Adrien Donnard

Sandra Guérard

Laurent Maheo
email: laurent.maheo@univ-ubs.fr

Philippe Viot

Gérard Rio

Multiaxial experiments with radial loading paths on a polymeric foam

Keywords:

Cellular materials such as polymeric foams in particular have been widely studied under uniaxial loading conditions. Many experimental studies have been focusing recently, however, on the responses of these foams to multiaxial loads. In the present study, a novel experimental hexapod device was used to perform combined uniaxial compression and simple shear tests. Using a post-processing method of analysis which can be used to study elementary mechanical behavior, the authors show the occurrence of non-proportional stress paths in the material under investigation although proportional kinematic paths were imposed. A failure limit criterion is presented for use with the foam of interest. The results of the present analysis yield useful information for meeting our future objective, namely to develop a numerical model for simulating multiaxial loading conditions.

Introduction

Cellular materials have been widely used for many years for manufacturing safety equipment and protective applications. Because of their considerable dissipative capacity and their weak force transmission properties, they are good candidates for protecting both goods and persons. The choice of cellular materials, which range from soft polymer foam to rigid aluminum foam, depends on the damping power required. Mills et al. [START_REF] Mills | Polymer foams for personal protection: cushions, shoes and helmets[END_REF], for instance, have studied the use of polymer foams for producing several types of personal protection, such as cushions, shoes and helmets. Fernandes and Alves de Sousa [START_REF] Fernandes | Motorcycle helmets-a state of the art review[END_REF] recently published a review focusing on motorcycle helmets and showed the importance of including polymeric foam liners in the helmet design. The authors of the previous study [START_REF] Fernandes | Motorcycle helmets-a state of the art review[END_REF] have also briefly addressed the topic of Finite Element models, citing several studies in which impact tests were simulated with a view to optimizing helmet design. In the present context of engineering applications, Yang and Shim [START_REF] Yang | A visco-hyperelastic constitutive description of elastomeric foam[END_REF] have recommended using a macroscopic description of the responses observed, taking elastomer foams to constitute a homogeneous continuum. Foam behavior can therefore be decomposed into elementary behavior such as hyperelasticity, viscosity and irreversible transformations.

When drawing up numerical models simulating foams subjected to mechanical loads, it is necessary to characterize the material. In most previous studies, foams have been characterized experimentally by performing uniaxial compression tests, which are easy to apply. The influence of the strain rate or the impact speed has often been studied to account for the normal conditions of use, as in Refs. [START_REF] Zhang | Constitutive modeling of polymeric foam material subjected to dynamic crash loading[END_REF][START_REF] Viot | Polymeric foam behavior under dynamic compressive loading[END_REF][START_REF] Ouellet | Compressive response of polymeric foams under quasi-static, medium and high strain rate conditions[END_REF][START_REF] Bouix | Polypropylene foam under dynamic loadings : strain rate, density and microstructure effects[END_REF]. However, the validity of these characterizations and models is questionable when the mechanical loads are no longer uniaxial loads. Some models such as Ogden's model [START_REF] Ogden | Fitting hyperelastic models to experimental data[END_REF] have taken several loading conditions into account. This model, which was developed for rubber-like materials, has been extended to compressible materials by including the changes of volume in addition to the shape change behavior. This distinction is a natural way of describing the hyperelastic behavior of rubber-like and cellular materials, and has been adopted in several models such as the Mooney-Rivlin [START_REF] Mooney | A theory of large elastic deformation[END_REF][START_REF] Rivlin | Large elastic deformations of isotropic materials iv. further developments of thegeneral theory[END_REF] model, which is based on tensor invariants, and the Ogden model, which is based on principal stretches.

Several authors have naturally performed multiaxial experiments to provide numerical models with data. Volume change behavior has been studied by performing hydrostatic tests [START_REF] Zhang | Constitutive modeling of polymeric foam material subjected to dynamic crash loading[END_REF][START_REF] Moreu | Rapid hydrostatic compression of low-density polymeric foams[END_REF][START_REF] Viot | Hydrostatic compression on polypropylene foam[END_REF][START_REF] Ozturk | Hydrostatic compression of anisotropic low density polymeric foamsunder multiple loadings and unloadings[END_REF][START_REF] Krundaeva | Dynamic compressive strength and crushing properties of expanded polystyrene foam for different strain rates and different temperatures[END_REF] and shape change behavior, by performing simple shear tests [START_REF] Zhang | Constitutive modeling of polymeric foam material subjected to dynamic crash loading[END_REF][START_REF] Kintscher | Stiffness and failure behaviour of folded sandwich cores under combined transverse shear and compression[END_REF][START_REF] Mosleh | Novel composite foam concept for head protection in oblique impacts[END_REF]. In both cases, the question of finding suitable methods for the post-processing analysis multiaxial data arises. In the case of large transformations, Criscione et al. [START_REF] Criscione | An invariant basis for natural strain which yields orthogonal stress response terms in isotropic hyperelasticity[END_REF] used an invariant basis for natural strains consisting of the amount-of-dilatation, the magnitude-of-distortion and the mode-of-distortion. These authors recommended using natural or Hencky strain and Cauchy stress tensors to study and compare the results of multiaxial loading tests. Combaz et al. [START_REF] Combaz | Yield surface of polyurethane and aluminium replicated foam[END_REF][START_REF] Combaz | Multiaxial yield behaviour of al replicated foam[END_REF] recently used these post-processing tools to analyze the results of tests on aluminum and polymeric foams subjected to multiaxial loads.

In the present study, quasi-static multiaxial experiments were performed with radial loading paths on a polymeric foam. This Polypropylene foam has been studied in detail by Viot et al. [START_REF] Viot | Polymeric foam behavior under dynamic compressive loading[END_REF][START_REF] Bouix | Polypropylene foam under dynamic loadings : strain rate, density and microstructure effects[END_REF][START_REF] Viot | Hydrostatic compression on polypropylene foam[END_REF] performing uniaxial and hydrostatic compression tests at several strain rates. In the present study, simple shear tests and tests involving simple shear combined with uniaxial compression were conducted using a novel hexapod device [START_REF] Maheo | Multiaxial behavior of foamsexperiments and modeling, 11th Int DYMAT Conference[END_REF] with which controlled shape changes and volume changes can be imposed. Using the method of analysis presented in Refs. [START_REF] Combaz | Yield surface of polyurethane and aluminium replicated foam[END_REF][START_REF] Combaz | Multiaxial yield behaviour of al replicated foam[END_REF], shape change and volume change processes were analyzed and compared between various imposed radial loading paths.

In the first part of this study (Section 2), the mechanical behavior of the Expanded Polypropylene foam under investigation is briefly described and the size of the specimens used is explained. The hexapod device and the post-processing method of analysis used are also presented. In the second part (Section 3), the results of basic experiments such as uniaxial compression and tension and simple shear tests are presented. The multiaxial loading experiments performed are described and the results are presented and discussed in Section 4. Lastly, the main results obtained are summarized in section 5.

Material and methods

Material

Polypropylene foam

This study was performed on a closed-cell expanded polypropylene (EPP) foam called Arpro supplied by the company JSP. This foam has been widely studied by the present authors, who have established that the strain rate and the density both affect the mechanical behavior of the material under uniaxial compression [START_REF] Viot | Polymeric foam behavior under dynamic compressive loading[END_REF] and hydrostatic compression [START_REF] Viot | Hydrostatic compression on polypropylene foam[END_REF] loading conditions. To extend this database, a new specimen of the same material with a mean density of 85.3 kg m -3 (standard deviation 4.3 kg m -3 ) was prepared.

Description of the samples

Two main characteristics have to be taken into consideration when performing mechanical tests. First, the specimen size has to be suitable for obtaining representative, fairly homogeneous elementary mechanical responses. Secondly, the shape of the specimen must correspond to the types of loading applied in the tests.

In the case of the present uniaxial tension tests, a dogbone specimen designed in line with standard NF EN ISO 1798 [START_REF] Nf | Matériaux polymères alvéolaires souples -Détermination de la résistance à la traction et de l'allongement à la rupture[END_REF] was used in order to concentrate the strain in the thinnest part of the specimen.

In the case of simple shear tests, the shape of the specimen greatly affects the results [START_REF] Wada | A method to measure shearing modulus of the foamed core for sandwich plates[END_REF][START_REF] Andrews | Size effects in ductile cellular solids. Part II : experimental results[END_REF]. Simple shear tests induce a tangential force and a normal force on the loaded faces. These two forces combined induce a compression load and a tensile load near the free edges [START_REF] Sell | Application of the plane simple shear test for determination of the plastic behaviour of solid polymers at large strains[END_REF][START_REF] Bouvier | Simple shear tests : experimental techniques and characterization of the plastic anisotropy of rolled sheets at large strains[END_REF], which could be regarded as a bending load. This process can be reduced by increasing the elongation ratio l h of the specimen, where l and h are the length and the height of the specimen, respectively. Bouvier et al. [START_REF] Bouvier | Simple shear tests : experimental techniques and characterization of the plastic anisotropy of rolled sheets at large strains[END_REF], for instance, used an elongation ratio of 10 on a metal sheet sample, G'Sell et al. [START_REF] Sell | Application of the plane simple shear test for determination of the plastic behaviour of solid polymers at large strains[END_REF] recommended a ratio of more than 15 in the case of polymers, and Mostafa et al. [START_REF] Mostafa | Effect of shear keys diameter on the shear perfor-mance of composite sandwich panel with PVC and PU foam core : FE study[END_REF] used a ratio of 13 in that of a foam sample. Specimens measuring 300 mm long, 20 mm high and 20 mm wide, having an aspect ratio l h of 15 were chosen here for performing both simple shear and uniaxial compression tests.

Methods

Experimental devices

A classical electromechanical Zwick Z250 Roell device was used to perform quasi-static uniaxial tensile tests in line with the procedure defined in the French standard NF EN ISO 1798 [START_REF] Nf | Matériaux polymères alvéolaires souples -Détermination de la résistance à la traction et de l'allongement à la rupture[END_REF]. A crosshead speed of 41.25 mm min -1 was imposed in order to obtain a strain rate of 0.0125 s -1 . Uniaxial stress was obtained using a 10 kN sensor and strain field with Digital Image Correlation (D.I.C.) VIC2D software from pictures recorded by a camera (CANON EOS 50D) at a frequency of 1 Hz.

Multiaxial experiments tests were performed using a hexapod facility. Fig. 1 shows the hexapod, which is a modified Gough-Stewart platform, a type of parallel robot constituted of a fixed and a moving platform. Thanks to 6 electromechanical jacks, the top plate can be moved independently in the six degrees of freedom, corresponding to three translation axes and three rotation axes. Its horizontal velocity can reach 1.4 m/s and the maximal vertical velocity is 1 m/s. For the multiaxial tests, a rigid arm is mounted perpendicular to the moving top platform. Specimens were glued between 2 steel plates, one of which was screwed onto a rigid arm and the other, onto a base frame. This arrangement made it possible to apply loads of all kinds to the sample, including even complex loads such as combinations of movements. The hexapod was able to reach speeds of up to 1 m/s in all directions. However, the present study focused simply on quasi-static loading performed at a speed of 0.25 mm/s. Strain rates of ε˙z z = 0.0125 s -1 and γ˙y z = 0.0125 s -1 were imposed at each test so as to be able to compare the results obtained without any involvement of the viscous contribution.

