The role of the sea-surface microlayer in the air–sea gas exchange of organochlorine compounds

Simultaneous measurements of organochlorine compounds (OCs) in seawater, the sea-surface microlayer and the atmosphere were conducted in June–July 2004 in the coastal marine environment of Singapore. Together, these measurements represent the first data on the flux of OCs between the ocean and atmosphere reported in the scientific literature that take into account the implication of the sea surface microlayer (SML) as a controlling boundary layer for the exchange of OCs. The average fluxes of ΣPCBs and ΣHCHs were 127.5 and −32.8 ng m−2 day−1 respectively using a modified two-layer model (negative flux indicates adsorption by the ocean). The average fluxes using a conventional approach, ignoring the SML as boundary layer (classical twolayer model), were 67.2 and −43.1 ng m−2 day−1 for ΣPCBs and ΣHCHs, respectively. However, the maximum difference in the flux calculation between the two approaches was up to 15-fold for individual compounds at high enrichment in the SML. It is shown that the SML plays an important role in the control of air–sea gas exchange of OCs, particular under a low prevailing wind regime and with an enrichment of OCs in the SML. The physical and chemical properties of OCs are critical factors in the control of the air–sea gas exchange process, and the effect of the SML on this process is more significant for more hydrophobic OCs.