Giant Particles and Magnetic Minerals in Atmospheric Dust

Introduction

Atmospheric dust has large-scale effects on planetary radiation, global climate, and biogeochemical cycles, and is therefore a critical component of the Earth’s climate. However, the dynamics and impact of the dust is poorly understood (e.g., [1]). Particularly two strong absorbers of solar energy, magnetic minerals and giant particles, have been neglected in aerosol and climate models (e.g., [2][3]). The effects of magnetic (nano)particles can be comparable to black and brown carbon, they promote ice nucleation and play a role in cloud formation (e.g., [4]). It has been recently discovered that strong winds are able to carry even the giant particles (≥ 100 μm) long distances, from Sahara to Iceland [5]. Despite their global importance, both the magnetic nanoparticles and the giant particles remain poorly described. The optical and light scattering properties and the exact mechanism by which these particles initiate ice nucleation are not yet understood.

Our project combines experimental and theoretical approaches to enhance our understanding of giant particles and magnetic minerals in atmospheric dust, utilising methods from both geosciences and physics. Ultimately, our work aims to contribute to characterising the particles and their source areas, long-range transport, and scattering effects, to be utilised in emission, transport, and deposition modelling, and in climate models.

Dust samples

The research material consists of Saharan dust that was deposited in Finland and collected as citizen science samples by the Finnish Meteorological Institute during 2021. The citizen science initiative yielded samples from 525 locations, with one or more samples collected from each site. The first results regarding some of the dust samples were published in 2023 [6]. The multidisciplinary study showed that the Saharan dust deposited in Finland originated from the Sahel desert (south of Sahara), based on the magnetic properties of the particles, and the System for Integrated modeLling of Atmospheric coMposition (SILAM) model (silam.fmi.fi). These results are an encouraging starting point for a more detailed analysis of the remaining > 500 samples.

Methods

This study begins by using mineralogical, geochemical, and magnetic methods to identify and characterise the particles in the Saharan dust samples. The particle grain-size and shape distribution are fundamental for understanding and predicting atmospheric residence times, optical properties, transport, and settling processes. Laser grain-size analyser and both dynamic and static image analyses will be used to determine the grain-sizedistributions and the particle shape (incl. sphericity, roundness, and aspect ratio). Bulk petrography and heavy mineral analysis provides the framework for the geological classification and yields information on the source, and both sedimentary and the pre-aeolian transport processes.

Magnetic mineral characterisation is fundamental for source discrimination and for understanding both atmospheric optical and cloud formation properties. Magnetic methods are non-destructive and powerful in characterising the type, particle size, and quantity of magnetic materials. Measurements will be carried out in order of increasing magnetic field: initial susceptibility with two frequencies, NRM demagnetisation, anhysteretic remanence, and isothermal remanence.

The research then focuses on the scattering and absorption of light by these particles, both experimentally and theoretically. The scattering matrix measurements will be conducted to analyse the physical and chemical characteristics, such as shape and composition, of the particles. The experimentally obtained information will then be used for developing the theoretical modelling of the particles, using numerical methods [7][8][9][10]. This is the first time when the scattering studies will culminate in an analysis of radiative effects of both the giant and magnetic particles in the Earth’s atmosphere.

For bulk material optical properties, the modular integrating-sphere spectrometer will be used to determine the reflection and absorption spectra of particles in the ultraviolet-visual-near-infrared wavelength range. The spectral directional scattering from a particle layer will be measured with a goniometer. For light scattering properties, the 4 × 4 scattering matrix of a dust particle relates the Stokes parameters (intensity with linear and circular polarisation) of the incident light to the Stokes parameters of the scattered light. In Helsinki, the unique acoustic levitator facility allows for measurements of the upper left-hand 2 × 2 block of the scattering matrix for a large particle in controlled position and orientation. The aim of the measurements is to develop profound shape, structure, and compositional models for these particles. The scattering matrix and bulk optical property measurements will be matched with particle models of varying sophistication and a study follows on the implications of the particle absorption and scattering properties in atmospheric radiative energy transfer.

References

[1] B.A. Maher et al. Earth Science Reviews, 99(1-2):61–97, 2010.
[2] A.A. Adebiyi and J.F. Kok. Science Advances, 6(15), 2020.
[3] N. Moteki et al. Nature Communications, 8(15329), 2017.
[4] B.A. Maher. Aeolian Research, 3(2):87–144, 2011.
[5] G. Varga et al. Scientific Reports, 11(11891), 2021.
[6] O. Meinander et al. Scientific Reports, 13(21379), 2023.
[7] K. Muinonen et al. Journal of Quantitative Spectroscopy & Radiative Transfer, 55(5), 1996.
[8] K. Muinonen et al. Journal of Quantitative Spectroscopy & Radiative Transfer, 110:1628–1639, 2009.
[9] H. Lindqvist et al. Journal of Quantitative Spectroscopy & Radiative Transfer, 217:329–337, 2018.
[10] T. Väisänen et al. Journal of Quantitative Spectroscopy & Radiative Transfer, 241, 2020.