By numerical modeling, the variability of the upward emerging spectral radiance is investigated due to the variability of the optically acting atmospheric constituents (no clouds) and due to the state of the earth's surface, in particular of wind-ruffled oceans. With the objective of determining the atmospheric turbidity (aerosol optical depth) from radiances at only one wavelength, those geometrical configurations between incoming and outgoing radiation are probed, where the variability of the radiance due to the variability of the perturbing optical parameters is minimal. This has been done for maritime aerosols and Sahara dust. It is found that with these aerosol types it is possible to determine the turbidity with an accuracy of better than ¿25% within cloud-free areas over aceans, if the limited measurement accuracy of the radiometers is not taken into account.