Effect of the accumulation of polycyclic aromatic hydrocarbons in the sea surface microlayer on their coastal air–sea exchanges

Several measurements of polycyclic aromatic hydrocarbons (PAHs) in coastal marine compartments (viz. atmosphere, sea surface microlayer, subsurface seawater, sinking particles and sediments), made nearly simultaneously at two stations in the north-eastern Mediterranean, were used to estimate the transport fluxes of individual and total PAHs through the air–seawater-sediment system. Diffusive air–sea exchange fluxes were estimated using both subsurface water (SSW) and sea surface microlayer (SML) concentrations. The air–SML fluxes ranged from 411 to 12,292 ng m− 2 d− 1 (absorption) and from − 506 to –13,746 ng m− 2 d− 1 (volatilisation) for total PAHs (Σ15). Air–seawater column transport of particle-associated PAHs was estimated from the analysis of particulate atmospheric and sediment interceptor trap materials. Air–sea particle deposition fluxes of total PAHs ranged from 13 to 114 ng m− 2 d− 1 and seawater particle settling fluxes (upper 5 m water column) ranged from 184 to 323 ng m− 2 d− 1.The results of this study indicate that both the magnitude and the direction of the calculated air–sea diffusive fluxes change when PAH concentrations in the SML are considered. As a result, PAHs accumulation in the SML could produce the so-called “flux capping effect”. However, the high variability in the coastal air–sea PAHs flux estimations, mainly due to the parameters uncertainty, requires further experimental approaches, including improvement of parameterisations.