Abstract
Abstract. 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), R 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 R value is 8–21 times higher for water than for the "dust" particles.
Highlights
Atmospheric aerosol particles absorb and scatter solar radiation and, have an effect on the Earth’s radiative budget and influence our climate
The dust particle measurements was conducted at a flow rate of 20 dm3 min−1, which corresponds to an average air exit velocity of ∼ 30 m s−1 and ∼ 50 μs average peak width
We use the visible-to-IR scattered light intensity ratio (R value) as an indicator to decide if the detected particle is a water droplet or not
Summary
Atmospheric aerosol particles absorb and scatter solar radiation and, have an effect on the Earth’s radiative budget and influence our climate. The intensity of the scattered light in a certain direction depends on the particle’s size, refractive index, shape, scattering angle and the wavelength of the incident light. The scattering behaviour of a homogenous, spherical particle can be described by Mie theory (Bohren and Huffman, 2004). It requires the wavelength of the incident light, the scattering angle, the refractive index of particle relative to the carrier medium (in our case it is air) and the particle’s diameter as input, and it provides an exact solution of the differential scattering cross section. The differential cross section as a function of the particle diameter shows a distinct pattern with a rapidly fluctuating structure called Mie oscillations in the range where the particle diameter is comparable to the wavelength of the incident light
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