## 3 Results and Discussion
### Chemical Characterization of Seawater  
PAHs detected in seawater (both in summer and winter samples) are summarized in Figure 1.  
It is interesting to observe that PAHs with a higher number of aromatic rings (i.e., BbFl, BaP, DBA, BP and Ind) were not detected in seawater, regardless of the season, in accordance with their higher hydrophobicity behavior (and hence lower solubility in water).  
In addition, an inter-seasonal variability of PAH concentrations (among summer and winter sampling campaigns) was also clearly present for all the detected analytes. Indeed, a higher number of congeners was detected in summer (8), rather than in winter (2), with a total PAH concentration about three times higher in summer than winter (2.2 ug/L and 0.8 ug/L, respectively).  
To explain these differences, both climatological and anthropogenic effects should be considered. To elucidate, one of the main aspects to consider is the seasonal variation of thermocline. Thermocline refers to the water layer below the surface, characterized by a massive temperature gradient which decreases up to 4 \({}^{\circ}\)C (or lower for deep waters). During summertime, surface waters increase in temperature while the deeper waters remain cold. Stratification causes rapid vertical changes in the density of water, which largely prevents the mixing of the water column [36]. This phenomenon could affect the sinking and the accumulation of micropollutants [37]. Conversely, during winter, temperatures of surface water can undergo those of deeper layers, which tends to rise due to its lower density (homothermic conditions). Consequently, the movement of water masses promotes a mixing and, therefore, a diffusion of dissolved compounds, such as pollutants, thus justifying their lower concentrations.  
In addition to the thermocline effect, the higher concentration of PAHs in the summer season could be addressed to the effects of the Northern current, the northern part of the cyclonic surface circulation of the North-Western Mediterranean Sea. Indeed, the Northern current exhibits its weakest intensities during summertime [38], thus promoting a possible accumulation of land and air derived pollutants, while during winter its higher intensities boosts their dispersion into the sea waters.  
Figure 1: Concentration of PAHs detected in seawater samples, both in summer (orange) and winter (green) sampling. Analytical conditions are detailed in the Experimental section.  
Finally, the intense touristic activities that characterize the Ligurian Sea during summertime (with private boat traffic, cruises, etc.) with respect to winter, could act as a possible source of PAHs, which can be released by thermic engines. Evidence of the effects of maritime traffic on the increment of PAH contamination were also investigated elsewhere [39]. In addition, the origin of PAHs (evaluated by studying the ration between congeners [40]) confirmed this hypothesis. In fact, both the ratio between the concentration of low (NaPh, AcPh, AcPY, Flu) and high molecular weight (Phe, Ant, FTh, BkFl) PAHs (0.92) and the ratio between phenanthrene and anthracene (0.30) are lower than 1, thus suggesting a dominant pyrogenic origin of the detected PAHs (i.e., transport activities, use of petroleum derived products, accumulation of small emissions of gasoline, engine oils, incomplete combustion of organic substances) [41,42].  
It should be noted that PAH concentrations detected in sea water within this study were in the same level of other Mediterranean areas [43], with a pollution classification from moderate to high, according to the indications proposed by [PERSON] and colleagues [44]. Concerning regulation limits, benzo[k]fluoranthene was the only congener detected among the compounds included in the regulated list, with an average concentration of 45 ng/L, which exceeds the maximum allowable concentrations (17 ng/L) [22].  
Concerning PCBs, only congeners 15 and 138 were detected, with concentrations below the quantitation limits.