Background Concentrations of Polychlorinated Dibenzo-p-Dioxins

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BACKGROUND CONCENTRATIONS OF POLYCHLORINATED DIBENZO-p-DIOXINS, DIBENZOFURANS AND BIPHENYLS IN THE GLOBAL OCEANIC ATMOSPHERE Laura Morales, Jordi Dachs, Belén González-Gaya, Gema Hernán, Manuela Abaos Abalos, and Esteban Abad Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es5023619 • Publication Date (Web): 01 Aug 2014 Downloaded from http://pubs.acs.org on August 11, 2014

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Environmental Science & Technology

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Submitted to Environmental Science & Technology

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BACKGROUND CONCENTRATIONS OF POLYCHLORINATED DIBENZO-p-

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DIOXINS, DIBENZOFURANS AND BIPHENYLS IN THE GLOBAL OCEANIC

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ATMOSPHERE

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Laura Morales, Jordi Dachs, Belén González-Gaya, Gema Hernán, Manuela Ábalos,

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Esteban Abad*

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Department of Environmental Chemistry, IDAEA-CSIC, Barcelona, Catalunya, Spain

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*Corresponding author email: [email protected]

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Abstract

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The remote oceans are among the most pristine environments in the world, away from sources

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of anthropogenic persistent organic pollutants (POP), but nevertheless recipient of atmospheric

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deposition of POPs that have undergone long range atmospheric transport (LRAT). In this work,

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the background occurrence of gas and aerosol phase polychlorinated dibenzo-p-dioxins and

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dibenzofurans (PCDD/Fs), and dioxin like polychlorinated biphenyls (dl-PCB) is evaluated for

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the first time in the atmosphere of the tropical and subtropical Atlantic, Pacific and Indian

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oceans. Thirty nine air samples were collected during the eight months Malaspina

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circumnavigation cruise on board the R/V Hespérides. The background levels of dioxins and dl-

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PCBs remained very low and in many cases very close or below the limit of detection.

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Expectedly, the levels of PCBs were higher than dioxins being PCB#118 the most abundant

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compound. In the particular case of dioxins, octachlorodibenzo-p-dioxin (OCDD) was the most

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abundant PCDD/F congener. Distribution of dl-PCB is dominated by the gas phase, while for

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PCDD/F the aerosol phase concentrations were higher, particularly for the more hydrophobic

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congeners. The Atlantic Ocean presented on average the highest PCDD/F and dl-PCB

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concentrations being lower in the southern hemisphere. The assessment of air mass back

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trajectories show a clear influence of continental source regions, and lower concentrations when

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the air mass has an oceanic origin. In addition, the samples affected by an oceanic air-mass are

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characterized by a lower contribution of the less chlorinated dioxins in comparison with the

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furans, consistent with the reported higher reaction rate constants of dibenzo-p-dioxins with OH

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radicals than those of dibenzofurans. The total dry atmospheric deposition of aerosol-bound

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∑PCDD/F and ∑dl-PCB to the global oceans was estimated to be 354 and 896 Kg/year,

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respectively.

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Introduction

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Persistent organic pollutants (POPs) are a diverse group of organic substances which persist in

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the environment, bioaccumulate and biomagnify in the food chain, are toxic, and prone to long-

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range atmospheric transport.1-4 Several families of POPs have been regulated by the Stockholm

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convention aiming at reducing or eliminating their production and use.5 Polychlorinated

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dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF), commonly known as

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dioxins, are among the most toxic POPs listed in the Stockholm Convention.6,7Some

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polychlorinated biphenyls (PCB), also included in the Stockholm Convention, have similar

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toxic properties to dioxins due to their similar structure and planarity, and are known as dioxin

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like PCB (dl-PCB). While PCDD/F are ubiquitous contaminants released into the environment

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as unintentional byproducts of chemical manufacturing and incineration processes,8-10 PCB were

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produced on purpose since 1929 during several decades for diverse industrial activities till they

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were generally banned over 30 years ago.11,12Although the use of PCBs is banned and PCDD/F

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emissions have been reduced during the last decade,13-16 their persistence and potential for long-

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range atmospheric transport (LRAT) account for the presence of PCDD/F and dl-PCB in remote

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locations away from source regions.12,17-24 The PCDD/F and dl-PCB occurrence in background