Forces were obtained using a piezoelectric 3D sensor (Kistler 9377C) mounted between the rigid arm and the sample. The local basis (X,Y,Z) of the sensor was defined as shown in Fig. 1b. The sensor was set at 2, 10, and 10 kN in the X, Y and Z directions, respectively. Electrical signals were then amplified and recorded at a frequency of 100 Hz, as can be seen in Fig. 2 in the particular case of a uniaxial compression applied on the Z-axis and a simple shear applied simultaneously on the YZ-plane.

As in the tensile tests, the displacements and strain fields were calculated using the D.I.C. technique. The region of interest in the D.I.C. was chosen so as to rule out the occurrence of boundary effects by excluding the areas on both sides of the specimen and those near the steel plates (see Fig. 3). A second camera was placed perpendicularly to the first one in order to detect any transversal strains occurring during the application of uniaxial compression and simple shear loads.

Post-processing of the multiaxial test data

In the case of a simple shear load or a uniaxial compression load or both loads applied simultaneously, the components of the symmetric Cauchy stress tensor expressed in the hexapod coordinate system can be written as in equation ( 1).

= ⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ == σ σ σσ σ F S σ F S [ ] 00 0 00 0 with and ij yz yz zz zz z yz y ( 1
)
where Fi is the force recorded in the i-direction, and S is the current z- normal section. It is important to note that in both experiments, although the shear components σxy and σxz were not measured, they were assumed to be null because EPP foam is generally reputed to be macroscopically homogeneous. The Fx force component recorded by the sensor was almost equal to 0. Even if it means that the average global value of σ xx is vanishingly small, the local component of σ xx may have non-zero value, because edge effects have not been taken into account. The same explanation can be given for the σ yy stress compo- nent. Even if the average value of σ yy can be considered as negligible, its local value is non-zero as specimen with large transversal dimensions under uniaxial compressive loadings involves local transversal stresses, except for null effective Poisson ratio materials. Because the present experiments do not allow obtaining these local variables, the assumption of zero value for σ xx and σ yy has been made. The logarithmic strain tensor obtained using the D.I.C. technique, which is presented in equation ( 2), involves only two dimensions. Assuming that we were dealing here with macroscopically homogeneous material, εxy and εxz could be taken to be equal to 0 under both loading conditions.

= ⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ ε ε εε εε [ ] 00 0 0 ij xx yy yz yz zz
(2)

A second camera was placed perpendicularly to the first one in order to measure the transversal strain. The transversal strain εxx did not exceed 0.6% in any of the simple shear tests, and was therefore not significant. For the sake of simplicity, the ε xx strain component was therefore set at 0 in the simple shear and uniaxial compression tests. Section 3.1 gives further information about the effective Poisson ratio in the case of tensile tests. Other components of the strain tensor (see equation ( 2)) were calculated from the results obtained using the D.I.C. technique.

Choice of the post-processing framework

Multiaxial experiments are complex tests from which it can be difficult to extract useful data. The classical stress-strain curve in Fig. 4 is not appropriate for comparing various loads or determining the responses of the material to multiaxial loads. One way of simplifying the analysis consists in studying the elementary mechanical behavior by separating the volume change responses from the shape change responses, in line with Combaz et al. [START_REF] Combaz | Yield surface of polyurethane and aluminium replicated foam[END_REF][START_REF] Combaz | Multiaxial yield behaviour of al replicated foam[END_REF]. These two contributions have been classically defined as the spherical and deviatoric parts of the stress and strain tensors, as long as suitable tensors are chosen. Due to the large transformations occurring in EPP foam, Criscione et al. [START_REF] Criscione | An invariant basis for natural strain which yields orthogonal stress response terms in isotropic hyperelasticity[END_REF] have recommended using the Cauchy stress tensor associated with the Hencky logarithmic strain tensor.

The volume and shape changes can be separated by performing a spherical/deviatoric decomposition:

=+ σ σσ ͠ ͠ ͠ sd and =+ εε ε , ͠ ͠ ͠ sd
where the superscripts . s and . d denote the spherical and deviatoric contributions, respectively. The invariants of the tensor a͠ used in the present study were:

The volume change contribution was assessed from the pressure p versus the relative volume V . Relations between these two variables can be expressed as follows, taking the first invariant of the stress tensor, I σ͠ , or the strain tensor, I ε͠ :

=- = = ( ) p I V v v I 3 ;e x p σ ε 0 ͠ ͠ (4)
where v and v0 are the current and initial volume, respectively.

The shape change contribution, which is complementary to the volume change contribution, can be described by:

the intensities of the deviatoric tensors, the deviatoric stress intensity Qσ and the deviatoric strain intensity Qε, based on the second invariant of their deviatoric tensors, II σ͠ d and II ε͠ d, respec- tively.

== - == - Q σ σ II Qε ε ε II :2 ; :2 ͠ ͠ ͠ ͠ σ dd σ dd ε͠ ͠ dd (5)
the directions in the deviatoric planes, which are given by the Lode angles φσ and φε defined in terms of the third and second invariants of the stress and strain deviatoric tensors, respectively.

= ⎛ ⎝ ⎜ - ⎞ ⎠ ⎟ = ⎛ ⎝ ⎜ - ⎞ ⎠ ⎟ φ III II φ III II 1 3 arccos 3 6 ( 2 )
;

1 3 arccos 3 6 ( 2 ) σ σ σ ε ε ε 3 2 3 2 ͠ ͠ ͠ ͠ d d d d (6)
One of the advantages of using mathematical tensor invariants is that this makes it possible to compare mechanically relevant variables which do not depend on the coordinate system used. In the case of the experiments presented in section 2.2.1, it was therefore possible to calculate the volume change and shape change variables from the experimental data as follows:

⎧ ⎨ ⎪ ⎪ ⎪ ⎩ ⎪ ⎪ ⎪ =- =+ = ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ ⎛ ⎝ + ⎞ ⎠ + ( ) p
⎧ ⎨ ⎪ ⎪ ⎪ ⎩ ⎪ ⎪ ⎪ =+ + =- + - + - + = ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ -+ -+ - + - + + -+ -+ - + ( ) Vε ε ε Qε ε ε ε ε εε φ exp( ) [ ( )( )( ) ] (7)
It is also worth defining two other geometric variables to make easier the understanding of strain and stress paths. The kinematic angle θε can be defined as the elevation of the volume related to the devia- toric strain plane. With the same manner, the stress angle θ σ can be defined as the elevation of the pressure related to the deviatoric stress plane, and corresponds to the triaxiality value. These two variables give meaningful information on the volume change compared to the shape change and will be defined in the following sections.

Basic experiments

Effective Poisson ratio

Mechanical characterization of cellular materials is usually conducted by performing uniaxial compression tests. To obtain failure data, uniaxial tensile tests can also be performed. The stress vs. strain graph obtained with the present EPP material is given by the full colored curves in Fig. 4. The compression response of the EPP material shows the presence of a large reversible contribution and a large hysteresis loop when loading -unloading was performed.

Another parameter often used to characterize cellular materials' behavior is the transversal changes occurring during uniaxial loading, which have been assimilated to an effective Poisson ratio. This parameter is known to be non-constant and often amounts to almost zero under uniaxial compression loading conditions [START_REF] Choi | Nonlinear properties of polymer cellular materials with a negative Poisson's ratio[END_REF][START_REF] Widdle | Measurement of the Poisson's ratio of flexible polyurethane foam and its influence on a uniaxial compression model[END_REF], which can be explained by micro-structural features of the material with the works of Gibson and Ashby [START_REF] Gibson | Cellular Solids. Structure and Properties[END_REF] or more recently with the numerical works of Mihai and Goriely [START_REF] Mihai | Finite deformation effects in cellular structures with hyperelastic cell walls[END_REF]. Indeed, under compressive loadings, the foam cells collapse, by a buckling process of the cell walls, in the free space of the cell and this involves a low effective Poisson ratio. Under tensile loading, the process is different. The foam cells are stretched and this involves a contraction of the cell in the transversal direction and a nonnull Poisson ratio. Under simple shear loading, foam cells are subjected to both compression and tension. Equilibrium of the foam under simple shear loading involves a competition between tension and compression processes in terms of transversal strain.

Based on the images recorded during the tests, transversal strains have been measured in order to obtain the effective Poisson ratio. As was to be expected and explained above, in line with findings made by Widdle et al. [START_REF] Widdle | Measurement of the Poisson's ratio of flexible polyurethane foam and its influence on a uniaxial compression model[END_REF], the effective Poisson ratio of the present EPP material varies during uniaxial loading (see the dotted colored curves in Fig. 4). It amounts to approximately 0.27 during uniaxial tension and ranges from 0.27 to 0.03 during uniaxial compression. When a specimen for hexapod device is used, the effective Poisson ratio is very different from a specimen for classical electromechanical device (see dotted green curve in Fig. 4). Indeed, the effective Poisson ratio is almost equal to 0.02 since the beginning of the compressive test. This is due to the size of the specimen that limits transversal displacements and also involves a stress increase (see full green curve in Fig. 4).

As explained in section 2.2.2, the assumption that εxx = 0 in the case of uniaxial compression and simple shear was adopted here for the sake of simplicity. However, this assumption cannot be made in the case of uniaxial tensile tests. The strain components εxx and εyy were therefore calculated using an effective Poisson ratio of 0.27 to be in line with the experimental values. All the strain invariants in equation ( 8) were therefore calculated on the basis of these assumptions.

Comparisons between basic experiments

The results of uniaxial compression, simple shear and uniaxial tension tests were expressed in the post-processing framework presented in section 2.2.3 in terms of the volume change and shape change contributions. Uniaxial compression and simple shear tests were performed using the hexapod device and the specimen described in section 2.1.2. Uniaxial tension tests were performed using a conventional electromechanical device and a specimen corresponding to French standard NF EN ISO 1798 [START_REF] Nf | Matériaux polymères alvéolaires souples -Détermination de la résistance à la traction et de l'allongement à la rupture[END_REF]. All these experiments were performed with a strain rate ε˙= 0.0125 s -1 .

The repeatability of all the tests was checked. Three specimens at least were tested in each case. As shown in Fig. 5, the results obtained in the experiments of each kind showed good repeatability. The slight variations observed were due to the density of the specimens, which affects the mechanical behavior of foam [START_REF] Gibson | Cellular Solids. Structure and Properties[END_REF] but did not invalidate the results of the analysis. The onset of rupture of the specimen is indicated by pink stars on all the graphs presented in this paper.

