Dust intrusion from the Sahara Desert as seen in a satellite image. Nasa

Dust arising from the Sahara desert cools the Earth's surface by reflecting back sunlight, more so when containing human pollutants, shows a study.

A team of researchers from Extremadura (Spain) and Portugal analysed the composition and radiative effect of desert aerosols during two episodes which simultaneously affected Badajoz (Spain) and Évora (Portugal) in two monitoring stations in the south-west of the Iberian Peninsula during August 2013.

They found that the amount of radiation that reached the surface was less than what would have been if the aerosols had not been there.

Although the second desert dust intrusion was more intense (with a greater concentration of PM10 particles), it was the earlier one which caused a greater degree of radiative cooling in both places.

This was due to the presence of absorbent aerosols arising 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," observed Ángeles Obregón, researcher in the physics department of the University of Extremadura (UEx) and the University of Évora (Portugal) and lead author of the study.

During the first event, the aerosols stayed close to the surface due to the presence of an anticyclone hovering over the region thus "reducing the amount of shortwave irradiance reaching the surface and causing greater radiative cooling", says Obregón.

However, they caution that the effects of each type of aerosol are different due to varying composition, size and properties.

Research into atmospheric aerosols is becoming increasingly important due to their opposing effects on the global temperature of Earth.

A mixed bag
Aerosols interact both directly and indirectly with the Earth's radiation budget and climate.

In the direct effect, the aerosols scatter sunlight directly back into space. Indirectly, aerosols that act as seed material for clouds can modify the size of cloud particles, changing how the clouds reflect and absorb sunlight, thereby affecting the Earth's energy budget.

Minute grains of sand and dirt from the desert surface are relatively large and would normally fall out of the atmosphere after a short flight unless blown to relatively high altitudes (15,000 ft and higher) by intense dust storms.

Again, as the dust is composed of minerals, the particles absorb sunlight as well as scatter it depending on the composition, says Nasa.

A recent study by the University of Edinburgh attributed a fall by 10% in rainfall over the last 50 years to anthropogenic aerosol emissions.

High levels of anthropogenic aerosols in the atmosphere caused heat from the sun to be reflected back into space, lowering temperatures on the earth's surface and reducing rainfall.

Atmospheric aerosols remain in the atmosphere for a short time and result from both natural and anthropogenic sources. While over 90% of aerosols are naturally produced in volcano eruptions and forest fires, humans contribute in many ways.

Fossil fuel combustion, biomass burning, automobiles, incinerators and smelters are among those that produce sulphates, nitrates, black carbon and other aerosols.