We see cloudier skies these days due to human aerosol pollution - as clouds tend to condense around small particles called aerosols in the form of sulfuric acid. Thus, scientists have generally assumed that Earth's ancient skies were much sunnier than they are now.
However, three new studies reveal that naturally emitted gases from trees can also form the seed particles for clouds. It indicates a cloudier past as well as a potentially cooler future: If Earth's climate is less sensitive to rising carbon dioxide (CO2) levels, as the study suggests, future temperatures may not rise as quickly as predicted.
"It's been long thought that sulfuric acid is really the key player [in cloud formation]," says atmospheric chemist Chris Cappa of the University of California, Davis, who was not involved in the research. The studies "show pretty convincingly that we don't need sulfuric acid around to allow new particles to grow."
Few scientists, who claim that CO2 and other gases from human activities are warming Earth disagree about how sensitive the planet's climate is to these changes. As sulfur dioxide that has increased seven times forms sulfuric acid leading to the formation of aerosol particles that seed cloud droplets. Since clouds reflect sunlight back into space, certainly extra clouds will increase the greenhouse gas warming.
An Experiment by CLOUD: The past may have been cloudier than scientists realized. To simulate ancient atmospheric conditions, one research group used CLOUD (Cosmics Leaving OUtdoor Droplets), a controlled chamber at CERN, Europe's particle physics facility near Geneva, Switzerland. Nearly as big as a bus, the chamber was filled with synthetically produced air, allowing precisely controlled chemical conditions. Jasper Kirkby, a CERN particle physicist, and his colleagues introduced a mixture of natural oxidants present in the air and an organic hydrocarbon released by coniferous plants. The hydrocarbon was rapidly oxidized. The only other ingredient allowed in the chamber was cosmic rays, high-energy radiation from outer space, which made the molecules clump together into aerosols. Sulfuric acid was not required. In fact, even when the researchers introduced low concentrations of sulfuric acid to the chamber such as might be found in unpolluted air, the aerosol formation rate was unaffected. In a second CLOUD experiment published simultaneously in Nature, researchers showed these same oxidized molecules could rapidly grow the particles to sizes big enough to seed cloud droplets.
To confirm the process in nature researchers made atmospheric measurements of aerosol formation at the Jungfraujoch high altitude research station, 3500 meters up in the Swiss Alps. Over the course of a year, they measured the changing concentrations of sulfuric acid and organic molecules in the air. They found more aerosols formed with more organic molecules around, and crucially observed formation of organic particles without sulfuric acid. They used exactly the same instruments as at CLOUD to analyze the aerosols: "The clusters were formed mainly by organics," says atmospheric chemist Federico Bianchi of the Paul Scherrer Institute in Villigen, Switzerland, who led the Jungfraujoch research published today in Science.
All the researchers stress sulfuric acid is still a major contributor to cloud formation on Earth today. "Today the purely plant-based pathway is much less important than it was preindustrially," Kirkby explains.
The researchers are working closely to understand how the newly discovered process affects predictions of the Earth's future climate.
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