The contribution of volume changes is presented in Fig. 5a in all the basic tests. Uniaxial compression resulted in a decrease in the volume of the specimen and an increase in the pressure (red curves in Fig. 5a). A pressure threshold can be clearly observed at a pressure p = 0.2 MPa and a volume ratio V = 0.98. This threshold matches the behavioral change observed on the microscopic scale between the elastic porosity stage and the stress plateau stage [START_REF] Gibson | Cellular Solids. Structure and Properties[END_REF]. Uniaxial tension resulted in an increase in the volume and a decrease in the pressure until failure occurred at a pressure p = -0.32 MPa (see pink stars on blue curves in Fig. 5a). Simple shear resulted in a drop of 0.08 MPa in the pressure, as shown by the green curves in Fig. 5a. This drop was due to the nature of the simple shear load, which was not a pure shear load. A slight decrease in the volume was also observed in the simple shear test, due to issues concerning the rigidity of the device. The effects of the rigidity of the device were corrected and reduced in all the shear tests by adjusting the movements of the hexapod platform. An arbitrary criterion defining valid tests was set at 2% of the maximum volume change.

The contribution of the shape changes is presented in Fig. 5b. In the case of uniaxial compression, a shear threshold was observed at a shear intensity Qσ = 0.5 MPa and a distortion intensity Qε = 0.02, which matches the pressure threshold observed in the case of the volume change behavior. The shape change behavior corresponding to a simple shear load was nearly the same as under uniaxial tension, but at a higher shear intensity. This difference can originate in the mechanical behavior of polymers that may be different in tension and in compression. But it mainly originates in specimen geometries that involve non-null local transversal stress σ yy , especially near the grip, for hex- apod device specimens and higher stiffness. Failure occurred at higher stress and strain intensities in the simple shear tests, Qσ = 1.1 MPa as compared to 0.8 MPa and Qε = 0.15 as compared to 0.07 under uni- axial tension, respectively.

Multiaxial experiments

Experimental procedure

We then focused on the influence of the volume change behavior on the shape change behavior, and vice-versa. Several multiaxial tests were performed in which radial loading paths were applied. In each of these tests, the kinematics imposed on the specimen using the hexapod followed a radial path, which can be defined as the kinematic angle θε between the ordinate axis Qε and the radial path (see equation ( 9)).

⎜ ⎟ ⎜ ⎟ = ⎛ ⎝ ⎞ ⎠ = ⎛ ⎝ ⎞ ⎠ θ V Q I Q arctan ln arctan ε ε ε ε (9)
For instance, the kinematic angle was equal to 0°in the simple shear tests and -90°or +90°in the triaxial compression and tension tests, respectively. Using equation ( 8), the volume ratio Iε and the distortion intensity Qε can be expressed as a function of the stretch value λ and the effective Poisson ratio α, giving θε under uniaxial loading conditions (see equation ( 10)).

= ⎛ ⎝ ⎜ ⎜ - + ⎞ ⎠ ⎟ ⎟ θ αλ αλ arctan (1 2 )ln (1 ) ln ε 2 3 ( 10
)
Two new radial paths in which simultaneous uniaxial compression and simple shear are combined can be arbitrary defined. The combined tests #1 and #2 followed radial loading paths. Their specific kinematic angles are defined between uniaxial compression and simple shear, and are equal to -42°and -31°, respectively. The characteristics of each test are summarized in Table 1.

Results

Kinematic behavior observed

The radial paths obtained in the D.I.C. analysis were presented in a diagram giving the distortion intensity vs. one of the volume variables, as shown in Fig. 6.

When the results were presented in a -QεV diagram (see Fig. 6a), the curves obtained were not completely straight because of the choice of variable = V Iε exp( ). However, when the results of the tests were presented as in the -QεI ε diagram in Fig. 6b, they exactly matched the radial paths applied although the Combined 1 test did not give a completely straight line. This was probably due to the large transformations imposed on the hexapod device, which had some difficulty in following the imposed loading path.

Failure of the specimens (pink stars on the graphs) occurred in all the tests except the uniaxial compression test. In tests with a low kinematic angle, failure in terms of the distortion intensity was delayed. These diagrams are useful means of defining failure limit curves because the failure process evolves very regularly with the kinematic angle. Linear and quadratic polynomial functions can therefore be used to fit the failure limit curve in both cases. Results of fitting procedure are presented in Table 2. It is worth noting that the use of a linear function gives an accurate approximation with a coefficient of determination R 2 greater than 0.99.

Volume change and shape change behavior

Three multiaxial experiments of each kind were performed to check the repeatability. Results are presented in terms of volume and shape changes in Fig. 7a andb, respectively. The scattering of the data was moderate and the main differences between results were due to differences between the density of the specimens.

Since it is proposed in the future to model the mechanical behavior of foams by separating the volume changes from the shape changes, it is worthwhile having some parameters which describe each of these contributions. In the above two figures, the curves look alike, and most of them are composed of two different slopes, corresponding to a threshold and a failure limit. To facilitate the observation and analysis of these curves, several parameters have been defined in Table 3.

In terms of the volume changes, the first slope K1 was nearly the same in all the tests in which uniaxial loading was applied. This slope differed in the case of the simple shear tests, however, because of issues concerning the rigidity of the experimental device, as mentioned in section 3.2. The kinematic angle was found to affect the pressure threshold Pσ0 and the second slope K 2, and hence the volume change behavior. When θ ε increased, the pressure threshold Pσ0 decreased and the second slope K 2 also decreased relatively. Except for the uniaxial compression tests, in which no failure occurred, the pressure at failure Pσf decreased when θε increased. All the results showing the effects of the kinematic angle on the volume change behavior are presented in Table 4.

In terms of the shape changes, the first slope µ1 was practically nearly the same in all the basic and multiaxial tests, except for the uniaxial tensile test. This difference in stiffness can be explained, firstly, by the different mechanical behaviors in tension and compression of

Q σ 0 Second slope K 2 µ 2 Failure V f and P σ f Qε f and Q σ f Table 4
Effects of the kinematic angle on the volume change behavior.

Loading (θ ε ) K1 [MPa] P σ 0 [MPa] K 2 [MPa] V f [-] P σ f [MPa]
Uniaxial compression (-51°) some polymers and, secondly, by the different specimen geometry used for these two loadings, as mentioned in section 3.2.

-
As regards the threshold and the second slope, it is worth noting that in all the tests including a uniaxial compression loading contribution, i.e., the uniaxial compression test, the combined #1 and the combined #2 tests, the threshold was approximately the same at about Q σ 0 = 0.5 MPa. In these three tests, the second slope subsequently increased when the kinematic angle increased. In a future objective of developing a new numerical model to describe EPP mechanical behavior or more generally foam mechanical behavior, a first process step is to define mathematical functions that can describe both volume change and shape change contributions. If the mathematical functions are the same whatever the kind of loadings (tension, compression, shear), the second process step which consisting in defining a hyperelastic potential with separation of spherical and deviatoric contributions will become easier. If we take this point further by increasing the kinematic angle, assuming the existence of a shear threshold of 0.5 MPa in all the tests, including the simple shear and tensile tests, the second slope will therefore increase reaching a maximum in the simple shear tests. This assumption might help to explain the shape of these two curves. Lastly, decreasing the kinematic angle delayed the failure in terms of the distortion intensity and the shear intensity. All the results showing the effects of the kinematic angle on the shape change behavior are presented in Table 5.

Stress response of the material

Just as the kinematic angle θε defines the kinematics imposed on the specimen, the stress angle θσ defined in equation [START_REF] Moreu | Rapid hydrostatic compression of low-density polymeric foams[END_REF] reflects the evolution of the loading in terms of the shear rate and the pressure, and corresponds almost to the triaxiality value.

⎜ ⎟ = ⎛ ⎝ ⎞ ⎠ θ p Q arctan σ σ (11)
For instance, in the case of a simple shear load, θσ = 0°under uniaxial loading, the stress angle can be calculated as in equation [START_REF] Viot | Hydrostatic compression on polypropylene foam[END_REF].

= ⎛ ⎝ ⎜ ⎜ -⎞ ⎠ ⎟ ⎟ ≃± °θ σ arctan 22 σ σ 3 2 3 (12)
It is worth presenting the evolution of the specimen under different loads in a 'Shear intensity Qσ -Pressure p' diagram such as that shown in Fig. 8.

It is worth noting that the uniaxial tension and compression loads gave straight paths. In the simple shear tests, the stress angle was not exactly null because the load applied was not a pure shear load (see section 3.2). In the combined tests #1 and #2, the initial slopes were equal to 18°and 13°, respectively. A sharp change then occurred at Q σ #1 = 0.49 MPa and Q σ #2 = 0.55 MPa at the same shear and pressure thresholds. From then on, the loading path made a change of direction, resembling that obtained with uniaxial tension or simple shear loads with stress angles of θ σ #1 = -4°and θ σ #2 = -10°. Only the combined test #2 gradually resulted in another change of direction, resembling that observed under uniaxial compression loading just before failure of the specimen occurred. One important point worth noting is that with straight imposed kinematic paths (see Fig. 6), the loading paths obtained were not all straight. The great differences in behavior observed between the uniaxial compression and simple shear tests (see section 3) resulted in changes in the loading paths when both types of loading were combined. This key point will be confirmed in the following subsection entitled 'Strain and stress Lode angles'.D e fining a failure criterion seems to be more difficult with the stress variables p and Qσ in Fig. 8 than with the strain variables V or Iε and Qε in Fig. 6.

Stress and strain Lode angles

The loading paths in the stress or strain deviatoric planes can be approached using the strain and stress Lode angles as defined in section 2.2.3 and shown in Fig. 9.

All the changes in the strain Lode angles shown in Fig. 9a corresponded to straight paths. Under uniaxial tension and compression, the Lode angles φε = 0°and φε = 60°, respectively, which is in agreement with the theoretical values. In the case of simple shear loads, the Lode angle calculated using the Digital Image Correlation method amounted to the expected value of 30°. In the combined tests #1 and #2, the strain Lode angles obtained were approximately equal to 45°and 50°, respectively.

For the same reasons as those mentioned in section 4.2.3, none of the stress Lode angles gave completely straight paths, except for those obtained with uniaxial tensile and compression loads. The stress Lode angle obtained in the simple shear test gradually departed from the shear angle, mainly because pure shear loads were not applied. In the combined tests, since the simple shear and uniaxial compressive contributions were very different, the stress Lode angle suddenly changed when the shear threshold was reached. This change of behavior was confirmed by the elevation related to the deviatoric stress plane (Fig. 8) and the Lode angle on the deviatoric stress plane (Fig. 9b). Failure, which may depend on either the stress or strain Lode angles, occurred earlier in the case of low Lode angles.

Conclusions

In this study, original multiaxial experiments were performed using a novel hexapod device, with which uniaxial compression and simple shear loads can be applied simultaneously to a polymeric foam to test its mechanical behavior with radial loading paths. The Cauchy stress tensor was obtained using a 3-D sensor and the logarithmic strain tensor using the Digital Image Correlation method. The evolution of the effective Poisson ratio was determined and the assumptions adopted in order to obtain inaccessible strain components are discussed.

