Forests Influence First Stage of Cloud Formation: How Trees Impact Our Climate

Clouds are crucial to determining what Earth's climate will be like in the future, but they're a major uncertainty in climate models. Now, scientists are taking a closer look at cloud formation, and have found that a lot of it has to do with trees.

Cloud droplets form in the atmosphere when water vapor condenses onto tiny particles. These particles are emitted directly from natural sources or human activity. This process of gas molecules clustering together to form particles is called nucleation, and produces more than half of the particles that seed cloud formation.

Despite knowing this, though, researchers have been unsure about the precise mechanisms underlying nucleation. In order to find that out, the researchers turned to the CLOUD experiment at CERN. This allowed them to reproduce a typical atmospheric setting inside of an essentially contaminant-free, stainless steel chamber.

The researchers filled the chamber with sulfure dioxide and pinnanediol, which is an oxidation product of alpha-pinene-an organic molecule given off by pine trees. The researchers then generated hodroxyl radicals, which are the dominant oxidant in Earth's atmosphere. The scientists then watched the oxidation chemistry unfold.

So what did they find? It turns out that the organic molecules given off by pine trees, called alpha-pinene, are chemically transformed multiple times in the highly oxidizing environment of the atmosphere. In addition, these oxidized organics take part in nucleation.

"It turns out that sulfuric acid and these oxidized organic compounds are unusually attracted to each other," said Neil Donahue, one of the researchers, in a news release. "This remarkably strong association may be a big part of why organics are really drawn to sulfuric acid under modern polluted conditions."

The findings reveal a little bit more about how clouds form. This could particularly help models of future climate scenarios by predicting nucleation rates. More specifically, the research reveals how forests and trees have a huge role in the very first stage of cloud formation.

The findings are published in the journal Science.