The effect of monomolecular surface films on the microwave brightness temperature of the sea surface

Airborne microwave radiometer measurements at 1·43 and 2·65 GHz over a sea surface covered with a monomolecular oleyl alcohol surface film and over adjacent slick sea surfaces are presented. The measurements show that at 2·65 GHz the brightness temperature T B is not affected by the slick, while at 1·43 GHz it drops from 93 K to a minimum value of almost O K. This implies that at 1·43 GHz the emissivity of the slick-covered sea surface is extremely small, similar to a metallic layer, and that this resonant-type phenomenon is confined to a narrow frequency band of width δƒ/ ƒ<0·6. The theoretical implications of these experimental findings are discussed in the framework of the Debye relaxation theory of polar liquids. It is conjectured that a thin layer of water molecules polarized by the surface film gives rise to an anomalous dispersion, which causes the large decrease in brightness temperature at 1·43 GHz. The modulus of the relative dielectric constant ε∗ is estimated to be ≥ 5·2 × 10−4 and the thickness of the emitting layer ≤1·9 × 10−4 m for 1·43 GHz. Furthermore, the film-induced surface activation energy is calculated to be 9·18 × 10−21 J. These values seem reasonable in the light of the theories on the physicochemical structure of surface layers.