Scientists' view of clouds is clearing up. Two new studies show that cloud-forming particles in the atmosphere, called aerosols, look different and make different clouds depending on their origins.

One study found that in one of the most pristine environments on Earth – above the treetops of the Amazon Rainforest – clouds mostly come from gas emitted by the plants. The entire rainforest is a self-sustaining engine in which "the plants cause the rainfall, and the rainfall causes the plants," said Harvard environmental chemist Scot Martin, a coauthor of the study.

The other, which was compiled from 15 years of data from airplanes flying through clouds, found that aerosols with human origins are larger, more numerous and contribute more to haze than biogenic particles.

This paper "demonstrates the importance of combustion-produced aerosols for controlling cloud-forming particles," commented cloud scientist Robert Wood of the University of Washington, who was not involved in either study.

Both studies, which appear in the September 17 Science, fill in "areas that so far have been nearly white spots on the landscape of aerosol information," said Urs Baltensperger of the Paul Scherrer Institute in Switzerland, who wrote a Perspectives article in Science. By providing a baseline to compare modern aerosol conditions against and a comprehensive look at what those conditions are, the two studies may provide a more complete and accurate way to investigate climate change.

Studying the air above the Amazon is about as close as you can get to studying the pre-industrial atmosphere, Martin and his colleagues argue, especially during the rainy season when human influence is minimal.

"That establishes a baseline so we can understand how human activities are changing things now," Martin said. "The Amazon is a laboratory for understanding the way things were, and therefore for measuring how humans have changed things."

Martin and his team built a 130-foot-high research tower deep in the forest north of Manus, Brazil, and collected aerosols from the atmosphere over a period of 10 days in March 2008. Using a wide range of techniques, some of which had never been used in the Amazon before, the team analyzed the samples both on site and back in their labs to determine the particles' size, concentration and origin.

"This is definitely an important paper," commented atmospheric chemist Joel Thornton of the University of Washington, who was not involved in the study. "It's the first study of its kind to make a set of comprehensive measurements of all the aerosol properties we can measure in the Amazon."

The team found that the aerosol concentration – the number of particles in a small volume of space – was 10 to 100 times lower above the Amazon than above more populated areas, even rural regions that are generally considered clean.

The data also show that the number of cloud droplets above the Amazon depends directly on the number of aerosols. This is in contrast to more polluted areas, where the number of cloud droplets depends on how quickly hot particles from burning fires or fossil fuels ascend into the atmosphere. In the cool regions of the upper atmosphere, water droplets condense onto hot aerosols like fog on a window.

These different limiting factors form different clouds, says atmospheric scientist Ulrich Poschl of the Max Planck Institute for Chemistry in Germany, lead author of the study. "Depending on aerosol particles, you form clouds with different properties," he said. "With different cloud properties, you can suppress rainfall so you have less frequent but more intense rain, especially under heavy pollution areas."

What exactly this difference means for the global climate is yet to be seen. But "we now have a clean background scenario to which we can compare our polluted environment," he said. "It’s a key for just benchmarking or validating climate models from which we want to learn how we are already influencing the environment, and how that will evolve in the future."

The team also found that clouds and rain in the region mostly came from the plants. Plants emit gases from their leaves and sap, which is one reason why they have distinctive smells. When those gases interact with sunlight, their chemistry changes such that they condense from diffuse gas to liquid droplets less than one micrometer – a thousandth of a millimeter – in size. These droplets then serve as the nucleus of a cloud.

Particles larger than one micrometer, which are important in forming ice crystals, also came from plant matter like pollen, fungus spores and bits of crumpled up leaf.

"The tight coupling that we're able to show between emissions from plants and hydrological cycle shows one area that could end up being quite sensitive to unintended consequences," Martin said.

The second study compiled data from 12 separate experiments conducted since 1995, in which atmospheric scientist Anthony Clarke of the University of Hawaii and colleagues collected atmosphere samples from airplanes over the Pacific Ocean.

"This paper impresses by its wealth of data, many of which stem from areas where very little data has been obtained so far," Baltensperger commented.

The study found that aerosol particles that result from human activity, from crop clearing to combustion engines, are just the right size to interact with light and build clouds. This means that, even if two regions have the same number of aerosols, the man-made aerosols will have a bigger impact.

"In cloud formation and in radiation transfer, larger particles that are sourced from combustion can play a more important role," Clarke said.

Particles of this size – a few hundred nanometers – are around the same size as the wavelength as visible light, which means light bounces off the particles and doesn't make it to the ground. The clouds they form also tend to be a brighter white, meaning they are more reflective. These properties could mean that combustion-based clouds could have a cooling effect on climate, even while greenhouse gases from the same combustion processes heat the planet.

"Of course everybody wants to know, what's the effects of all this?" Clarke said. "Unfortunately that's not easily done without very complex models. But there is data out there now for modelers."

All that data may be the ultimate legacy of these two studies.

"In that sense they are highly complimentary," Poschl said. These studies represent "a major benchmark in advancing the models in this direction, that we really can understand these processes in cloud and rainfall formation."

Images: 1) View of the clouds from a research airplane. Anthony Clarke. 2) The research tower in the Amazon rainforest where atmospheric chemists collected samples. Science/AAAS. 3) Science/AAAS 4) Anthony Clarke

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