Laboratory Studies of Scattering Properties of Polluted Cloud Droplets: Implications for FSSP Measurements

Laboratory experiments were conducted in the Mainz vertical wind tunnel to study the effects of pollutants dissolved or suspended in cloud droplets on the droplet size measurements of a Forward Scattering Spectrometer Probe (FSSP). The FSSP is a widely used instrument to derive microphysical properties of atmospheric clouds. Individual droplets of different well-defined sizes were freely falling at their terminal velocities in the wind tunnel while the intensity of radiation emitted by the He-Ne laser of the FSSP and scattered by the droplets was measured. For this purpose, the FSSP was adapted and mounted to the wind tunnel. The intensity of radiation scattered by the droplets in the FSSP measurement is principally used to derive the droplet size. The droplets contained soluble ammonium sulfate or suspended absorbent graphite particles as pollutants in concentrations that were higher than usually found in atmospheric cloud droplets. The results of the measurements and corresponding simulations indicate that for high pollutant concentrations, the scattered laser radiation detected by the FSSP depends significantly on the refractive index of the droplet (i.e., on the concentration of soluble or insoluble pollutants). However, for the lower pollutant concentrations usually observed in atmospheric cloud droplets, the need for correcting the droplet sizes measured with the FSSP for the effects of the pollutions can be avoided.