Bi-directional air-sea exchange and accumulation of POPs ( PAHs , PCBs , OCPs and 1 PBDEs ) in the nocturnal marine boundary layer 2 3

1 PBDEs) in the nocturnal marine boundary layer 2 3 Gerhard Lammel, Franz X. Meixner, Branislav Vrana, Christos Efstathiou, Jiři 4 Kohoutek, Petr Kukučka, Marie D. Mulder, Petra Přibylová, Roman Prokeš, Tatsiana P. 5 Rusina, Guo-Zheng Song, Manolis Tsapakis 6 7 1 Masaryk University, Research Centre for Toxic Compounds in the Environment, Brno, 8 Czech Republic 9 2 Max Planck Institute for Chemistry, Multiphase Chemistry Dept., Mainz, Germany 10 3 Max Planck Institute for Chemistry, Biogeochemistry Dept., Mainz, Germany 11 4 Hellenic Centre for Marine Research, Institute of Oceanography, Gournes, Greece 12 * lammel@recetox.muni.cz 13 14 Abstract 15 As a consequence of long-range transported pollution air-sea exchange can become a major 16 source of persistent organic pollutants in remote marine environments. The vertical gradients 17 in air of 14 species i.e., 4 parent polycyclic aromatic hydrocarbons (PAHs), 3 polychlorinated 18 biphenyls (PCBs), 3 organochlorine pesticides (OCPs) and 2 polybrominated diphenylethers 19 (PBDEs) in the gas-phase were quantified at a remote coastal site in the southern Aegean Sea 20 in summer. Most vertical gradients were positive (Δc/Δz > 0) indicating downward (net 21 depositional) flux. Significant upward (net volatilisational) fluxes were found for 3 PAHs, 22 mostly during day-time, and for 2 OCPs, mostly during night-time, as well as for 1 PCB and 1 23 PBDE during part of the measurements. While phenanthrene was deposited, fluoranthene 24 (FLT) and pyrene (PYR) seem to undergo flux oscillation, hereby not following a day/night 25 cycle. Box modelling confirms that volatilisation from the sea surface has significantly 26


Introduction
The marine atmospheric environment is a receptor for persistent organic pollutants (POPs) which are advected from sources on land, primary and secondary, such as volatilization from contaminated soils.This is a concern as these substance bioaccumulate along marine food chains (e.g., Lipiatou and Saliot, 1991;Borgå et al., 2001).Primary sources do not exist in the marine environment, except for polycyclic aromatic hydrocarbons (PAHs, ship engines).
Long-range transport from urban and industrial sources on land are the predominant sources of PAHs and polychlorinated biphenyls (PCBs) in the global oceans (Atlas and Giam,1986) and in the Mediterranean (Mandalakis et al., 2005;Tsapakis and Stephanou, 2005;Tsapakis et. al, 2006;Iacovidou et al., 2009;Mulder et al., 2015).
However, the sea surface itself can turn into a secondary source of POPs provided concentrations build up in surface waters.Such studies are still rare.Re-volatilisation was observed for hexachlorocyclohexane (HCH) and PAHs, not only in coastal waters (Lohmann et al., 2011), but also in the open sea (Jantunen and Bidleman, 1995;Lakaschus et al., 2002) including the Mediterranean (Castro-Jiménez et al., 2012;Mulder et al., 2014).After longterm accumulation of declining emissions (even after phase-out), reversal of air-sea exchange may result at some point, as indicated by global modelling for organochlorine pesticides (OCPs; Stemmler and Lammel, 2009).The seasonality of on-going emissions on the other hand may trigger a seasonal reversal of air-sea exchange, as indicated for retene, a PAH emitted from biomass burning in the Mediterranean (summer maximum; Mulder et al., 2014).
Similarly, PAHs emitted in fossil fuel combustion in residential heating (winter maximum) may re-volatilise seasonally from the sea surface in receptor areas.
The direction of diffusive air-surface exchange flux of organics can be identified by comparing the fugacities and can be quantified based on the Whitman two-film model (Bidleman and Connell, 1995;Schwarzenbach et al., 2003) or micro-meteorological techniques.The latter have so far only rarely been used to quantify air-water (Perlinger et al., 2005;Rowe and Perlinger, 2012;Sandy et al., 2012;Wong et al., 2012) or air-soil (Parmele et al., 1972;Majewski et al., 1993;Kurt-Karakus et al., 2006) gas exchange fluxes.
