By Claire Ryder
Mineral dust aerosol particles are regularly lifted into the atmosphere in arid regions, such as deserts, and transported over thousands of kilometres by the wind, such as from the Sahara desert to the Caribbean Sea, as shown in the satellite image in Figure 1.
Figure 1: Dust crosses the Atlantic from the Sahara desert on 23 July 2005 captured by the SeaWIFS satellite instrument. credit: NASA
Recent fieldwork forming part of the Fennec project (Washington et al., 2012; Ryder et al., 2015), where aircraft observations were made of in situ Saharan dust properties over the remote desert (Ryder et al., 2013b) have shown that close to dust sources concentrations are very high, as expected, but that unexpectedly, the concentrations of coarse particles (larger than 1 micron diameter) and giant particles (larger than 10 microns diameter) are also higher than expected (see Figure 2), since giant particles would be expected to fall back to the surface within hours due to their larger weight. However, Ryder et al. (2013b) measured particles sized up to 100 microns up to altitudes of 5 km, indicating that they are not so readily deposited from the atmosphere, and giving them potential to travel large distances.
Figure 2: Dust volume size distributions from various aircraft fieldwork campaigns. Fennec, over remote desert, is shown in black. (Taken from Ryder et al., 2013b).
So why does it matter that coarse and giant particles are present to a such a great extent?
Firstly, dust interacts with both solar and terrestrial radiation, perturbing the atmospheric radiation balance. In the solar spectrum, dust causes cooling at the surface by reflecting and absorbing radiation, warming in the atmospheric column, and either cooling or warming at the top of atmosphere (TOA) depending on the brightness of the surface type beneath and the properties of the dust. In the terrestrial spectrum, dust absorbs outgoing longwave radiation and causes a warming at the surface, a cooling in the atmosphere, and a warming at the TOA. The balance of these processes can impact surface temperatures, sensible and latent heat fluxes, regional atmospheric circulation and precipitation, and lead to a small net cooling or warming of the climate system at the TOA.
Dust size has a major impact on this radiative effect, as illustrated by Figure 3, where we see that for larger particles with an effective radius (re) of 9 microns, at solar wavelengths (e.g. 0.5 microns) the single scattering albedo (controlling the amount of absorption exerted by dust), drops substantially, while at terrestrial wavelengths (e.g. 12 microns) the extinction efficiency increases substantially for larger particles. Thus knowing the sizes of dust particles in the atmosphere is crucial in determining how they will alter the radiative balance of the atmosphere.
Figure 3 – Impacts of dust size on optical properties, from Tegen & Lacis (1996). Left panel: single scattering albedo (controlling how absorbing dust is – low values indicate high absorption). Right panel: extinction efficiency (controlling how much radiation is extinguished by dust).
Another important aspect of dust is its impact on the biosphere, by depositing nutrients to ocean and rainforest ecosystems (Jickells et al., 2005). Deposited dust mass is dominated by the largest particles, and therefore understanding the transport of coarse and giant particles is crucial to pinning down the impacts of dust on the biogeochemical cycles.
Finally, coarse and giant dust particles appear to be transported further in the atmosphere than we would expect (Ryder 2013a). Thus it is not surprising that dust models, both in numerical weather prediction and climate models struggle to adequately represent the transport of both coarse and giant dust particles (Kok et al., 2017; Ansmann et al., 2017) and their associated impacts on weather and climate.
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