Bidirectional air–sea exchange and accumulation of POPs (PAHs, PCBs, OCPs and PBDEs) in the nocturnal marine boundary layer

Abstract

As a consequence of long-range transported pollution, air–sea exchange can become a major source of persistent organic pollutants in remote marine environments. The vertical gradients in the air were quantified for 14 species, i.e. four parent polycyclic aromatic hydrocarbons (PAHs), three polychlorinated biphenyls (PCBs), three organochlorine pesticides (OCPs) and two polybrominated diphenylethers (PBDEs) in the gas-phase at a remote coastal site in the southern Aegean Sea in summer. Most vertical gradients were positive (Δc∕Δz > 0), indicating downward (net depositional) flux. Significant upward (net volatilisational) fluxes were found for three PAHs, mostly during daytime, and for two OCPs, mostly during night-time, as well as for one PCB and one PBDE during part of the measurements. While phenanthrene was deposited, fluoranthene (FLT) and pyrene (PYR) seem to undergo flux oscillation, hereby not following a day–night cycle. Box modelling confirms that volatilisation from the sea surface has significantly contributed to the night-time maxima of OCPs. Fluxes were quantified based on eddy covariance. Deposition fluxes ranged from −28.5 to +1.8 µg m−2 day−1 for PAHs and −3.4 to +0.9 µg m−2 day−1 for halogenated compounds. Dry particle deposition of FLT and PYR did not contribute significantly to the vertical flux.

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Atmos. Chem. Phys., 16, 6381–6393, 2016
www.atmos-chem-phys.net/16/6381/2016/
doi:10.5194/acp-16-6381-2016
© Author(s) 2016. CC Attribution 3.0 License.
Bidirectional air–sea exchange and accumulation of POPs (PAHs,
PCBs, OCPs and PBDEs) in the nocturnal marine boundary layer
Gerhard Lammel1,2, Franz X. Meixner3, Branislav Vrana1, Christos I. Efstathiou1, Jiˇ
ri Kohoutek1, Petr Kukuˇ
cka1,
Marie D. Mulder1, Petra Pˇ
ribylová1, Roman Prokeš1, Tatsiana P. Rusina1, Guo-Zheng Song3, and Manolis Tsapakis4
1Masaryk University, Research Centre for Toxic Compounds in the Environment, Brno, Czech Republic
2Max Planck Institute for Chemistry, Multiphase Chemistry Dept., Mainz, Germany
3Max Planck Institute for Chemistry, Biogeochemistry Dept., Mainz, Germany
4Hellenic Centre for Marine Research, Institute of Oceanography, Gournes, Greece
Correspondence to: Gerhard Lammel (lammel@recetox.muni.cz)
Received: 13 November 2015 – Published in Atmos. Chem. Phys. Discuss.: 1 February 2016
Revised: 3 May 2016 – Accepted: 9 May 2016 – Published: 25 May 2016
Abstract. As a consequence of long-range transported pollu-
tion, air–sea exchange can become a major source of persis-
tent organic pollutants in remote marine environments. The
vertical gradients in the air were quantified for 14 species, i.e.
four parent polycyclic aromatic hydrocarbons (PAHs), three
polychlorinated biphenyls (PCBs), three organochlorine pes-
ticides (OCPs) and two polybrominated diphenylethers (PB-
DEs) in the gas-phase at a remote coastal site in the south-
ern Aegean Sea in summer. Most vertical gradients were
positive (1c/1z > 0), indicating downward (net deposi-
tional) flux. Significant upward (net volatilisational) fluxes
were found for three PAHs, mostly during daytime, and for
two OCPs, mostly during night-time, as well as for one PCB
and one PBDE during part of the measurements. While
phenanthrene was deposited, fluoranthene (FLT) and pyrene
(PYR) seem to undergo flux oscillation, hereby not follow-
ing a day–night cycle. Box modelling confirms that volatil-
isation from the sea surface has significantly contributed
to the night-time maxima of OCPs. Fluxes were quanti-
fied based on eddy covariance. Deposition fluxes ranged
from −28.5 to +1.8 µg m−2day−1for PAHs and −3.4 to
+0.9 µg m−2day−1for halogenated compounds. Dry particle
deposition of FLT and PYR did not contribute significantly
to the vertical flux.
1 Introduction
The marine atmospheric environment is a receptor for per-
sistent organic pollutants (POPs), which are advected from
primary and secondary sources on land. This is a concern
as these substance bioaccumulate along marine food chains
(e.g. Lipiatou and Saliot, 1991; Borgå et al., 2001). Pri-
mary sources do not exist in the marine environment, ex-
cept for polycyclic aromatic hydrocarbons (PAHs; from 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 (At-
las 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, fed by riverine or atmospheric deposition input. Such
studies are still rare. Revolatilisation was observed for hex-
achlorocyclohexane (HCH) and PAHs, not only in coastal
waters (Lohmann et al., 2011), but also in the open sea (Jan-
tunen and Bidleman, 1995; Lakaschus et al., 2002), including
the Mediterranean (Castro-Jiménez et al., 2012; Mulder et
al., 2014). After long-term accumulation of declining emis-
sions (even after phase-out), a 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 ongoing emissions on the other
hand may trigger a seasonal reversal of air–sea exchange, as
Published by Copernicus Publications on behalf of the European Geosciences Union.