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terrestrial locations is widely documented,25-29however, there is a dearth of assessments forthe

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marine environment. Previous studies have reported on the atmospheric PCDD/F occurrence in

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coastal areas,30-32 and oceanic islands.33 To the best of our knowledge, only one study reported

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on dioxins in the open Mediterranean Sea atmosphere, 34 and there are only two previous studies

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on the dioxin atmospheric occurrence and fate in Atlantic Ocean north-south transects.16,35

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Therefore, the occurrence of PCDD/F and dl-PCBs in the global oceanic atmosphere needs to be

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comprehensively assessed in order to determine the background levels in the planetary

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atmospheric boundary layer, and better understand the major drivers of the LRAT of dioxins.

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The LRAT of semivolatile and hydrophobic POPs is affected by a number of processes, notably

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degradation with OH radicals and atmospheric deposition.36 It is well accepted that oceanic

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biogeochemical processes, such as the biological pump, play a critical role in controlling the

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diffusive air-water exchange of PCBs and presumably dioxins.37-40However, most PCDD/Fs are

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found mainly associated to the aerosol phase and thus are deposited to the ocean by dry and wet

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deposition.34,41The aims of this study are: i) to assess for the first time the global atmospheric

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background occurrence of gas- and aerosol-phase PCDD/F and dl-PCB in the Pacific, Indian

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and Atlantic Oceans, ii) investigate on potential drivers of their LRAT, iii) estimate the

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atmospheric dry deposition of PCDD/F to the ocean.

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Materials and methods

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Sample collection

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High volume air samples were collected onboard the R/V Hespérides during the Malaspina

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oceanographic circumnavigation campaign from December 2010 to July 2011. A total of thirty

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nine air samples were collected from the tropical and subtropical oceans between 40oN and

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35oS: 17 samples from the Atlantic Ocean, 8 from the Indian Ocean and 14 from the Pacific

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Ocean. Sample locations are shown in Figure 1. The aerosol and gas phases were sampled

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separately on quartz fiber filter (QFF) and polyurethane foams (PUF), respectively, using high

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volume air samplers (MCV, Collbató, Spain) placed above the bridge deck (15m above sea

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level).The sampled air volumes varied from 600 to 1300 m3. Wind direction was taken into

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account in order to avoid ship contamination (exhausts fumes from the chimney) with a vane

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which turned on the samplers when the relative wind direction was between 0º and 180º from

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the direction of the ship (ship emissions located at 270º). PUFs and QFFs were kept wrapped in

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aluminum foil and in two zip bags and stored frozen at -20ºCuntil their analysis in the

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laboratory.

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Chemical analysis

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Gas and aerosol phase were separately analyzed from 19 atmospheric samples, while only the

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gas phase was processed from the remaining 20 samples. Prior to extraction, samples were

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spiked with known amounts of the surrogate mixtures 13C12 PCDD/F (EN-1948-ES, Wellington

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Labs., Guelph, Canada) and 13C12 dl-PCB (P48-W-ES LCS, Wellington Labs., Guelph, Canada).

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Two h after spiking, the samples were Soxhlet extracted for ~24 h using 800 mL of toluene as

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solvent. The organic extract was concentrated in a rotary evaporator and then transferred to n-

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hexane. The clean-up and fractionation process was based on the sequential use of open

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chromatographic multilayer silica, basic alumina and carbon columns. The first column of

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multi-layer silica (top: 12 g acid silica modified with 44% sulphuric acid; bottom: 6 g basic

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silica modified with 33% sodium hydroxide) was followed by an alumina column. Columns in

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tandem were pre-eluted with 100 ml of n-hexane. After adding 90 mL n-hexane (Fraction 1) in

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the tandem, multilayer silica column was removed and samples were eluted from the alumina

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column with 75 mL of hexane:dichloromethane (9:1) (Fraction 2) containing dl-PCB and 120

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mL of hexane:dichloromethane (1:1) (Fraction 3) containing PCDD/F. Fraction 3 was purified

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using a carbon column (Carbopack C 80/100 at 18% with Celite 545), and dioxins were eluted

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with 75 mL of toluene. The extracts were rotary concentrated, reduced to dryness by a gentle

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stream of nitrogen and reconstructed in a known amount of mixtures of 1948-IS, Wellington Labs., Guelph, Canada) and

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C12 PCDD/F (EN-

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C12-dl-PCB (P48-RS, Wellington Labs.,

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Guelph, Canada) used as internal standards ( C12 PCDD/F and dl-PCB added, listed in table S).

