Atmospheric Occurrence and Deposition of Polycyclic Aromatic

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Environ. Sci. Technol. 2007, 41, 5608-5613

Atmospheric Occurrence and Deposition of Polycyclic Aromatic Hydrocarbons in the Northeast Tropical and Subtropical Atlantic Ocean SABINO DEL VENTO AND JORDI DACHS* Department of Environmental Chemistry, IIQAB-CSIC, Jordi Girona 18-24, Barcelona 08034, Catalunya, Spain

Atmospheric transport and deposition of polycyclic aromatic hydrocarbons (PAHs) over tropical oceans has not yet been studied even though tropical oceans account for 35% of the global oceans. Here we show the results from measurements of gas- and aerosol-phase PAHs and dry deposition samples in the NE tropical Atlantic Ocean atmosphere, a region between 26° and 21° N, off shore of the Saharan desert. The results show that PAHs concentrations are high at the coastal ocean (15-20 ng m-3 for sum of 27 gas-phase individual PAHs) and decrease by a factor of 2-3 at open ocean (26° W). The spatial variability observed is consistent with dilution, reaction, and deposition during transport. Atmospheric dry deposition velocities ranged between 0.1 and 0.3 cm s-1 with higher deposition velocities for the more volatile PAHs. Outbreaks of Saharan Dust significantly increase the deposition rates of PAHs. The occurrence and deposition of PAHs in tropical regions is complex and results from the interplay of a number of processes, such as wind speed, aerosol loads, important sources from West Africa, processes controlling the diurnal variability, and sequestration driven by high primary productivity regions. The measured average atmospheric residence times of gas- and aerosolphase PAHs are 3.7 and 3.5 days, respectively.

Introduction Long range atmospheric transport and deposition of organic pollutants are important processes to understand the regional and global transport of pollutants and their impact to remote ecosystems such as oceanic waters. The few studies that have focused on atmospheric occurrence of pollutants such as polycyclic aromatic hydrocarbons (PAHs) over the oceans are based on either north-south latitudinal transects or studies in temperate regions (1). Atmospheric transport in tropical regions has received little attention even though they account for 35% of the global oceans. Furthermore, few studies have focused on the variability of atmospheric PAHs during transport. Depletion of atmospheric concentrations can be due to a number of processes such as dilution, atmospheric deposition, and degradation. Furthermore, the magnitude of these processes depends on many factors such as wind speed regime, primary productivity of underlying ocean, aerosol load, temperature, etc. The Northeast Subtropical/Tropical Atlantic Ocean is a region receiving important atmospheric input of Saharan * Corresponding author e-mail: [email protected]. 5608

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dust and characterized by strong gradients in primary productivity (2) which may influence the regional fate of organic pollutants (3). Furthermore, it is characterized by high temperatures and a wind regime dominated by trade winds that facilitate its study due to lack of significant changes in wind direction, etc. The objectives of this study are (i) to report the occurrence of PAHs during atmospheric transport in the subtropical/tropical atmosphere of the NE Atlantic, (ii) quantify atmospheric dry deposition of PAHs, and (iii) elucidate the factors driving the deposition and atmospheric residence time of PAHs.

Materials and Methods Sampling and Site Characterization. Shipboard atmospheric samples were collected in the northeast subtropical/tropical Atlantic Ocean, during a cruise on board the R/V Hespe´rides from May 16 to June 8, 2003. The cruise track, a box delimited by the West African coast and meridian 26° W, and parallels 26° N and 21° N, is shown in Figure 1.1 in the Supporting Information, Annex I. Gas and aerosol phase samples and dry deposition atmospheric samples were taken according to the following procedures. Air Samples. A total of 27 pairs of atmospheric samples (gas + aerosol phase) were collected using a modified highvolume air sampler (MCV, Collbato´, Spain), placed on the bow of the ship, to minimize contamination from the ship’s exhausts. The air sampler operated at a calibrated flow rate of approximately 40 m3 h-1, so that the air volume ranged ∼450-880 m3 per sample. The air stream passed first through quartz microfiber filters (QM-A, Whatman International Ltd., Maidstone, England), to collect particulate phase, and then through polyurethane foam adsorbents (PUF), which retain gas phase. PUF plugs had been pre-extracted with a mixture of hexane/acetone (1:1, v/v) for 48 h, wrapped in aluminum foil, vacuum-dried in desiccators, and stored in double-sealed plastic bags. QM-A filters (20.3 × 25.4 cm; 2.2 µm pore size) had been combusted at 400 °C overnight and sealed in aluminum foil prior to use. Before and after sampling, filters were weighted to obtain total suspended particle (TSP) concentrations. Air samples were collected continuously both during stations and transit between two sampling stations. Usually a 24 h sampling was carried out, except for the last 4 samples in the south transect where 12 h day/night samples were taken. Dry Deposition Atmospheric Samples. During the cruise, 4 measurements (2 along parallel 26° N, 1 along longitudinal transect at 26° W and 1 in south transect at 21° N) were conducted of PAHs dry deposition fluxes (FDD, ng m-2 d-1). Results are given for PAHs with molecular weight >202 for minimizing sampling artifacts (4). Empirical deposition velocities (vD, m d-1) were calculated from FDD ) vDCA, where CA is the average PAHs suspended particle concentration for the measurement period. Dry deposition was measured on two stainless steel trays filled with pre-filtered seawater (GFF filter, 2 filtrations) which were left on the bow of the ship. After 3-5 days of sampling, without rain events, the seawater of each tray was first filtered through a GFD filter (Whatman, 47 mm diameter, mesh size 2.7 µm) to collect the large size aerosols. The filtered seawater was then filtered through a GFF filter (Whatman, 47 mm diameter, mesh size 0.7 µm) to collect aerosols in the 0.7-2.7 µm size range. All filters were previously combusted at 400 °C overnight and sealed in aluminum foil prior to use. More details about the methodological procedure can be found elsewhere (4). Meteorological Data. Wind speed and direction, humidity, and air and water temperature were monitored continuously 10.1021/es0707660 CCC: $37.00