We studied the vertical fluxes of POPs at sea surface level with a gradient method at a remote coastal site in the eastern Mediterranean.The measurements were done in the context of a coordinated multi-site campaign on POP cycling in the region (Lammel et al., 2015).The POP concentration in surface seawater was determined, too, such that the direction of air-sea exchange could be addressed by a second method.

Site and sampling
The site selected for atmospheric measurements was Selles Beach, at the northern coast of Crete, 35.2°N/25.4°E,very close (4 km) to the Finokalia observatory.This is a remote site, some 70 km east of major anthropogenic emissions (Iraklion, a city of 100000 inhabitants with airport and industries; Mihalopoulos et al., 1997;Kouvarakis et al., 2000).The Mediterranean region includes urban and industrial areas and is adjacent to to source regions (i.e.western, central and eastern Europe).Exposure of the study area to long-range transported pollution from central and eastern Europe is highest in summer (Lelieveld et al., 2002).Organic substances were collected 3-13 July in the gaseous and particulate atmospheric phases using low volume samplers (F ≈ 2.3 m³ h -1 , Leckel LVS, PM 10 inlet) equipped with quartz fibre filter (QFF, Whatman QMA 47 mm, baked at 320°C prior to usage) and 2 polyurethane foam (PUF) plugs (Molitan, density 0.030 g cm -3 , 5.5 cm diameter, total depth 10 cm, cleaned by extraction in acetone and dichloromethane, 8 h each) in series.Two of these samplers collected gases and particles at different heights (inlets at z 1 = 1.05 and z 2 = 2.8 m), about 0.5 m apart in the horizontal.Day-time (9-20 h EEDST) and night-time (21-8 h EEDST) sampling was conducted from 2 July in the evening until 13 July 2012 in the evening.During part of the measurements, from 6 July in the morning until 10 July 2012 in the evening, a third sampler was used to collect replica of gaseous samples (PUF plugs only) at z 1 .For the concentration at z 1 , c z1 , replica concentrations (mean of 2 measurements) were used whenever possible.The samplers were placed on a rocky beach.The horizontal distance between the samplers and the water was ≈3 m, while the vertical distance between the rock and water surfaces was 0.1-0.3m, varying due to tide and waves.After exposure, filters and PUFs were packed in Al foil and zip-bags, stored and transported in a cool box to the laboratory.
Free dissolved contaminants in seawater were sampled using silicone rubber (SR) sheets (Altec, Great Britain) as passive water samplers (PWS).Quantification of trace organics from PWS is sensitive and validated (Rusina et al., 2010).Uncertainties in results obtained by application of partition based passive samplers are believed to range around a factor of two depending on the level of experience of the laboratory (Allan et al., 2009).Different aspects of uncertainty are discussed in Lohmann et al. (2012).At two localities, distanced 0.8 and 2.2 km west of Selles Beach, each two SR PWS were deployed in parallel.Each sampler consisted of six sheets (55 × 90 × 0.5 mm).Before exposure SR sheets were cleaned by Soxhlet extraction in ethyl acetate (96 h) followed by methanol (48 h, shaken), and spiked by a mix of 15 performance reference compounds (PRCs; D 10 -biphenyl and 13 PCB congeners not occurring in the environment) according to the procedure (Booij et al., 2002).Samplers were deployed 3 July -2 August 2012 in water mounted on stainless steel wire holders at 1 m depth using buoy and rope.After exposure, samplers were stored and transported in original vials and brought in a cool box to the laboratory.

Meteorological parameters and vertical flux calculations
Boundary layer (BL) depth is needed for interpretation of the variation of the concentrations in air.BL depth data are taken from simulations of the Lagrangian dispersion model FLEXPART, version 9 (Stohl et al., 1998).These were run in forward direction, and based on analysed wind fields (ECMWF, 0.5° resolution).The model BL height is calculated according to Vogelezang and Holtslag (1996) using the critical Richardson number.According to wind direction during sampling we allocate BL depths at upwind locations well off shore (70-100 km) or inland (≈20 km), respectively, as the relevant BL depth for interpretation of atmospheric concentrations at the coastal site.The mean BL depth during sampling intervals is used.
