Laboratory measurements of mass transfer of carbon dioxide and water vapour for smooth and rough flow conditions

Mass transfer rates of carbon dioxide (CO₂) and water (H₂O) have been measured in a 32-m wind-wave tunnel, with a beach in the middle providing 2 sections of 16 m fetch. The wind speed (referred to 10 m) range explored was 1 to 24 m/s, covering a full range of aerodynamic conditions from smooth to fully rough. A pronounced minimum in the Dalton number for both CO₂ and H₂O is revealed. The minimum Dalton number, at wind speeds between 2 and 3 m/s, indicates a characteristic common point in the transfer mechanism for both gas and aqueous phase controlled constituents. This point corresponds to the occurrence of the first wind generated wavelets. At higher winds, the transfer rates of CO₂ increase more rapidly than those of H₂O, reflecting the increased importance of wave-generated turbulence for the ventilation of water phase limited compounds at the air-water interface.