Composition and mixing state of atmospheric aerosols determined by electron microscopy: method development and application to aged Saharan dust deposition in the Caribbean boundary layer

Abstract

Abstract. The microphysical properties, composition and mixing state of mineral dust, sea salt and secondary compounds were measured by active and passive aerosol sampling, followed by electron microscopy and X-ray fluorescence in the Caribbean marine boundary layer. Measurements were carried out at Ragged Point, Barbados during June–July 2013 and August 2016. Techniques are presented and evaluated, which allow for statements on atmospheric aerosol concentrations and aerosol mixing state based on collected samples. It became obvious that in the diameter range with the highest dust deposition the deposition velocity models disagree by more than 2 orders of magnitude. Aerosol at Ragged Point was dominated by dust, sea salt and soluble sulfates in varying proportions. The contribution of sea salt was dependent on local wind speed. Sulfate concentrations were linked to long-range transport from Africa and Europe, and South America and the southern Atlantic Ocean. Dust sources were located in western Africa. The dust silicate composition was not significantly varied. Pure feldspar grains were 3 % of the silicate particles, of which about a third were K-feldspar. The average dust deposition observed was 10 mg m−2 d−1 (range of 0.5–47 mg m−2 d−1), of which 0.67 mg m−2 d−1 was iron and 0.001 mg m−2 d−1 phosphorus. Iron deposition was mainly driven by silicate particles from Africa. Dust particles were mixed internally to a minor fraction (10 %), mostly with sea salt and less frequently with sulfate. It was estimated that the average dust deposition velocity under ambient conditions is increased by the internal mixture by 30 %–140 % for particles between 1 and 10 µm dust aerodynamic diameter, with approximately 35 % at the mass median diameter of deposition (7.0 µm). For this size, an effective deposition velocity of 6.4 mm s−1 (geometric standard deviation of 3.1 over all individual particles) was observed.

Sun photometer retrievals of Saharan dust properties over Barbados during SALTRACE

“…For instance, the mean real part of the refractive index (440 nm) during SALTRACE retrieved with sun photometer data is 1.47 for mineral dust (Table 2), which is much lower than the value reported from in situ observations, about 1.55-1.61 . The problems encountered to retrieve the correct dust refractive index using a spheroid model were also highlighted by Kemppinen et al (2015). Conversely, the single-scattering albedo and its spectral dependence (Table 2) are very similar for dust using sun photometer and in situ optical instruments.…”

Differences and Similarities of Central Asian, African, and Arctic Dust Composition from a Single Particle Perspective

“…Feldspar and K-feldspar indices are defined (Appendix A), showing the similarity of the measured particle composition to a feldspar composition. As it was found that index values >0.8 identify a feldspar particle with high probability [42], this threshold value is used here to assess the contribution of feldspar particles to the total deposition. Note that for example a layer of clay minerals covering a feldspar grain might therefore prohibit the classification as (pure) feldspar.…”

“…However, these are usually trace to minor species [31,53]. As an estimate for Ca atmospheric processing, single particle apparent relative ion balances can be defined [42]. Here, we use the Ca-Mg-S relative ion balance, defined as…”

“…Particles were analyzed in the size range of 0.7-50 µm projected area diameter. Details on the technique can be found elsewhere [42] and references therein. For the present study, only particles with dust compositions ("dust component") were regarded.…”

“…Based on the single particle composition quantification, an element index for the element X was introduced [36,42] as…”