Colors of contrails from fuels with different sulfur contents

During a recent flight experiment, contrails were produced from fuels with normal and very high sulfur content. A color effect was observed [Schumann er al., 1996]. When observed with the Sun in the back, the low-sulfur contrail appeared grey with weak contrast against the cloud deck underneath, whereas the high-sulfur contrail appeared slightly brown with higher contrast. In order to give an explanation, we compute the optical properties of the soot-containing droplets in the contrails and perform a simple radiative transfer computation. We find that the degree of activation of the soot particles is the basic parameter for the explanation of color and contrast of the contrails. It determines the size and number concentration of the droplets, which both in turn determine the optical properties of the contrail as a whole. For the visual appearance of the relatively optically thin contrails (tau less than or similar to 1.2) the soot volume fraction in the droplets plays only a minor role.