 2007 American Chemical Society Published on Web 07/14/2007

at every minute onboard during the entire duration of the cruise. Analytical Procedures. All samples were spiked with perdeuterated PAHs surrogate standards (anthracene-d10 and perylene-d12, Cambridge Isotopes, Cambridge, UK) before extraction. Air Samples. PUFs (gas phase) and QM-A (particle phase) were Soxhlet extracted for 24 h in hexane/acetone (1:1, v/v) (Merck, Darmstadt, Germany) and dichloromethane/methanol (2:1, v/v) (Merck, Darmstadt, Germany), respectively. The extracts were concentrated by rotary evaporation to 1 mL and for QM-A samples were solvent-exchanged to hexane. All samples were then fractionated on a 3% deactivated alumina column (3 g) with a top layer of anhydrous sodium sulfate. Each column was eluted with a first fraction of 5 mL of hexane, and a second fraction of 12 mL of dichloromethane/hexane (2:1, v/v). The second fraction containing PAHs was concentrated in a rotary evaporator and solventexchanged to isooctane under a gentle stream of nitrogen. Dry Deposition Atmospheric Samples. Extraction of GFD and GFF filters containing deposited aerosols was performed by sonication of the spiked filters with dichloromethane/ methanol (2:1, v/v) (Merck, Darmstadt, Germany) (3 × 20 mL) for 20 min. The recovered extracts were combined, concentrated by rotary evaporation to around 1 mL, and solvent-exchanged to hexane. All samples were then cleanedup on a 3% deactivated alumina column (1.5 g) and eluted with 5 mL of dichloromethane/hexane (2:1, v/v). Further details for methods and GC-MS analyses are described elsewhere (5, 6) and in the SI, Annex III . A total of 27 individual PAHs were quantified. Throughout the text, the following abbreviations are used: phenantrene (Phen), anthracene (Ant), dibenzothiophene (DBT), methydibenzothiophenes (MDBTs, sum of three compounds), methylphenantrenes (MPhens, sum of four compounds), dimethylphenantrenes (DMPhens, sum of five compounds), fluoranthene (Fla), pyrene (Py), benz(a)anthracene (BaA), chrysene (Chry), benzo(b)fluoranthene and benzo(k)fluoranthene (Bb+kF, as sum of both compounds), benzo(e)pyrene (BeP), benzo(a)pyrene (BaP), perylene (Per), indeno(1,2,3-cd)pyrene (IP), dibenz(a, h)anthracene (DBA), and benzo(ghi)perylene (BghiP). Quality Assurance. Mean PAH surrogate recoveries in PUFs samples were 92% and 59% and in QM-A filters were 77% and 88% for anthracene-d10 and perylene-d12, respectively. Reported values are not recovery corrected. Perdeuterated PAHs (pyrene-d10 and benz[a]anthracene-d12, Cambridge Isotopes Cambridge, UK) were used as internal standards. Field blank masses for PUF and QFF accounted for 0.5-12.3% of the total PAHs (27 compounds) mass in gas-phase samples and 0.4-3.4% for QMA. Three split PUFs were used to quantify breakthrough of gas phase. The second half of the split PUFs accounted for between 3 and 7% of the total mass collected on the entire PUF, and thus there is not a significant breakthrough of PAHs. A fraction of the QM-A were analyzed to determine atmospheric organic (OC) and elemental (EC) carbon aerosol concentrations by thermal-optical transmittance in a Sunset Laboratory carbon analyzer using the NIOSH temperature protocol (7). The low EC concentrations (0.72 ( 0.59 µg m-3) and the higher OC/EC ratios (8) suggest no contamination from emissions from the ship’s funnel. Besides, samples from the north-south transect are not considered here.