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Finally, high resolution gas chromatography coupled to high resolution mass spectrometry

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(HRGC-HRMS) was used for instrumental analysis. All analysis were performed on a Trace GC

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ultra gas chromatograph (Thermo Fisher Scientific, Milan, IT) fitted with a 60m x 0.25 mm i.d.

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x 0.25 µm film thickness DB-5ms fused silica column for PCDD/F and a 60m x 0.25 mm i.d. x

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0.25 µm film thickness DB-XLB fused silica column for dl-PCB coupled to a high resolution

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mass spectrometer (DFS, Thermo Fisher Scientific) with a Xcalibur data acquisition system.

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The HRGC-HRMS was used with a positive electron ionization (EI+) source operating in the

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multiple ion detection (MID) mode at 10.000 resolution (10% valley definition). Quantification

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was carried out by isotopic dilution method, a common method in mass spectrometry42-45 and

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extensively detailed in well accepted method such EPA 1613.46

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Quality assurance and quality control (QA/QC)

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All material employed for the sampling was rigorously cleaned and properly stored before the

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sampling cruise. All sampling devices were also rinsed with acetone after and before sampling

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to avoid potential contamination between different samples. Laboratory blanks covering the

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whole methodology and field blanks, which include the transport and the storage during the

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oceanographic campaign, were performed in parallel to the field samples. Expectedly, field

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blanks presented slightly higher levels in comparison to those achieved for laboratory blanks,

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especially for dl-PCB congeners. In most cases, analyte’s concentration in the blanks(field and

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laboratory blanks) were below LOD, and only 5 over the 17 analyzed PCDD/Fs congeners were

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detected in some blanks, corresponding to trace amounts of octa- and hepta- chlorinated dioxins

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and furans (see Annex II for details).For the 5 dioxins detected in blanks, the levels in samples

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were above the blank levels for 93 % of the samples PCB#118, PCB#105 and PCB#156 were

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also detected in the blank samples. The levels of the few detected PCDD/Fs congeners in blanks

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were considerably lower than the mean levels found in the oceanic samples. Blank

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concentrations were around 25% of sample mean PCDD/F concentrations. Blank concentrations

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were around 12% of the mean samples dl-PCB concentrations. Therefore, there is a lack of

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contamination during handling, storage and analysis for most of the compounds and samples.

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Sample congener concentrations below mean blank congener were considered below detection

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limit and not reported. For dl-PCB, this affected mainly the aerosol phase samples due to the

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low levels in this matrix. Individual LODs achieved for all samples are provided as Supporting

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Information in TablesS2a and S2b.The overall reliability of the analytical procedure was

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assured by using labeled compounds in each sample to identify unequivocally the different

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congeners and also obtain the surrogate recoveries (TablesS2c and S2d of Supporting

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Information) which had a median recovery value of 92%. All the samples satisfied the minimum

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requirements described in well accepted methods such as UNE-EN 1948 or EPA 1613.46,47 The

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efficiency of the method was also checked by analyzing certified reference materials (CRM) for

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PCDD/F and internal reference materials for dl-PCBs. Complementary, the laboratory is in

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continuous participation with inter-laboratory studies to ensure the quality of the protocols

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used.48,49

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Results and discussion

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Occurrence of PCDD/F in the global oceanic atmosphere

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Figures 1 and 2 show the global distribution of PCDD/Fs, and Table S3a summarizes the

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PCDD/F congener specific mean and range of concentrations, and toxic equivalents, in both the

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gas and aerosol phases for all the oceans. Figure 3 shows the characteristic patterns of

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PCDD/Fs. In the gas phase, concentrations ranged between 1 and 27 fg/m3 for ∑PCDFs, and

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between 3 and 41 fg/m3 for ∑PCDDs. The toxic equivalent of ∑PCDD/Fs was between 0.1 and

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7.2 fg I-TEQ/m3. Octachlorodibenzo-p-dioxin (OCDD) was the most abundant congener, and it

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was detected in all, except one, of the analyzed samples. The highest concentration of OCDD

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was found in the North Pacific with gas phase concentrations reaching 25 fg/m3. The maximum

