Scientists part the clouds on how droplets form

Scientists part the clouds on how droplets form

The current understanding of how cloud droplets form involves water vapor that encounters cooler air, often at higher altitudes and lower pressure. The vapor then condenses into small droplets of water or ice crystals that comprise clouds.

But the real catalyst in this process is the condensation of water on aerosol particles. These particles, known as cloud condensation nuclei, seed the formation of the cloud droplets. The details surrounding this microphysical process remain unclear, but the belief took hold among many atmospheric scientists and meteorologists that the main factor of significance when cloud droplets formed was the solubility of the aerosol.

These microscopic interactions could have macroscopic effects. The size of the droplets in a cloud affect its brightness. The smaller and more numerous the droplets, the more light gets scattered. Reflecting more light has the effect of cooling Earth's surface.

Certain inorganic particles, like sea salt, dissolve easily in water, but the atmosphere is typically a complex mixture of organic and inorganic aerosols. Sources of organic aerosols include diesel and gasoline emissions, forests, wildfires and even algal blooms in the ocean.

To account for this mix of particles, the Berkeley Lab researchers conducted experiments using custom-built equipment to model cloud droplet formation. They used dicarboxylic acids, a type of organic compound, and ammonium sulfate, an inorganic salt. They measured the size of the droplets formed when the particles were exposed to water vapor under typical cloud-forming conditions.

"We were finding that the cloud droplets were 50 to 60 percent larger than predicted using standard models that relied upon how easily the particles could dissolve," said the author. "That's when we realized something else was going on, so we created a new model."

By factoring in the effects of surface tension depression, the researchers were able to correctly predict the size of the droplets formed.

"The role of inorganic and organic aerosols in cloud formation has been a highly contentious issue that's been argued about for many years," said another author. "Based on the paper's findings, I would say that these surface interactions play a central role in cloud droplet formation, and that they should be considered in climate models."