For characterization of the local meteorological conditions, continuous measurements (5 min averages) of air temperature, relative humidity, wind speed and wind direction were accomplished by three automatic weather stations (model WMT520;Vaisala,Helsinki,Finland) which have been placed at the beach in ≈200 m distance (2) and ≈100 m inland (1), respectively, from the sampling location.For characterization of the atmospheric surface layer's thermodynamic stratification, vertical profiles of wind speed, wind direction, air temperature and relative humidity were determined by continuous measurements at four levels (0.34, 0.70, 1.45, and 3.00 m above ground).Data was recorded by 2D ultrasonic wind sensors (model WMT701; Vaisala, Helsinki, Finland) and aspirated temperature and relative humidity sensors (model MP103A; Rotronic, Bassersdorf, Switzerland) in 10 s intervals, which were averaged to 30 min means for further data processing.For determination of key micrometeorological quantities (e.g., sensible heat flux, friction velocity; see SI, S1.3), fast response measurements of the 3D wind vector and air temperature have been performed by a 3D ultrasonic anemometer (CSAT-3, Campbell Scientific Inc., Logan, USA) on a small mast, 4 m above ground and about 7 m ESE of the profile mast.Corresponding data were continuously recorded with a sampling frequency of 20 Hz by a suitable logger (model CR3000; Campbell Scientific Inc., Logan, USA).Key micrometeorological quantities were derived from fast response 3D wind and air temperature data (20 Hz) according to the eddy covariance (EC) method; 20 Hz data were processed by the TK3 algorithm (Mauder and Foken, Department of Micrometeorology, University of Bayreuth, Germany), and the results were averaged every 30 minutes.Only periods with wind direction between 270° and 40° (i.e., onshore winds) were considered to calculate vertical fluxes of gaseous organics (more details in the SI, S2.1).
The turbulent vertical gaseous organics flux, F c (ng m -2 s -1 ), has been calculated according to the aerodynamic method as the product of the vertical difference of concentration, ∆c z (ng m - 3 ), and the turbulent transfer velocity, v tr (m s -1 ): where z 2 and z 1 are the heights of inlets of gaseous organics' sampling (1.05 m and 2.80 m, see 2.1, above).The transfer velocity is a measure of the vertical turbulent (eddy) diffusivity.
Details of the underlying formulation and the calculation scheme are given in the SI, S1.3.
The temperature program was 80°C, 15°C min -1 to 180°C, 5°C min -1 to 310°C.The injection volume was 1 μL in splitless mode at 280°C, with He used as a carrier gas at constant flow of 1.5 mL min -1 .
A sulphuric acid modified silica gel column was used for the PCB/OCP and PBDE cleanup.
PBDEs were analysed using GC-HRMS (gas chromatography with high resolution mas spectrometry) on a Restek RTX-1614 column (15 m × 0.25 mm × 0.1 µm).The resolution was set to > 10000 for BDE 28-183, and > 5000 for BDE 209. 13C BDEs 77 and 138 were used as injection standards.The MS was operated in EI+ mode at the resolution of >10000.
The temperature program was 80°C (1 min hold), then 20°C min -1 to 250°C, followed by 1.5°C min -1 to 260°C and 25°C min -1 to 320°C (4.5 min hold).The injection volume was 3 μL in splitless mode at 280°C, with He used as a carrier gas at constant flow of 1 mL min -1 .
Recovery of native analytes varied between 72 and 102% for PAHs, between 88 and 103% for PCBs, and between 75 and 98% for OCPs.The results for PAHs, OCPs and PCBs were not recovery corrected.For PBDEs, isotopic dilution method was used, the average recoveries ranged 78-128%.