6382 G. Lammel et al.: Bidirectional air–sea exchange and accumulation of POPs
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 residen-
tial heating (winter maximum) may revolatilise seasonally
from the sea surface in receptor areas.
The direction of diffusive air–surface exchange flux of or-
ganics can be identified by comparing the fugacities and can
be quantified based on the Whitman two-film model (Bidle-
man and McConnell, 1995; Schwarzenbach et al., 2003) or
micrometeorological 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 sur-
face seawater was determined too, such that the direction of
air–sea exchange could be addressed by a second method.
1.1 Site and sampling
The site selected for atmospheric measurements was Selles
Beach on 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 100 000 inhabitants with airport and indus-
tries; Mihalopoulos et al., 1997; Kouvarakis et al., 2000). The
Mediterranean region includes urban and industrial areas and
is adjacent to source regions (i.e. western, central and east-
ern Europe). Exposure of the study area to long-range trans-
ported pollution from central and eastern Europe is highest
in summer (Lelieveld et al., 2002).
Organic substances were collected during 3–13 July in
the gaseous and particulate atmospheric phases using low-
volume samplers (F≈2.3 m3h−1, Leckel LVS, PM10 in-
let) equipped with quartz fibre filters (QFF, Whatman QMA
47 mm, baked at 320◦C prior to usage) and two polyurethane
foam (PUF) plugs (Molitan, density 0.030 g cm−3, 5.5 cm di-
ameter, total depth 10 cm, cleaned by extraction in acetone
and dichloromethane, 8 h each) in series. Two of these sam-
plers collected gases and particles at different heights (inlets
at z1=1.05 and z2=2.8 m), about 0.5 m apart in the hori-
zontal. According to the Monin–Obukhov similarity theory
of turbulent transport in the surface layer (see Sect. S1.3 in
the Supplement), vertical concentration profiles are expected
to be logarithmic, while – due to the vertically non-linear be-
haviour of the eddy diffusivity – the resulting vertical flux is
per definitionem constant. For a given surface source (sink)
of gases or particles, corresponding negative (positive) verti-
cal concentration gradients (∂c/∂z) will be maximal close to
the source (sink). Therefore, over aerodynamically smooth
surfaces, the lower inlet height should be preferably in the
order of a few tenths of a metre, while the distance to the
upper inlet height should be maximised to yield large (and
consequently statistically significant) vertical concentration
differences. However, the choice for the upper inlet height is
limited by the horizontal extension of the so-called “fetch”,
i.e. the upwind surface homogeneity (in terms of orography,
structure and vegetation), which should be at least 100 ×z2
(e.g. Foken, 2008). In the case of our study, the choice of
the lower inlet height (z1=1.05 m) is just due to the de-
sign of the aerosol sampler, which does not allow sampling
below 1.05 m above ground, while the upper inlet height
(z2=2.80 m) accounts for the limited surface homogeneity
(200 m) of the Selles Beach site.
Daytime (09:00–20:00 EEDST and night-time (21:00–
08:00 EEDST) sampling was conducted from 2 July in the
evening to 13 July 2012 in the evening. During part of the
measurements, from 6 July in the morning to 10 July 2012
in the evening, a third sampler was used to collect replica
of gaseous samples (PUF plugs only) at z1. For the concen-
tration at z1,cz1, replica concentrations (mean of two mea-
surements) 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 dis-
tance between the rock and water surfaces was 0.1–0.3 m,
varying due to tide and waves. After exposure, filters and
PUFs were packed in aluminium foil and zip-bags, stored
and transported in a cool box to the laboratory.
Free dissolved contaminants in seawater were sampled us-
ing silicone rubber (SR) sheets (Altec, Great Britain) as pas-
sive water samplers (PWSs). Quantification of trace organics
from PWSs is sensitive and validated (Rusina et al., 2010a).
Uncertainties in results obtained by application of partition-
based passive samplers are believed to range around a fac-
tor of 2, depending on the level of experience of the labo-
ratory (Allan et al., 2009). Different aspects of uncertainty
are discussed in Lohmann et al. (2012). At two localities, at
distances of 0.8 and 2.2 km west of Selles Beach, two SR
PWSs 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; D10-biphenyl
and 13 PCB congeners not occurring in the environment) ac-
cording to the procedure (Booij et al., 2002). Samplers were
deployed from 3 July to 2 August 2012 in water mounted
on stainless steel wire holders at 1 m depth using buoys and
rope. After exposure, samplers were stored and transported
in original vials and brought in a cool box to the laboratory.
Daily mean temperature was 28.2 (22.4–34.5) ◦C and wind
velocity was 4.8 (0.6–7.7) m s−1(hourly data). No precipita-
tion occurred. The meteorological situation is described in
the Supplement, Sect. S2.1.
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G. Lammel et al.: Bidirectional air–sea exchange and accumulation of POPs 6383
1.2 Meteorological parameters and vertical flux
calculations
Boundary layer (BL) depth is needed for interpreting the
variation of concentrations in air. BL depth data are taken
from simulations of the Lagrangian dispersion model FLEX-
PART, version 9 (Stohl et al., 1998). These were run in a for-
ward direction and based on analysed wind fields (ECMWF,
0.5◦resolution). The model BL height is calculated ac-
cording to Vogelezang and Holtslag (1996) using the criti-
cal 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) as the relevant BL
depth for interpretation of atmospheric concentrations at the
coastal site. The mean BL depth during sampling intervals is
used.
For characterisation of the local meteorological condi-
tions, continuous measurements (5 min averages) of air tem-
perature, relative humidity, wind speed and wind direc-
tion were accomplished by three automatic weather stations
(model WMT520; Vaisala, Helsinki, Finland) which have
been placed at the beach at distances of ≈200 m (2) and
≈100 m inland (1) from the sampling location. For charac-
terisation of the atmospheric surface layer’s thermodynamic
stratification, vertical profiles of wind speed, wind direc-
tion, 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 were recorded by 2-D ultra-
sonic wind sensors (model WMT701; Vaisala, Helsinki, Fin-
land) and aspirated temperature and relative humidity sen-
sors (model MP103A; Rotronic, Bassersdorf, Switzerland)
in 10 s intervals, which were averaged to 30 min means for
further data processing. For determining key micrometeoro-
logical quantities (e.g. sensible heat flux, friction velocity;
see Supplement, Sect. S1.3), fast response measurements of
the 3-D wind vector and air temperature have been performed
by a 3-D ultrasonic anemometer (CSAT-3, Campbell Scien-
tific 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 Sci-
entific Inc., Logan, USA). Key micrometeorological quanti-
ties were derived from fast response 3-D wind and air tem-
perature 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, Uni-
versity of Bayreuth, Germany), and the results were averaged
every 30 min. 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 Sup-
plement, Sect. S2.1).
The turbulent vertical gaseous organics flux, Fc
(ng m−2s−1), has been calculated according to the aero-
dynamic method as the product of the vertical difference
of concentration, 1cz(ng m−3)and the turbulent transfer
velocity, vtr (m s−1):
Fc= −vtr1cz= −vtr [c(z2)−c(z1)],(1)
where z2and z1are the heights of inlets of gaseous organics’
sampling (1.05 and 2.80 m, see Sect. 2.1, above). The trans-
fer velocity is a measure of the vertical turbulent (eddy) dif-
fusivity. As long as the vertical concentration gradient is sta-
tistically significant, and the choice of the upper inlet height
accounts for the homogeneity of the upwind so-called fetch
(see above), the resulting flux, calculated from the transfer
velocity and the vertical concentration gradient (see Eq. 1), is
the vertical turbulent net flux of the corresponding gas, repre-
sentative for the fetch area and equal to the overall deposition
flux (if c(z2)−c(z1) > 0).
Details of the underlying formulation and the calculation
scheme are given in the Supplement, Sect. S1.3.
1.3 Chemical analysis
For organic analysis all samples were extracted with
dichloromethane during ≈1 h in an automatic extractor
(Büchi B-811). Surrogate extraction standards (D8-
naphthalene, D10-phenanthrene, D12 -perylene, PCB30,
PCB185, 13C BDEs 28, 47, 99, 100, 153, 154, 183 and
209) were spiked on each PUF and QFF prior to extraction.
The volume was reduced after extraction under a gentle
nitrogen stream at ambient temperature, and fractionation
was achieved on a silica gel column. Samples were analysed
using a GC-MS (gas chromatograph coupled with a mass
spectrometer) Agilent 7890 coupled to Agilent 7000B
with a J&W Scientific fused silica column DB-5MSUI
(60 m ×0.25 mm ×0.25 µm) for 2–4-ring PAHs (naphtha-
lene (NAP), acenaphthylene (ACY), acenaphthene (ACE),
fluorene (FLN), phenanthrene (PHE), anthracene (ANT),
fluoranthene (FLT), pyrene (PYR), benzo(a)anthracene
(BAA) and chrysene (CHR)). Terphenyl was used as in-
jection standard. The temperature programme was 80 ◦C,
15 ◦C min−1to 180 ◦C, 5 ◦C min−1to 310 ◦C. The injection
volume was 1 µL in splitless mode at 280 ◦C, with He used
as a carrier gas at a constant flow of 1.5 mL min−1.
A sulphuric acid modified silica gel column was used for
the PCB/OCP and PBDE clean-up. Samples were analysed
using a GC-MS/MS Agilent 7890 coupled to Agilent 7000B
with a SGE HT-8 column (60 m ×0.25 mm ×0.25 µm)
for α-HCH, β-HCH, γ-HCH, δ-HCH, o, p0- and p, p0-
DDE (dichlorodiphenyldichloroethylene), -DDD and -DDT,
penta- and hexachlorobenzene (PeCB, HCB). PCB 121
was used as injection standard for chlorinated substances.
The temperature programme was 80 ◦C (1 min hold),
40 ◦C min−1to 200 ◦C, 5 ◦C min−1to 305 ◦C. The injection
volume was 3 µL in splitless mode at 280 ◦C, with He used
as a carrier gas at constant flow of 1.5 mL min−1.
PBDEs were analysed using GC-HRMS (gas chromatog-
raphy with high-resolution mass spectrometry) on a Restek
RTX-1614 column (15 m ×0.25 mm ×0.1 µm). The resolu-
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6384 G. Lammel et al.: Bidirectional air–sea exchange and accumulation of POPs
tion was set to >10 000 for BDE 28–183 and >5000 for
BDE 209. 13C BDEs 77 and 138 were used as injection stan-
dards. The MS was operated in electron impact (+) mode at
the resolution of >10 000. The temperature programme was
80 ◦C (1 min hold), then 20 ◦C min−1to 250 ◦C, followed by
1.5 ◦C min−1to 260 ◦C and 25 ◦C min−1to 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 in the range of 78–
128 %.
The mean of four 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
less than the limit of quantification (LOQ). Field blank val-
ues of most analytes in air samples were below the instru-
ment limit of quantification (ILOQ), which corresponded
to 6–34 pg m−3for PAHs, 7–23pg m−3for PCB and OCPs
and 0.003–0.04 pg m−3for PBDEs (Supplement, Table S2).
Higher LOQs were determined for analytes in gaseous air
samples, namely 0.18 and 0.50 ng m−3for FLN and PHE,
and typically 28 pg m−3for HCB. In the particulate phase a
higher LOQ resulted for PHE, i.e. 170 pg m−3. The break-
through in PUF samples was estimated and, as a conse-
quence, NAP, FLN, HCB and PeCB results are not consid-
ered as the sampled air volume (typically ≈25 m3for PUFs)
expectedly lead to breakthrough under the prevailing temper-
atures (Melymuk et al., 2015).
Free dissolved water concentrations of analytes in PWSs
were calculated from amounts accumulated in SRs using
the exponential uptake model described in Smedes (2007).
The required sampling rates were estimated by fitting perfor-
mance reference compounds dissipation data from sampler
to the model described by Booij and Smedes (2010). ILOQ
corresponded to 0.5–4.2 pg L−1for PAHs (but 9 pg L−1for
NAP), 0.05–0.5 pg L−1for PCB and OCPs and 0.0003–
0.037 pg L−1for PBDEs (Supplement, Table S2). Site spe-
cific LOQs were 1–10 pg L−1for PAHs (but 400 pg L−1for
NAP), 0.1–0.8 pg L−1for PCB, 0.1–1.4 pg L−1for OCPs
(but 2.8 pg L−1for α-HCH) and 0.01–0.11 pg L−1for PB-
DEs (but 0.59 pg L−1for BDE209).
1.4 Vertical gradients of trace organics’ concentration
in air
Air–sea gas exchange can be studied by determining the ver-
tical 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 mea-
surements, c, and twice as much as the uncertainty of concen-
tration differences, 1cz. Values of concentrations and verti-
cal concentration differences (gradients) not exceeding these
thresholds are considered to be insignificant. This applies to 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).
1.5 Air–water fugacity ratio
The direction of diffusive air–sea gas exchange can be de-
rived from the fugacity ratio calculation, based on the Whit-
man two-film model (Bidleman and McConnell, 1995). The
fugacity ratio, fa/fw, is calculated:
fa/fw=caRTa/(cwHTw,salt ), (2)
with gas-phase concentration ca(ng m−3), dissolved aque-
ous concentration cw(ng m−3), universal gas constant R
(Pa m3mol−1K−1), both sea-surface temperature (SST or
Tw(K)) and salinity corrected Henry’s law constant HTw,salt
(Pa m3mol−1; see Sect. S1.1 for details) and air temperature
Ta(K). Tawas adopted from the on-site measurement (see
above). cwis derived as the average of the results at two lo-
calities, with two replicas each (see above, Sect. 2.1). SST
data, measured on the sampling day and in the area, were
downloaded from the respective database (see Sect. S1.4 for
details). Air and water sampling were not totally in phase:
sampling in air occurred over 12 days (2–13 July), while SR
exposure occurred over 28 days (3–30 July), i.e. collection
was done 10 days after the air sample collection. Conse-
quently, for those substances which are quickly equilibrated
(within a few days) in PWS, i.e. HCH and 3-ring PAHs, no
simultaneous measurements in air and water were done (see
Sect. 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, HTw,salt, and mea-
sured concentrations and temperature changes during sam-
pling (e.g. Bruhn et al., 2003; Castro-Jiménez et al., 2012).
Substance property data are taken from the literature (Sup-
plement, Table S1). This conservative uncertainty margin is
also adopted here, while fa/fw>3.0 indicates net deposi-
tion and fa/fw<0.3 net volatilisation.
1.6 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; Mul-
der et al., 2014) in order to test the hypothesis that the di-
urnal 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 at-
mospheric mixing depth, while advection (long-range trans-
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G. Lammel et al.: Bidirectional air–sea exchange and accumulation of POPs 6385
Table 1. Statistics of (a) concentrations at ground level, z1=1.05 m and (b) vertical gradients, 1cz, over 1z =1.75 m of gaseous PAHs and
halogenated POPs. Mean ±standard deviation (n, min–max), ng m−3, except PBDEs: pg m−3). All data are included (exceeding or below
uncertainty thresholds). Individual data: see Tables S3, S4. For mean and standard deviations, values <LOQ were replaced by LOQ/2.
(a) all samples night-time daytime
ACE 0.091 ±0.021 (0.059 to 0.12) 0.11 ±0.006 (5, 0.11 to 0.12) 0.076 ±0.012 (5, 0.059 to 0.091)
PHE 1.28 ±0.89 (0.21 to 3.54) 1.00 ±0.52 (8, 0.21 to 1.46) 1.47 ±1.10 (10, 0.25 to 3.54)
FLT 0.35 ±0.26 (0.044 to 0.89) 0.29 ±0.24 (8, 0.046 to 0.76) 0.40 ±0.29 (10, 0.044 to 0.89)
PYR 0.23 ±0.20(0.09 to 0.86) 0.18 ±0.09 (6, 0.13 to 0.34) 0.28 ±0.25 (8, 0.09 to 0.86)
PCB 28 0.020 ±0.009 (0.006 to 0.037) 0.021 ±0.010 (5, 0.008 to 0.033) 0.020 ±0.009 (10, 0.006 to 0.037)
PCB 52 0.012 ±0.007 (0.003 to 0.024) 0.014 ±0.008 (6, 0.003 to 0.024) 0.011 ±0.005 (10, 0.003 to 0.023)
PCB101 0.008 ±0.002 (0.005 to 0.011) 0.009 ±0.002 (4, 0.006 to 0.011) 0.006 ±0.001 (5, 0.005 to 0.008)
α-HCH 0.038 ±0.021 (0.008 to 0.078) 0.056 ±0.013 (6, 0.039 to 0.076) 0.031 ±0.020 (10, 0.008 to 0.078)
γ-HCH 0.106 ±0.072 (0.007 to 0.245) 0.136 ±0.078 (8, 0.030 to 0.245) 0.089 ±0.067 (11, 0.007 to 0.242)
p, p0-DDE 0.007 ±0.004 (0.003 to 0.015) 0.008 ±0.003 (4, 0.006 to 0.012) 0.007 ±0.004 (8, 0.003 to 0.015)
BDE 47 0.327 ±0.119 (0.20 to 0.51) 0.265 ±0.046 (3, 0.228 to 0.317) 0.36 ±0.14 (5, 0.20 to 0.51)
BDE 99 0.218 ±0.062 (0.14 to 0.31) 0.195 ±0.024 (4, 0.161 to 0.214) 0.234 ±0.077 (5, 0.137 to 0.313)
(b) all samples night-time daytime
ACE 0.008 ±0.047 (−0.026 to 0.10) −0.001 ±0.027 (4, −0.024 to 0.037) 0.019 ±0.072 (3, −0.026 to 0.10)
PHE 1.24 ±2.10 (−0.047 to 7.35) 1.19 ±1.96 (4, −0.047 to 5.55) 1.29 ±2.32 (6, 0.009 to 7.35)
FLT 0.19 ±0.43 (−0.46 to 0.95) 0.33 ±0.44 (6, −0.25 to 0.95) 0.073±0.40 (6, −0.46 to 0.73)
PYR 0.081 ±0.21 (−0.42 to 0.45) 0.18±0.17 (6, 0.038 to 0.45) 0.004 ±0.22 (6, −0.42 to 0.32)
PCB 28 0.024 ±0.046 (−0.013 to 0.155) 0.013 ±0.040 (7, −0.011 to 0.103) 0.032 ±0.050 (10, −0.013 to 0.155)
PCB 52 0.016 ±0.036 (−0.011 to 0.120) 0.009 ±0.036 (7, −0.011 to 0.089) 0.020 ±0.038 (10, −0.004 to 0.120)
PCB101 0.012 ±0.018 (−0.005 to 0.044) 0.007 ±0.019 (4, −0.005 to 0.035) 0.018 ±0.018 (4, −0.001 to 0.044)
α-HCH 0.079 ±0.188 (−0.048 to 0.675) 0.025 ±0.131 (5, −0.048 to 0.258) 0.107 ±0.212 (10, −0.019 to 0.675)
γ-HCH 0.107 ±0.273 (−0.159 to 0.890) 0.018 ±0.222 (7, −0.159 to 0.510) 0.164 ±0.297 (11, −0.073 to 0.890)
p, p0-DDE 0.001 ±0.005 (−0.006 to 0.009) 0.002 ±0.004 (4, −0.001 to 0.007) 0.001 ±0.006 (5, −0.006 to 0.009)
BDE 47 −0.229 ±0.072 (−0.280 to −0.178) no data −0.229 ±0.072 (2, −0.280 to −0.178)
BDE 99 −0.042 ±0.017 (−0.059 to 0.025) −0.058 (1) −0.034 ±0.014 (2, −0.044 to −0.025)
port) is less significant (horizontal homogeneity of air mass;
Lammel et al., 2003). This 2-box model predicts concen-
trations by integration of two coupled ordinary differential
equations that solve the mass balances for the two compart-
ments, namely the atmospheric marine BL and seawater sur-
face mixed layer. Processes considered in air are dry (par-
ticle) deposition, removal from air by reaction with the hy-
droxyl radical and air–sea mass exchange flux (dry gaseous
deposition), while in seawater export (settling) velocity, de-
position flux from air, air–sea mass exchange flux (volatili-
sation) and degradation (as a first-order process) are consid-
ered. Input parameters are listed in the Supplement, Table S3.
2 Results and discussion
2.1 Day–night variation of concentrations in air
Four PAHs (ACE, PHE, FLT, PYR), three OCPs (α- and γ-
HCH, p, p0-DDE), three PCB congeners (PCB28, -52 and
-101) and two PBDE congeners (BDE47 and -99) were quan-
tified in gas-phase samples, while the other species were
found <LOQ in all or most samples (Figs. 1a, 2a, Ta-
ble 1a, b). This is a consequence of limited air sample vol-
ume (≈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 (Kamari-
anos et al., 2002; Mandalakis and Stephanou, 2002; Tsapakis
and Stephanou, 2005; Cetin and Odabasi, 2008; Halse et
al., 2011; Lammel et al., 2010, 2011; Castro-Jiménez et al.,
2012; Berrojalbiz et al., 2014; Mulder et al., 2014, 2015). To
our best knowledge, the DDE levels are the lowest reported
from the region. This confirms the remote character of the
site. Influence from local sources, not expected at this remote
site (Iacovidou et al., 2009), is sometimes indicated by an
anti-correlation between wind speed and atmospheric con-
centration. At Selles Beach, dilution by higher wind speed
is indeed indicated for one contaminant, ACE (by significant
anti-correlation, p < 0.05 confidence level, ttest). This is ex-
pected because of its short atmospheric lifetime.
BL depths ranged between 160 and 500 m during night-
time and 270 and 760 m during daytime (mean of sam-
pling intervals, i.e. 11 h). Day–night variation of contami-
nants’ atmospheric concentrations, often related to mixing
and local sources, was not clear: for PAHs the mean ratio
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6386 G. Lammel et al.: Bidirectional air–sea exchange and accumulation of POPs
Figure 1. Gaseous (a) PAH and (b) halogenated substances’ concentrations at ground level, z1=1.05 m (upper), vertical concentration
differences, 1cz=cz2−cz1, over 1z =1.75m (middle) and vertical fluxes, Fc= −vtr1cz(positive indicates upward, negative indicates
downward; lower). Only significant data (exceeding uncertainty thresholds) are shown; gaps represent no data.
of day–night concentrations (gaseous only), cday/cnight , was
0.67 (0.54–0.85) for ACE, while cday/cnight ranged wider,
at 0.25–3.35 for PHE, FLT and PYR, with mean values of
cday/cnight =1.41–1.53. Also for PBDEs cday /cnight >1 is
found (1.20 and 1.37). The low value for ACE can be ex-
plained by its short photochemical lifetime (Keyte et al.,
2013). cday/cnight >1 was previously observed for PAHs at
the same site and explained by temperature-driven volatili-
sation from surfaces overcompensating for photochemistry
(Lee et al., 1998; Tsapakis and Stephanou, 2007). For chlo-
rinated substances, we find mean cday/cnight <1, namely
0.56–0.66 for HCH isomers and 0.68–0.94 for PCBs and
DDE, while the individual values range widely from 0.04
to 4.8. However, there was a clear day–night trend, with
mostly night-time maxima of both PAHs (Fig. 1a) and chlori-
nated species (Fig. 1b) during a period of continuous onshore
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G. Lammel et al.: Bidirectional air–sea exchange and accumulation of POPs 6387
winds, 6–10 July. 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, ttest). This results in night-
time accumulation. Diel variation, apart from mixing, is re-
lated to advection and air–sea mass exchange and studied in
more detail in Sect. 3.2.
2.2 Diffusive air–sea exchange
The variation of air concentrations (with night-time maxima)
during a period of northerly flow without change of air mass
is predicted using the 2-box model (Sect. 2.6). For PCB28,
-52, FLT and BDE47 air concentrations are qualitatively well
captured (Fig. 2). These are maintained by dry gaseous de-
position alone (PCB52, FLT) or by oscillating fluxes (HCB,
PCB28: upward, Fig. 2; PYR: mostly downward, Fig. S4a).
The model-predicted fluxes are in good agreement with the
observed values (Sect. 3.3, Table S5) except for each day-
time sampling interval of FLT and BDE47 (upward fluxes)
and for one daytime interval of PCB28 (downward flux; in
total four agreements, three disagreements). The modelling
results support that (during advection of one air mass) the
diel variation of contaminant concentrations in air and, par-
ticularly the night-time accumulation, was explained by the
combination of volatilisation from the sea surface and atmo-
spheric mixing depth. Volatilisation from the sea surface has
significantly contributed to the night-time maxima of HCB
(upward flux, Fig. 2) and PCB28 (Fig. 2), as well as of PYR
during one night (Fig. S4a; Table S5). This, to our knowl-
edge, had never been observed before.
fwis derived from the mean concentrations in seawa-
ter at two locations (see Supplement, Table S6, for individ-
ual data). For the measurement period 2–13 July 2012, the
comparison of air–water fugacity ratios (Sect. 2.5) suggests
net deposition (prevailing downward fluxes, fa/fw>3) of
gaseous FLT, PYR, BDE47 and -99, net volatilisation (pre-
vailing upward fluxes , fa/fw<0.3) of gaseous PCB28 and
-101 close to phase equilibrium (0.3< fa/fw<3) for p, p0-
DDE and PCB52 (Table 2). These results are the same as
those 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 fu-
gacity calculations is consistent with what was indicated by
the correlation of air concentrations and BL depth during on-
shore winds (Supplement, Sect. S2.5).
2.3 Vertical concentration gradients in air
PAH vertical gradients mostly indicated deposition,
1c/1z > 0, found in 28 cases (14 during the day, 14 at
night), while negative gradients were found in 10 cases
(8 during the day, 2 at 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)
Table 2. Concentrations in air (gaseous, ground level, z=1.05 m,
if >LOQ for most samples; ng m−3, except PBDEs: pg m−3)and
surface seawater (pg L−1)and fugacity ratios, fa/fw.
cacwfa/fw
FLT 0.35 18.2 19
PYR 0.23 4.4 59
PCB 28 0.020 4.65 0.15
PCB 52 0.012 0.54 0.94
PCB 101 0.0076 0.84 0.34
p, p0-DDE 0.0072 0.84 0.76
BDE 47 0.33 0.15 2600
BDE 99 0.22 0.038 16140
1c/1z tended to be clearly lower during daytime, indicating
that volatilisation of PAHs from the sea surface was stronger
during daytime. This could be explained by a higher fugacity
from seawater, fw, which increases with HTw,salt (see
above, Sect. 2.5), which in turn increases with sea-surface
temperature, Tw. Similarly, for the halogenated substances,
significant positive gradients, 1c/1z > 0, indicating depo-
sition were more frequent than significant negative gradients,
i.e. 37 cases (15 PCBs, 22 OCPs, 30 during the day, 7 in
one night only) and 20 cases (2 PCBs, 17 OCPs, 1 PBDE,
5 during the day, 15 at night). 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. Diel variation of PCB52
air–sea exchange flux direction is well reflected by the
model (Fig. S4b). This trend is most significant for the HCH
isomers for which a stronger volatilisation flux from the sea
surface is found at night than during daytime (1c/1z < 0)
or even for deposition during daytime (1c/1z > 0 on 6 and
9 July). Hence, volatilisation from the sea surface may have
contributed to and 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. daytime mean was typically 0.5–1.5 K
warmer than night-time mean temperature. Even somewhat
lower upward fluxes, Fc, of HCH at night than during the
day, caused by a slightly lower sea-surface temperature,
may have caused cday/cnight <1 in combination with the
day–night variation of the BL depth (on average 50% deeper
for daytime sampling periods). PBDE daytime maxima may
indicate local volatilisation from soil, which are enhanced
during daytime. 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 daytime (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). To summarise, average significant
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6388 G. Lammel et al.: Bidirectional air–sea exchange and accumulation of POPs
Figure 2. Predicted (pink) and observed (black) concentrations (ng m−3)and predicted diffusive air–sea exchange fluxes, Fc, (lower: red
upward and blue downward, ng m−2day−1)of selected contaminants, (a) HCB, (b) PCB28, (c) FLT and (d) BDE47 during 6–10 July 2012.
Atmos. Chem. Phys., 16, 6381–6393, 2016 www.atmos-chem-phys.net/16/6381/2016/