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OCDD concentration for the North Atlantic was 13 fg/m3 (Table S3a), consistent with the

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concentration of10.6fg/m3reported at Barbados in 1999,33 but one order of magnitude lower

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than the maximum OCDD found at the same latitudes in 1998 (230fg/m3).35It is possible that

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PCDD/F concentrations over the open ocean have decreased by more than one order of

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magnitude during the last decade due to reduced emissions in source regions as suggested

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previously.16The lowest concentration of PCDD/Fs was measured for TCDD in the South

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Atlantic (0.1 fg/m3). TCDD and pentachloro dibenzo-p-dioxin (PCDD) could only be quantified

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in 31% of the analyzed gas phase samples due to levels below detection limits.

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In the aerosol phase, the concentration varied from 2 to 21 fg/m3 for ∑PCDFs and from 3 to 92

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fg/m3 for ∑PCDDs fg/m3, while the toxic equivalent was between 0.3 and 5.6 fg I-TEQ/m3.

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Three congeners were detected in almost all of the aerosol phase samples: OCDD,

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Heptachlorodibenzo-p-dioxin (HpCDD) and 1,2,3,4,6,7,8-heptachlorodibenzofuran (HpCDF).

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The highest concentration for OCDD was reported in the South Atlantic (64 fg/m3), with a mean

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concentration for the global ocean of 14 fg/m3. This is comparable with the aerosol phase

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concentrations of 8.4fg/m3 reported at Barbados.33These concentrations were 2 orders of

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magnitude lower than those reported previously, with values as high as 210fg/m3 for OCDD for

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samples taken in 1998.35 Tetrachlorinated dibenzofuran (TCDF) and tetrachlorinated dibenzo-p-

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dioxin (TCDD) could only be detected in 20% of the aerosol phase samples; their

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concentrations were generally low, with a minimum concentration range of 0.2-0.6 fg/m3 for

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TCDD, and 0.3-0.8 fg/m3for TCDF.

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The global background atmospheric concentration (gas + aerosol phase) of ∑PCDD/Fs over the

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ocean is of 43fg/m3, or 3.8 fg I-TEQ/m3, and these varied from 15 to 126 fg/m3 for ∑PCDD/Fs,

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and from 1to 10fg I-TEQ/m3. There are only few studies reporting the atmospheric

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concentrations of ∑PCDD/Fs over the ocean, and these only cover the Atlantic Ocean16,35and

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the Mediterranean Sea.34Mean, median and range of ∑PCDD/Fs(gas + aerosol phase) in

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Atlantic Ocean studies were collected in the table S3i from the Supporting Information.

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Malaspina concentrations were comparable to those measured in Nizzetto et al. (2008),16 but

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one order of magnitude higher than those measured in 1998.35The ∑PCDD/Fs levels reported

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here are one order of magnitude lower than those described for the open Mediterranean Sea in

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2006-2007 (60-1040fg/m3),34 which is highly influenced by the proximate highly populated

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regions. Therefore, it is possible that there has been a decrease of concentrations of PCDD/F

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during the late 90’s15 and the following decade, but that this decreasing trend has been stopped.

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This could be consistent with a lack of further decrease in the primary sources of PCDD/Fs, and

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that secondary sources of PCDD/Fs are becoming increasingly important buffering the decrease

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of PCDD/Fs. Time trends of PCDD/F in the Baltic Sea, thus close to primary sources, also show

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a lack of significant decrease during the last years.50

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The pattern of PCDD/F in the gas and aerosol phase (Figure 3) showed profiles consistent with

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those reported for rural and remote areas28,32,51-53 for most of the samples. In the particular case

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of samples affected by air masses coming from proximate land (samples 120, 121, 41), a

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slightly different pattern was observed with a lower OCDD contribution to the total

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concentration contribution. Classical anthropogenic sources.54,55use to have a profile without

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OCDD levels standing out so much above the rest in contrast with those reported for rural areas

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with a major OCDD contribution to the total concentration. These samples also showed a higher

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contribution of the less chlorinated congeners, which are degraded by OH radicals during

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atmospheric transport. As expected, the aerosol phase is enriched with the most hydrophobic

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congeners such as those having 7-8 Chlorines. 1,2,3,7,8,9-Hexachlorodibenzofuran presented

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the highest percentages in the aerosol phase (average of 90%). Otherwise, tetrachlorinated

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dioxins and furans were predominantly found in the gas phase, with an average of 80% for

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TCDF. The overall levels of ∑PCDD/F were similar for the gas and aerosol phases, except in

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some regions such as off the coast of Brazil, where the aerosol phase concentrations were the

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highest (see below for a discussion on air-mass back trajectories).