The mean of 4 field blank values was subtracted from the air sample values.Values below the mean + 3 standard deviations of the field blank values were considered to be <LOQ.Field blank values of most analytes in air samples were below the instrument limit of quantification (ILOQ), which corresponded to 6-34 pg m -3 for PAHs, 7-23 pg m -3 for PCB and OCPs and 0.003-0.04pg m -3 for PBDEs (SI, Table S2).Higher LOQs were determined for analytes in gaseous air samples, namely 0.18 and 0.50 ng m -3 for FLN and PHE, and typically 28 pg m -3 for HCB.In the particulate phase a higher LOQ resulted for PHE, i.e. 170 pg m -3 .The breakthrough in PUF samples was estimated, and as a consequence, NAP, FLN, HCB and PeCB results are not considered as the sampled air volume (typically ≈25 m³ for PUFs) expectedly lead to breakthrough under the prevailing temperatures (Melymuk et al., 2016).

Vertical gradients of trace organics' concentration in air
Air-sea gas exchange can be studied by determining the vertical concentration gradients of trace gases in air (Doskey et al., 2004;Else et al., 2008).
Three standard deviations of field blank concentrations are considered as the absolute uncertainty of concentration measurements, c, and twice that much as the uncertainty of concentration differences, ∆c z .Values of concentrations and vertical concentration differences (gradients) not exceeding these thresholds are considered insignificant.This applied for a large fraction of gradients, namely OCP (34 out of 70), PCB (27 out of 44), PBDE (4 out of 5), and PAHs (17 out of 46) (Table S3).

Air-water fugacity ratio
The direction of diffusive air-sea gas exchange can be derived from the fugacity ratio calculation, based on the Whitman two-film model (Bidleman and McConnell, 1995).The fugacity ratio, f a /f w , is calculated as: f a /f w = c a RT a / (c w H Tw,salt ) with gas-phase concentration c a (ng m -3 ), dissolved aqueous concentration c w (ng m -3 ), universal gas constant R (Pa m 3 mol -1 K -1 ), sea surface temperature (SST), T w , and both T w and salinity corrected Henry's law constant H Tw,salt (Pa m 3 mol -1 ; see S1.1 for details), and air temperature T a (K).T a was adopted from the on-site measurement (see above).c w is derived as the average of the results at two localities, 2 replicas each (see above, 2.1).SST data, measured on the sampling day and in the area, were downloaded from respective database (see S1.4 for details).Air and water sampling was not totally in phase: sampling in air was over 12 days (2-13 July), while SR exposure was during 28 days (3-30 July) i.e., collection was done 10 days after air sample collection.Consequently, for those substances which are quickly equilibrated (within a few days) in PWS i.e., HCH and 3-ring PAHs, no simultaneous measurement in air and water was done (see section 2.1).Although the seawater concentrations of HCH and 3-ring PAHs might have been stable over 28 days, no such evidence exists and we refrain from relating the fugacities.Values 0.3 < FR < 3.0 are conservatively considered to not safely differ from phase equilibrium, as propagating from the uncertainty of the Henry's law constant, H Tw,salt , and measured concentrations and temperature changes during sampling (e.g., Bruhn et al., 2003;Castro-Jiménez et al., 2012).
Substance property data are taken from the literature (SI, Table S1).This conservative uncertainty margin is also adopted here, while f a /f w > 3.0 indicates net deposition and f a /f w <0.3net volatilisation.

Non-steady state 2-box model
The air-sea mass exchange flux of several OCPs and PAHs are simulated by a non-steady state zero-dimensional model of intercompartmental mass exchange (Lammel, 2004;Mulder et al., 2014) in order to test the hypothesis that the diurnal variation of contaminant concentrations in air during a period of on-shore advection of one air mass is explained by the combination of volatilisation from the sea surface and atmospheric mixing depth, while advection (long-range transport) is less significant (horizontal homogeneity of air mass; Lammel et al., 2003).This 2-box model predicts concentrations by integration of two coupled ordinary differential equations that solve the mass balances for the two compartments, namely the atmospheric marine BL and seawater surface mixed layer.Processes considered in air are dry (particle) deposition, removal from air by reaction with the hydroxyl radical, and air-sea mass exchange flux (dry gaseous deposition), while in seawater export (settling) velocity, deposition flux from air, air-sea mass exchange flux (volatilisation), and degradation (as first order process) are considered.Input parameters are listed in the SM, Table S3.