Results and Discussion Site Characterization During the Sampling Period. The region off the NW African coast is influenced by the Canary Current. The coastal upwelling region and open ocean zone differ both in structure and dynamics. Primary production gradients encompass highly productive waters especially off

the NW African coast (3.5 µg Chl a L-1) and highly oligotrophic waters near the subtropical gyre (0.05 µg Chl a L-1) (2). All year long northeasterly winds sustain the coastal upwelling (9, 10). Since there is usually an active POP cycling between air and water, the samples taken can be separated into two groups according to water masses’ origin (10, 11). The first, representative of open ocean conditions, includes samples taken west of meridian 23° W. The second, at the costal upwelling region, proximate upwind anthropogenic and continental sources (more exposed to land influence), includes those samples collected east of meridian 18° W, close to the Canary Islands and NW African coast. Different water masses and metabolic processes usually occur in the two longitudinal transects where samples were taken. Heterotrophic processes occur in the north transect. Conversely, net autotrophic metabolism processes dominate the costal upwelling of cold water in the southern transect (10, 12). In order to reflect these features, results are reported for both transects, referred as north (26° N) and south (21° N) transect, respectively, and for open ocean and costal zone (SI, Annex III). This division is also supported by back trajectories analysis (see discussion below). Winds speeds (as measured on the ship) were always high (on average 10 m s-1) and from the northeast direction. Air mass origins for the cruise samples were established using the NOAA HYbrid Single-Particle Lagrangian Integrated Trajectory Model (HYSPLIT) (13). The 48 h back trajectories (BTs) were traced at 15, 100, 500, 2000, and 3000 m above sea level (masl). Based on boundary layer BTs (at 15, 100, and 500 masl), coastal samples had a certain land influence from NW African coast, the Canary Islands and, to a lesser extend, the Iberian Peninsula. Air masses were therefore impacted by continental emissions even during periods of marine flow. The performed BTs for open ocean show that these trajectories are clearly oceanic, as air had crossed the Atlantic Ocean for at least 2 days before the sampling period (SI, Annex I). Sea-level BTs reflect only the influence of trade winds in the marine boundary layer. BTs at higher altitude are useful to study the Saharan dust convey, as the main stream of dust transport takes place between 1500 and 7000 m asl (14, 15). Since large dust aerosols can deposit with high deposition speeds, BTs at different altitudes provide more comprehensive information than those at sea level. African dust outbreaks occur with different frequency, intensity, and duration all year long (16, 17). The campaign took place during a period of the year when Sahara air mass did not intrude the trades wind inversion layer (14, 15). Therefore, sea-level BTs reflect only the influence of trade winds in the marine boundary layer (MBL). High altitude transport can be detected by BTs calculated at 2000-3000 masl (SI, Annex II). They clearly show different pathways from BTs at lower altitude (SI, Annex I) and air mass’ origin is from Africa. Therefore, this suggests that deposited particles proceeding from higher levels of the atmosphere could contribute to sampled aerosols. PAHs Occurrence in the NE Subtropical Atmosphere. Average, minimum, and maximum PAHs gas and particulate concentrations are given in Supporting Information Annex III for north and south transects (see also Figure 1). Gaseous average PAHs total concentrations (Σ27 PAHs) reached 15.11 and 19.79 ng m-3 in the north and south transect, respectively. Average total PAH concentrations were higher at coastal ocean (19.70 ng m-3) than that at open sea (14.59 ng m-3). Conversely, Σ27 PAHs aerosol phase concentrations were 0.42 and 1.05 ng m-3 in open ocean and costal areas, and 0.67 and 1.06 ng m-3 in north and south transect, respectively. Low molecular weight (LMW) PAHs with three and four rings (from Phe to Py) were the most abundant in gas phase. Phe was the dominant compound, followed by MPhens, MDBTs, VOL. 41, NO. 16, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Dry Deposition Flux (ng m-2 d-1), Overall Dry Deposition Velocity (cm s-1), Total Deposited Matter (mg m-2 d-1), and Total Suspended Matter (µg m-3)a dry deposition flux (ng m-2 d-1) fine fraction (0.7 - 2.7µm) 26° N 15° W

26° N 18° W

coarse fraction (>2.7 µm) 21° N 23° W

26° N 15° W

26° N 18° W

overall dry deposition velocity (cm s-1) 21° N 23° W

fluoranthene pyrene benz(a)anthracene chrysene benzo(b)+(k)fluoranthene benzo(e)pyrene benzo(a)pyrene perylene indeno(1,2,3-cd)pyrene dibenz(a,h)anthracene benzo(ghi)perylene Σ12 PAHs