G. Lammel et al.: Bidirectional air–sea exchange and accumulation of POPs 6389
daytime vertical gradients, 1c/1z, 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 iso-
mers on a half-day basis (sequential sampling periods) for
FLT in less than two days (Table S4). Changing directions
of net air–sea mass exchange had been observed in the re-
gion from a ship cruise for OCPs, PCBs and one alkylated
PAH, dimethylphenanthrene (Castro-Jiménez et al., 2012;
Berrojalbiz et al., 2014) in 2006 and 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 mod-
elling; Mulder et al., 2014). Earlier, in 2000–2002 air–sea
exchange of PAHs was found to be depositional for all mem-
bers (Tsapakis et al., 2006).
These observations of an increasing number of pollutants
attaining phase equilibrium and bidirectional flux may indi-
cate a long-term trend from deposition towards “reversal”,
i.e. volatilisation of these pollutants in the marine environ-
ment of the eastern Mediterranean and more generally of re-
ceptor seas regions located in the outflow of regions emitting
long-lived semivolatile pollutants, such as most POPs. For
substances close to phase equilibrium (attained in a long-
term trend), the direction of air–sea exchange may change
with a high frequency, as found here. Implicitly, dry depo-
sition is difficult to budget. However, fluctuation may also
occur in response to seasonal trends: in summer 2010 FLT
and PYR were found close to phase equilibrium in the east-
ern Mediterranean, while retene (RET) was found mostly
volatilisational. A model simulation had revealed that sea-
sonal primary emissions and subsequent deposition of RET
(from open fires in the region) are triggering seasonal flux re-
versal, which over many weeks, however, is fluctuating with
a high frequency (<24 h; Mulder et al., 2014). Obviously,
longer observations are needed to assess the prevailing verti-
cal flux direction. Extrapolation of the observations to annual
fluxes is not justified, as day–night fluctuations may be part
of a more complex temporal pattern. The seawater surface
as a secondary source of pollution should be assessed based
on flux measurements during several seasons and over longer
time periods.
2.4 Quantification of vertical fluxes of gaseous
contaminants
Vertical fluxes, Fc, can be quantified for periods with trans-
fer velocity, vtr, determined, which varied between 3.3 and
8.4 cm s−1, on average it was 5.3 ±1.9 cm s−1. The time cov-
erage of this parameter was 70 %; however, satisfying time
coverage of sampling intervals was achieved in 14 out of
20 sampling intervals during 11 days (on average 5.5cm s−1)
and 3 nights (on average 3.5 cm s−1)(Figs. 1a, 2c, Table S5).
The fluxes of PAHs could be determined based on 8 peri-
ods of daytime and 2 periods of night-time sampling. 15 PAH
fluxes were downward (Fc= −3.6±7.0 µg m−2day−1), 8
upward (ranging Fc=0.8±0.6 µg m−2day−1)and 13 in-
significant (|Fc|< (0.7±0.7)µg m−2day−1). Both direc-
tions were observed for three species, while the flux of PHE
was downward (on average Fc= −7.3 µg m−2day−1)when-
ever significant (Table S5a).
vdep = −Fc/cg(3)
The above equation corresponded to a mean deposition ve-
locity for gaseous PHE of vdep =0.0043 ±0.0031 cm s−1,
still significantly deviating from zero (1 standard deviation
criterion, based on measurement at z1=1.05 m). 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 (Tas-
demir and Esen, 2009). During the first days of the campaign
FLT and PYR were volatilised and later deposited too.
For 11 periods of daytime and 2 of night-time sam-
pling, the fluxes of 8 halogenated substances were down-
ward in 12 cases (Fc= −0.58 ±0.87 µg m−2day−1)and
upward in 6 cases (Fc=0.30 ±0.31 µg m−2day−1; these
were 0.11–0.25 for γ-hexachlorocyclohexane (HCH) and
0.91 µg m−2day−1for BDE47) and insignificant in 15 cases
(|Fc|0.19 ±0.45 µg m−2day−1)(Table S5b). The fluxes cor-
responded to mean deposition velocities which were not
distinguishable from zero, e.g. 0.020 ±0.032 cm s−1for α-
HCH and 0.011 ±0.015 cm s−1for PCB28 (1 standard devi-
ation criterion).
Air–sea exchange fluxes had been estimated earlier based
on measurements in air and seawater in the Aegean Sea as
well as from application of the 2-film model in 2001–2002
(Tsapakis et al., 2006) for PAHs 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, kol, strongly dependent on wind veloc-
ity and sea-surface temperature (Jurado et al., 2004; Man-
dalakis et al., 2005). For both PCBs and PAHs, widely vary-
ing kol values have been estimated (Gigliotti et al., 2002;
Mandalakis et al., 2005). The corresponding mean Fc(five
sampling periods, just one in the case of ACE; Table S5)
found in our study in 2012 has the same direction but exceeds
the previous findings for PHE (downward) and FLT (down-
ward in 2001–2002, upward in 2006) by more than 1 and 2
orders of magnitude respectively. For PYR the opposite di-
rection (now upward, downward in 2001–2002 and 2006) is
found. The flux direction found for the PCBs is unchanged
compared to the 2006 measurements, then found close to
phase equilibrium (Berrojalbiz et al., 2014).
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6390 G. Lammel et al.: Bidirectional air–sea exchange and accumulation of POPs
2.5 Particulate phase concentrations and total
deposition
Only PAHs and PBDEs were found to exceed LOQ in the
particulate phase. Their day–night variation was minimal
(Table S3a, b), on average cday/cnight =0.90–1.18 for par-
ticulate 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 reason. While cday/cnight =1.20 and 1.37 for
gaseous PBDEs suggests volatilisation from the ground dur-
ing the day, the absence of cday/cnight >1 for the particulate
phase may indicate that the species are not in gas-particle
phase equilibrium. This has been pointed out based on previ-
ous 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
because of small amounts of precipitation. Dry deposition
is the sum of particle deposition and diffusive depositional
fluxes (part of air–sea exchange, see Sect. 3.2). The dry par-
ticle deposition flux, Fp dep, can be estimated.
Fp dep = −vdepcp,(4)
with vdep being determined by particle size and wind speed
and cpdetermined close to the sea surface. Dry particle de-
position 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 submicrome-
ter range (Lipiatou and Saliot, 1991), also during the mea-
surements reported here (own, unpublished data measured
simultaneously). For particles of 0.5 µm, aerodynamic size
vdep ≈0.1 cm s−1can be expected for the mean wind ve-
locity at Selles Beach, i.e. 5 m s−1(Slinn and Slinn, 1980).
Adopting vdep =0.1 cm s−1would suggest Fp dep ≈ −0.023,
−0.016 and −0.010 µg m−2day−1for PHE, FLT and PYR
(cp=0.26, 0.19 and 0.11 ng m−3respectively; mean of the
same five daytime sampling intervals in the period 3–10 July
for which Fcwas determined, Table S5a). This means that
the contribution of Fp dep to dry deposition of PHE was
negligible (Fc≈1000×Fp dep;Fc= −26 µg m−2day−1)and
Fp dep negligibly compensated for net-volatilisation of FLT
and PYR in diffusive air–sea exchange (Fc= +0.91 and
+0.79 µg m−2day−1for FLT and PYR respectively). The
values of vdep integrated over the entire size spectrum may
differ considerably from values of vdep for MMD (Ruij-
grok et al., 1995). Furthermore, the mass median diame-
ter 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, par-
ticle deposition is unlikely to have significantly compen-
sated for net-volatilisation, as even for particles grown to
1.5 µm Fp dep would be higher by not more than a factor
of 10 (Slinn and Slinn, 1980). For PHE, FLT and PYR,
Fp dep = −0.021, −0.018 and −0.009 µg m−2day−1respec-
tively were determined experimentally at Finokalia Observa-
tory in 2001 (mean of 25 weeks between March and October;
Tsapakis et al., 2006). This means that within measurement
uncertainties, the particle deposition fluxes found in 2012 are
the same as one decade earlier, in both absolute and relative
(three PAH members) terms. These fluxes are also in agree-
ment with what was estimated in the Aegean Sea in summer
2006, namely Fp dep = −0.010 to −0.015 µg m−2day−1for
the same PAHs assuming vdep ≈0.2 cm s−1(Castro-Jiménez
et al., 2012).
A similar calculation for the BDEs for one daytime sam-
pling interval (cp=0.16 and 0.20 ng m−3for BDE47 and
BDE99 respectively; 6 July for which Fcwas determined,
Table S5b) suggests that the contribution of Fp dep to dry de-
position of BDE47 was also negligible (Fc≈100 ×Fp dep;
Fc= +3.0 µg m−2day−1), while no direct comparison can
be made for BDE99 (|Fc|3.8 µg m−2day−1; Table S5b).
Hereby, vdep =0.05–0.3 cm s−1was adopted to account for
mass median diameters in the range 0.5–1.5 and mass trans-
fer kinetic limitations for redistribution during long-range
transport (Cetin and Odabasi, 2008; Luo et al., 2014; and in
agreement with own, unpublished data measured simultane-
ously in a short distance).
Significant vertical concentration differences in the partic-
ulate phase, 1cp/1z > 0 and 1cp/1z < 0, were found. No-
tably during one daytime and sequential night-time sampling
(6–7 July) and during daytime of 9 July, all significant gradi-
ents determined for particulate phase contaminants were neg-
ative, i.e. higher concentrations at the lower level, z1(PHE
and FLT each 1, PYR 2 cases; PBDEs each 1 case; Table S3),
while the opposite gradient was found for other nights and
days. Marine aerosol contains organic matter (OM), mostly
in the accumulation mode, in particular over biologically pro-
ductive surface waters and under low wind speeds (Gantt et
al., 2011; Albert et al., 2012). A considerable part of OM is
water insoluble (O’Dowd et al., 2004; Facchini et al., 2008).
Hence, the marine aerosol contains traces of POPs, which
previously were either in dissolved form or associated with
suspended OM. Vertical particle gradients may be sustained
by turbulent diffusion (Pryor et al., 2008). While average
wind speed was highest during daytime of 9 July, it was av-
erage during 6–7 July. No fluxes can be derived from the gra-
dients determined in this study, downwards or upwards.
3 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 volatilisa-
tion from the sea surface. Volatilisation from the sea surface
Atmos. Chem. Phys., 16, 6381–6393, 2016 www.atmos-chem-phys.net/16/6381/2016/