Day/night variation of concentrations in air
4 PAHs (ACE, PHE, FLT, PYR), 3 OCPs (α-and γ-HCH, p,p'-DDE), 3 PCB congeners (PCB28, -52 and -101) and 2 PBDE congeners (BDE47 and -99) were quantified in gas-phase samples, while the other species were found <LOQ in all or most samples (Fig. 1a, 2a, Table 1a, b).This is a consequence of limited air sample volume (≈25 m³).PAHs and PBDEs were also found in the particulate phase.The levels observed (Table 1a) are at the lower end of what had been reported from marine, rural and remote sites in the region in the previous ≈15 years, in particular with regard to the chlorinated species (Kamarianos et al., 2002;Mandalakis and Stephanou, 2002;Tsapakis and Stephanou, 2005;Cetin and Odabasi, 2008;Halse et al., 2011;Lammel et al., 2010 and2011;Castro-Jiménez et al., 2012;Berrojalbiz et al., 2014;Mulder et al., 2014 and2015).To our best knowledge, the DDE levels are the lowest reported from the region.This confirms the remote character of the site.Influence of local sources, not expected at this remote site (Iacovidou et al., 2009), is sometimes indicated by an anti-correlation between wind speed and atmospheric concentration.At Selles Beach, dilution by higher wind speed is indeed indicated for one contaminant, ACE (by significant anti-correlation, p < 0.05 confidence level, t-test).This is expected, because of its short atmospheric lifetime.
BL depths ranged 160-500 m during night-time and 270-760 m during day-time (mean of sampling intervals i.e., 11 h).Day/night variation of contaminants' atmospheric concentrations, often related to mixing and local sources, was not obvious: For PAHs the mean ratio of day/night concentrations c day /c night = 0.67 for ACE, while it was c day /c night = 1.4-1.55 for PHE, FLT and PYR.Also for PBDEs c day /c night > 1 is found (1.20 and 1.37).The low value for ACE can be explained by its short photochemical lifetime (Keyte et al., 2013).c day /c night > 1 was previously observed for PAHs at the same site and explained by temperature driven volatilisation from surfaces overcompensating for photochemistry (Lee et al., 1998;Tsapakis and Stephanou, 2007).For chlorinated substances, we find c day /c night < 1, namely 0.56-0.66for HCH isomers, and 0.68-0.95for PCBs and DDE.However, there was a clear day/night, with mostly night-time maxima trend during a period of continuous on-shore winds, 6-10 July (Fig. 1).Apparently, contaminants' concentrations were influenced by BL depth, as indicated by anti-correlation with PAHs and OCPs (except DDE; significant for α-HCH on the p < 0.05 confidence level, t-test).This, apart from mixing, is related to advection and air-sea mass exchange and studied in more detail in Section 3.5.

Diffusive air-sea exchange
The variation of air concentrations (with nighttime maxima) during a period of northerly flow without change of air mass is predicted using the 2-box model (Section 2.6).For PCB28, -52, FLT and BDE47 air concentrations are qualitatively well captured (Fig. 2, S4).These are maintained by dry gaseous deposition alone (PCB52, FLT) or by oscillating fluxes (HCB, PCB28: mostly upward, PYR: mostly downward; Fig. S5).The model predicted fluxes are in good agreement with the observed values (Section 3.3, Table S5) except for each one daytime sampling interval of FLT and BDE47 (upward fluxes), and for one day-time interval of PCB28 (downward flux; in total 4 agreements, 3 disagreements).The modelling results support that (during advection of one air mass) the diurnal variation of contaminant concentrations in air was explained by the combination of volatilisation from the sea surface and atmospheric mixing depth.Volatilisation from the sea surface has significantly contributed to the night-time maxima of HCB and PCB28, as well as of PYR during 1 night (night-time upward fluxes; Table S5).This, to our knowledge, had never been observed before.
f w is derived from the mean concentrations in seawater at two locations (see SI, Table S6, for individual data).The comparison of air-water fugacity ratios (Section 2.5) suggests for the  2).These results are the same as determined based on passive air sampling at several locations along the shore at and near Selles Beach (Lammel et al., 2015).
The direction of DDE and PCB fluxes derived from fugacity calculations is consistent with what was indicated by the correlation of air concentrations and BL depth during on-shore winds (SI, S2.5).