2.45 2.72 1.13 3.00 6.52 3.62 1.99 0.42 2.30 0.08 3.04 27.27

0.757 0.867 0.134 0.40 0.78 0.592 0.16 3.69

1.11 3.81 NA NA NA NA NA NA NA NA NA 4.93

4.64 5.52 2.31 3.91 11.53 6.04 3.68 0.00 5.90 10.11 53.62

1.63 1.94 0.56 1.26 2.17 0.81 0.58 0.37 0.71 0.94 10.96

56.23 40.94 19.07 46.76 64.27 24.29 15.73 0.00 10.91 0.56 8.94 287.69

total deposited matter

138.03b

156.76b

212.22b

227.94b

261.14b

294.54b

TSP a

- ) not found; NA ) not available.

b

mg

m-2

d-1. c

Average.

d

µg

9

26° N 18° W

21° N 23° W

0.18 0.20 0.23 0.16 0.09 0.09 0.23 0.02 0.07 0.01 0.10 0.12c

0.10 0.13 0.16 0.10 0.09 0.06 0.16 0.14 0.03 0.04 0.10c

0.73 0.52 0.54 0.43 0.29 0.21 0.38 0.07 0.04 0.04 0.33c

98.78d

57.95d

37.35d

m-3.

and DMPhens. All PAHs were detected in aerosol phase. MPhens, DMPhens, and MDBTs were the most abundant compounds in aerosol followed by B(ghi)P, IP, Py, Phe, BeP, and Cry. PAHs concentrations reported here are high and comparable to those reported for coastal/semi-rural locations and lower than urban sites (6, 18-21). PAHs concentrations in the northeast subtropical Atlantic Ocean are comparable with those reported for Eastern Mediterranean for the gas (3.6-56.3 ng m-3) and aerosol phase (4.1-57.2 ng m-3), where air masses originated mainly from North Europe (20). LMW PAHs gaseous concentrations are similar to those described from the Northwest Mediterranean coast, whereas particulate concentrations are an order of magnitude lower (6) but this is consistent with the low EC concentrations measured in the NE Atlantic (SI, Annex V). Phe, Fluo, Py, and Chry gaseous concentrations were in the high range of those measured at Tenerife Island below the atmospheric inversion layer (6.0, 3.6, 1.4, and 0.07 ng m-3, respectively) (22). Mean gas concentrations along a north-south Atlantic transect (1) were 2-3-fold lower of those found in this study, even though this study did not report data for the latitudinal range studied here. Jaward et al. reported minimum concentrations in the South Hemisphere oceanic samples, and these are comparable to minimum values measured in this study along south transect. However, studies such as that of Jaward et al. show that there is a considerable regional and global variability of PAH concentrations over the global oceans and higher concentrations are found in tropical areas (1). Aerosol-phase PAHs concentrations in this study were in the same range as shipboard measurements over the Sea of Japan during ACE-Asia campaign (21), but an order of magnitude higher than those reported in the North Atlantic Ocean (23) and in the tropical Atlantic Ocean 25 years ago (24). The issue of high atmospheric concentrations of POPs in tropical areas is interesting and merits some attention. Higher values of other variables such as nitrogen, metals, and OC, measured in this region, in this (11, 25) and other cruises (26, 27), suggest the influence of sources from proximate North Africa and up-wind Europe. Other factors which could contribute to explain high levels of POPs are high temperature, which can displace equilibrium toward air, and low soil and vegetation reservoir capacity of regional continents 5610

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and islands to retain POPs. Due to land ecosystems with very low biomass vegetation, low soil organic carbon, and high temperatures such as those of the arid and desert region of NW Africa, these have little capacity to hold POPs (28). In any case, the high concentrations found in coastal-influenced atmospheres in the tropical Atlantic is an issue that needs further attention. Atmospheric Dry Deposition. Different techniques and surface receiving mediums have been used for dry deposition measurements (19, 29-31). So far, all measurements have been performed on land and oceanic deposition rates are extrapolated from those made at coastal sites. As far as we know, this is the first time that direct dry deposition measures are carried out onboard a research vessel. Deposition fluxes and velocities are presented in Table 1 according to the mean latitude and longitude during each deposition period along the two longitudinal transects. A Saharan dust outbreak occurred at the beginning of the campaign (north transect, east of longitude 22° W). Desert dust collected in the samples was even visually detectable due to its characteristic brown color. As a consequence of the Sahara dust outfall, a TSP local maximum was recorded during the first of the dry deposition measurements, which was reflected in high aerosol fluxes (350 mg m-2 d-1). As previously discussed, air masses at high altitudes have African origin, thus Sahara dust outfall from high altitude adds to suspended particles in the marine boundary layer. Significantly lower aerosol deposition fluxes were observed at open sea (147-218 mg m-2 d-1) and in the southern transect (165 mg m-2 d-1). Details on these aerosol fluxes and their nutrient and metal content can be found elsewhere (11, 25). Conversely the higher PAH FDD (293 ng m-2 d-1) was measured in the southern transect measurement, a period with higher wind speed. Increased wind speed accelerates deposition of aerosols (30) and the effect is more pronounced for accumulation mode aerosols, which include most of the PAHs. Concurrent maximum fluxes of other compounds, e.g., total nitrogen, phosphorus, and total organic carbon were also observed (11). PAH concentrations in suspended aerosols (CP, ng g-1) sampled simultaneously to the three deposition sampling periods are shown in Figure 2a. Off-shore aerosol concentrations in the north transect, corresponding to the second data set (3687 ng g-1), were lower than those in the coastal