G. Lammel et al.: Bidirectional air–sea exchange and accumulation of POPs 6391
has significantly contributed to the night-time maxima of (at
least) HCB, HCH, PCB28, PCB52 and PYR. Apart from
long-range transport across the Aegean Sea, local sources
were indicated for PBDEs: PBDE cycling was characterised
by volatilisation and transport from the island during the day
and deposition to the sea surface.
We successfully quantified the diffusive air–sea exchange
flux of four 3–4-ring PAHs (in the upper pg m−3concentra-
tion range), three OCPs, three PCBs and two PBDEs (in the
lower pg m−3concentration range) at a remote coastal site
using a gradient in combination with the eddy 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 1 order
of magnitude.
Both flux directions were observed (fluctuation) for the
OCPs studied, as well as for three PAHs (ACE, FLT, PYR)
and one PCB (PCB52), not determined by the day–night cy-
cle. Fluctuation of more substances addressed might have
been hidden by gaps in the time series of fluxes (limited by
the uncertainties of sampling and analysis) or the time res-
olution (limited by the sensitivity of the analytical method).
Hence, the mean flux direction on one hand and observations
during part of the time for the trace substances may differ.
For example, volatilisation of BDE47 (observed in one night
only) may have been the exception. In general, longer ob-
servations and observations across seasons of the flux are
needed to assess dry deposition fluxes and the state of air–
sea exchange of those anthropogenic trace substances which
have been approaching phase equilibrium historically (Jan-
tunen and Bidleman, 1995; Stemmler and Lammel, 2009;
Berrojalbiz et al., 2014) or seasonally (Mulder et al., 2014).
Information about the Supplement
Detailed methodological information (substance properties,
analytical quality assurance parameters, micrometeorologi-
cal technique, two-box model) and results (meteorological
situation, transfer velocity, atmospheric concentration and
flux data).
The Supplement related to this article is available online
at doi:10.5194/acp-16-6381-2016-supplement.
Acknowledgements. We thank Giorgos Kouvarakis and Niko-
las Mihalopoulos, University of Crete, Iraklion; Günther
Schebeske, MPIC, for on-site support and Dušan Lago, MU,
for air mass back-trajectory modelling. This research was sup-
ported by the Granting Agency of the Czech Republic (project
No. 312334), the Czech Ministry of Education, Youth and
Sports (LO1214 and LM2015051), and the European Union FP7
(No. 262254, ACTRIS).
Edited by: L. Zhang
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Supplement of Atmos. Chem. Phys., 16, 6381–6393, 2016
http://www.atmos-chem-phys.net/16/6381/2016/
doi:10.5194/acp-16-6381-2016-supplement
© Author(s) 2016. CC Attribution 3.0 License.
Supplement of
Bidirectional air–sea exchange and accumulation of POPs (PAHs, PCBs,
OCPs and PBDEs) in the nocturnal marine boundary layer
Gerhard Lammel et al.
Correspondence to: Gerhard Lammel (lammel@recetox.muni.cz)
The copyright of individual parts of the supplement might differ from the CC-BY 3.0 licence.

