Some Saharan dust falls back to Earth before it leaves Africa. Some of it streams out over the Atlantic Ocean or Mediterranean Sea, carried on the wind as far away as South America and the Southeastern United States. All of it has an as-yet unmeasured impact on Earth's energy budget and the climate by reflecting sunlight back into space.
"The people who build climate models make some assumptions about dust and its impact on the climate," said Dr. Sundar Christopher, a professor of atmospheric science at UAHuntsville. "We want to learn more about the characteristics of this dust, its concentrations in the atmosphere and its impact on the global energy budget so we can replace those assumptions with real data."
Dust is one kind of particle, or aerosol, that floats around in the atmosphere. Most of the recent research into aerosols has focused on particles made by humans, such as smoke, soot or other types of pollution.
"There has been a lot of research looking at the climate effects of man-made aerosols," Christopher said. "Particles from smoke and burning fossil fuels are tiny, sub-micron size. These tiny particles cool the atmosphere because they reflect sunlight back into space before it has a chance to heat the air. That means less solar energy is available at the surface to heat the planet."
Because they are so small, pollution aerosols don't have a significant effect on heat energy. That's why they usually have a net cooling effect on the atmosphere.
Dust particles, on the other hand, weighing in at a hefty 10+ microns (a human hair is about 100 microns in diameter) do absorb some solar radiation, convert it to heat and release that heat into the air. They also reflect some radiation back into space, so dust both heats and cools the atmosphere.
More importantly they have a significant effect on heat energy in the air. Dust absorbs thermal energy rising from the ground and re-radiates it either toward space (and colder temperatures) or back toward the surface.
"One thing we want to do is calculate how reflective dust is, because not all dust is created equal," Christopher said. "We're trying to calculate reflectivity so we can say with precision how much sunlight is being reflected."
The composition and shape of dust particles is very complex. They are not spherical, which makes calculating their energy budget challenging and time consuming. Also, the composition of dust varies depending on which part of the Sahara the dust comes from. Some of it absorbs more solar energy than others.
"Climate models are not very sophisticated in the way they handle dust," Christopher said. "And the long-wave or infrared part is something that has been ignored for a long time. We want to nail down those values."
Why start with the Sahara? First, the Sahara contributes about half of all of the dust carried into Earth's atmosphere every year. The Saharan dust is also more "pristine" than dust from U.S. or Asian deserts. Dust from U.S., Chinese or Mongolian deserts frequently mixes with pollution to create an aerosol stew, which can make it difficult to study just the dust.
Studying the Saharan dust is enough of a challenge, in part because it is made of the same stuff as the desert underneath. That means the dust in the atmosphere looks very much like the surface below it. Only in the past few years have new instruments and new techniques been developed that help scientists "see" which is dust and which is desert.
Christopher has received a grant of almost $500,000 through NASA's CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) program to support the research for the next three years. The CALIPSO satellite's instruments include a LIDAR, which shoots a laser into the atmosphere then catches light that bounces off particles in the air to learn more about aerosols.
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