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
<p>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...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-09-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/13429/2018/acp-18-13429-2018.pdf |
| Summary: | <p>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<sup>−2</sup> d<sup>−1</sup> (range of 0.5–47 mg m<sup>−2</sup> d<sup>−1</sup>), of
which 0.67 mg m<sup>−2</sup> d<sup>−1</sup> was iron and 0.001 mg m<sup>−2</sup> d<sup>−1</sup>
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<sup>−1</sup> (geometric standard deviation of 3.1 over all
individual particles) was observed.</p> |
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| ISSN: | 1680-7316 1680-7324 |