Optical, microphysical and compositional properties of the Eyjafjallajökull volcanic ash

Abstract

Abstract. Better characterization of the optical properties of aerosol particles are an essential step to improve atmospheric models and satellite remote sensing, reduce uncertainties in predicting particulate transport, and estimate aerosol forcing and climate change. Even natural aerosols such as mineral dust or particles from volcanic eruptions require better characterization in order to define the background conditions from which anthropogenic perturbations emerge. We present a detailed laboratorial study where the spectral optical properties of the ash from the April-May (2010) Eyjafjallajökull volcanic eruption were derived over a broad spectral range, from ultra-violet (UV) to near-infrared (NIR) wavelengths. Samples of the volcanic ash taken on the ground in the vicinity of the volcano were sieved, re-suspended, and collected on filters to separate particle sizes into fine and mixed (coarse and fine) modes. We derived the spectral mass absorption efficiency α abs [m 2 g −1 ] for fine and mixed modes particles in the wavelength range from 300 to 2500 nm from measurements of optical reflectance. We retrieved the imaginary part of the complex refractive index Im(m) from α abs , using MieLorenz and T-matrix theories and considering the size distribution of particles obtained by scanning electron microscopy (SEM), and the grain density of the volcanic ash measured as ρ = 2.16 ± 0.13 g cm −3 . Im(m) was found to vary from 0.001 to 0.005 in the measured wavelength range. The dependence of the retrieval on the shape considered for the particles were found to be small and within the uncertainties estimated in our calculation. Fine and mixed modes were also analyzed by X-ray fluorescence, exhibiting distinct elemental composition supporting the optical differences we found between the modes. This is a comprehensive and consistent characterization of spectral absorption and imaginary refractive index, density, size, shape and elemental composition of volcanic ash, which will help constrain assumptions of ash particles in models and remote sensing, thereby narrowing uncertainties in representing these particles both for short-term regional forecasts and long-term climate change.

Development and application of a backscatter lidar forward operator for quantitative validation of aerosol dispersion models and future data assimilation

“…The average asymmetry factor was 1.8 for small particles (< 0.5 µm) and 2.0 of larger particles (Schumann et al, 2011). Electron microscope measurements of Rocha-Lima et al (2014) showed that the asymmetry factor of the volcanic ash fine fraction was between 1.2 and 1.8.…”

“…From the findings of Rocha-Lima et al (2014), the average aspect ratio of volcanic ash is known but there is no information about a distribution function of particle shapes and real volcanic ash particles have an infinite variety of particle shapes. Consequently, the spherical shape was used as reference even if the real volcanic ash particles are known to be fractal and complex shaped.…”

A detailed characterization of the Saharan dust collected during the Fennec campaign in 2011: in situ ground-based and laboratory measurements

“…The functional relationship between σ and the reflectance ratio is a power law of the logarithm, where b is an empirical power law coefficient determined to be 1.218 and G is a geometrical factor determined to be equal 1 for a large range of geometries of the setup (angle between light incidence and detection), including the one used in this work. This method was previously derived and calibrated by Martins et al (2009) using Monarch 71 black carbon particles manufactured by the Cabot Corporation, it was compared and showed good agreement against other absorption techniques by Reid et al (1998) and applied to volcanic ash samples (Rocha-Lima et al, 2014). This technique is based on the assumptions that the reflectance of the filter decreases as particles are loaded on it and that the reduction of the light reflected by the filter is due to absorption only by the aerosol particles.…”

“…In situ measurements of Saharan dust were complemented with laboratory analyses for the characterization of their optical properties using the methods presented in Martins et al (2009), Rocha-Lima et al (2014), and Rocha-Lima (2015. Size and aspect ratio distributions of the dust particles were obtained by scanning electron microscopy (SEM).…”

A Backscatter Lidar Forward Operator for Particle-Representing Atmospheric Chemistry Models

“…The average asymmetry factor was 1.8 for small particles (<0.5 µm) and 2.0 of larger particles (Schumann et al, 2010). Electron microscope measurements of Rocha-Lima et al (2014) showed that the asymmetry factor of the volcanic ash fine fraction has a value between 1.2 and 1.8.…”