FIGURE 1. Longitudinal trends for selected PAHs along the north transect (26° N) and south transect (21° N). Solid lines: gas phase; dotted lines: aerosol phase. (a) phenantrene in north transect; (b) phenantrene in south transect; (c) benz(a)anthracene in north transect; and (d) benz(a)anthracene in south transect.

FIGURE 2. PAHs total suspended concentrations (ng g-1) (a) and dry deposition fluxes (FDD, ng g-1) for coarse (b) and fine particles (c) at mean latitude and longitude during each experiment. area (8828 ng g-1). Although higher concentrations (33 235 ng g-1) were registered in the south transect at 21° N 23° W, corresponding to the higher PAH deposition fluxes. Only a small fraction of aerosol-phase PAH concentrations (CP ng g-1) finally deposit. PAHs concentrations in deposited particles (ng g-1) are smaller in comparison to those in suspended aerosol (Figure 2b and c) by a factor of 20-50. Most of the suspended particles are accumulation mode particles with high PAH concentrations, while deposited particles are much larger and thus have consistently lower PAH concentrations. As observed in previous studies (32), fine particles are responsible for only a small fraction of the dry deposition flux. PAHs in deposited aerosols smaller than 2.7 µm (with already a fraction of coarse aerosols) account for between 25% and 34% of total PAHs deposition flux. PAHs

in suspended aerosol-phase concentration and deposition samples show similar profiles (Figure 2), especially for PAHs with more than four aromatic rings. LMW PAHs were more abundant in large-sized aerosols consistent with earlier works that have shown that more volatile PAHs are associated with coarse particles (33, 34). Empirical deposition velocity (vD) follows a similar pattern of dry deposition flux: higher vD values (average 0.33 cm s-1) were recorded in the measurement in the south transect than in the northern transect (average 0.11 cm s-1). Values reported here are consistent with previous studies of aerosolphase PAH deposition velocities in coastal or lake environments which usually ranged from 0.1 to 0.8 cm s-1 (35, 36), or from 0.01 to 0.8 cm s-1 in modeling studies (30). These velocities are up to 1 order of magnitude lower than VOL. 41, NO. 16, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Correlation of overall dry deposition velocity (vD, cm s-1) vs log super-cooled liquid vapor pressure (vP, Pa). Diamonds represent experiment at 26° N 15° W, squares at 26° N 18° W and circles at 21° N 23° W. Values of vP from ref 39, except Py, BaA, and BeP are from ref 40. deposition velocity for total suspended matter (average 3.8 cm s-1) (11). As PAHs tend to associate to fine particles (33), they are expected to have lower vD. The vd values for individual PAHs show lower values for heavier compounds, as reported in other works (34). To investigate the relationship between vD and physical-chemical properties, vD was plotted against vapor pressure (vp, Pa) (Figure 3). vD positively correlates with logarithm of vapor pressure (vp, Pa) (Figure 3) with higher vD for the more volatile PAHs. This variability is statistically significant for two of the three measurements (Figure 3). At open sea, where fluxes are much lower, there is not a significant correlation between vD and vapor pressure. Longitudinal Spatial Trends. Previous studies on the spatial variability of atmospheric concentrations of PAHs and other POPs have focused on latitudinal transects, but little work has been done on longitudinal gradients which may provide useful information on changes in atmospheric concentrations during transport. Longitudinal trends for selected PAHs (Phe, Py, BaA, and B(ghi)P) are plotted in Figure 1 and SI Annex IV for north and south transects, respectively. North transect PAHs gas and aerosol concentrations were coupled and decreased by a factor of 2 or 3 as the ship cruised off-shore. The decrease can be the consequence of a number of factors such as dilution, atmospheric deposition (airwater exchange, wet and dry deposition), and possible reactions with hydroxyl radical (37) and ozone (20), but points clearly to a significant source of PAHs upwind. The fact that gas and aerosol phase are coupled indicates that the dynamic gas-aerosol partitioning is faster than the other processes, including those driving the decrease in concentrations, and/ or equilibration of dust aerosol deposited from higher air masses. It is possible to estimate atmospheric residence times (ARTs) from north transect samples (see SI, Annex VI) which are 3.7 days for the gas phase, and 3.5 days for the aerosol phase considering only those PAHs that were also observed in the gas phase. While gas-phase ART is constant for all PAHs, for aerosol phase PAHs it goes from 1 to 2 days for phenanthrene-pyrene to 6 days for benzofluoranthenes. ART for heavier compounds is even longer (see SI). This again is presumably related to different association of lighter PAHs to larger aerosols and being more available for dynamic gasparticle partitioning, while the heavier PAHs would be recalcitrant to degradation and deposition processes due to their association with accumulation mode soot aerosols, with lower deposition rates. These ARTs are significantly shorter than those obtained by considering dry aerosol deposition only (7.9 days), thus indicating that dilution, degradation, and air-water exchange play a role as drivers of ART. Conversely, gas and aerosol phase in the south transect are partially uncoupled (Figure 1 and SI, Annex IV), indeed 5612