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Occurrence of dl-PCB in the global oceanic atmosphere

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Figure 4 shows the global distribution of dl-PCBs, and Table S3b summarizes the mean and

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range of the concentrations for each one of the 12 dl-PCB congeners. Most of the dl-PCB were

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detected in all the gas phase samples. ∑dl-PCB concentrations in the gas phase ranged from 385

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to 9227 fg/m3. Expressed as TEQ units, concentrations were between 0.1 and 8.4 fg WHO-

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TEQ2005/m3. PCB118 was the most abundant congener, with a maximum concentration of 3330

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fg/m3, followed by PCB105, PCB156, PCB77 and PCB156. Conversely, PCB81 was detected

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only in 23% of the analyzed gas phase samples. The lowest concentration was obtained for

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PCB169, 1fg/m3 in a gas phase sample from the South Pacific.

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As expected, the concentrations of dl-PCB were significantly lower in the aerosol-phase than in

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the gas phase consistent with other reports of PCBs in the oceanic atmosphere.40,56,57Theaerosol

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phase concentrations of ∑dl-PCB ranged from 1 to 2675 fg/m3, while their associated TEQ

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values varied between 0.01 and 4.5 fg WHO-TEQ2005/m3.Similarly to the gas phase, the most

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abundant PCB congeners were PCB#118, PCB#105 and PCB#156, while PCB#81 was the less

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abundant, only detected in 15% of the aerosol phase samples, followed by PCB#126 and

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PCB#114 which were detected in less than 25% of the samples. The highest aerosol phase

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concentrations of dl-PCBs were found in the South Atlantic Ocean close to the Brazilian coast,

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and in the south Western Indian Ocean close to South Africa (Figure 4).

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The profiles of dl-PCB in both the gas and aerosol phase do not vary significantly for the

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different oceanic regions, and the congener distribution is similar to those reported in the

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literature for the continental/coastal atmosphere32,51-53 being the PCB#118 the predominant,

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followed by PCB#105 and PCB#156 (Figure 3). To the best of our knowledge, this is the first

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coherent data set on the atmospheric oceanic occurrence of dl-PCB, with the exception of a

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report for the Mediterranean Sea atmosphere.56 Previous efforts35on dl-PCB occurrence in

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oceanic atmosphere were affected by ship contamination58. Only PCB#118 (commonly reported

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as ICES PCB as well as dl-PCB)has a notable body of literature on their occurrence in the

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tropical-subtropical oceans since ICES PCB were reported in some oceanic studies.59-63The

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mean atmospheric concentrations (gas + aerosol) of PCB#118 are similar but in the lower range

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than those reported previously (See Table S3j for the comparison).

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Influence of air mass back trajectories on PCDD/F and dl-PCB occurrence

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There is a notable variability of the atmospheric concentrations of both PCDD/F and dl-PCB

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globally (Figures 1, 2 and 4). Concentrations in the Pacific Ocean lie generally in the low-end of

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the global background levels, except for the samples south west of California (USA), Mexico

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and Central America. The variability in concentrations is higher in the other oceanic regions.

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Significant differences between oceans can be stated if we compare the more abundant

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congeners from samples from the different oceans (excluding those influenced by land-based air

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masses). In general, South Pacific Ocean has the lower PCDD/F concentrations while North

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Atlantic has the higher dl-PCB concentrations. The higher concentrations East of Brazil, close

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to South Africa, Australia and off West Africa (Figures 1, 2 and 4) suggest that the air-mass

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back-trajectories (BTs) could influence the observed spatial patterns by driving higher

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concentrations in areas influenced by adjacent source regions. The BTs were obtained from the

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Hysplit model from Air Resources Laboratory of the National Oceanic and Atmospheric

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Administration (NOAA).64The sample specific BTs during the Malaspina circumnavigation

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cruise reflect the general oceanic pattern of a clock-wise subtropical gyre in the north Atlantic

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and north Pacific, and counter-clock wise gyre in the Southern Atlantic, Pacific and Indian