Contents
S1 Methodology
S1.1 Substance properties
S1.2 Analytical quality assurance parameters
S1.3 Vertical flux calculations by micrometeorological techniques
S1.4 Non-steady state two-box model
S2 Results
S2.1 Meteorological situation
S2.2 Transfer velocity
S2.3 Atmospheric concentration and flux data
S2.4 Seawater concentration data
S2.5 Model predicted concentrations and air-sea exchange flux
References
S1 Methodology
S1.1 Substance properties
For the fugacity ratio calculation based on the Whitman two-film model (Bidleman and
McConnell, 1995), the Henry’s law constant was corrected for the sea water temperature and
the salt water (by the Setschenow constant, e.g., Zhong et al., 2012).
Table S1. References of physico-chemical properties used. H = Henry's law constant, dUaw =
enthalpy of water-air phase change, KS = salting-out (Setschenow) constant. Substances
addressed: see main text.
PAHs
PCBs
OCPs
PBDEs
H
Bamford et al.,
1999
Li et al., 2003
Cetin et al., 2006
Cetin and Odabasi,
2005; Tittlemier et al.,
2002
dUaw
Bamford et al.,
1999
Li et al., 2003
Cetin et al., 2006
Cetin and Odabasi,
2005
KS
Jonker and Muijs,
2010 (a)
Rowe et al., 2007
Lohmann et al.,
2012; Cetin et al.,
2006
Jonker and Muijs, 2010 (b)
(a) assumed to be given by value for isomer in case of lack of data
(b) adopted estimate