The post-processing framework based on tensor invariants used by Combaz et al. [START_REF] Combaz | Yield surface of polyurethane and aluminium replicated foam[END_REF][START_REF] Combaz | Multiaxial yield behaviour of al replicated foam[END_REF] was adopted for comparing the results of basic and multiaxial experiments.

The results obtained in this study can be summarized as follows:

1. The mechanical behavior depends on the kinematic angle, and hence on the contribution of the simple shear to combined loads.

The results of a detailed study show the effects of this parameter on the various steps in the mechanical response of the material, decomposed into the volume and shape change contributions. In short, -the elasticity of microscopic cell walls, as defined by Gibson and Ashby [START_REF] Gibson | Cellular Solids. Structure and Properties[END_REF], is not subject to any variations depending on the kinematic angle; -the pressure threshold decreases with the kinematic angle, whereas the shear threshold does not show any changes; -the pressure resistance observed during the stress plateau evolves in the opposite way from the shear resistance. 2. Failure occurred later in the case of combined loads involving relative small simple shear contributions. The failure limit can be easily determined in terms of the strain path using a distortion intensity-to-volume ratio diagram. 3. The radial strain paths imposed did not systematically result in straight stress paths. Since the mechanical behavior of the material differed considerably between uniaxial compression and simple shear tests, the stress paths observed in the combined tests were not radial in terms of either the pressure-to-shear intensity ratio or the stress Lode angle.

In conclusion, the results obtained in this study increase the database available on this widely used foam, which will also be extremely useful in our future project, in which it is proposed to model these experiments. In particular, the possibility to get the loading path in the stress space related to the strain space brings useful information. Since the novel hexapod device can be used to perform multiaxial tests, it is also intended to perform tests of this kind with various loading paths in order to determine how they affect the mechanical behavior of the material of interest.

Fig. 1 .

 1 Fig. 1. The hexapod device. A black 300 × 20 × 20 mm sample of EPP foam was glued between two steel plates.

Fig. 2 .Fig. 3 .

 23 Fig. 2. Forces recorded under simultaneous uniaxial compression and simple shear loads.

Fig. 4 .

 4 Fig. 4. Uniaxially loaded EPP foam at a strain rate of 0.0125 s -1 .

Fig. 5 .

 5 Fig. 5. Results of basic experiments. Pink stars indicate failure of the specimen. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 6 .

 6 Fig. 6. Radial paths obtained with the D.I.C. correlation method. Pink stars indicate failure of the specimens. The dotted line gives the failure limit. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 7 .

 7 Fig. 7. Comparison between multiaxial and basic experiments. Pink stars indicate failure of the specimens. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 8 .

 8 Fig. 8. Loading paths obtained with the 3D sensor. Pink stars stand for failure of the specimen. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 9 .

 9 Fig. 9. Strain and stress Lode angles. Pink stars stand for failure of specimen. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Table 1

 1 Controlled radial path experiments.

	Loading	Kinematic angle θ ε
	Uniaxial compression ( = α 0)	-51°C
	ombined 1	-42°C
	ombined 2	-31°S
	imple shear	0°U
	niaxial tension ( = α 0.27)	24°A

Table 2

 2 Failure limit functions.

	Polynomial	Equation				Coefficient of
	functions							determination R 2
	Linear	Q ε	=-	V 1.702	+	1.808		0.9929
		=-εε Q I 1.004	+	0.131		0.9991
	Quadratic	Q ε	=-VV 0.870 2.851 2	+	2.103	0.9999
		Q ε	=-	-II 0.146 1.178 ε ε 2	+	0.123

Table 3

 3 Definition of parameters.

	Description	Parameters Volume change	Parameters Shape change
	First slope	K 1	µ 1
	Threshold	P σ 0

Table 5

 5 Effects of the kinematic angle on the shape change behavior.

	Loading (θ ε )	µ1 [MPa] Q σ 0 [MPa] µ2 [MPa] Q ε f [-] Q σ f [MPa]
	Uniaxial compression	34.1	0.51	0.66	--
	(-51°)
	Combined 1 (-42°)	31.0	0.46	1.17	1.43	1.81
	Combined 2 (-21°)	30.0	0.55	1.83	0.43	1.11
	Simple shear (0°)	32.7	(0.47)	(11.15)	0.14	1.08
	Uniaxial tension (24°) 22.3	(0.55)	(7.33)	0.08	0.86

Acknowledgments

The French Fonds Unique Interministériel (FUI number F 14 10 009 M) is gratefully acknowledged for funding the New Generation of Airliner Seats (Genosia) project.

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Javier Castro-Jimenez
email: javier.castro-jimenez@mio.osupytheas.fr

Badreddine Barhoumi

Andrea Paluselli

Marc Tedetti

Begoña Jiménez

Juan Muñoz-Arnanz

Henri Wortham

Mohamed Ridha Driss

Richard Sempere

Javier Castro Jiménez

Richard Sempéré

Javier Castro-Jiménez

à la diffusion de documents scientifiques de niveau

Keywords: PCB, flame retardant, dioxin, marine pollution, environmental risk TOC

recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Occurrence, loading and exposure of atmospheric particle-bound POPs at the African and European edges of the western Mediterranean Sea.

Introduction

Atmospheric particulate matter has been recognized as a major environmental airborne pollutant impacting human health and ecosystems. [START_REF] Who | Health Aspects of air Pollution. Results from the WHO Project "Systematic Review of Health Aspects of air Pollution in Europe[END_REF] A recent study performed in the Mediterranean Sea supports the hypothesis that atmospheric aerosols can elicit a number of toxic effects in marine organisms due to the presence of hydrophobic contaminants intimately associated to the atmospheric particle phase. [START_REF] Mesquita | Toxicity assessment of atmospheric particulate matter in the Mediterranean and Black Seas open waters[END_REF] High molecular weight (HMW) polycyclic aromatic hydrocarbons (PAHs) and generally persistent organic pollutants (POPs) such as polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polychlorinated biphenyls (PCBs) are among the contaminants suspected to produce these hazardous effects. [START_REF] Mesquita | Toxicity assessment of atmospheric particulate matter in the Mediterranean and Black Seas open waters[END_REF] In addition, POPs associated to the atmospheric particle phase can be more resistant to atmospheric degradation processes (e.g. photodegradation) increasing therefore their atmospheric residence times, subsequent long-range atmospheric transport (LRAT) and potential expossure. [START_REF] Mesquita | Toxicity assessment of atmospheric particulate matter in the Mediterranean and Black Seas open waters[END_REF] A first step in order to elucidate the potential role of particle-bound POPs in toxic pathways and to estimate their atmospheric dry deposition, considered as the main vector for the introduction of the most hydrophobic contaminants in aquatic ecosystems, is their accurate qualitative (congener specific distribution) and quantitative (environmental concentration) determinations in the atmospheric aerosol. The legacy PCDD/Fs and PCBs may lead to complex mixtures in the environment. However, the seventeen 2,3,7,8-chlorine substituted PCDD/Fs (i.e. seven PCDDs or "dioxins" + ten PCDFs or "furans") and the twelve dioxin-like PCBs ( DL PCBs) named CBs-81, -77, -105, -114, -118, -123, -126, -156, -157, -167, -169, -189, jointly considered as dioxin-like pollutants, have received most of the attention due to their accumulation and toxic effects in biota and humans. [START_REF] Safe | Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs): biochemistry, toxicology and mechanism of action[END_REF][START_REF] Safe | Polychlorinated biphenyls (PCBs), dibdenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and related compounds: environmental and mechanistic considerations which support the development of toxic equivalency factors (TEFs)[END_REF][START_REF] Van Den Berg | A source inventory and budget for chlorinated dioxins and furans in the United Kingdom environment[END_REF][6] In addition, a set of non-dioxin-like PCBs, commonly referred as indicator PCBs ( ID PCBs), containing CBs-28, -52, -101, -138, -153 and -180 are often monitored due to their higher environmental concentrations and potential toxic effects by different mechanisms of action. [7][8][9] Other POPs with recognized toxicity too, such as polybrominated diphenyl ethers (PBDEs) can add up to the "contaminantaerosol cocktail". 10,11 These pollutants have different sources in the environment.

PCDD/Fs can occur as unintentional by-products in a number of industrial processes and domestic heating, 12,[START_REF] Bruzy | Global mass balance for polychlorinated dibenzo-p-dioxins and dibenzofurans[END_REF] while current emissions of PCBs are urban/industrial centers, open burning of products containing PCBs, waste incinerations, accidental fires and revolatilization from environmental reservoirs. [START_REF] Nizzetto | Present, and Future Controls on Levels of Persistent Organic Pollutants in the Global Environment[END_REF][START_REF] Breivik | Towards a global historical emission inventory for selected PCB congeners -a mass balance approach: 2[END_REF] PBDE were mostly used (and still used in some regions) as flame retardants and, contrary to PCDD/Fs and PCBs, are considered as a first generation of "emerging contaminants" (only recently restricted).

Three PBDE commercial formulations were extensively used worldwide: the pentaformulation (with predominance of BDE-47 and -99), the octa-BDE (with PBDE-183 as one of the major components) and the deca-BDE (with BDE-209 predominating). 16 These contaminants are globally banned by the Stockholm Convention of POPs (including all PBDE formulations) and regulated by the Water and Marine Strategy

