A Saharan dust storm blowing dust out across the Atlantic. During an event such as this in May 2005 we discovered the phenomenon of dust orientation. Observations with a new highly sensitive polarimeter  at the La Palma observatory in the Canary islands showed that the dust polarized light passing through it. This can only occur if the dust grains align with a preferred orientation. The position angle of polarization indicated that the preferred alignment is with the long axis of the dust grain vertical. This orientation is opposite to the horizontal orientation known to occur for ice crystals in cirrus clouds (as a result of hydrodynamic forces). Such orientation has not previously been reported for mineral dust aerosols.

The vertical alignment has been explained as being due to a vertical electric field. The electric field is likely to be enhanced by the charging of individual dust grains. The vertical alignment significantly alters the radiative properties of the dust and may therefore have impacts on the radiative balance of tthe atmosphere when dust is present. Dust charging may also affect the transport of dust in the atmosphere.

Publications:

Bailey, J. et al., 2008, MNRAS, 386, 1016-1022.

Ulanowksi, Z. et al., 2007, Atmos. Chem. Phys., 24, 6161-6173.


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