Study finds climate-influencing cirrus clouds form around mineral dust and metallic aerosols; little evidence of black carbon
An interdisciplinary team from MIT, the National Oceanic and Atmospheric Administration (NOAA), and elsewhere has identified the major particles upon which cirrus clouds form. A paper on their results is published in the journal Science.
Formation of cirrus clouds depends upon the availability of ice nuclei to begin condensation of atmospheric water vapor. While it is known that only a small fraction of atmospheric aerosols are efficient ice nuclei, the critical ingredients that make those aerosols so effective has not been established. We have determined in situ the composition of the residual particles within cirrus crystals after the ice was sublimated. Our results demonstrate that mineral dust and metallic particles are the dominant source of residual particles, while sulfate/organic particles are underrepresented and elemental carbon and biological material are essentially absent. Further, composition analysis combined with relative humidity measurements suggest heterogeneous freezing was the dominant formation mechanism of these clouds.—Cziczo et al.
Cirrus clouds influence global climate, cooling the planet by reflecting incoming solar radiation and warming it by trapping outgoing heat. Understanding the mechanisms by which these clouds form may help scientists better predict future climate patterns.
The team sampled cirrus clouds using instruments aboard high-altitude research aircraft, analyzing particles collected during multiple flights over a nine-year period. They found that the majority of cloud particles freeze, or nucleate, around two types of particles: mineral dust and metallic aerosols. Contrary to what many lab experiments have found, the team observed very little evidence of biological particles, such as bacteria or fungi, or black carbon (BC) emitted from automobiles and smokestacks.
Cirrus clouds typically form at altitudes higher than most commercial planes fly. To sample at such heights, the team enlisted three high-altitude research aircraft from NASA and the National Science Foundation (NSF): a B-57 bomber, a DC-8 passenger jet, and a G-V business jet, all of which were repurposed to carry scientific instruments.
From 2002 to 2011, the team conducted four flight missions in regions of North America and Central America where cirrus clouds often form. Before takeoff, the team received weather forecasts, including information on where and when clouds might be found.
For each mission, Dan Cziczo, associate professor of atmospheric chemistry at MIT, and Karl Froyd, of NOAA’s Earth System Resource Laboratory, mounted one or two instruments to the nose of each plane: a single particle mass spectrometer and a particle collector.
Each flight followed essentially the same protocol: As a plane flew through a cloud, ice particles flowed through a specialized inlet into the nose of the plane. As they flowed in, the particles thawed, evaporating most of the surrounding ice. What’s left was a tiny kernel, or seed, which was then analyzed in real time by the onboard mass spectrometer for size and composition. The particle collector stored the seeds for further analysis in the lab.
After each flight, Cziczo and his colleagues analyzed the collected particles in the lab using high-resolution electron microscopy. They compared their results with analyses from the onboard mass spectrometer and found the two datasets revealed very similar cloud profiles: More than 60% of cloud particles consisted of mineral dust blown into the atmosphere, as well as metallic aerosols.
Cziczo notes that while mineral dust is generally regarded as a natural substance originating from dry or barren regions of the Earth, agriculture, transportation and industrial processes also release dust into the atmosphere.
Cziczo’s team also identified a “menagerie of metal compounds,” including lead, zinc and copper, that may 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 is also emitted in the exhaust of small planes.
Froyd says knowing what particles are absent in clouds is just as important as knowing what’s present: Such information, he says, can be crucial in developing accurate models for climate change.
There’s been a lot of research efforts spent on looking at how these particle types freeze under various conditions. 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.—Karl Froyd
Daniel J. Cziczo, Karl D. Froyd, Corinna Hoose, Eric J. Jensen, Minghui Diao, Mark A. Zondlo, Jessica B. Smith, Cynthia H. Twohy, and Daniel M. Murphy (2013) Clarifying the Dominant Sources and Mechanisms of Cirrus Cloud Formation. Science doi: 10.1126/science.1234145