Framework Directives at European level (except for the octa-and deca-PBDE formulations). However, still very little or no information exists on current base line levels and stocks for many marine areas, even in environments under important anthropogenic pressure like the Mediterranean Sea. This semi-enclosed environment of high ecological and socio-economic relevance is largely impacted by POPs and related contaminants. 17,18,19 PCDD/Fs, PCBs and PBDEs are among the POPs found in the atmospheric compartment from various NW Mediterranean coastal environments (from rural to large cities), [START_REF] Valle | Estimation of PCDD/F distribution and fluxes in the Venice Lagoon, Italy: Combining measurement and modelling approaches[END_REF][START_REF] Jaward | Passive air sampling of PCBs, PBDEs and organochlorine pesticides across Europe[END_REF][START_REF] Castro-Jiménez | PCDD/F and PCB multi-media ambient concentrations, congener patterns and occurrence in a Mediterranean coastal lagoon (Etang de Thau, France)[END_REF][START_REF] Castro-Jiménez | Atmospheric concentrations, occurrence and deposition of persistent organic pollutants (POPs) in a Mediterranean coastal site (Etang de Thau, France)[END_REF][START_REF] Mari | Air concentrations of PCDD/Fs, PCBs and PCNs using active and passive air samplers[END_REF][START_REF] Gambaro | Organic micropollutants in wet and dry depositions in the Venice Lagoon[END_REF][START_REF] Garcia-Flor | Surface waters are a source of polychlorinated biphenyls to the coastal atmosphere of the North-Western Mediterranean Sea[END_REF] and the central/Eastern basin [START_REF] Mandalakis | Study of atmospheric PCB concentrations over the eastern Mediterranean Sea[END_REF][START_REF] Mandalakis | Mass budget and dynamics of polychlorinated biphenyls in the eastern Mediterranean Sea. Global biogeochem[END_REF][START_REF] Mandalakis | Particle-size distribution and gas/particle partitioning of atmospheric polybrominated diphenyl ethers in urban areas of Greece[END_REF][START_REF] Iacovidou | Occurrence and diurnal variation of polychlorinated biphenyls and polybrominated diphenyl ethers in the background atmosphere of Eastern Mediterranean[END_REF][START_REF] Mosca | Evaluation of a simultaneous sampling method of PAHs, PCDD/Fs and dl-PCBs in ambient air[END_REF][START_REF] Lammel | Air and seawater pollution and air-sea gas exchange of persistent toxic substances in the Aegean Sea: spatial trends of PAHs, PCBs, OCPs and PBDEs[END_REF][START_REF] Pozo | Assessing persistent organic pollutants (POPs) in the Sicily Island atmosphere, Mediterranean, using PUF disk passive air samplers[END_REF] , but also farther from the coast in the entire Mediterranean Sea. [START_REF] Iwata | Distribution of persistent organochlorines in the oceanic air and surface seawater and the role of ocean on their global transport and fate[END_REF][START_REF] Castro-Jiménez | Atmospheric occurrence and deposition of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the open Mediterranean Sea[END_REF][START_REF] Berrojalbiz | Atmospheric occurrence, transport and deposition of polychlorinated biphenyls and hexachlorobenzene in the Mediterranean and Black seas[END_REF] The European Mediterranean coast is by far the most studied and only few measurements (focussing in the SE Mediterranean) have been carried out in the African Mediterranean edge. [START_REF] Cetin | Ambient concentrations and source apportionment of PCBs and trace elements around an industrial area in Izmir, Turkey[END_REF][START_REF] Cetin | Particle-Phase Dry Deposition and Air-Soil Gas-Exchange of Polybrominated Diphenyl Ethers (PBDEs) in Izmir, Turkey[END_REF] . The atmospheric occurrence of POPs over the NW Mediterranean African coast still largely unknown. In addition, parallel observations at both the African and European margins of the Mediterranean Sea have never been attempted to the best of our knowledge.

We report here results from 22 aerosol samples (total suspended particles, TSP)

simultaneously

Materials and Methods

Sampling and study areas

Atmospheric aerosols samples (TSP) were simultaneously collected from March 2015

to January 2016 in two representative coastal locations under strong anthropogenic pressure at both the African and European W Mediterranean Sea edges: the cities of Bizerte (Tunisia) and Marseille (France) (Figure S1). The air was drawn through precombusted quartz fiber filters (QFFs) placed in high volume air samplers (Tisch Environmental, Inc, USA). The sampling volume raged from ~ 1800 to ~5000 m 3 depending on the site and sampling event (Table S1).

Marseille: Samples were collected at the roof (~10m S3). Laboratory blanks (1 blank per five-sample batch)

showed lower or similar levels to field blanks, so no contamination during sampling, storage and analysis occurred. Results were blank corrected. Chromatographic peaks were only considered when the ratio between the two monitored ions was within ±15% of the theoretical value, and the signal-to-noise (S/N) ratio>3 (instrumental limit of detection, LOD). Limits of quantification (LOQs) corresponded to S/N≥10. Calibration curves were daily checked. Median LODs ranged from ~ 0.3 to ~ 1.1 pg (PCDD/Fs), from 2.1 to ~ 8.3 pg (PCBs) and from ~ 1.1 to 32.5 pg (PBDEs) depending on the compound and sample (Table S4). Median method recoveries (extraction-cleanupanalysis) varied from 70 to 97 % (PCDD/Fs), from 83 to 110 % (PCBs) and from 60 to 120 % (PBDEs) (Table S5). Results were corrected by recoveries.

TOC, EC determinations and statistical analysis

Total organic carbon (TOC) content was determined in filter subsamples by high temperature combustion (CHN analyser). 46 For the elemental carbon (EC) determination, additional filter sub-samples were pre-combusted at 340 °C during 2 h under oxygen flow 47 and then analysed as above indicated. STATA/SE 12.1 software was employed for the statistical data analysis.

Results and discussion

Concentrations in the atmospheric aerosol

Dioxin-like POPs ∑2,3,7,8-PCDD/F concentrations in the aerosols over Bizerte area (Tunisia) ranged from 32 to 1100 fg m -3 (222 fg m -3 , median) along the sampling period and were generally higher than those over Marseille coastal area (France), varying from 31 to 1280 fg m -3 (83 fg m -3 , median) (Figure 1A). The highest differences were found for the months of April, July and September 2015 (3, 11, ~5-fold higher levels in Bizerte, respectively). However, these values were not significantly different (Mann-Whitney test, p=0.28) for the sum of 2,3,7,8-PCDD/Fs most probably due to the high annual variability, in particular in Bizerte site, and the limited number of samples analysed.

Significant differences were only found for two furan congeners (i.e. 2,3,4,6,7,8-HxCDF and 1,2,3,4,6,7,8-HpCDF) (Mann-Whitney test, p=0.04) (Figure S2, Table S6).

A general trend showing higher PCDD/F levels in the coldest months of the year was observed at both sites. This effect has been previously reported in other Mediterranean coastal areas and has been attributed to the combined effect of the increase of diffusive combustion sources (e.g. domestic heating and road traffic) and a less efficient dispersion of the local emissions during cold conditions. [START_REF] Castro-Jiménez | Atmospheric concentrations, occurrence and deposition of persistent organic pollutants (POPs) in a Mediterranean coastal site (Etang de Thau, France)[END_REF] The ∑2,3,7,8-PCDD/F was clearly dominated by the contribution of PCDDs in Marseille whereas a more even contribution of PCDDs and PCDFs was found for the aerosols of Bizerte area (Figure 1A), pointing to different PCDD/F sources at both sites. This is the first time that PCDD/Fs have been measured in atmospheric aerosols from the Marseille area and in a coastal site at the African Mediterranean edge. The concentrations found at both sites are within the range of those previously reported for a coastal area in the Gulf of Lion (50-1500 fg m -3 ) [START_REF] Castro-Jiménez | Atmospheric concentrations, occurrence and deposition of persistent organic pollutants (POPs) in a Mediterranean coastal site (Etang de Thau, France)[END_REF] , urban/industrial sites from Barcelona area (570-1162 fg m -3 ) [START_REF] Mari | Air concentrations of PCDD/Fs, PCBs and PCNs using active and passive air samplers[END_REF] and in the open NW Mediterranean (94-990 fg m -3 ). [START_REF] Castro-Jiménez | Atmospheric occurrence and deposition of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the open Mediterranean Sea[END_REF] Reported values from a cruise transect close to the NW Mediterranean African coast (38 fg m -3 ) are lower than those measured in Bizerte coastal area. [START_REF] Castro-Jiménez | Atmospheric occurrence and deposition of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the open Mediterranean Sea[END_REF] Concentrations measured in a background site in Barcelona area (230 fg m -3 ) are lower too than those in Marseille [START_REF] Mari | Air concentrations of PCDD/Fs, PCBs and PCNs using active and passive air samplers[END_REF] . No data for comparison was found in the central/eastern Mediterranean Sea.

Recent PCDD/F measurements carried out in the atmospheric aerosol over remote areas from the Atlantic, Pacific and Indian oceans revealed 6 to 11-fold lower background levels (5 to 113 fg m -3 ). 48 The ∑ 18 PCB concentrations measured in the aerosols from Bizerte, ranging from 0.7 to ~4.0 pg m -3 (2.1 pg m -3 , median) were significantly higher (Mann-Whitney test, p=0.03) than those measured in the aerosols over Marseille, which varied from 0.5 to 2.7 pg m -3

(1.0 pg m -3 , median). ID PCBs dominated the total levels at both sites with ∑ ID PCB concentrations significantly higher too in Bizerte (0.6 to 3.4 pg m -3 , median value of ~ 2.0 pg m -3 ) than in Marseille (0.3 to 1.8 pg m -3 , median value of 0.8 pg m -3 ) (Mann-Whitney test, p=0.01) (Figure 1B). ∑ DL PCB concentrations, which represented a minor contribution to the total PCB atmospheric stock at both sites, were not statistically different (0.1 -0.5 pg m -3 in Bizerte compared to 0.1 -0.8 pg m -3 in Marseille).

However, the concentrations of some of the most toxic DL PCBs (i.e. CB-81, -126, -169) as well as the CBs-167, -189 were significantly higher in Bizerte than in Marseille coastal area (2-4 fold-higher median values depending on the congener) (Figure S3, Table S7).

∑ 18 PCB (and ∑ ID PCB) concentrations exhibited different annual trends in both sites.

While a general increase of concentrations was observed in winter months compared to spring and summer in Marseille, a more pronounced seasonality was observed for Bizerte, showing clearly higher concentrations in spring-early summer and late autumnwinter months, with minimum levels in August, September and October (Figure 1B). As a result, 2 to 8-fold higher ∑ 18 PCB levels were measured in Bizerte than in Marseille from March to July while the concentration differences between the two sites in winter months were less important, and in some months (e.g. November) higher for Marseille.

Interestingly, higher ∑ DL PCB concentrations were generally measured from August 2015 to January (except in December) in Marseille. These facts highlight the differential environmental behaviour, seasonality and related exposure of individual PCBs at both sites.

