Electron microscopy of particles collected at Praia, Cape Verde, during the Saharan Mineral Dust Experiment: particle chemistry, shape, mixing state and complex refractive index

Abstract

A B S T R A C TA large field experiment of the Saharan Mineral Dust Experiment (SAMUM) was performed in Praia, Cape Verde, in January and February 2008. The aerosol at Praia is a superposition of mineral dust, sea-salt, sulphates and soot. Particles smaller than 500 nm are mainly mineral dust, mineral dust-sulphate mixtures, sulphates and soot-sulphate mixtures. Particles larger then 2.5 µm consist of mineral dust, sea-salt and few mineral dust-sulphate mixtures. A transition range exists in between. The major internal mixtures are mineral dust-sulphate and soot-sulphate. Mineral dust-sea-salt mixtures occur occasionally, mineral dust-soot mixtures were not observed. The aspect ratio was 1.3-1.4 for dry particles smaller than 500 nm and 1.6-1.7 for larger ones. Parameterizations are given for dry and humid state. Although the real part of the refractive index showed low variation (1.55-1.58 at 532 nm), a multi-modal imaginary part was detected as function of particle size, reflecting the complex composition. Soot mainly influences the absorption for wavelengths longer than the haematite absorption edge, whereas for shorter wavelengths dust is dominating. The refractive index of the aerosol depends on the source region of the mineral dust and on the presence/absence of a marine component.

Properties of cloud condensation nuclei (CCN) in the trade wind marine boundary layer of the western North Atlantic

“…5I.b, only an indication of a thin layer remains on the spot where the particle used to be. Similar types of particles have also been observed on Cape Verde with the same technique, and the remaining thin layer was suggested to be comprised of refractory organic compounds (Kandler et al, 2011). The remaining thin layer is often observed in the present study.…”

“…However, the temperature conditions can be considered to be similar for all measurements, as identical instrument settings were used. From a chemical analysis it becomes obvious that nitrate and sulfate evaporated, while sodium chloride, all mineral dust components and soot remained stable (Kandler et al, 2011). From the decomposition behaviour and the absence of sodium chloride melting, we concluded that the temperature is between 200 and 800 • C, most likely higher than 300 • C (Kiyoura and Urano, 1970;Lide, 2009).…”

Long-term observations of cloud condensation nuclei in the Amazon rain forest – Part 2: Variability and characteristic differences under near-pristine, biomass burning, and long-range transport conditions

“…The system is able to record characteristic X-ray emissions for all elements with Z > 5. Obviously, in the present work Si could not be quantified due to the silicon substrate (Kandler et al, 2011).…”

Online differentiation of mineral phase in aerosol particles by ion formation mechanism using a LAAP-TOF single-particle mass spectrometer

“…Field studies show that most dust in the atmosphere is dominated by varying quantities of quartz, K-feldspar, plagioclase, calcite, hematite, kaolinite and the illite-smectite group of clay minerals (Kaufman et al, 2005;Kandler et al, 2007Kandler et al, , 2009Kandler et al, , 2011Formenti et al, 2008;Jeong, 2008;Kaufmann et al, 2016). At a single-particle level, particle size dependence is observed with an abundance of quartz and feldspar grains in the coarse fraction and a fine fraction (< 2 µm) dominated by clay minerals.…”