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there is a clear variability in the gas-phase concentrations that is not always followed by the aerosol-phase PAHs. The strong variability in gas-phase concentrations is especially evident for the 2 days where day and night time periods were sampled separately. Jaward et al. first reported evidence of diurnal cycling for POPs over the ocean during a northsouth Atlantic transect and suggested that it occurs only in remote areas, such as South Atlantic tropical Ocean (1, 38). PAHs profiles in south transect (Figure 1 and SI, Annex IV) show a similar variation, especially for LMW compounds in gas phase, such as Phen and Py. Interestingly, this variability, although less enhanced, is also observed in the aerosol phase. Jaward et al. discussed the potential processes that could drive this diurnal variability, even though much uncertainty remains on the significant drivers (1, 38). In fact, the dynamic coupling of water column processes and atmospheric occurrence of POPs has been suggested in a number of studies (1, 3). In addition to the diurnal cycles, higher PAHs concentrations in off-shore than close to the African coast were observed for transect at 21° N. This is consistent with this coastal oceanic region being under the influence of the strong up-welling area. Enhanced air-waterphytoplankton exchange and thus sequestration fluxes due to settling particles (3) is consistent with these lower concentrations in the upwelling influenced region. The present study suggest that factors driving the occurrence of PAHs, and presumably other POPs, are complex in tropical regions in terms of sources, occurrence, transport, and fate.

Acknowledgments We gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY website (http://www.arl.noaa.gov/ready.html) used in this publication. S.D.V. acknowledges Ph.D. fellowship from the Catalan Government. This research is part of DEPOCEC projects, founded by the Spanish Ministry of Education and Science. S. Agustı´ is acknowledged for leading the COCA cruise.

Supporting Information Available Information about gas and aerosol concentrations, BTs EC/ OC, and residence time. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Jaward, F. M.; Barber, J. L.; Booij, K.; Jones, K. C. Spatial distribution of atmospheric PAHs and PCNs along a northsouth Atlantic transect. Environ. Pollut. 2004, 132 (1), 173-181. (2) Calleja, M. L.; Duarte, C. M.; Navarro, N.; Agusti, S. Control of air-sea CO2 disequilibria in the subtropical NE Atlantic by planktonic metabolism under the ocean skin. Geophys. Res. Lett. 2005, 32 (8), 1-4. (3) Dachs, J.; Eisenreich, S. J.; Hoff, R. M. Influence of eutrophication on air-water exchange, vertical fluxes, and phytoplankton concentrations of persistent organic pollutants. Environ. Sci. Technol. 2000, 34 (6), 1095-1102. (4) Del Vento, S.; Dachs, J. Influence of the surface microlayer on atmospheric deposition of aerosols and polycyclic aromatic hydrocarbons. Atmos. Environ. 2007, 41 (23), 4920-4930. (5) Dachs, J.; Glenn, I.; Thomas, R.; Gigliotti, C. L.; Brunciak, P.; Totten, L. A.; Nelson, E. D.; Franz, T. P.; Eisenreich, S. J. Processes driving the short-term variability of polycyclic aromatic hydrocarbons in the Baltimore and northern Chesapeake Bay atmosphere, USA. Atmos. Environ. 2002, 36 (14), 2281-2295. (6) Perez, S.; Dachs, J.; Barcelo, D. Sea Breeze Modulated Volatilization of Polycyclic Aromatic Hydrocarbons from the Masnou Harbor (NW Mediterranean Sea). Environ. Sci. Technol. 2003, 37 (17), 3794-3802. (7) Birch, M. E.; Cary, R. A. Elemental carbon-based method for monitoring occupational exposures to particulate diesel exhaust. Aerosol Sci. Technol. 1996, 25 (3), 221-241. (8) Dachs, J.; Calleja, M. L.; Duarte, C. M.; del Vento, S.; Turpin, B.; Polidori, A.; Herndl, G. J.; Agusti, S. High atmosphere-ocean

(9)

(10)

(11)

(12) (13)

(14)

(15)

(16) (17) (18) (19)

(20) (21)

(22)

(23)