S1.2 Analytical quality assurance parameters
Table S2: Instrument limits of quantification (ILOQ) for various
types of sample, given as masses and concentrations
Analyte
Mass
concentration
(pg)
air
(pg m-3)
water
(pg L-1)
PAHs
160-840
6-34
0.5-4.2
PCBs and OCPs
50-510
7-23
0.05 - 0.5
PBDEs
0.083-0.953
0.003-0.304
0.0003 - 0.037
S1.3 Vertical flux calculations by micrometeorological techniques
Two micrometeorological methods, the aerodynamic and the eddy covariance technique, have
been applied to derive turbulent vertical gaseous organics fluxes (Fc, in ng m-2 s-1). According
to the aerodynamic method (Hicks et al., 1987; Kuhn et al., 2007), Fc is the product of the
vertical difference of gaseous organics concentration,

cz (ng m-3), and the turbulent transfer
velocity, vtr (m s-1):
Fc =  vtr

cz =  vtr [ c(z2)  c(z1) ] (S1)
where z2 and z1 are the heights of inlets of gaseous organics' sampling (1.05 m and 2.80 m, see
section 2.1). The transfer velocity, a measure of the vertical turbulent (eddy) diffusivity
between z2 and z1, is simply the inverse of the aerodynamic resistance, Ra (s m-1), against the
vertical transport:
Ra = vtr1 = (

u)-1 [ ln(zr,2 / zr,1) 

H(zr,2 / L) 

H(zr,1 / L)] (S2)
where

is the von Karman constant (= 0.4), u is the friction velocity (in m s−1), zr,i = zi−d is
the relative height (in m), and d = 0.34 m is the zero-plane displacement height for the Selles
Beach site, derived by the eddy covariance technique. Applying the Monin-Obukhov similari-
ty theory (Monin and Obukhov, 1954) for the mathematical description of turbulent transport
in the atmospheric surface layer, a characteristic length scale, the Obukhov length L (in m;
Obukhov, 1948) is the quantitative measure of the relation between dynamic (friction) and
thermal (buoyancy) forces which drive the turbulent transport,

L =  u3 [

g Tair1 H (cp

air)1 ] 1 (S3)
where Tair is the air temperature (K), cp is the specific heat of air at constant pressure (1004.7
m2 s-2 K-1), g the acceleration of gravity (9.807 m s−2), and H the turbulent sensitive heat flux
(W m−2). When applying the eddy covariance technique, the key micrometeorological
quantities u and H were derived from fast response (20 Hz) measurements of the three spatial
components of the 3D wind vector and the air temperature. For control of the atmospheric sur-
face layer's thermodynamic stratification vertical gradients of wind speed (u) and air
temperature (Tair) have been used which in turn have been derived from continuous mea-
surements of wind speed and air temperature at four levels (0.34, 0.70, 1.45, and 3.00 m above
ground) at Selles Beach. The dimensionless integrated similarity functions (or integrated
stability correction functions)

H(zr,2/L) and

H(zr,1/L) for heat were calculated after Paulson
(1970).
S1.4 Non-steady state two-box model
A non-steady state 2-box model was applied to test the hypothesis that the diurnal variation of
POP concentrations in air during 6-10 July is explained by local processes, namely the
combination of volatilisation from the sea surface and atmospheric mixing depth.
The model simulations for the period 6-10 July 2012 were initialised by observed surface
seawater concentrations, cw (Table 2), and modelled marine boundary layer depths
(Lagrangian dispersion model; see Section 2.2). Temperature and wind speed data were taken
from measurements at the site (Cretan north coast). Winds were on-shore throughout the
simulated period. Input data are listed in Table S3. Gaseous air and seawater concentrations
and the air-sea exchange flux, Faw, are output.
Substances for which input data were incomplete or insufficient observational data were
available (model evaluation) were not simulated. Upon input, simultaneous measurements of
air and seawater concentrations are lacking for HCH and 3-ring PAHs (see Section 2.5) and
some physic-chemical properties are lacking for PeCB. Insufficient air concentration and flux
measurements during 6-10 July are available for DDE. Input parameters for the 2-box model
are listed in Table S3. The biogeochemical parameters had been used earlier to simulate the
air-sea exchange of a PAH in the same region (Mulder et al., 2014).

Table S3. Input parameters for the 2-box model, (a.) environmental, (b.) substance specific.
a.
Parameter
Unit
Value adopted or
mean (min-max)
Reference
OH concentration in air
molec
cm-3
1.5×106 during day-time,
0 during nighttime
climatological data
(Spivakovsky et al., 2000)
Dissolved organic carbon
concentration in seawater
µM
61.5
Pujo-Pay et al., 2011
Atmospheric mixing height
m
522 (131-1295)
Modelled (see section 2.2, Fig. S1)
Mixing depth in ocean
m
40
d'Ortenzio et al., 2005
Concentration of particulate
organic carbon in surface seawater
µM
3.08
Pujo-Pay et al., 2011
Air temperature
K
302.1 (299.1 – 305.1)
Measured
Surface seawater temperature
(SST)
K
297.4 (297.0 – 297.9)
satellite-retrieved data a
Export (settling) velocity of
particle-sorbed molecule in
seawater
m s-1
8×10-7
Schwarzenbach et al., 2003, divided
by a factor of 10 (Mulder et al.,
2014, upper estimate parameter set)
Deposition velocity of particle-
sorbed molecule in air
m s-1
6.5×10-5
Franklin et al., 2000
a AVHRR (Advanced Very High Resolution Radiometer), 1.5 × 1.5 km resolution; source: eoweb.dlr.de:8080

b.
Parameter
Unit
Value adopted or mean
(min-max)
Reference
Henry coefficient
Pa m-3
mol-1
FLT/PYR: 1.96/1.71
PCB28/PCB52: 18.1/14.8
PBDE47/PBDE99:
0.85/0.60
Bamford et al., 1999;
Li et al., 2003;
Cetin and Odabasi, 2005
1st order degradation rate
coefficient in seawater
10-9 s-1
FLT/PYR: 4.2/2.8
PCB28/PCB52: 2.2/1.2
PBDE47/PBDE99: 0/0
Value for freshwater (USEPA, 2009)
divided by 10; Value for water (Beyer
et al., 2000; Wania and Daly, 2002)
divided by 10; T dependence: EU,
1996; assumed to be 0 in lack of data
Gas-phase reaction rate
coefficient with OH
10-12
cm³
molec-1
s-1
FLT/PYR: 11/50
PCB28/PCB52:
0.934/0.0162
PBDE47/PBDE99: 1.5/0.80
Keyte et al., 2013;
Anderson and Hites, 1996; USEPA,
2009;
Raff and Hites, 2007
Octanol/water partitioning
coefficient Kow
log
FLT/PYR: 5.16/4.88
PCB28/PCB52: 5.66/5.91
PBDE47/PBDE99:
6.11/6.61
Calvert et al., 2002;
Li et al., 2003;
Hayward et al., 2006
Dissolved organic
carbon/water partition
coefficient
L g-1
0.411× Kow
Karickhoff, 1981
Particulate organic
carbon/water partitioning
coefficient
L g-1
Kow
assumed to be given by Kow
(Rowe et al., 2009)
Setschenow constant
L mol-1
FLT/PYR: 0.364/0.354
PCB28/PCB52: 0.3/0.3
PBDE47/PBDE99:
0.35/0.35
Jonker and Muijs, 2010;
Cetin et al., 2006; Rowe et al., 2007;
Jonker and Muijs, 2010
Particulate mass fraction in
air
0.05 (0.02 - 0.14)
Measured
Table S4. Measured sea surface temperature 6-10 July 2012 (°C), AVHRR, 12 h means,
resolution 1.5 × 1.5 km (Valavanis et al., 2004), input for simulation of ca
Date
Day
Night
20120706
24.749
23.837
20120707
24.202
23.972
20120708
24.626
24.536
20120709
24.115
24.297
20120710
23.91
23.914

Fig. S.1. Modelled atmospheric mixing depth (m) 6-10 July 2012 (UTC), input for simulation
of ca
0
200
400
600
800
1000
1200
1400
00:00:00 00:00:00 00:00:00 00:00:00 00:00:00 00:00:00
S2 Results
S2.1 Meteorological situation
During 2-11 July 2012 the Aegean was mostly influenced by northerly, and in its northern part
easterly advection over the Marmara Sea as part of a cyclonic system which resided over
Romania during 1-3 July and over western Russia during 4-10 July. The sky was cloud-free all
the time. No frontal passage occurred, such that for all samples taken in the study region the
hypothesis of horizontal homogeneity of air mass collected can be applied. Under the
influence of a strong westerly flow towards Europe the flow in the northern part of the Aegean
switched to westerly during the night 11-12 July, such that air which was residing over the
SW Balkans was advected as well as air from beyond, i.e. central Italy and the NW
Mediterranean Sea and the Iberian Peninsula.
The local meteorological situation for the entire sampling campaign (213 July 2012) is given
in Fig. S2, showing the temporal course of wind speed and direction measured at the position
of one of the automatic weather stations at Selles Beach. Surprisingly, >80% of the campaign
(and >95% of the gradient measurements i.e., horizontal bars in Fig. S2) experienced wind
speeds > 4 m s-1. Under such conditions a local wind system (land-sea breeze) is most
unlikely. Consequently, as soon as the wind speed breaked down to < 3 m s-1, land-sea breeze
occurred (nights 5-6 July, 13-14 July, Fig. S2) Under these conditions the atmospheric

surface layer is usually very well mixed due to the dynamic forces (friction), such that thermal
stability corrections in eq. (S2) can be neglected (