PCB data comparability may be tricky since different number and type of PCBs may be considered. Therefore, for comparative purposes we focus only on ID PCBs. Σ ID PCB concentrations measured at both edges of the NW Mediterranean Sea were generally lower than previous coastal measurements performed in the NW Mediterranean in the Gulf of Lion (0.5 to 7.2 pg m -3 ) [START_REF] Castro-Jiménez | Atmospheric concentrations, occurrence and deposition of persistent organic pollutants (POPs) in a Mediterranean coastal site (Etang de Thau, France)[END_REF][START_REF] Garcia-Flor | Surface waters are a source of polychlorinated biphenyls to the coastal atmosphere of the North-Western Mediterranean Sea[END_REF] and those carried out close to Barcelona area (9 -26 pg m -3 ). [START_REF] Garcia-Flor | Surface waters are a source of polychlorinated biphenyls to the coastal atmosphere of the North-Western Mediterranean Sea[END_REF] Cruise measurements encompassing the Mediterranean Sea and the SW Black Sea in 2006 and 2007 revealed higher concentrations too for Σ ID PCB (0.4-18 pg m -3 ), but PCB-118 was included in the sums. [START_REF] Berrojalbiz | Atmospheric occurrence, transport and deposition of polychlorinated biphenyls and hexachlorobenzene in the Mediterranean and Black seas[END_REF] However, concentrations measured in Bizerte and Marseille were generally higher that those reported for a coastal site (considered as a background location) in the Eastern Mediterranean 27 consistent with previous studies indicating higher PCB levels in the W Mediterranean. [START_REF] Berrojalbiz | Atmospheric occurrence, transport and deposition of polychlorinated biphenyls and hexachlorobenzene in the Mediterranean and Black seas[END_REF] Most of the previous studies correspond to measurements performed more than a decade ago (as for PCDD/Fs) so comparison must be done carefully. Particle-bound PCBs have received little attention since then in the Mediterranean basin compared to gas phase PCBs, most probably due to the dominant role of vapor PCBs driving total PCB ambient levels (especially for low MW congeners) and the increasing applications of passive sampling techniques to POP atmospheric measurements. Thus, recent studies in the Mediterranean Sea considered only the gas phase or the assemble gas+particle phases. [START_REF] Lammel | Air and seawater pollution and air-sea gas exchange of persistent toxic substances in the Aegean Sea: spatial trends of PAHs, PCBs, OCPs and PBDEs[END_REF][START_REF] Pozo | Assessing persistent organic pollutants (POPs) in the Sicily Island atmosphere, Mediterranean, using PUF disk passive air samplers[END_REF] ∑ DL PCB background measurements in the atmospheric aerosol over remote areas from the Atlantic, Pacific and Indian oceans revealed up to 500-fold lower levels (1 -2675 fg m -3 ). 48 These figures confirmed that the NW Mediterranean Sea is considerably exposed to dioxin-like POPs, and might act in turn as a potential secondary source, in particular of PCBs, for other Mediterranean regions (less exposed) and for the most pristine areas of 296 the major oceans of the Earth. It has been reported that the principal control on the 297 levels of legacy POPs in active circulation in the global environment is currently in a 298 state of transition (from primary to secondary source control). [START_REF] Nizzetto | Present, and Future Controls on Levels of Persistent Organic Pollutants in the Global Environment[END_REF] The role of the NW (~ 9.0 pg m -3 , median) (Figure 1C). A general trend of higher levels in Marseille area was found, with the exception of the samples collected in spring in Bizerte, in particular the one of March 2015 (which exhibited 3 to ~50-fold higher PBDE levels than the rest of samples collected in Bizerte). This concentration peak can be attributed to an unusual higher concentration of medium-high MW PBDEs in that sample compared to the others, in particular BDE-183 (Table S8), likely due to the combined effect of the air mass back trajectories (BTs) and potential local sources during this sampling event (see discussion below). However, the concentrations at both sites did not show a statistical difference (Mann-Whitney test, p>0.05). Still, some individual congeners exhibited statistical differences but with contrasting trends, like BDE-47 with 3-fold higher median concentrations in Marseille than Bizerte (Mann-Whitney test, p=0.01), while BDE-3, -7, -71, -77, -126, and -156 showing higher levels in the aerosols from Bizerte (p˂0.01 -0.04) (Figure S4, Table S8). PBDE-209 was the major contributor to the total Σ 27 PBDE for most of the samples at both sites (Figure 1C).

Σ 27 PBDE concentrations measured in Marseille and Bizerte were generally within the range of those reported for a coastal site in the NW Mediterranean (2-18 pg m -3 , Σ 8 PBDE) [START_REF] Castro-Jiménez | Atmospheric concentrations, occurrence and deposition of persistent organic pollutants (POPs) in a Mediterranean coastal site (Etang de Thau, France)[END_REF] , higher than levels in a coastal background site in central/E Mediterranean (0.5-2 pg m -3 , Σ 15 PBDE) [START_REF] Iacovidou | Occurrence and diurnal variation of polychlorinated biphenyls and polybrominated diphenyl ethers in the background atmosphere of Eastern Mediterranean[END_REF] but in the lower end of values reported for aerosols collected over large cities in the E Mediterranean like Athens (Greece) (15-23 pg m -3 , Σ 12 PBDE) [START_REF] Mandalakis | Particle-size distribution and gas/particle partitioning of atmospheric polybrominated diphenyl ethers in urban areas of Greece[END_REF] and Izmir (Turkey) (27-62 pg m -3 , Σ 7 PBDE). 38

POPs patterns in the atmospheric aerosol

The PCDD/F atmospheric pattern (congener relative contribution) for most of Bizerte direct transport from the Fos-Berre area to Marseille is rarely observed. Such transport could occur when the wind is a composition of land/sea breeze and light Mistral but the air mass will go towards the sea before to come back to Marseille which corresponds to a half-range atmospheric transport. Furthermore, these wind conditions correspond generally to sunny period inducing strong photochemical processes. [START_REF] El Haddad | Isotope dilution analysis of polychlorinated biphenyls (PCBs) in transformer oil and global commercial PCB formulations by high resolution gas chromatography-high resolution mass spectrometry[END_REF][40][41]43 The PCB atmospheric pattern was dominated in general by the HMW ID PCBs at both sites. A clear predominance of PCB-180 in Bizerte (accounting for the 35.0±8.7 % of the ∑ 18 PCBs) was observed compared to Marseille area (19.2±5.1 % of the ∑ 18 PCBs).

A similar pattern was observed in the two sites for DL PCB, but PCB-118 and -105 predominated in Marseille (12.6±3.2% and 5.7±1.8%, respectively) compared to Bizerte (4.6±2.0% and 1.9±0.5%, respectively). The different abundance of these three major PCBs may be either the reflect of different past usage of PCB commercial mixtures at both sites or distinct emissions rates to the environment at present. For example, PCB-180 is one of the major constituent of Aroclor 1260 and 1262, while CB-105, 118 were relatively more abundant in the Aroclor 1254 formulation. One of the major uses of these Aroclor formulations was as industrial oils destined to transformers. 51 It has been very recently reported that a large number of transformers containing PCBs are still used or stored in unsatisfactory conditions in Bizerte area with high probability of oil leaks. 44 This could explain the higher contribution of PCB-180 in Bizerte compared to Marseille. focus on the differences in the atmospheric pattern found in Bizerte (dominated by a characteristic "source pattern", n=7) and in Marseille (dominated by a characteristic "sink" pattern, n=9).

The predominance of BDE209 was reflected also in the atmospheric congener pattern (Figure 3) accounting for the 72.3±18.6% and 65.5±18.0% of the Σ 27 PBDE in Marseille and Bizerte, respectively. The important abundance of this BDE denotes the past and maybe current usage of deca-PBDE formulations at both edges of the NW Mediterranean Sea. 16 In addition, the predominance of BDE-47 and BDE-99 in the pattern from Marseille samples too, together with their higher abundance (6.8±7.3% and 5.3±4.5% for BDE-47 and -99, respectively) compared to the pattern in Bizerte (2.0 ±1.9 % and 2.8±2.5 % for BDE-47 and -99, respectively), point first to a major usage of

Factors affecting temporal and spatial variability

The finest fractions of the atmospheric TSP, like TOC and EC, have been reported to efficiently sorb POPs-like contaminants. [START_REF] Mandalakis | Particle-size distribution and gas/particle partitioning of atmospheric polybrominated diphenyl ethers in urban areas of Greece[END_REF][START_REF] Dachs | Adsorption onto Aerosol Soot Carbon Dominates Gas-Particle Partitioning of Polycyclic Aromatic Hydrocarbons[END_REF][START_REF] Koelmans | Black carbon: The reverse of its dark side[END_REF] Generally higher median concentrations of TSP (66.0 ug m -3 ), TOC (7.5 ug m -3 ) and EC (2.0 ug m -3 ) were measured in the aerosols over Bizerte compared to Marseille area, exhibiting 48.0, 6.4 and 0.9 ug m -3 of TSP, TOC and EC, respectively. However, only significant differences (Mann-Whitney test, p=0.03-0.006) were found for the TSP and EC (Figure S5). No correlations were found for PCDD/F concentrations and TSP levels in any of the sites and only ∑ 18 PCB, ∑ ID PCBs and one PCB congener correlated (p=0.01-0.04) with TSP concentration in Bizerte (Figure S6). ∑PCDD/F and ∑ DL PCB (as well as most of PCDD/Fs and a certain number of DL PCBs) concentrations positively correlated with TOC concentration S7). The fact that PCDD/Fs did not correlate with EC is somehow unexpected since this correlation has been suggested in other atmospheric studies. [START_REF] Yu | Particle-bound polychlorinated dibenzo-p-dioxins and dibenzofurans in the atmosphere of Guangzhou, China[END_REF][START_REF] Lohmann | Assessing the importance of ab-and adsorption to the gas-particle partitioning of PCDD/Fs[END_REF] The narrow range of variation of EC concentrations in the samples analysed could have resulted in this lack of correlation. Regarding TOC, the correlations mostly observed for Marseille could be the result of different TOC sources (different sorption properties) in both sites (e.g. more related to urban activities in Marseille and predominantly industrial activities in Bizerte).

Our results indicate that the TOC and/or EC contents in the atmospheric aerosol may have a stronger effect than the TSP as a whole on the spatial-temporal variability of the studied POPs. Concentrations normalized by TOC slightly decreased the ∑2,3,7,8-PCDD/F, ∑ 18 PCB ∑ ID PCBs and ∑ 27 PBDE annual variability at both sites (except for the month of January 2016) and confirmed the similar spatial variability with no significant differences for PCDD/Fs (p=0.67) and PBDEs (p=0.66) and higher levels of ∑ 18 PCB (although not significant, p=0.07) and ∑ ID PCBs (p=0.03) in Bizerte (Figure S8). The higher concentrations in January 2016 resulted in 4 to 9-fold higher levels in Bizerte (412, 1050, 154 and 3210 ng g TOC -1 ) compared to Marseille (80,113, 51 and 811 ng g TOC -1 ) for ∑2,3,7,8-PCDD/F, ∑ ID PCB, DL PCBs and ∑ 27 PBDE, respectively. In addition, an increase of the concentration differences (normalized by TOC) compared to their respective volumetric concentrations was found for ∑ DL PCBs in March 2015.

Concentrations normalized by EC (Figure S9) showed predominant levels of ∑2,3,7,8-PCDD/F for Marseille in March 2015 and January 2016 (where the lowest concentration of EC were measured) and confirmed the ∑ 18 PCB and ∑ ID PCB higher levels in Bizerte during spring-early summer compared to Marseille, but pointed to generally higher concentrations in Marseille from September to winter. Interestingly, a statistically significant (p<0.01) predominance of ∑ DL PCB levels in Marseille resulted from the normalization by EC, which was not verified as volumetric or TOC normalized concentrations. The same effect (p˂0.001) was observed for the ∑ 27 PBDE, exhibiting higher levels in Marseille for all months (7.4 ug g EC -1 , median value) than in Bizerte (3.0 ug g EC -1 , median value) with clear peak differences in August and September.

Four-day BTs were calculated (50, 500 and 1000m AGL) by using the HYSPLIT model [START_REF] Draxler | HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) Model access via NOAA ARL READY Website[END_REF] (Figure S10). The lowest ∑2,3,7,8-PCDD/F levels were registered in the months of June, July and August 2015 in Marseille, where air masses had a clear Atlantic influence, while the highest level was measured in the month of January 2016 with air masses showing continental influence (Figure S9). A similar min-max temporal pattern was observed in Bizerte, with lower levels in summer months (except for July) than in January. However, the BTs in July revealed a more important continental particle transport over some regions in the NW and E Mediterranean Sea [START_REF] Berland | Spatial extent of new particle formation events over the Mediterranean Basin from multiple ground-based and airborne measurements[END_REF][START_REF] Kalivitis | Particle size distributions in the Eastern Mediterranean troposphere[END_REF] . In addition, PBDEs have been described to be mostly associated to the finest particles of the atmospheric aerosol [START_REF] Mandalakis | Particle-size distribution and gas/particle partitioning of atmospheric polybrominated diphenyl ethers in urban areas of Greece[END_REF] , consistent with the correlations found with the TOC and/or EC contents (more abundant and stronger) compared to the TSP as discussed above.