(24) (25)

exchange of organic carbon in the NE subtropical Atlantic. Geophys. Res. Lett. 2005, 32 (21), 1-4. Eglinton, T. I.; Eglinton, G.; Dupont, L.; Sholkovitz, E. R.; Montluc¸ on, D.; Reddy, C. M. Composition, age, and provenance of organic matter in NW African dust over the Atlantic Ocean. Geochem. Geophys. Geosyst. 2002, 3 (8). Pelegri, J. L.; Aristegui, J.; Cana, L.; Gonzalez-Davila, M.; Hernandez-Guerra, A.; Hernandez-Leon, S.; Marrero-Diaz, A.; Montero, M. F.; Sangra, P.; Santana-Casiano, M. Coupling between the open ocean and the coastal upwelling region off northwest Africa: Water recirculation and offshore pumping of organic matter. J. Mar. Syst. 2005, 54 (1-4 Spec. Iss.), 3-37. Duarte, C. M.; Dachs, J.; Llabres, M.; Alonso-Laita, P.; Gasol, J. M.; Tovar-Sanchez, A.; Sanudo-Wilhemy, S.; Agusti, S. Aerosol inputs enhance new production in the subtropical northeast Atlantic. J. Geophys. Res. 2006, 111 (G4), 4006-4006; art. no. G04006. Helmke, P.; Romero, O.; Fischer, G. Northwest African upwelling and its effect on offshore organic carbon export to the deep sea. Global Biogeochem. Cycles 2005, 19 (4). Draxler, R. R.; Rolph, G.D. HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) Model access via NOAA ARL READY Website (http://www.arl.noaa.gov/ready/hysplit4.html). NOAA Air Resources Laboratory: Silver Spring, M, 2003. Chiapello, I.; Bergametti, A. G.; Chatenet, A. B.; Bousquet, A. P.; Dulac, A. F.; Soares, A. E. S. Origins of African dust transported over the northeastern tropical Atlantic. J. Geophys. Res. 1997, 102 (D12), 13701-13709. Viana, M.; Querol, X.; Alastuey, A.; Cuevas, E.; Rodriguez, S. Influence of African dust on the levels of atmospheric particulates in the Canary Islands air quality network. Atmos. Environ. 2002, 36 (38), 5861-5875. Arimoto, R. Eolian dust and climate: relationships to sources, tropospheric chemistry, transport and deposition. Earth-Sci. Rev. 2001, 54 (1-3), 29-42. Goudie, A. S.; Middleton, N. J. Saharan dust storms: Nature and consequences. Earth-Sci. Rev. 2001, 56 (1-4), 179-204. Gigliotti, C. L.; Dachs, J.; Nelson, E. D.; Brunciak, P. A.; Eisenreich, S. J. Polycyclic aromatic hydrocarbons in the New Jersey coastal atmosphere. Environ. Sci. Technol. 2000, 34 (17), 3547-3554. Gigliotti, C. L.; Totten, L. A.; Offenberg, J. H.; Dachs, J.; Reinfelder, J. R.; Nelson, E. D.; Glenn, T. R., IV; Eisenreich, S. J. Atmospheric concentrations and deposition of polycyclic aromatic hydrocarbons to the Mid-Atlantic East Coast region. Environ. Sci. Technol. 2005, 39 (15), 5550-5559. Tsapakis, M.; Stephanou, E. G. Polycyclic aromatic hydrocarbons in the atmosphere of the Eastern Mediterranean. Environ. Sci. Technol. 2005, 39 (17), 6584-6590. Simoneit, B. R. T.; Kobayashi, M.; Mochida, M.; Kawamura, K.; Lee, M.; Lim, H. J.; Turpin, B. J.; Komazaki, Y. Composition and major sources of organic compounds of aerosol particulate matter sampled during the ACE-Asia campaign. J. Geophys. Res. D 2004, 109 (19), D19S10 1-22. Ribes, S.; Van Drooge, B.; Dachs, J.; Gustafsson, O.; Grimalt, J. O. Influence of Soot Carbon on the Soil-Air Partitioning of Polycyclic Aromatic Hydrocarbons. Environ. Sci. Technol. 2003, 37 (12), 2675-2680. Crimmins, B. S.; Dickerson, R. R.; Doddridge, B. G.; Baker, J. E. Particulate polycyclic aromatic hydrocarbons in the Atlantic and Indian Ocean atmospheres during the Indian Ocean Experiment and Aerosols99: Continental sources to the marine atmosphere. J. Geophys. Res. D 2004, 109 (5), D05308 1-12. Marty, J. C.; Tissier, M. J.; Saliot, A. Gaseous and particulate polycyclic aromatic hydrocarbons (PAH) from the marine atmosphere. Atmos. Environ., Part A 1984, 18 (10), 2183-2190. Tovar-Sanchez, A.; San ˜ udo-Wilhelmy, S. A.; Kustka, A. B.; Agusti, S.; Dachs, J.; Hutchins, D. A.; Capone, D. G.; Duarte, C. M. Effects

