Air–sea exchange and gas–particle partitioning of polycyclic aromatic hydrocarbons in the Mediterranean

Abstract. Polycyclic aromatic hydrocarbon (PAH) concentration in air of the central and eastern Mediterranean in summer 2010 was 1.45 (0.30–3.25) ng m⁻³ (sum of 25 PAHs), with 8 (1–17)% in the particulate phase, almost exclusively associated with particles < 0.25 μm. The total deposition flux of particulate PAHs was 0.3–0.5 μ g m⁻² yr⁻¹. The diffusive air–sea exchange fluxes of fluoranthene and pyrene were mostly found net-depositional or close to phase equilibrium, while retene was net-volatilisational in a large sea region. Regional fire activity records in combination with box model simulations suggest that seasonal depositional input of retene from biomass burning into the surface waters during summer is followed by an annual reversal of air–sea exchange, while interannual variability is dominated by the variability of the fire season. One-third of primary retene sources to the sea region in the period 2005–2010 returned to the atmosphere as secondary emissions from surface seawaters. It is concluded that future negative emission trends or interannual variability of regional sources may trigger the sea to become a secondary PAH source through reversal of diffusive air–sea exchange.

Capsule: In late summer the seawater surface in the Mediterranean has turned into a temporary secondary source of PAH, obviously related to biomass burning in the region.