Top Human-caused emissions create new cloud-forming particles

That's the conclusion of a new study from University of Utah scientists and colleagues, which finds that at a laboratory on a mountaintop in Colorado, new aerosol particles are forming in the air on average every other day and that those particles, likely formed from gases emitted by nearby power plants, can grow until they're big enough for water to condense around, forming clouds.

The study draws an important scientific link, using newly developed statistical methods, that link aerosol growth to measured cloud condensation nuclei—which are the critical ingredients for accurately modeling the role of aerosols and clouds in climate change.

"Storm Peak Laboratory is a remote site in the Intermountain West," says Gannet Hallar, professor of atmospheric sciences. "Just because we only measure there doesn't mean it's not happening at all other remote sites in the Intermountain West."

The study is published in Atmospheric Chemistry and Physics.

How clouds form

Let's begin with the process of cloud formation. To form, clouds need something in the air, such as a speck of dirt or salt, around which to begin condensing water vapor. These somethings, about a tenth of the diameter of spider silk, are particles called "cloud condensation nuclei."

It's known that increased aerosols in the atmosphere lead to more cloud formation, and more reflective clouds. But it's not known whether new aerosol particles, much smaller than cloud condensation nuclei and formed with human-caused emissions, can grow into cloud condensation nuclei. And it's also not known how to incorporate the relationship between aerosols and clouds in climate models. As you might imagine, clouds play a large role in the climate, reflecting solar energy and moving water from one place to another. So being able to realistically model those clouds could help increase the accuracy of the models forecasting changes in our climate.

To understand the relationship between the formation of new aerosol particles and cloud condensation nuclei, recent graduate Noah Hirshorn, Hallar and colleagues dove into a 15-year record of aerosol measurements at Storm Peak Laboratory, a University of Utah facility directed by Hallar and located on the Steamboat ski resort in Colorado at an elevation of more than 10,500 ft (3210 m).

"Given the remote, mountaintop location of Storm Peak Lab," the authors write, "clean atmospheric conditions are common at the laboratory."

Upwind of the laboratory, however, are several power plants emitting sulfur dioxide, a gas that changes in the atmosphere to sulfuric acid, which can form into particles and cause new particle formation events.

For nearly 20 years, new particle formation has been measured by people looking at three-dimensional charts showing measurements of aerosol particles over time. Certain patterns in the appearance of bursts of new particles, as well as persistent growth of those particles, formed the criteria for designating new particle formation events.

But Hirshorn, Hallar and other researchers, including U graduates Lauren Zuromski and Christopher Rapp, developed an automatic method, based on statistics, to classify new particle formation events. The new method improves event detection accuracy and efficiency and agrees well with manual detection methods.