(26)

(27) (28)

(29)

(30)

(31) (32) (33) (34) (35)

(36)

(37)

(38)

(39) (40)

of dust deposition and river discharges on trace metal composition of Trichodesmium spp. in the tropical and subtropical North Atlantic Ocean. Limnol. Oceanogr. 2006, 51 (4), 17551761. Baker, A. R.; Jickells, T. D.; Biswas, K. F.; Weston, K.; French, M. Nutrients in atmospheric aerosol particles along the Atlantic Meridional Transect. Deep-Sea Res. Part II 2006, 53 (14-16), 1706-1719. Baker, A. R.; Kelly, S. D.; Biswas, K. F.; Witt, M.; Jickells, T. D. Atmospheric deposition of nutrients to the Atlantic Ocean. Geophys. Res. Lett. 2003, 30 (24), OCE 11-1-OCE 11-4. Dalla Valle, M.; Jurado, E.; Dachs, J.; Sweetman, A. J.; Jones, K. C. The maximum reservoir capacity of soils for persistent organic pollutants: Implications for global cycling. Environ. Pollut. 2005, 134 (1), 153-164. Van Drooge, B. L.; Grimalt, J. O.; Torres-Garcia, C. J.; Cuevas, E. Deposition of Semi-volatile Organochlorine Compounds in the free troposphere of the eastern north Atlantic ocean. Mar. Pollut. Bull. 2001, 42 (8), 628-634. Jurado, E.; Jaward, F. M.; Lohmann, R.; Jones, K. C.; Simo, R.; Dachs, J. Atmospheric dry deposition of persistent organic pollutants to the Atlantic and inferences for the global oceans. Environ. Sci. Technol. 2004, 38 (21), 5505-5513. Sun, P.; Blanchard, P.; Brice, K. A.; Hites, R. A. Trends in polycyclic aromatic hydrocarbon concentrations in the Great Lakes atmosphere. Environ. Sci. Technol. 2006, 40 (20), 6221-6227. Arimoto, R.; Duce, R. A.; Ray, B. J.; Tomza, U. Dry deposition of trace elements to the western North Atlantic. Global Biogeochem. Cycles 2003, 17 (1), 10-1. Offenberg, J. H.; Baker, J. E. Aerosol size distributions of Polycyclic Aromatic Hydrocarbons in urban and over-water atmospheres. Environ. Sci. Technol. 1999, 33 (19), 3324-3331. Vardar, N.; Odabasi, M.; Holsen, T. M. Particulate dry deposition and overall deposition velocities of polycyclic aromatic hydrocarbons. J. Environ. Eng. 2002, 128 (3), 269-274. Holsen, T. M.; Noll, K. E.; Fang, G.-C.; Lee, W.-J.; Lin, J.-M. Dry deposition and particle size distributions measured during the Lake Michigan urban air toxics study. Environ. Sci. Technol. 1993, 27 (7), 1327-1333. Duce, R. A.; Liss, P. S.; Merrill, J. T.; Atlas, E. L.; Buat-Menard, P.; Hicks, B. B.; Miller, J. M.; Prospero, J. M.; Arimoto, R.; Church, T. M.; Ellis, W.; Galloway, J. N.; Hansen, L.; Jickells, T. D.; Knap, A. H.; Reinhardt, K. H.; Schneider, B.; Soudine, A.; Tokos, J. J.; Tsunogai, S.; Wollast, R.; Zhou, M. The atmospheric input of trace species to the world ocean. Global Biogeochem. Cycles 1991, 5 (3), 193-259. Brubaker, W. W., Jr.; Hites, R. A. OH reaction kinetics of polycyclic aromatic hydrocarbons and polychlorinated dibenzo-p-dioxins and dibenzofurans. J. Phys. Chem. A 1998, 102 (6), 915921. Jaward, F. M.; Barber, J. L.; Booij, K.; Dachs, J.; Lohmann, R.; Jones, K. C. Evidence for dynamic air-water coupling and cycling of persistent organic pollutants over the open Atlantic Ocean. Environ. Sci. Technol. 2004, 38 (9), 2617-2625. Lei, Y. D.; Chankalal, R.; Chan, A.; Wania, F. Supercooled liquid vapor pressures of the polycyclic aromatic hydrocarbons. J. Chem. Eng. Data 2002, 47 (4), 801-806. Hinckley, D. A.; Bidleman, T. F.; Foreman, W. T.; Tuschall, J. R. Determination of vapor pressures for nonpolar and semipolar organic compounds from gas chromatographic retention data. J. Chem. Eng. Data 1990, 35 (3), 232-237.

Received for review March 31, 2007. Revised manuscript received May 31, 2007. Accepted June 8, 2007. ES0707660

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