Spectral light extinction to characterize fast fog formation

Two supplementary methods for time-dependent droplet sizing, both based on the spectral dependence of light extinction, are applied to an adiabatically expanding vapor in which droplets are formed as a result of heterogeneous condensation. First, by measuring the extinction coefficients at three different wavelengths, we obtain time-dependent values of the modal radius, the size variance, and the droplet number density, with a typical time resolution of 1 µs. The shape of the size-distribution function is investigated by a second method. Using a white-light source in combination with a spectrometer and a CCD array, we obtain the full visible light attenuation spectrum with a time resolution of 1.5 ms. By applying an inversion technique based on trial size distributions, we find that the zeroth-order log-normal distribution describes the fog adequately. Both methods yield the same droplet growth curves and droplet number densities.