Amazon Rainforest Clouds Created By Salt
September 11, 2012

Researchers Study How Clouds Form In The Amazon

April Flowers for - Your Universe Online

Early morning, deep in the Amazon jungle, fog drifts through the still air. The sun rises and clouds appear to float across the forest canopy.

Where do these clouds and mists come from? Water vapor needs soluble particles to condense on, and airborne particles are the seeds of liquid droplets in fog, mist and clouds.

To learn how aerosol particles form in the Amazon, an international team of scientists analyzed samples of naturally formed aerosols collected above the forest floor, deep in the rainforest. The research team included Mary Gilles of the Chemical Sciences Division at the U.S. Department of Energy´s Lawrence Berkeley National Laboratory, David Kilcoyne of the Lab´s Advanced Light Source (ALS), Christopher Pöhlker, Meinrat Andreae and Ulrich Pöschl of Germany´s Max Planck Institute for Chemistry (MPIC).

The results from the ALS analysis were combined with results from other facilities to provide essential clues to the evolution of fine particles around which Amazon clouds and fog condense. The most important of these particles turned out to be potassium salts.

At ALS beamline, the researchers performed scanning transmission x-ray microscopy (STXM) to determine the near-edge x-ray absorption fine structure (NEXAFS) of particles collected during the wet season in the remote, pristine forest northeast of Manaus, Brazil.

“Through absorption of soft x-rays by an atom´s core electrons, and subsequent emission of photons, the identity and exact location of the elements in the aerosol samples can be identified,” says Kilcoyne. “The essence of STXM is that it not only tells you if carbon is present but how this carbon is bound to other elements within the aerosol particles. This allows us to distinguish between soot, which is graphitic, and organic carbon.”

The study, published in Science, showed three different types of organic aerosol particles were found, all similar to laboratory-generated reference samples: oxidation products based on precursor chemicals emitted in the gas phase by trees, including terpenes (the major component of turpentine) from tree resin, and isoprene, another organic compound abundantly released through leaves.

“In the beginning we focused on the carbon, oxygen, and nitrogen contents of the organic materials,” says Pöhlker. “But then, to our surprise, we found very high potassium levels, up to 20 percent.”

All but three of the 77 Amazonian aerosol samples showed remarkably strong signals for potassium in the form of salts.

The smaller the aerosol, the greater the proportion of potassium — those collected early in the morning were the smallest and richest in potassium. Larger particles contain more organic materials, but not much potassium, suggesting that potassium salts generated during the night acted as seeds for gas-phase products to condense onto, forming aerosols of different kinds.

“Biomass burning is also a rich source for potassium-containing aerosols in forested regions, but potassium from forest fires is correlated with the presence of soot, a graphitic form of carbon,” Gilles says. “Before and during the collection period there were no documented fires that could have affected the biosphere where the samples were collected, and no evidence of soot was observed in the samples. Hence the source of potassium could only have been natural forest organisms.”

The prime suspect for this release of potassium salts are fungal spores. Some fungi launch spores by building up water pressure through osmosis in sacs (asci) that contain the spores. When the pressure is high enough, the ascus bursts and squirts the spores into the air along with fluid containing potassium, chloride, and sugar alcohol. Other types of fungi fire "ballistospores" when water vapor in the atmosphere condenses and causes a sudden release of restraining surface tension, also ejecting potassium, sodium, phosphates, sugars and sugar alcohol.

Other biogenic mechanisms also release salts into the early morning mists that cover the forest, including salts dissolved in water by transpiration during the day and, at night, the oozing of sap rich in sugars, minerals, and potassium from the edges of leaves.

Thus a key role in the formation of aerosols in the rainforest is played by invisible tiny grains of potassium salts, generated by natural plants and other living things at night and early in the morning.

Terpenes and isoprenes are primarily released in the gas phase by plants in the jungle, and once in the atmosphere they react with water, oxygen, organic compounds, acids and other chemicals exuded by indigenous plants. These reaction products are less volatile and initiate the condensation within the low-lying forest biosphere. Since the smallest particles are typically the most important in condensation, potassium salts fill the role. As the day goes on, gas-phase products continue to condense and the particles continue to grow.

Cloud cover, precipitation, water cycle, and the climate of the Amazon basin and beyond can be traced back to salts from fungi and plants in the undisturbed jungle all during the rainy seasons. These salts provide the precursors of natural cloud-condensation nuclei and directly influence how fog and clouds form and evolve in the rainforest.