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oceans (Figure S1b). In the Northern Atlantic, the ship traversed the trade winds (from ~30ºN to

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near the inter-tropical convergence zone, ITCZ) with winds coming from the northeast (samples

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4 and 11) or the East-north East(samples126-142). Samples 4 and 11 (NE Atlantic) show high

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concentrations of dl-PCBs, and to less extent PCDD/F. This is consistent with the high

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concentrations of PCBs, and other pollutants, described for this region reflecting inputs from

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source regions in North West Africa and South Eastern Europe.59,60,62 In the rest of the samples

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from the north Atlantic (samples 126-142), the levels of dl-PCB and PCDD/F are low for those

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located in the north Atlantic subtropical gyre, while higher levels are observed in the Caribbean

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due to proximate land or coastal seawater as sources. In the North Pacific, levels are generally

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low due to BT showing that the air mass had travelled over the ocean for several days (samples

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92-120), except some sampling events showing higher concentrations of PCDD/F, which are

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affected by air mass BT originating in Mexico and central America, especially in sample 121,

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but also other samples in this region (samples 121-125).

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Other samples with high PCDD/F concentrations in the gas phase were sample 41, and samples

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41 and 46 for the aerosol phase, with air masses coming from south of Cape of Good Hope

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(South Africa), thus reflecting the influence of proximate land. For the aerosol phase, the

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highest concentrations of PCDD/F and dl-PCB were achieved in samples taken off-shore the

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Brazilian coast (samples 23 and 24). High levels of polycyclic aromatic hydrocarbons have also

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been described for these samples.6 The BT for the lower atmosphere show an oceanic influence

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(from the East); however, the air masses for the upper boundary layer (800-100 m height) show

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an influence from the proximate Brazilian coast. Therefore, it is possible that the aerosol phase

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concentration of PCDD/F and dl-PCB is affected by aerosol particles deposited from this upper

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air mass. This would also explain the fact that such an increase is not observed for the gas phase

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PCDD/F and dl-PCB concentrations of-shore Brazil.

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In the southern hemisphere and in the open ocean, the ship crossed regions with trade winds

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from the southeast (samples 16-35, 58-63, 81-88). Between 30 and 35 degrees south, there are

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the subtropical highs, a region under a ridge pressure which has variable winds mixed with calm

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periods (samples 39-51, 67-71) (Figure S1bBTs graphics in the Supporting Information). At

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Open Ocean, the PCDD/F and dl-PCB concentrations in the gas and aerosol phases are

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generally low reflecting the oceanic origin of the BT for the few days before the sampling event.

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Exceptionally, in some cases it is not possible to justify from the BT for the low atmospheric

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boundary layer the unexpected relative higher levels for PCDD/F such as in sample from Indian

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Ocean. The BT in the upper boundary layer are more variable, with episodes of air masses

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coming from the NE65and particles may have reached the lower boundary layer by deposition.

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In addition, there is presumably a volatilization of PCDD/F and PCBs in this region, enhanced

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by the high wind speeds during and before these sampling events. Volatilization of PCBs has

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been described for the oligotrophic oceanic gyres.16,61Therefore, regarding the influence of BT

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on the global distribution of dl-PCB, the different patterns between the gas and aerosol

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phasesare remarkable (Figure3). In the gas phase, the Atlantic Ocean had the highest values of

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dl-PCB and a decreasing tendency from North to South is observed as a reflection of historic

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emissions impact and the influence of emissions from SW Europe and NW Africa. The Pacific

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and Indian Oceans presented generally the lowest dl-PCB levels except for 2 samples previously

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discussed. Besides, the aerosol phase levels are also generally low (background) with the

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exception of the commented influence of Brazil and South Africa on the proximity of the

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sampling sites which are influenced by BT in the lower and at the upper levels of the boundary

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layers.