H(zr,2/L) =

H(zr,1/L)  0).
The local wind direction, however, was almost thoroughly from the west (270°), within a
quite narrow range (259°; averages during individual sampling periods given in
Fig. S2). Only winds between 270° and 40° (condition for onshore winds) could be considered
for the evaluation of turbulent vertical fluxes, Fc, from and to seawater. All individual
sampling periods with more than 10% of the time outside this wind sector were rejected
(nights 3-4 July, 4-5 July, 6-7 July, 7-8 July, 8-9 July, 9-10 July, 10-11 July).
Fig. S2: Wind speed (magenta) and wind direction (grey) observed on Selles Beach during 2-
14 July 2012. White circles represent central values of sampling intervals. Horizontal bars
indicate the length of intervals. Green shaded area = on-shore winds. For flux calculations
only intervals with on-shore wind were used.
S2.2 Transfer velocity
The time series of turbulent transfer velocity, vtr, at Selles Beach, derived from micrometeoro-
logical measurements by eddy covariance technique (sections 2.4, S1.3), is shown in Fig. S3.
Data represent temporal averages of 30 min vtr values over each individual sampling period
(mostly 11 h), horizontal bars indicate the length of the sampling period, and vertical error
bars correspond to ± 1 of the respective vtr mean. Since only those data from periods with
0
2
4
6
8
10
12
14
16
02-Jul
12:00 03-Jul
12:00 04-Jul
12:00 05-Jul
12:00 06-Jul
12:00 07-Jul
12:00 08-Jul
12:00
wind speed [m s-1]
0
45
90
135
180
225
270
315
360
wind direction [deg]
0
2
4
6
8
10
12
14
16
08-Jul
12:00 09-Jul
12:00 10-Jul
12:00 11-Jul
12:00 12-Jul
12:00 13-Jul
12:00 14-Jul
12:00
wind speed [m s-1]
0
45
90
135
180
225
270
315
360
wind direction [deg]

winds from the sea (onshore winds between 270° and 0°) could be considered for suitable
fetch conditions, data of the nights 3-4 July and in the period 6-11 July have not been used for
calculations of vertical fluxes. Values of transfer velocity range between approx. 2.5 and 7.5
cm s-1, and – since turbulent transport at Selles Beach was dominated by dynamic forces, vtr
data mirror more or less those of horizontal wind speed (see Fig. S2).
Fig. S3: Turbulent transfer velocities (cm s-1) during the field campaign at Selles Beach, Crete
(35.2°N, 24.4°E) during 2-13 July 2012. Eddy covariance technique and calculations have
been applied to derive these data from measurements of fast response (20 Hz) measurements
of key micrometeorological parameters (details, see S1.3).
2
3
4
5
6
7
8
9
10
02-Jul
00:00 04-Jul
00:00 06-Jul
00:00 08-Jul
00:00 10-Jul
00:00 12-Jul
00:00 14-Jul
00:00
transfer velocity, vtr [cm s-1]
daytime
nighttime

S2.3 Atmospheric concentration and flux data
Table S3. Concentrations at ground level, z1 = 1.05 m, of (a.) gaseous and particulate PAHs
(ng m-3), and (b.) gaseous OCPs (ng m-3), PCBs (ng m-3) and gaseous and particulate PBDEs
(pg m-3) from 3 July 2012 day-time (D) until 13 July 2012 day-time. N = night-time. Upper
limits: Insignificant data (<3 standard deviations of field blank concentrations). Data from 6
July D until 10 July D based on 2 replica measurements (mean).
a.
ACE
PHE
FLT
PYR
g
p
g
p
g
p
g
p
3 July D
<0.058
<0.004
2.933
0.159
0.893
0.117
0.367
<0.003
3-4 July N
<0.096
<0.004
1.058
0.294
0.759
0.198
0.344
<0.003
4 July D
<0.058
<0.004
3.543
0.158
0.782
0.162
0.858
<0.003
4-5 July N
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
5 July D
<0.072
<0.004
2.366
0.126
0.634
<0.019
0.237
<0.003
5-6 July N
0.274
n.d.
4.507
n.d.
0.828
n.d.
0.930
n.d.
6 July D
0.068
<0.004
1.363
0.672
0.549
0.354
0.276
0.111
6-7 July N
0.119
<0.004
1.375
0.374
0.301
0.208
0.152
0.064
7 July D
0.081
<0.004
1.047
0.290
0.248
0.152
0.140
0.056
7-8 July N
0.116
<0.004
1.339
0.309
0.265
0.159
0.149
0.065
8 July D
0.091
<0.004
1.693
0.260
0.261
0.152
0.145
0.056
8-9 July N
0.107
<0.004
2.339
0.234
0.271
0.134
0.132
<0.003
9 July D
0.081
n.d.
0.591
n.d.
0.239
n.d.
0.090
n.d.
9-10 July N
0.110
<0.004
1.864
0.266
0.289
0.144
0.136
0.060
10 July D
0.059
<0.004
1.256
0.200
0.263
0.124
0.130
<0.003
10-11 July N
0.083
n.d.
0.756
n.d.
0.264
n.d.
0.133
n.d.
11 July D
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
11-12 July N
<0.072
<0.004
0.308
<0.065
0.046
<0.019
<0.10
<0.003
12 July D
<0.072
<0.004
0.246
<0.065
0.044
<0.019
<0.10
<0.003
12-13 July N
<0.072
<0.004
0.215
<0.065
0.059
<0.019
<0.10
<0.003
13 July D
<0.072
<0.004
0.567
<0.065
0.092
<0.019
<0.10
<0.003

b.

-HCH

-HCH
PCB28
PCB52
PCB101
p,p‘-
DDE
PBDE47
PBDE99
g
g
g
g
g
g
g
p
g
p
3 July D
0.078
0.242
0.037
0.023
0.008
0.015
<0.38
n.d.
<0.30
n.d.
3-4 July N
0.055
0.194
0.024
0.016
0.009
0.012
<0.38
n.d.
<0.30
n.d.
4 July D
0.008
0.007
0.006
0.003
0.005
0.003
<0.38
n.d.
<0.30
n.d.
4-5 July N
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
5 July D
0.028
0.105
0.015
0.009
0.006
0.006
<0.38
n.d.
<0.30
n.d.
5-6 July N
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
6 July D
0.017
0.042
0.021
0.010
<0.012
<0.008
0.500
0.161
0.313
0.203
6-7 July N
0.053
0.123
0.021
0.012
0.003
<0.008
<0.38
0.110
0.214
0.166
7 July D
0.029
0.065
0.022
0.010
<0.012
<0.008
0.509
0.104
0.299
0.145
7-8 July N
0.039
0.115
0.023
0.014
0.006
0.006
0.317
0-108
0.161
0.140
8 July D
0.024
0.074
0.020
0.010
<0.012
<0.008
0.345
0.132
0.264
0.192
8-9 July N
0.055
0.189
0.032
0.024
0.011
0.007
0.251
0.113
0.209
0.174
9 July D
0.019
0.046
0.024
0.010
<0.012
0.005
0.203
0.114
0.137
0.147
9-10 July N
0.076
0.245
0.033
0.024
0.011
0.007
0.228
0.135
0.197
0.152
10 July D
0.042
0.154
0.022
0.014
0.005
0.004
<0.38
0.134
<0.30
0.159
10-11 July N
0.014
0.066
<0.015
0.005
<0.012
<0.008
<0.38
0.110
0.141
0.137
11 July D
0.027
0.066
0.030
0.013
<0.008
0.010
0.262
0.105
<0.30
0.134
11-12 July N
<0.015
0.057
0.008
0.005
<0.012
<0.008
<0.38
n.d.
0.249
n.d.

12 July D
<0.015
0.038
0.008
0.003
<0.012
<0.008
<0.38
n.d.
<0.30
n.d.
12-13 July N
<0.015
0.030
0.008
0.003
<0.012
<0.008
<0.38
n.d.
<0.30
n.d.
13 July D
0.042
0.145
0.016
0.013
<0.012
0.004
<0.38
n.d.
<0.30
n.d.