Vertical concentration gradients in air
PAH vertical gradients mostly indicated deposition, Δc/Δz > 0, found in 28 cases (14 during day, 14 during night), while negative gradients were found in 10 cases (8 during day, 2 during night).The vertical gradient of PAHs was insignificant in 17 cases.When volatilisation was observed (3-5 July for FLT and PYR, 6-9 July for ACE) Δc/Δz tended to be clearly lower during day-time, indicating that volatilisation of PAHs from the sea surface was stronger during day-time.This could be explained by a higher fugacity from seawater, f w , which increases with H Tw,salt (see above, 2.5), which, in turn, increases with sea surface temperature, T w .Similarly, for the halogenated substances, significant positive gradients, Δc/Δz > 0, indicating deposition were more frequent than significant negative gradients i.e., 37 cases ( 15PCBs, 22 OCPs, 30 during day, 7 during one night only) and 20 cases (2 PCBs, 17 OCPs, 1 PBDE, 5 during day, 15 during night), respectively.For these substance classes, a vertical gradient was insignificant in 65 cases (according to the measurement uncertainties).During at least some nights of the period 6-10 July night-time maxima of HCH, and PCB52 in air coincided with negative vertical gradients, i.e. emissions from the sea surface.This trend is most significant for the HCH isomers for which a stronger volatilisation flux from the sea 13 Atmos.Chem. Phys. Discuss., doi:10.5194/acp-2015-926, 2016 Manuscript under review for journal Atmos.Chem.Phys.Published: 1 February 2016 c Author(s) 2016.CC-BY 3.0 License.
surface is found during the nights than during day time (Δc/Δz < 0), or even deposition during day-time (Δc/Δz > 0 on 6 and 9 July).Hence, volatilisation from the sea surface may have contributed to and may even have caused the night-time maxima of the atmospheric concentrations of HCH and PCB52 (see above and Table 1a): The diel variation of air temperature was small i.e., day-time mean was typically 0.5-1.5 K warmer than night-time mean temperature.Even somewhat lower upward fluxes, F c , of HCH during night than during day, caused by a slightly lower sea surface temperature, may have caused c day /c night < 1 in combination with the day/night variation of the BL depth (by average 50% deeper for daytime sampling periods).PBDE day-time maxima may indicate local volatilisation from soil, enhanced during day-time.Again, this is consistent with the positive correlation of air concentrations with BL depth (above).Only one BDE concentration gradient was significant, which was volatilisational and during day-time (Fig. 1b, Table 1b).Fluctuating PCB fluxes are in line with the observation that PCBs were close to phase equilibrium in the Aegean in 2006(Berrojalbiz et al., 2014)).Summarizing, average significant day-time vertical gradients, Δc/Δz, of all contaminants exceeded average significant night-time gradients, except for FLT and PYR.
The direction of the gradient, hence, of air-sea exchange is found to have changed for ACE, PYR and the HCH isomers on a half-day basis (sequential sampling periods), for FLT during less than 2 days (Table S4).Changing directions of net air-sea mass exchange had been observed in the region along a ship cruise for OCPs, PCBs and one alkylated PAH, dimethylphenanthrene (Castro-Jiménez et al., 2012;Berrojalbiz et al., 2014) in 2006and for FLT and PYR in 2010(Mulder et al., 2014).Fast fluctuation of the direction of air-sea exchange throughout large parts of the year had been found for one alkylated PAH, retene, in the sea region following biomass burning emissions (based on box modelling; Mulder et al., 2014).Earlier, in 2000-02 air-sea exchange of PAHs was found depositional for all members (Tsapakis et al., 2006).Both directions were observed for 3 species, while the flux of PHE was downward (by average F c = -7.3µg m -2 d -1 ) whenever significant (Table S5a).With v dep = -F c / c g , this corresponded to a mean deposition velocity for gaseous PHE of v dep = 0.0043±0.0031cm s -1 , still significantly deviating fom zero.Even 3-ring PAHs' deposition is dominated by the particulate phase and a wide range has been reported (0.001-10 cm s -1 ; Zhang et al., 2015), also based on measurements in the region (Tasdemir and Esen, 2009).During the first days of the campaign FLT and PYR were volatilised, later deposited, too.
