Air–sea exchange of semivolatile organic compounds — Wind and/or sea surface temperature control of volatilisation studied using a coupled general circulation model

Volatilisation from the ocean surface plays an important role in the large-scale cycling of semivolatile organic compounds. Air–sea exchange is thought to be controlled by wind speed, temperature, relative contaminant levels in air and water and the extent of association of semivolatile organic substances (SOCs) with particulate matter in the atmosphere and water column. The nonlinear relationship between sea surface temperature (SST), wind speed and volatilisation rate suggests that the influence of these parameters on the volatilisation rate will vary for different temperature and wind speed regimes, hence in different climatic zones. The global multicompartment chemistry-transport model MPI-MCTM is used to determine the sensitivity of volatilisation of dichlorodiphenyltrichlorethan (DDT) and dichlorodiphenyldichloroethylene (DDE) to SST and wind speed changes. Parameters controlling the volatilisation of DDT from the ocean surface, none of them independent, and their degree of control are identified and mapped by a correlation analysis on the seasonal time scale and by an empirical orthogonal functions (EOF) analysis on the inter-annual timescale. Seasonal variations of volatilisation are shown to be controlled by either wind speed or SST in different sea regions. On this time scale wind is more dominant than SST in the global ocean. The main pattern of inter-annual variability (up to decades), however, is shown to be explained by SST. The results suggest that large-scale spatial averaging, including zonally averaging leads to underestimates of the volatilisation rate and the long-range transport potential of SOCs.

Research Highlights
► Volatilisation of SOCs from the ocean is studied with a global GCM.
► The impact of zonal averaging of SST and wind speed is assessed.
► On the seasonal time-scale wind speed and SST control regimes are mapped.
► On interannual time-scales sensitivity of volatilisation is studied by an EOF analysis.