that FHC will be more efficiently scavenged during atmospheric transport because of their higher association to coarser particles, whereas PHC preferentially undergo soot-mediated long-range dispersion. This is in agreement with the observation of a greater proportion of UCM in deposited particles with respect to suspended particles, along with the opposite
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Literature Cited
FIGURE 3. Pyrogenic (filled circles) and methylated (open squares) PAH vs soot content in marine aerosols from Mallorca (western Mediterranean). Pyrogenic PAH encompass benzofluoranthenes, benzo[e]pyrene, indeno[1,2,3-c,d]pyrene, benzo[g,h,i]perylene, and coronene. Photoreactive pyrogenic PAH have been excluded for calculations. Methylated PAH include the C1 and C2 derivatives of phenanthrene, anthracene, fluoranthene, and pyrene. proportions of non-alkylated PAH, in a Swedish rural area (25). Thus, we conclude that source reconciliation based on FHC/PHC relative distributions in aged aerosols should be done with caution. Our observations are also relevant to the understanding of the formation processes of organic cloud condensation nuclei (CCN), since submicron carbonaceous aerosols are a major source of tropospheric CCN (4, 26). Aging enhances CCN activity of particles as their surface becomes hygroscopic by uptake of water-soluble substances such as sulfuric acid (4, 26). The loss of the hydrophobic FHC coating suggested herein would also contribute to combustion aerosol hygroscopy and, hence, to their transformation into CCN.
Acknowledgments We are thankful to M. Colom-Alte´s, M. Aceves, X. Querol, J. L. Ferna´ndez-Turiel, and A. Conde for providing samples and/ or sampling assistance and to S. Riera, who performed soot measurements. Support by CEC through ENVIRONMENT (EROS-2000 project) and SCIENCE (SC1*-CT92-0824) programs is acknowledged.
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Received for review December 2, 1996. Revised manuscript received April 21, 1997. Accepted May 12, 1997.X ES960994M X
Abstract published in Advance ACS Abstracts, June 15, 1997.
