On the application of scattering matrix measurements to detection and identification of major types of airborne aerosol particles: Volcanic ash, desert dust and pollen. (September 2021)

• Record Type: Journal Article
• Authors: Gómez Martín, Juan Carlos; Guirado, Daniel; Frattin, Elisa; Bermudez-Edo, Maria; Cariñanos Gonzalez, Paloma; Olmo Reyes, Francisco José; Nousiainen, Timo; Gutiérrez, Pedro J.; Moreno, Fernando; Muñoz, Olga
• Is Part Of: Journal of quantitative spectroscopy & radiative transfer. Volume 271(2021)
• Publication Date: 2021-09
• Subjects: Spectrum analysis -- Periodicals; Radiation -- Periodicals; Analyse spectrale -- Périodiques; Rayonnement -- Périodiques; Radiation; Spectrum analysis; Periodicals; 543.0858
• DOI: 10.1016/j.jqsrt.2021.107761
• Languages: English
• ISSNs: 0022-4073

Abstract

Highlights: Measurements of the scattering matrix at 488 nm and 633 nm of two airborne desert dust samples collected in Spain and China, and at 488 nm of a volcanic ash sample from the 2010 Eyjafjallajökull eruption are presented. The complete scattering matrix at 514 nm of a pollen sample (cypress) is also reported for the first time. Volcanic ash samples collected around the world are remarkably similar in terms of size, morphology and optical constants, which results in very similar scattering matrix elements and justifies calculating an average scattering matrix of volcanic ash. Similarly, an average scattering matrix of desert dust has been constructed. Measurements of the backscattering depolarization ratio do not enable identifying these major types of aerosols. Morphological differences at the sub-micron scale between irregular mineral aerosol particles are mainly registered in the scattering angle dependence of the F 22 / F 11 and F 44 / F 11 elements. Cypress pollen, with a very narrow size distribution, can be distinguished from mineral particles by a characteristic angular dependence of the degree of linear polarization. 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 δ L (180°) are used to obtain information about non-sphericity and discrimination between fine and coarse aerosol. However, the average δ L (180°) for the three types of aerosols considered in this work in the visible spectral range is δ L (180°) = 0.40 ± 0.05. This shows that δ L (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.