Air-sea exchange fluxes had been estimated earlier based on measurements in air and seawater in the Aegean Sea and application of the 2-film model for PAHs in 2001-02 (Tsapakis et al., 2006) and in spring 2006 for PAHs, HCB and PCBs (Castro-Jiménez et al., 2012).Hereby, the flux is calculated proportional to a substance specific mass transfer coefficient, k ol , strongly dependent on wind velocity and sea surface temperature (Jurado et al., 2004;Mandalakis et al., 2005).For both PCBs and PAHs widely varying k ol values have been estimated (Gigliotti et al., 2002;Mandalakis et al., 2005).The corresponding mean F c (5 sampling periods, just 1 in the case of ACE; Table S5) found in our study, 2012, has the same direction but exceeds by more than one and two orders of magnitude, respectively, the previous findings for PHE (downward) and FLT

Particulate phase concentrations and total deposition
Only PAHs and PBDEs were found exceeding LOQ in the particulate phase.Their day-night variation was minimal (Table S3a, b), by average c day /c night = 0.90-1.18for particulate PAHs, 1.03 and 1.07 for the PBDEs.This supports the perception that particulate PAH is not attacked by the hydroxyl radical, but 'shielded' by the particle matrix (e.g.Zhou et al., 2012).
The same had been observed previously at the same site (Tsapakis and Stephanou, 2007).
Effective photochemistry can also be excluded for particulate PBDEs for the same reasonWhile c day /c night = 1.20 and 1.37 for gaseous PBDEs suggests volatilisation from ground during the day, the absence of c day /c night > 1 for the particulate phase may indicate that the species are not in gas-particle phase equilibrium.This has been pointed out based on previous PBDE measurements in the region (Cetin and Odabasi, 2008).However, the data set discussed here is limited and gas-particle partitioning was not the subject of this study.
Total deposition is the sum of dry and wet deposition, the latter not being significant in the Mediterranean in summer.Dry deposition is the sum of particle deposition and diffusive depositional fluxes (part of air-sea exchange, see 3.2).The dry particle deposition flux, F p dep , can be estimated based on F p dep = -v dep c p with v dep being determined by particle size and wind speed.Dry particle deposition to the sea surface is most efficient under high wind speeds (Williams, 1982).The mass median diameter of PAHs at remote sites has been mostly found in the submicrometer range (Lipiatou and Atmos.Chem. Phys. Discuss., doi:10.5194/acp-2015-926, 2016 Manuscript under review for journal Atmos.Chem.Phys.Published: 1 February 2016 c Author(s) 2016.CC-BY 3.0 License., 1991), also during the measurements reported here (own, unpublished data measured simultaneously).For particles of 0.5 µm aerodynamic size v dep ≈ 0.1 cm s -1 can be expected for the mean wind velocity at Selles Beach, i.e. 5 m s -1 .Actually, v dep is not very sensitive to wind speed for this particle size range.(Slinn and Slinn, 1980) Adopting v dep = 0.1 cm s -1 would suggest F p dep ≈ -0.023, -0.016 and -0.010 µg m -2 d -1 for PHE, FLT and PYR (c p = 0.26, 0.19 and 0.11 ng m -3 , respectively, mean of the same 5 day-time sampling intervals in the period 3-10 July for which F c was determined, Table S5a).This means that the contribution of F p dep to dry deposition of PHE was negligible (F p dep ≈ 1000 × F p dep ; F c = -26 µg m -2 d -1 ) and F p dep negligibly compensated for net-volatilization of FLT and PYR in diffusive air-sea exchange (F c = +0.91 and +0.79 µg m -2 d -1 for FLT and PYR, respectively).The mass median diameter of the semivolatile PAHs FLT and PYR might well be larger than 0.5 µm as a consequence of redistribution in the aerosol along transport.However, even then, particle deposition could not have significantly compensated for net-volatilization, as for 1 µm particles F p dep would be higher by approximately a factor of 3 (Slinn and Slinn, 1980).For PHE, FLT and PYR F p dep = -0.021,-0.018 and -0.009 µg m -2 d -1 , respectively, were determined experimentally at Finokalia Observatory in 2001 (mean of 25 weeks between March and October; Tsapakis et al., 2006).This means that within measurement uncertainties for the same PAHs based on assuming v dep ≈ 0.2 cm s -1 (Castro-Jiménez et al., 2012).