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Atmospheric deposition and degradation affecting the background occurrence of PCDDF/

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and dl-PCBs

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The background oceanic concentrations of PCDD/F reported here are not only among the lowest

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ever reported in the literature consistent with the remoteness of the studied regions, but the first

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PCDD/F data for large regions of the world ocean. During atmospheric transport, PCDD/F and

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dl-PCB will be subject to a number of removal processes, basically atmospheric deposition and

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OH degradation. Diffusive air-water exchange is usually considered the main atmospheric

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deposition process for PCBs, but not for most dioxins.20,34,37-41 However, because PCDD/Fs and

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dl-PCBs were not measured in the dissolved phase, the magnitude and direction of this process

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cannot be elucidated here. Nevertheless, Jurado et al. have estimated that for most PCDD/Fs,

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atmospheric dry deposition is the main depositional process for the open ocean,38,41 and Castro-

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Jiménez and collaborators have reached the same conclusions from measurements performed in

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the Mediterranean Sea.34 The dry aerosol deposition fluxes (FDD, pg/m2d) can be estimated from

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the measured aerosol phase concentrations by

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[1]

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Where vD is the overall aerosol dry deposition velocity (cms-1), and Cp is the concentration of

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the compound in the aerosol phase (pg/m3).33,66 During the Malaspina Cruise, the dry deposition

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fluxes of PAHs were measured and an empirical parameterization has been suggested to predict

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the vD values of semivolatile organic compounds at the open ocean,65

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(2)

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Where U10 is the wind speed at 10m above sea level (m s-1), Chls is the surface chlorophyll

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concentration (mg m-3), and PL is the compound’s vapor pressure (Pa), which was calculated as

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suggested elsewhere.67 The vD values for PCDD/F and dl-PCB during the Malaspina cruise

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ranged from 0.01 to0.2 cms-1, which is of the same order of magnitude than those estimated

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from models(0.05 to 0.5 cms-1) by Jurado et al.38 The uncertainty associated to the estimation of

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vD by equation 2 is of a factor of 3 (335%) as estimated by Gonzalez-Gaya et al.(2014).65

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Dry deposition fluxes of PCDD/F and dl-PCBs to the global oceans show a large geographical

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variability depending on aerosol size distribution, atmospheric turbulence and atmospheric

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stability.38 Figure 5 shows the average dry deposition fluxes of PCDD/F and dl-PCBs for the

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different oceanic basins. The dry deposition fluxes of PCDD/F are lower than those reported for

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the Mediterranean Sea, a difference driven by the lower PCDD/F concentrations in the open

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ocean, and the lower vD values in this study. The Atlantic and Pacific Oceans receive most of

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the dry deposition input of PCDD/F to the global ocean, 130 and 170 kg/yr respectively, while

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60 Kg PCDD/F/yr were estimated for the Indian Ocean. Overall, the integrated dry deposition

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flux of ∑PCDD/F to the global oceans is of 360 Kg/yr. This figure is significantly lower than

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the 2400 Kg /yr estimated previously,68 but which was based from the higher PCDD/F

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concentrations measured in 1998.35

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As expected, mean dry aerosol deposition fluxes for dl-PCB for the oceans (Figure5) were

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considerably higher than PCDD/F deposition fluxes. Atlantic Ocean received the highest input

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with more 630 Kg of dl-PCB per year accounting up to 70% of the total dry dl-PCB deposition.

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Secondly, 190 Kg/yr were estimated for Indian Ocean followed by Pacific Ocean with only 80

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Kg/yr. The total dry aerosol deposition of ∑dl-PCB to the main oceans is estimated to be 900

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Kg/yr. It is likely that this represents a small fraction of the dl-PCBs exchange between the

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atmosphere and ocean, because diffusive exchange may be one order of magnitude higher than

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dry deposition in the case of PCBs.

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It has been suggested that atmospheric degradation, due to reaction with OH radicals, is the

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main removal mechanism of dioxins from the atmosphere.68 This removal process will affect

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mainly the PCDD/F found in the gas phase. The reaction rate constants of PCDD/F with OH

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radicals are higher for the less chlorinated congeners, and are significantly higher for PCDD

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than for PCDFs.68-70 Therefore, the comparison of the ratio TCDF/TCDD in the gas-phase could

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provide information on whether this loss process induces a change in the pattern of PCDD/F by

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removing some congeners faster than others during atmospheric transport. Figure 6 shows the

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TCDF/TCDD ratio, together with the BTs during the sampling period for all samples in which

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both compounds were detected (11 samples out of the 39 analyzed gas phase samples). A

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remarkable finding is that samples affected by oceanic air masses are quite different from those

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with air masses coming from land or marine coastal regions. The TCDF/TCDD ratio in samples

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from oceanic air masses presented higher values of TCDF/TCDD (>1) than the samples affected

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by land air masses (