Organic molecular markers in marine aerosols over the Western Mediterranean Sea

A scientific campaign was undertaken along the Western sector of the Mediterranean Sea in the summer 2015 (26th Jun to 13th Jul), with the goal of gathering information about organic contaminants affecting marine aerosol over the Italian seas and with a special focus on changes in composition due to sources. 24 PM₁₀ atmospheric samples in total were chemically characterized, including polycyclic aromatic hydrocarbons (PAHs), aliphatic hydrocarbons (n-alkanes) and phthalate esters. Contemporarily, regulated gaseous toxicants (i.e. ozone, nitrogen oxides and sulfur dioxide) and meteorological parameters were recorded. Samplings were carried out inshore in front of harbors (N = 7) and along the cruise, both during the vessel shipping (N = 11, transects) and at its stops offshore (N = 6). Total PAH concentrations ranged from 0.03 to 1.94 ng/m³ and raised close to harbors and coastal sites, confirming that continental sources were responsible for the strong increase of pollution levels there compared to offshore. The percent composition and diagnostic ratio rates of PAHs were different for harbors, while transects were in agreement with offshore stops, possibly due to the different impact of pollution sources. n-Alkanes (C₂₁C₃₈) and the corresponding carbon preference index rates (CPI) were assessed; their values ranged 8.7-90 ng/m³ and 1.1-2.9 respectively, which suggested that fossil fuel combustion was the dominant source, though biogenic emission could contribute. Alkyl phthalates revealed wide variability in concentrations among aerosol samples. Moreover, long-range atmospheric transport and particle ageing effect induced by photo-oxidants were important factors controlling the composition of organic aerosols in the Mediterranean Sea air.