Monocyclic aromatic hydrocarbons (phthalates and BTEX) and aliphatic components in the SE Mediterranean costal Sea-surface microlayer (SML): Origins and phase distribution analysis

This study assesses for the first time, the origin of monocyclic aromatic hydrocarbons (MAH) and aliphatic SVOC, as well as their distribution pattern in the SML and the sea sub surface layer (SSL) of the Haifa (Israel) coastal waters during 2015. Seasonal analysis of the SML and SSL showed high prevalence of several SVOC members, including BTEX, phthalates, trimethyl-benzene and monoterpenes. Concentrations of the BTEX group were elevated in the SSL particulate (>0.2 μm) and the SML free fractions (<0.2 μm), while only minor to nil levels were found in the SSL free fraction. Total phthalates were recorded in all sea layers, yet were found in higher concentrations in the SML's particulate and free fractions. The distribution of these molecules was linked to the presence of SML polysaccharides that showed elevated concentrations in the rainy season. Phthalates are rapidly transported to the SML in this area and have the potential of either aggregating or dissolving in the water column. We conclude that when examining the dominant MAH distribution, low-molecular weight phthalates enrichment takes place in the SML despite their higher solubility compared to the BTEX group. BTEX molecules have potentially elevated emission rates from SML particulate matter, with low adsorption to particulate matter and very low water solubility in the SSL. These observations may be linked to the low affinity of BTEX molecules to local particulate matter and to the low Henry coefficient (KH). Molecules like low-molecular weight phthalates (higher KH than BTEX), can be found in all water layers, but tend to concentrate in the SML and may also become airborne (though to a lower extent in the gas phase) via aerosol forming processes. Finally, while atmospheric depositions were found to be the major source of BTEX components in 2015, phthalates originated from urban runoff, the Kishon River as well as atmospheric depositions. These results may explain the long term presence of phthalates in aqueous form and the chronic BTEX evaporation in this coastal area.