Tiny particles formed from trace gases can seed open ocean clouds

It doesn't take much to seed a cloud in the atmosphere above the open ocean, according to a new study published Friday in the journal Nature Communications.

When sunlight reacts with trace gas molecules in the marine boundary layer, the half-a-mile-thick layer of atmosphere that sits above the open ocean, tiny aerosols are forged -- a process called new particle formation.

"When we say 'new particle formation,' we're talking about individual gas molecules, sometimes just a few atoms in size, reacting with sunlight," study co-author Chongai Kuang said in a news release.

"It's interesting to think about how something of that scale can have such an impact on our climate -- on how much energy gets reflected or trapped in our atmosphere," said Kuang, a climate scientists and researcher at the Energy Department's Brookhaven National Laboratory.

Previously, scientists assumed the marine boundary layer rarely hosted new particle formation, but the latest models suggest otherwise.

Cloud formation is one of the biggest blind spots in climate modeling, undermining the certainty of predictions posited by even the most high-resolution simulations.

All cloud formation begins with aerosols -- airborne particles.

To accurately model the formation of clouds, scientists need to understand the supply of cloud-seeding aerosols in the atmosphere.

Scientists decided the open ocean would provide a pristine environment for studying aerosol generation and cloud formation.

The research team married data collected at a weather station on Graciosa Island in the Azores, a small archipelago in the middle of the North Atlantic Ocean, with observations acquired by an airplane outfitted with 55 different atmospheric instrument systems.

The research plane followed a porpoise flight path, repeatedly ascending and descending, to capture vertical profiles of the marine boundary layer surrounding the Azores.

Scientists expected both the ground-based measurements and boundary layer profiles to reveal relatively small concentrations of trace gases and limited particle formation.

"But there were particles that we measured at the surface that were larger than newly formed particles, and we just didn't know where they came from," Kuang said.

The vertical profiles acquired by the research plane showed particle formation was happening at the top of the marine boundary.

Scientists found evidence that trace gases like dimethyl sulfide were being carried aloft and reacting with sunlight, yielding aerosols capable of seeding new clouds.

"Then, once these aerosol particles form, they attract additional gas molecules, which condense and cause the particles to grow to around 80 to 90 nanometers in diameter. These larger particles then get transported downward -- and that's what we're measuring at the surface," Kuang said.

"The surface measurements plus the aircraft measurements give us a really good spatial sense of the aerosol processes that are happening," he said.

Eventually, these particles get big enough to pull in water vapor. Upon contact, the water vapor condenses to form tiny droplets -- a cloud seedling.

Clouds, depending on their size, composition and altitude, can have a variety of impacts on climate patterns. They can trap heat, but also reflect sunlight and cast shade.

Understanding more precisely how clouds form and where they come from can help scientists improve climate models.

"We'll take this knowledge of what is happening and make sure this process is captured in simulations of Earth's climate system," Kuang said.

In addition to findings ways to incorporate the latest aerosol data into broader cloud and climate models, Kuang said he and his colleagues are also keen on tracking down the source of the surprisingly large concentration of trace gases lurking in the marine boundary layer.

"There are some important precursor gases generated by biological activity in the ocean -- e.g., dimethyl sulfide -- that may also lead to new particle formation. That can be a nice follow-on study to this one -- exploring those sources." Kuang said.

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