Saliot
A similar calculation of for the BDEs for one day-time sampling interval (c p = 0.16 and 0.20 ng m -3 for BDE47 and BDE99, respectively, for day-time 6 July for which F c was determined, Table S5b) suggests that the contribution of F p dep to dry deposition of BDE47 was negligible, too (F p dep ≈ 100 × F p dep ; F c = +3.0µg m -2 d -1 ), while no direct comparison can be made for Atmos.Chem. Phys. Discuss., doi:10.5194/acp-2015-926, 2016 Manuscript under review for journal Atmos.Chem.Phys.Published: 1 February 2016 c Author(s) 2016.CC-BY 3.0 License.BDE99 (│F c │≲ 3.8 µg m -2 d -1 ; Table S5b).Hereby, v dep = 0.3 cm s -1 was adopted to account for mass median diameters close to 1 µm near the sources and mass transfer kinetic limitations for re-distribution during long-range transport (Cetin and Odabasi, 2008;Luo et al., 2014; and in agreement with own, unpublished data measured simultaneously in a short distance).
Significant vertical concentration differences in the particulate phase, Δc p /Δz > 0 and Δc p /Δz < 0, were found.Notably during one day-time and sequential night-time sampling (6-7 July) and during day-time of 9 July all significant gradients determined for particulate phase contaminants were negative, i.e. higher concentrations at the lower level, z 1 (PHE and FLT each 1, PYR 2 cases; PBDEs each 1 case; Table S3), while the opposite gradient was found in other nights and days.Apart from particle sources at the ground (not relevant here), vertical particle gradients may be sustained by turbulent diffusion (Pryor et al., 2008).While average wind speed was highest during day-time of 9 July, it was average during 6-7 July.No fluxes can be derived from the gradients determined in this study, downward or upward.

Conclusions
The diurnal variation of contaminant concentrations in air at a remote coastal site in the Aegean Sea was explained by the combination of atmospheric mixing depth and volatilisation from the sea surface.Volatilisation from the sea surface has significantly contributed to the night-time maxima of PCB28.Apart from long-range transport across the Aegean Sea, local sources were indicated for PBDEs: PBDE cycling was characterized by volatilization and transport from the island during the day and deposition to the sea surface.
We successfully quantified the diffusive air-sea exchange flux of 4 3-4 ring PAHs (in the upper pg m -3 concentration range), 3 OCPs, 3 PCBs and 2 PBDEs (in the lower pg m -3 concentration range) at a remote coastal site using a gradient in combination with the eddy Both flux directions were observed (fluctuation) for the OCPs studied, as well as for 3 PAHs (ACE, FLT, PYR) and 1 PCB (PCB52), not determined by the day-night cycle.Fluctuation of more substances might have been hidden by the method's uncertainties.Hence, the mean flux direction on one hand side and observations during part of the time of the trace substances may differ.E.g. volatilisation of BDE47 (observed in 1 night only) may have been the exception.In general, longer observations and across seasons of the flux is needed to assess the state of air-sea exchange of those anthropogenic trace substances, which have been approaching phase equilibrium historically (Jantunen and Bidleman, 1995;Stemmler and Lammel, 2009;Berrojalbiz et al., 2014) or seasonally (Mulder et al., 2014).
the particle deposition fluxes found in 2012 are the same than one decade earlier, in both absolute and relative (3 PAH members) terms.These fluxes are also in agreement with what was estimated in the Aegean Sea in summer 2006, namely F p dep = -0.010--0.015µg m -2 d -1 Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2015-926,2016 Manuscript under review for journal Atmos.Chem.Phys.Published: 1 February 2016 c Author(s) 2016.CC-BY 3.0 License.covariance technique.Many vertical gradients were insignificant and concentrations of other analytically targeted PAHs, PCBs, OCPs and PBDEs remained <LOQ.More substances could have been included using high-volume sampling, by which the sampled air volume could have been increased by one order of magnitude.