British people may not like blood rain but Sahara Desert dust is not traveling 2,000 miles over an ocean just to make their cars dirty - it also helps cool things down. 

Researchers have analyzed the composition and radiative effect of desert aerosols during two episodes which simultaneously affected Badajoz (Spain) and Évora (Portugal) in August 2012 and found that it caused radiative cooling of the Earth's surface. Atmospheric aerosols (solid or liquid particles suspended in the atmosphere) are difficult to examine for various reasons - they remain in the atmosphere for only a short time and their cause may be natural or anthropogenic.

But though solar radiation is the main factor in global temperature of the Earth, atmospheric aerosols are important also. For that reason, a group of researchers from Extremadura and Portugal has analyzed the radiative effect of a type of natural aerosol - dust from desert areas - because the Iberian Peninsula is relatively close to the Sahara desert.


Dust intrusion from the Sahara Desert. Credit: NASA 

During two desert aerosol intrusions which occurred in August 2013 and that affected two monitoring stations in the south-west of the Iberian Peninsula, "the amount of radiation that reached the surface was less than that which would done if these aerosols had not been there," Mª Ángeles Obregón, researcher in the Physics department of the University of Extremadura (UEx) and the University of Évora (Portugal) and lead author of this study, explained via SINC.

By studying the precise measurements of these intrusions, the team determined that although the second desert dust intrusion was more intense (with a greater concentration of PM10 particles), it was in fact the earlier one which caused a greater degree of radiative cooling in both places. As the study underlines, this is due to the presence of absorbent aerosols from anthropogenic pollution.

"It was not only a mix of desert aerosol and pollution but there were also surface and column measurements. This is not always possible, given that the measurements are often contaminated by cloud cover and are fragmented, or simply do not exist because the instruments are calibrating themselves." 

During this event, the aerosols stayed close to the surface due to the presence of a anticyclone hovering over the study region at sea-level, "reducing the amount of shortwave irradiance reaching the surface and causing greater radiative cooling," stated Obregón, who likens the effects of desert dust with those resulting from certain forest fires or episodes of high pollution.

Citation: Obregón, M.A et al. "Aerosol radiative effects during two desert dust events in August 2012 over the Southwestern Iberian Peninsula" Atmospheric Research 153(404-415) DOI: 10.1016/j.atmosres.2014.10.007 febrero de 2015