Evidence that satellites sense the cross-sectional area of suspended particles in shelf seas and estuaries better than their mass

Quantitative algorithms for the remote sensing of suspended sediments in shelf seas and estuaries have traditionally sought to relate the signal sensed by the satellite radiometer to the mass of particles in suspension. In situ measurements presented in this paper, however, show that the irradiance reflection coefficient at the sea surface is more closely related to the cross-sectional area of particles in suspension than to their mass. Other optical properties (which in turbid water depend on particle area) can therefore be related directly to the reflection coefficient. To interpret satellite data in terms of mass concentration (for testing numerical models of suspended sediment transport, for example), it is necessary to know the relationship between the area and mass of the particles. For the 2008-2009 dataset presented here for western UK coastal waters, the particle cross-sectional area varies by a factor of 5 for particles of a given size and mass. The causes of this variation are currently poorly understood.