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A new method is presented in this paper which analyses the scattered  light of individual aerosol particles simultaneously at two  different wavelengths in order to retrieve information on the  particle type. We show that dust-like particles, such as volcanic  ash, can be unambiguously discriminated from water droplets on  a single-particle level. As a future application of this method, the  detection of volcanic ash particles should be possible in a humid  atmosphere in the presence of cloud droplets. The characteristic behaviour of pure water's refractive index  can be used to separate water droplets and dust-like particles which  are commonly found in the micrometre size range in the ambient  air. The low real part of the water's refractive index around  2700–2800 nm results in low scattered light intensities  compared to e.g. the visible wavelength range, and this feature can  be used for the desired particle identification.    The two-wavelength measurement set-up was theoretically and  experimentally tested and studied. Theoretical calculations were  done using Mie theory. Comparing the ratio of the scattered light at  the two wavelengths (visible-to-IR (infrared), <i>R</i> value) for water droplets and different dust  types (basalt, andesite, African mineral dust, sand, volcanic ash,  pumice) showed at least 9-times-higher values (on average 70 times)  for water droplets than for the dust types at any diameter within  the particle size range of 2–20  μm. The envisaged  measurement set-up was built up into a laboratory prototype and was  tested with different types of aerosols. We generated aerosols from  the following powders, simulating dust-like particles: cement dust,  ISO 12103-1 A1 Ultrafine Test Dust and ash from the 2012 eruption of  the Etna volcano. Our measurements verified the theoretical  considerations; the median experimental <i>R</i> value is 8–21 times higher  for water than for the "dust" particles

Dual-wavelength light-scattering technique for selective detection of volcanic ash particles in the presence of water droplets