This will result in higher atmospheric residence times and a higher potential of LRAT.

These facts may support the hypothesis of the potential "jumping" of PBDE local atmospheric stocks from the NW European Mediterranean edge to the NW African coast (where lower PBDE ambient levels has been generally measured) under favorable conditions. Indeed, the two sites are relatively close (around 780 km straight line).

However, this hypothesis requires further confirmation with more sites at both edges of the NW Mediterranean. In addition, a better characterization of PBDE local sources (emission inventory) at the African edge is needed. The BT analysis reveals too other potential hot spots farther west in the N African coast which find their way to Bizerte area passing through the Gibraltar strait and along the African cost, as observed in the month of May 2015. The three POP families exhibited higher levels in Bizerte area compared to Marseille (with a different air mass circulation pattern) in this month. This situation has been previously described for PCDD/Fs [START_REF] Castro-Jiménez | Atmospheric occurrence and deposition of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the open Mediterranean Sea[END_REF] and PAHs 18 in the same region.

BTs could only explain part of the annual and spatial variability for the studied POPs, most probably due to their different environmental sources and current local emissions patterns (mostly for legacy POPs) at the studied sites as discussed above, which may prevail over the BT influence at a local/regional scale in different seasons of the year. In addition, other parameters affecting the atmospheric gas-particle partitioning of POPs (e.g. temperature, relative humidity) may have an important influence at local scale.

Atmospheric dry deposition and exposure

Dry deposition fluxes (F DD, pg m -2 d -1 ) of PCDD/Fs, PCBs and PBDEs were calculated as:

F DD = v d C A [1]
where C A is the POP volumetric concentration in the aerosol (pg m -3 ) and v d (cm s -1 ) is the particle deposition velocity. A reference value of 0.2 cm s -1 for v d was adopted for both sites. This value is within the range of v d experimentally measured [START_REF] Del Vento | Influence of the surface microlayer on atmospheric deposition of aerosols and polycyclic aromatic hydrocarbons[END_REF] or previously used in coastal NW Mediterranean Sea [START_REF] Castro-Jiménez | Atmospheric concentrations, occurrence and deposition of persistent organic pollutants (POPs) in a Mediterranean coastal site (Etang de Thau, France)[END_REF][START_REF] Castro-Jiménez | Atmospheric occurrence and deposition of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the open Mediterranean Sea[END_REF][START_REF] Berrojalbiz | Atmospheric occurrence, transport and deposition of polychlorinated biphenyls and hexachlorobenzene in the Mediterranean and Black seas[END_REF] for the estimation of dry deposition fluxes of POPs. However, a degree of uncertainty is associated to this calculation due to the lack of v d measurements during the sampling events.

Overall, the atmospheric loading at both coastal areas is dominated by PBDEs, ranging from 70 to ~ 3400 ng m -2 y -1 , followed by PCBs (30 to ~ 250 ng m -2 y -1 ) and PCDD/Fs

(2 to 80 ng m -2 y -1 ) as expected from their relative atmospheric concentrations and the assumptions considered in these calculations (Table 1, S9-S11). These fluxes were generally within the range of previously reported dry deposition estimations in the Mediterranean and SW Black Sea for the same POPs [START_REF] Castro-Jiménez | Atmospheric concentrations, occurrence and deposition of persistent organic pollutants (POPs) in a Mediterranean coastal site (Etang de Thau, France)[END_REF][START_REF] Mandalakis | Mass budget and dynamics of polychlorinated biphenyls in the eastern Mediterranean Sea. Global biogeochem[END_REF][START_REF] Castro-Jiménez | Atmospheric occurrence and deposition of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the open Mediterranean Sea[END_REF] . However, some differences were found in the atmospheric loading at both edges of the Mediterranean Sea. While a higher PBDE median loading is estimated for Marseille area (~ 550 ng m -2 y -1 ) compared to Bizerte (~400 ng m -2 y -1 ), the median dry deposition fluxes of PCBs (135 ng m -2 y -1 , and PCDD/Fs (14 ng m -2 y -1 ) in Bizerte were higher than those of Marseille, ~60 and 5 ng m -2 y -1 , respectively. Indeed, a 3-fold higher loading of dioxin-like pollutants was estimated for Bizerte as toxic equivalents (TEQ) [START_REF] Van Den Berg | The 2005 world health organization reevaluation of human and mammalian toxic equivalency factors for dioxins and dioxin-like compounds[END_REF] (400 pg TEQ m -2 y -1 , median) compared to Marseille (137 pg TEQ m -2 y -1 , median) due to a higher contribution of PCDFs and DL PCBs in Bizerte (Table 1). These figures indicate a higher potential exposure to these toxic chemical at the African edge of the W Mediterranean. A human health risk assessment for the whole set of dioxin-like POPs was performed based on the inhalation exposure. We follow here a similar approach as a very recent assessment based on only two dioxin-like PCBs performed in Bizerte area 44 . Briefly, concentrations expressed as WHO 05 TEQ were calculated using the latest available toxic equivalency factors (TEFs). [START_REF] Van Den Berg | The 2005 world health organization reevaluation of human and mammalian toxic equivalency factors for dioxins and dioxin-like compounds[END_REF] A daily air inhalation volume of 15 m 3 for an adult of 60 kg was assumed. [START_REF] Gregoris | Gas-particle distributions, sources and health effects of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and polychlorinated naphthalenes (PCNs) in Venice aerosols[END_REF] The ∑2,3,7,8-PCDD/F and Σ DL PCB concentrations in the aerosols over Bizerte area, ranged from 0.7 -70.2 and 0.07 -2.2 WHO 05 TEQ fg m -3 , respectively, whereas in Marseille levels varied from 0.4 -23.0 and 0.03 to 0.3 WHO 06 TEQ fg m -3 , respectively. Median TEQ values (for all dioxin-like congeners) of 6.3 and 2.2 WHO 05 TEQ fg m -3 (Bizerte and Marseille) would result to potential intakes of 0.002 and 0.001 pg TEQ kg -1 day -1 , respectively. Both levels are three orders of magnitude lower than the daily intake threshold of 1-4 pg TEQ kg -1 day -1 propose by WHO [START_REF] Van Leeuwen | Dioxins: WHO's tolerable daily intake (TDI) revisited[END_REF] . The atmospheric particle phase PCDD/F concentration can be considered as a good descriptor of the total atmospheric concentration due to their general predominance in this compartment after their emissions to the environment. [START_REF] Castro-Jiménez | PCDD/F and PCB multi-media ambient concentrations, congener patterns and occurrence in a Mediterranean coastal lagoon (Etang de Thau, France)[END_REF][START_REF] Castro-Jiménez | Atmospheric concentrations, occurrence and deposition of persistent organic pollutants (POPs) in a Mediterranean coastal site (Etang de Thau, France)[END_REF][START_REF] Castro-Jiménez | Atmospheric occurrence and deposition of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the open Mediterranean Sea[END_REF]49 Contrary, PCBs show a general preferential partitioning (mostly low MW congeners) to the vapour phase. [START_REF] Berrojalbiz | Atmospheric occurrence, transport and deposition of polychlorinated biphenyls and hexachlorobenzene in the Mediterranean and Black seas[END_REF]48 However, according to filed measurements performed in various urban, coastal and remote Mediterranean environments and inland aquatic ecosystems the Σ DL PCB gas phase concentrations are not expected to exceed more than 100 fold the measured particulate phase concentrations. [START_REF] Castro-Jiménez | Atmospheric concentrations, occurrence and deposition of persistent organic pollutants (POPs) in a Mediterranean coastal site (Etang de Thau, France)[END_REF][START_REF] Mandalakis | Study of atmospheric PCB concentrations over the eastern Mediterranean Sea[END_REF][START_REF] Berrojalbiz | Atmospheric occurrence, transport and deposition of polychlorinated biphenyls and hexachlorobenzene in the Mediterranean and Black seas[END_REF][START_REF] Gregoris | Gas-particle distributions, sources and health effects of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and polychlorinated naphthalenes (PCNs) in Venice aerosols[END_REF][START_REF] Castro-Jiménez | Monitoring atmospheric levels and deposition of dioxin-like pollutants in sub-alpine Northern Italy[END_REF] Therefore, the worst case Σ DL PCB TEQ concentration could be estimated as 100 fold the TEQ calculated considering only the particle-bound PCB concentrations, resulting in a corrected daily intake (for all dioxin-like congeners) of 0.01 and 0.004 pg TEQ kg -1 day -1 in Bizerte and Marseille areas, respectively. This new value stills far below the daily intake threshold proposed by WHO and therefore a risk by the inhalation route is excluded at present in the studied areas. It should be noted thought that the daily intake threshold proposed by WHO is based on toxicity studies evaluating dose-response through oral exposure.

However, absorption and tissue partitioning could be different via the inhalation route and the local toxicity effect to lungs is not considered, so there is still an uncertainty degree in this assessment.

There are no TEFs associated to PBDEs and according to the European Chemicals Agency, the risks of deca-BDE and lower brominated transformation products to the humans via the environment cannot be adequately addressed in a quantitative way due to the high uncertainties regarding long-term exposure and effects. [START_REF]) ether. Committee for Risk Assessment (RAC)/ Committee for Socio-economic Analysis (SEAC)[END_REF] Similarly to PCDD/Fs, PBDE particle-bound atmospheric concentrations can be a good indicator of the total atmospheric levels, particularly considering that one of the most abundant PBDEs is the HMW PBDE-209 (almost exclusively in the particle phase). For example, the PBDE particle-bound fraction in a large Mediterranean city (Athens, Greece)

exhibiting similar atmospheric TSP concentration as Marseille and Bizerte accounted from more than the 70% of the total airborne PBDE concentrations. [START_REF] Mandalakis | Particle-size distribution and gas/particle partitioning of atmospheric polybrominated diphenyl ethers in urban areas of Greece[END_REF] Following the above calculations, median intakes of 1.6 and 2.2 pg kg -1 day -1 were estimated for Bizerte and Marseille, respectively. As a reference, three order of magnitude higher (~7.0 ng kg -1 day -1 ) PBDE intakes has been reported for American adults. [START_REF]An exposure assessment of polybrominated diphenyl ethers[END_REF] However, the exposure to PBDE has been mostly associated to indoors environments (mainly dust ingestion and dermal contact) [START_REF]An exposure assessment of polybrominated diphenyl ethers[END_REF] and inhalation is not considered as the main route of concern for these substances.

equivalency factors (TEFs) for PCBs, PCDDs, PCDFs for humans and wildlife.

