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

1 ACS Paragon Plus Environment


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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.

298 299

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-

310

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

311

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

312

the measured aerosol phase concentrations by

313

[1]

314

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

315

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

316

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

317

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

318

(2)

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

320

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

321

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

324

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

332

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

333

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

334

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

335

concentrations measured in 1998.35

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

337

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.

339

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


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340

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

342

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

356

by land air masses (

Background Concentrations of Polychlorinated Dibenzo-p-Dioxins

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