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

Abstract

Abstract Coastal waters are subject to many sources of organic matter, such as land runoff, industrial and urban waste, rivers and atmospheric depositions . Semi-volatile organic compounds (SVOC) are frequently found next to coastal and urban areas. Depending on the specific physical properties of each compound in the SVOC group, these compounds may be adsorbed to particulate matter , evaporate to the atmosphere or accumulate in the sea's uppermost layer - the Sea surface microlayer (SML). Since many of these compounds present health risks, understanding the mechanisms that control the SVOC environmental concentration is crucial, especially in coastal waters. 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 ( 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 (K H). 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.