Environ. Health Perspec. 1998, 106, 775-792.

  collected (once per month) in two urban coastal sites at both the African and European edges of the W Mediterranean. The samples have been analysed for 62 toxic chemicals associated to the atmospheric aerosol. The main objectives of this study are: (1) to establish base line atmospheric levels of three of the most toxic POPs families (i.e. PCDD/Fs, PCBs, and PBDEs) in two reference urban coastal sites in the European and African W Mediterranean and (2) to compare the status of pollution, to estimate the atmospheric loading and potential exposure of those chemicals at both edges of the W Mediterranean coast.

299CFigure 1 .

 1 Figure 1. Temporal variability of PCDD/F (A), PCB (B) and PBDE (C) concentrations

  samples showed an important contribution of furans, with 1,2,3,4,6,7,8-HpCDF and OCDF accounting for the 16.3±3.7 % and 12.2±7.3 % of the ∑2,3,7,8-PCDD/F, values almost reaching the relative contribution of 1,2,3,4,6,7,8-HpCDD (17.0±4.0 %) and OCDD (25.5±4.5 %) which are the two most abundant PCDDs (Figure2A-1). However, the PCDD/F pattern in samples from Marseille was dominated by a higher contribution of 1,2,3,4,6,7,8-HpCDD (21.2±6.8 %) and OCDD (53.0±4.0 %) and lower contribution of 1,2,3,4,6,7,8-HpCDF and OCDF accounting for the 5.8±3.0 % and 6.0±2.8 % of the ∑2,3,7,8-PCDD/F, respectively (Figure2A-2). A higher contribution of PCDFs in the PCDD/F atmospheric pattern compared to PCDDs has been associated in previous studies to 'fresh' emissions (sources). A situation in which the lower chlorinated congeners, in particular PCDFs, have insufficient time to experience significant atmospheric degradation / depletion processes.[START_REF] Castro-Jiménez | Atmospheric concentrations, occurrence and deposition of persistent organic pollutants (POPs) in a Mediterranean coastal site (Etang de Thau, France)[END_REF][START_REF] Castro-Jiménez | Atmospheric occurrence and deposition of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the open Mediterranean Sea[END_REF]49,50 Contrary, a predominance of higher chlorinated PCDDs and little contribution of PCDFs is considered as a 'sink' signal where the lighter congeners were 'weathered' in their travel from the source to the sampling sites due to atmospheric processes (e.g. photodegradation).These patterns suggest current local PCDD/F emission inBizerte in combination with a minor contribution of long/medium-range atmospheric transport of PCDD/Fs to the area. Contrary, the long/medium range atmospheric transport seems to drive the atmospheric occurrence of PCDD/Fs in Marseille area with a minor contribution of local sources. The possibility of short-range atmospheric transport from the adjacent industrial areas (Fos-Berre) to Marseille with known sources of PCDD/Fs (e.g. from incinerators, metallurgical and petrochemical facilities) is not excluded. Nevertheless, because of the surrounding topography of Marseille, bordered by the Mediterranean from the SW and enclosed from N, E and S by mountain ranges up to ~700m ASL,

Figure 2 .

 2 Figure 2. Average atmospheric pattern (n=11) of PCDD/F (A) and PCBs (B) in

  penta-PBDE compared to the octa-PBDE formulations in Marseille. Second, to a different emission and usage pattern compared to Bizerte, where a major usage of octa-PBDE formulations may be the most likely scenario represented by a higher BDE-183, -206, -207 predominance in Bizerte reaching up to 31.6, 9.1 and 13.8% of the Σ 27 PBDE, respectively.

Figure 3 .

 3 Figure 3. Average atmospheric pattern (n=11) of PBDEs in Marseille and Bizerte

  influence for Bizerte compared to Marseille. ∑ 18 PCB concentrations had minimum level in April 2015 over Marseille, with air masses mostly circulating over the W Mediterranean waters and peaked also in January 2016. The lowest ∑ 18 PCB concentrations were measured in October in Bizerte, whereas the higher levels correspond to April 2015 (with air masses circulating over the Bizerte area and the African continent) consistent with suspected current local PCB emissions. Interestingly, ∑ 27 PBDE exhibited the lowest concentrations in the months of March 2015 over Marseille, with air masses showing an important component coming from the N African coast, while the highest concentrations were measured in the same month in Bizerte, with air masses showing the 50m height component coming from the Gulf of Lion area. Recent reports have shown evidences of the North to South atmospheric

Considering 1 .

 1 6 x10 10 m 2 as the surface of the Gulf of Lion[START_REF] Alekseenko | Seasonal dynamics and stoichiometry of the planktonic community in the NW Mediterranean Sea: a 3D modeling approach[END_REF] and an equivalent surface in the surroundings of Bizerte in the African coast, from 9 to 10 kg of POPs can be annually loaded (median value) to surface waters at both edges of the Mediterranean associated to the atmospheric aerosol. The sensitive environment of Bizerte coastal lagoon (1.28 x10 8 m 2 ) 62 could receive up to 0.5 kg of POPs yearly. This loading of toxic chemicals may have implications for the aquatic organisms but this evaluation cannot be performed with the present data. However, it should be further investigated.

( 6 )

 6 Partzefall, W. Risk assessment of dioxin contamination in human food. Risk assessment of dioxin contamination in human food. Food Chem. Toxicol. 2002, 40, 1185e1189. (7) Opinion of the scientific panel on contaminants in the food chain on a request from the Commission related to the presence of non-dioxin-like PCBs in feed and food. The EFSA Journal (2005) 284, 1 -137 (8) Faroon, O., Jones, D., De Rosa, C. Effects of polychlorinated biphenyls on the nervous system. Toxicol Ind Health. 2000, 16, 305-333. (9) Monosson, E. Reproductive and developmental effects of PCBs in fish: a synthesis of laboratory and field studies. Rev. Toxicol. 1999, 3, 25-75. (10) De Wit, C. An overview of brominated flame retardants in the environment. Chemosphere 2002, 46, 583-624 (11) Vonderheidea, A.P., Mueller, K.E., Meija, M., Welshd, G.L. Polybrominated diphenyl ethers: Causes for concern and knowledge gaps regarding environmental distribution, fate and toxicity. Sci. Total. Environ. 2008, 400, 425-436. hydrocarbons and organochlorines in atmospheric suspended particles from a north African coastal city (Bizerte, Tunisia). J. Hazard. Mater.( Submitted) (45) US-EPA, Method 1613. Tetra-through Octa-Chlorinated Dioxins and Furans by Isotope Dilution HRGC/HRMS; U.S. Environmental Protection Agency: Washington, DC, October 1994 (46) Raimbault P., N. Garcia, Cerrutti, F. Distribution of inorganic and organic nutrients in the South Pacific Ocean. Evidence for long-term accumulation of organic matter in nitrogen-depleted waters. Biogeosciences, 2008, 5, 281-298. (47) Cachier H., Bremond, P-M., Buat-Menard, P. Determination of atmospheric soot carbon with a simple thermal method. Tellus, 1989, 41B, 379-390. (48) Morales, L.; Dachs, J.; González-Gaya, B.; Hernán, G.; Ábalos, M.; Abad E. Background Concentrations of Polychlorinated Dibenzo-p-Dioxins, Dibenzofurans, and Biphenyls in the Global Oceanic Atmosphere. Environ. Sci. Technol. 2014, 48, 10198-10207. (49) Lohmann, R.; Jones, K. C. Dioxins and furans in air and deposition: a review of levels, behaviour and processes. Sci. Total Environ. 1998, 219, 53-81. (50) Brubaker, W.W. JR.; Hites, R. A. Polychlorinated dibenzo-pdioxins and dibenzofurans: gas-phase hydroxyl radical reactions and related atmospheric removal. Environ. Sci. Technol. 1997, 31, 1805-1810.

  (p=0.004-0.03) but not with EC levels in Marseille. Only 2,3,7,4-TCDD positively correlated with TOC concentrations in Bizerte and no correlations were observed for DL PCBs and TOC in Bizerte. Interestingly, PCDD/F concentrations were not correlated with EC levels in any of the sites, while ∑ DL PCBs, ∑ 18 PCB and ∑ ID PCBs were correlated with EC mostly in Bizerte (p=0.002-0.01), most probably due to the higher EC levels in this site. Σ 27 PBDE concentrations did not significantly correlate with TSP, TOC or EC in any of the sites. However, while only two individual PBDEs correlated with TSP levels (Bizerte) a considerable number of PBDE congeners were correlated with TOC concentrations in Marseille and with EC levels in Bizerte (Figure

		100
	Relative distribution (%)	60 70 80 90 4 6 8 10 12 14	MARSEILLE (FR) MARSEILLE (FR) BIZERT (TN) BIZERTE (TN)
		2
		0

Table 1 .

 1 Atmospheric dry deposition fluxes of PCDD/Fs, PCBs and PBDEs in Marseille and Bizerte coastal areas

		Median	Mean	Range	Median	Mean	Range	Median	Mean	Range	Median	Mean	Range
	Σ 7 PCDDs	4.4	12.1	1.5 -71.2	5.8	10.0	1.7 -40.1	60.9	179.7	9.7 -941.0	122.5	421.2	28.2 -1955.8
	Σ 10 PCDFs	1.5	2.5	0.5 -9.6	8.0	9.2	0.3 -29.0	84.0	139.9	16.9 -507.8	214.2	602.9	16.5 -2473.1
	Σ 17 PCD/Fs	5.2	14.6	2.0 -80.8	14	19.2	2.0 -69.2	128.6	319.7	26.5 -1448.8	375.3	1024.1	45.0 -4428.9
	∑ 12DL PCBs	12.1	17.7	8.7 -52.2	16.1	16.9	5.4 -32.2	8.40	10.10	2.0 -21.6	23.7	37.8	4.1 -136.7
	∑ 6ID PCBs	49.0	55.3	20.81-115.2	118.7	124.1	35.9 -216.1
	∑ 18 PCBs	61.1	73.0	30.8 -167.4	134.8	141.0	41.3 -246.2
	∑ 27 PBDEs	547.90	586.90 141.0 -1110.3 405.00 695.80 69.4 -3386.0

ACS Paragon Plus Environment Environmental Science & Technology

Acknowledgements

This work was funded by the Labex OT-Med (no. ANR-11-LABX-0061) -MEDPOP, funded by the French Government "Investissements d'Avenir" (ANR) through the A*MIDEX project (no ANR-11-IDEX-0001-02) and takes place in the MERMEX/MISTRALS program. It also received the financial support from the IRD French-Tunisian International Joint Laboratory (LMI) "COSYS-Med", the MIO-IRD Action Sud project "AEROBIZ" and the PACA region project "Particule". Catherine Guigue is acknowledged for her help on the sample preparation for the TOC/EC determinations. We thank Patrick Raimbault for the TOC analyses, Xavier Mari and Benjamin Guinot for the preparation of samples for EC determinations and Genevieve Deviller (DERAC) for her contribution in the chemical risk assessment section.

Supporting information available

Additional data on the sampling and analytical procedures, QA/QC, compound-bycompound atmospheric levels and spatial distribution (box-plots) and deposition fluxes are presented in this section. This information is available free of charge via the Internet