Table S4. Vertical concentration differences, cz = cz2-cz1, of gaseous (a.) gaseous and particulate PAHs (ng m-3), and (b.) gaseous OCPs (ng m-3),
PCBs (ng m-3) and gaseous and particulate PBDEs (pg m-3). z = 1.75 m. Upper limits: Insignificant values of concentration differences (<6
standard deviations of field blank concentrations). cz1 data from 6 July D until 10 July D based on 2 replica measurements (mean).
a.
ACE
PHE
FLT
PYR
g
p
g
p
g
p
g
P
3 July D
<0.058
<0.007
7.35
<±0.129
-0.46
<±0.037
-0.15
n.d.
3-4 July N
<0.096
<0.007
5.55
<±0.129
-0.25
0.047
0.11
n.d.
4 July D
<0.058
<0.007
0.65
<±0.129
-0.24
<±0.037
-0.42
n.d.
4-5 July N
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
5 July D
<0.072
<0.007
0.53
<±0.129
-0.39
0.048
-0.13
n.d.
5-6 July N
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
6 July D
0.10
<±0.007
1.48
-0.248
<±0.076
-0.111
<±0.076
-0.041
6-7 July N
-0.024
<±0.007
<±0.37
<±0.129
0.18
<±0.037
<±0.076
-0.006
7 July D
-0.026
<±0.007
<±0.37
<±0.129
<±0.076
<±0.037
<±0.076
<±0.005
7-8 July N
<±0.013
<±0.007
0.62
<±0.129
0.37
<±0.037
0.14
<±0.005
8 July D
-0.019
<±0.007
0.76
<±0.129
0.41
<±0.037
0.16
<±0.005
8-9 July N
<±0.013
<±0.007
0.97
<±0.129
0.89
<±0.037
0.40
0.056
9 July D
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
9-10 July N
0.037
<±0.007
0.80
<±0.129
0.95
<±0.037
0.45
<±0.005
10 July D
n.d.
<±0.007
0.51
<±0.129
0.73
<±0.037
0.32
0.059
10-11 July N
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
11 July D
n.d.
n.d.
<±0.37
n.d.
0.15
n.d.
<±0.075
n.d.
11-12 July N
n.d.
n.d.
<±0.37
n.d.
0.12
n.d.
<±0.075
n.d.
12 July D
n.d.
n.d.
<±0.37
n.d.
<±0.076
n.d.
<±0.075
n.d.
12-13 July N
n.d.
n.d.
<±0.37
n.d.
0.44
n.d.
0.18
n.d.
13 July D
n.d.
n.d.
7.35
n.d.
-0.46
n.d.
-0.15
n.d.

b.

-HCH

-HCH
PCB28
PCB52
PCB101
p,p‘-
DDE
PBDE47
PBDE99
g
g
g
g
g
g
g
p
g
p
3 July D
0.675
0.890
0.155
0.120
0.044
<±0.008
n.d.
n.d.
n.d.
n.d.
3-4 July N
0.258
0.510
0.103
0.089
0.035
<±0.008
n.d.
n.d.
n.d.
n.d.
4 July D
0.187
0.483
0.080
0.052
0.016
<±0.008
n.d.
n.d.
n.d.
n.d.
4-5 July N
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
5 July D
0.147
0.313
0.060
0.035
0.011
<±0.008
n.d.
n.d.
n.d.
n.d.
5-6 July N
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
6 July D
0.078
0.189
0.029
0.012
n.d.
n.d.
-0.28
<±0.076
<±0.33
<±0.099
6-7 July N
-0.036
-0.057
<±0.016
<±0.007
n.d.
n.d.
n.d.
<±0.076
n.d.
<±0.099
7 July D
<±0.020
-0.041
<±0.016
<±0.007
n.d.
n.d.
n.d.
<±0.076
n.d.
<±0.099
7-8 July N
-0.025
-0.048
<±0.016
<±0.007
<±0.008
<±0.008
n.d.
<±0.076
n.d.
<±0.099
8 July D
<±0.020
-0.018
<±0.016
<±0.007
n.d.
n.d.
<±0.24
<±0.076
<±0.33
<±0.099
8-9 July N
-0.025
-0.082
<±0.016
-0.0089
<±0.008
<±0.008
n.d.
<±0.076
<±0.33
<±0.099
9 July D
<±0.020
0.037
<±0.016
<±0.007
n.d.
n.d.
n.d.
-0.114
n.d.
-0.147
9-10 July N
-0.048
-0.159
<±0.016
-0.011
<±0.008
<±0.008
n.d.
<±0.076
n.d.
<±0.099
10 July D
<±0.020
-0.052
<±0.016
<±0.007
<±0.008
0.0091
n.d.
<±0.076
n.d.
<±0.099
10-11 July N
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
11 July D
<±0.020
0.088
<±0.016
<±0.007
n.d.
<±0.008
n.d.
-0.105
n.d.
-0.134
11-12 July N
n.d.
-0.030
<±0.016
<±0.007
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
12 July D
n.d.
<±0.019
<±0.016
<±0.007
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
12-13 July N
n.d.
<±0.019
<±0.016
<±0.007
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
13 July D
<±0.020
-0.073
0.017
<±0.007
n.d.
<±0.008
n.d.
n.d.
n.d.
n.d.

Table S5. Vertical fluxes Fc = -vtr cz of gaseous (a.) PAHs (µg m-2 d-1), and (b.) OCPs (µg m-2 d-1), PCBs (µg m-2 d-1) and PBDEs (ng m-2 d-1).
Positive = upward, negative = downward. Empty fields = no data. Insignificant data (<6 standard deviations of field blank concentrations) given
as upper limits.
a.
ACE
PHE
FLT
PYR
3 July D
-28.46
1.80
0.59
3-4 July N
4 July D
-2.41
0.94
1.56
4-5 July N
5 July D
-1.33
0.97
0.33
5-6 July N
6 July D
-0.34
-4.84
<±0.25
<±0.24
6-7 July N
7 July D
0.16
<±2.26
<±0.47
<±0.46
7-8 July N
8 July D
0.12
-4.72
-2.57
-1.02
8-9 July N
9 July D
9-10 July N
10 July D
-2.06
-2.97
-1.32
10-11 July N
11 July D
11-12 July N
<±1.44
-0.59
<±0.29
12 July D
<±1.57
-0.51
<±0.32
12-13 July N
<±1.03
<±0.21
<±0.21
13 July D
<±0.89
-1.06
-0.44

b.

-HCH

-HCH
PCB28
PCB52
PCB101
p,p‘-DDE
PBDE47
PBDE99
3 July D
-2.61
-3.45
-0.60
-0.47
-0.17
<±0.03
3-4 July N
4 July D
-0.69
-1.79
-0.30
-0.19
-0.06
<±0.03
4-5 July N
5 July D
-0.37
-0.78
-0.15
-0.09
-0.03
<±0.02
5-6 July N
6 July D
-0.26
-0.62
-0.09
-0.04
0.91
<±1.09
6-7 July N
7 July D
<±0.12
0.25
<±0.10
<±0.04
7-8 July N
8 July D
<±0.12
0.11
<±0.10
<±0.04
<±1.47
<±2.08
8-9 July N
9 July D
<±0.13
0.23
<±0.10
<±0.04
9-10 July N
10 July D
<±0.08
-0.21
<±0.06
<±0.03
<±0.03
0.04
10-11 July N
11 July D
<±0.06
0.26
<±0.05
<±0.02
<±0.02
11-12 July N
-0.12
<±0.06
<±0.03
12 July D
<±0.08
<±0.07
<±0.03
12-13 July N
<±0.05
<±0.04
<±0.02
13 July D
<±0.05
0.18
-0.04
<±0.02
<±0.02

S2.4 Seawater concentration data
Table S6. Concentrations in surface seawater at two localities west of Selles Beach and
arithmetic mean (pg L-1).
cw at locality 1
cw at locality 2
mean cw
FLT
21.2
15.3
18.2
PYR
6.3
2.6
4.4
PCB 28
4.73
4.58
4.65
PCB 52
0.53
0.54
0.54
PCB 101
0.84
0.85
0.84
p,p‘-DDE
0.75
0.93
0.84
BDE 47
0.12
0.18
0.15
BDE 99
0.039
0.037
0.038
S2.5 Model predicted concentrations and air-sea exchange flux
During the period 6 ̶ 10 July, Crete was under influence of a constant northerly flow during
day and night without change of air mass (see above, S2.1).
Many pollutants showed pronounced night-time maxima: cday/cnight = 0.3 ̶ 0.5 for HCH
isomers, 0.6 ̶ 0.7 for ACE, PHE, DDE and PCB52. For other species this was less pronounced
or no day/night trend was observed (cday/cnight = 0.8–0.9 for PCB28, FLT, PYR, 1.1 ̶ 1.3 for
PBDEs) (Fig. 1).
During on-shore advection many contaminants’ concentrations were influenced by BL depth,
as indicated by anti-correlation with PAHs and OCPs (except DDE; r = -0.76 ̶ -0.37 i.e.,
significant for α-HCH on the p < 0.05 confidence level, t-test). BL depth was not correlated
with PCBs’ and DDE concentrations (│r│ < 0.45). These findings indicate sea surface
sources.
Using the 2-box fugacity model (above, S1.4), air–sea mass exchange fluxes, Fem, in the range
-1000 ̶ +10 ng m-2 h-1 (positive defined upward) and atmospheric concentrations fed by Fem
only in the range 0.01 ̶ 2.2 ng m-3 are simulated (Table S7).

Table S7. Predicted concentrations in the marine atmospheric BL and surface seawater (mean
(min-max), ng m-3) and air–sea mass exchange fluxes, Fem (mean (min-max), ng m-2 h-1)
ca (ng m-3)
cw (ng m-3)
Fem (ng m-2 d-1)
FLT
0.51 (0.13–2.27)
20.6 (18.2–22.0)
-3.20 (-13.6–0.00)
PYR
0.13 (0.01–1.13)
5.41 (4.44–5.60)
-0.96 (-9.32–0.08)
PCB28
0.037 (0.011–0.11)
4.60 (4.55–4.65)
0.061 (-0.009–0.14)
PCB52
0.016 (0.005–0.049)
0.54 (0.54–0.54)
-0.007 (-0.041–0.007)
BDE47
0.45 (0.12–2.04)
4.00 (0.15–6.41)
-5.22 (-20.0–0.00)
BDE99
0.25 (0.051–1.27)
3.34 (0.49–4.89)
-3.69 (-15.3–0.00)
Fig. S4. Predicted vertical flux, Fc (red upward and blue downward, ng m-2 d-1), of selected
pollutants i.e. (a) PYR, (b) PCB52, (c) BDE99 during 6-10 July 2012 (see also Fig. 2).
a. b.
c.

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