Cirrus Cloud Formation Depends Mainly On Mineral Dust, Metallic Particles
May 10, 2013

Mineral Dust Is The Dominant Driving Force Behind Cirrus Cloud Formation

April Flowers for - Your Universe Online

At any given time, nearly one-third  of the planet is covered by cirrus clouds — the thin wisps of vapor that trail across the sky — which coalesce in the upper layers of the troposphere, as much as 10 miles or higher above the surface of the Earth.

Cirrus clouds cool the planet by reflecting incoming solar radiation, and warm it by trapping outgoing heat. In this way, they influence global climate, so understanding the mechanisms by which these clouds form may help scientists better predict future climate patterns.

An interdisciplinary team of researchers, led by MIT and the National Oceanic and Atmospheric Administration (NOAA), has identified the major seeds on which cirrus clouds form by sampling the clouds using instruments aboard high-altitude research aircraft. They analyzed particles collected during multiple flights over a nine-year period, finding that a majority of cloud particles freeze, or nucleate, around two types of seeds: mineral dust and metallic aerosols.

The researchers found the absence of certain particles in the clouds interesting as well. In the laboratory, researchers have found that substances like black carbon and fungal spores readily form cloud particles. In the upper atmosphere, however, the team detected barely a trace of these particles.

"We think we're really looking at the seed, the nucleus of these ice crystals," says Dan Cziczo, an associate professor of atmospheric chemistry at MIT. "These results are going to allow us to better understand the climatic implications of these clouds in the future."

To sample the cirrus clouds, which generally form at higher altitudes than most commercial planes fly, the researchers enlisted the help of three high-altitude research aircraft from NASA and the National Science Foundation (NSF). The three aircraft - a B-57 bomber, a DC-8 passenger jet, and a G-V business jet — had all been repurposed to carry scientific instruments.

The team conducted four flight missions from 2002 to 2011 in the regions of North America and Central America where cirrus clouds often form. The team received weather forecasts before takeoff, including information on where and when clouds might be found.

"More often than not, the forecast is solid, and it's up to the pilot to hit a cloud," Cziczo says. "If they find a good spot, they can call back on a satellite phone and tell us if they're inside a cloud, and how thick it is."

Cziczo and Karl Froyd, of NOAA´s Earth System Research Laboratory, mounted one or two instruments to the nose of each plane for each mission. The instruments were a particle mass spectrometer and a particle collector.

The same protocol was followed for each flight. As each plane flew through a cloud, ice particles were funneled through a specialized inlet on the nose of the plane. The particles thawed as they flowed inward, evaporating most of the surrounding ice. The kernel, or seed, that was left was then analyzed in real time by the onboard mass spectrometer for size and composition. The seeds were stored for further laboratory analysis by the particle collector.

After each flight, the team analyzed the particles in the lab using high-resolution electron microscopy, comparing their results with analyses from the onboard mass spectrometer. The two datasets revealed very similar cloud profiles, with more than 60 percent of cloud particles consisting of mineral dust blown into the atmosphere, as well as metallic aerosols.

Mineral dust is generally regarded as a natural substance originating from dry or barren areas of the planet. Cziczo notes, however, agriculture, transportation, and industrial processes also release dust into the atmosphere.

"Mineral dust is changing because of human activities," Cziczo says. "You may think of dust as a natural particle, but some percentage of it is manmade, and it really points to a human ability to change these clouds."

Some global-modeling studies predict higher dust concentrations in the future, Cziczo adds, due to desertification, land-use changes and changing rainfall patterns due to human-induced climate effects.

The team identified what they called a "menagerie of metal compounds," which included lead, zinc and copper. These could also point to a further human effect on cloud formation. "These things are very strange metal particles that are almost certainly from industrial activities, such as smelting and open-pit burning of electronics," Cziczo adds. Lead could also be from the exhaust emissions of small planes.

Despite other lab findings, the team observed very little evidence of biological particles, such as bacteria or fungi, or black carbon emitted from automobiles and smokestacks. Knowing what particles are not in the clouds is just as important as knowing which ones are, according to Froyd. Such information can be crucial in developing accurate climate change models.

"There's been a lot of research efforts spent on looking at how these particle types freeze under various conditions," Froyd says. "Our message is that you can ignore those, and can instead look at mineral dust as the dominant driving force for the formation of this type of cloud."

The team published their findings in a recent issue of Science.