Abstract
Atmospheric aerosols play key roles in climate and have important impacts on human activities and health. Hence, much effort is directed towards developing methods of improved detection and discrimination of different types of aerosols. Among these, light scattering-based detection of aerosol offers several advantages including applications in both in situ and remote sensing devices. In this work, new scattering matrix measurements for two samples of airborne desert dust collected in Spain and China are reported. The average extrapolated scattering matrices of airborne desert dust and of volcanic ash at two wavelengths have been calculated and compared with the aim of finding criteria to distinguish these two types of aerosol. Additionally, the scattering matrix of cypress pollen has been measured and extrapolated to explore differences with mineral dust that can be exploited in atmospheric detection. Field measurements of the backscattering linear depolarization ratio δL(180°) are used to obtain information about non-sphericity and discrimination between fine and coarse aerosol. However, the average δL(180°) for the three types of aerosols considered in this work in the visible spectral range is δL(180°) = 0.40 ± 0.05. This shows that δL(180°) is not informative about the composition or morphology of irregular particles. By contrast, measurements of scattering matrix elements or depolarization ratios at different scattering angles may provide information about the structural differences of particles, and in particular may enable to differentiate airborne volcanic ash from desert dust, which are otherwise similar in terms of size and optical constants. Cypress pollen shows a characteristic degree of linear polarization curve that is very different from that of polydisperse irregular mineral dust. Light scattering field instruments and remote sensing methods could extract more information about the characteristics of aerosol particles if modifications were introduced to measure the phase curves of several scattering matrix elements or depolarization ratios.
Highlights
Mineral particles constitute a large fraction of primary atmospheric aerosol and they are known to affect climate directly by kilometers away from their origin
We have reviewed scattering matrix measurements of volcanic ash samples compiled at the GranadaAmsterdam Light Scattering Database [60] and we have carried out new measurements of the scattering matrix of long-range airborne desert dust samples and a common pollen sample in the Cosmic Dust Laboratory (CODULAB) at IAA-CSIC [59,62]
Average scattering matrices of volcanic ash and desert dust have been constructed from existing and new measurements carried out with particle samples collected in the field and representative of natural airborne aerosols
Summary
Mineral particles constitute a large fraction of primary atmospheric aerosol and they are known to affect climate directly by kilometers away from their origin. The total emissions of volcanic ash into the troposphere by smaller volcanic eruptions (VEI ≤ 4), which are the most frequent, are estimated to be 20 Tg yr−1 [7], i.e. two orders of magnitude lower than the source strength of soil mineral dust. Another important aspect of the presence of volcanic ash clouds in the mid- and upper troposphere is the danger that they pose to aviation [8]
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