Evolution in Air

Aerosol optical tweezers advance understanding of chemical evolution of airborne particles

Two studies led by faculty in Carnegie Mellon University’s Center for Atmospheric Particle Studies show how aerosol optical tweezing can allow scientists to scrutinize the components of the atmosphere with new precision.

“What this allows us to do, really for the first time, is directly probe and understand how particles evolve in the atmosphere,” said Ryan Sullivan, associate professor of chemistry and mechanical engineering, who is the first scientist in North America to make use of optical tweezer technology to study aerosol particles suspended in air

Optical tweezers take advantage of the small forces exerted by light to trap and gently manipulate small particles or droplets. Arthur Ashkin won the 2018 Nobel Prize in Physics for developing this technique. In aerosol optical tweezing (AOT), individual particles are gently levitated, or “tweezed,” in a laser beam, while a Raman vibrational spectrum of the particle is collected using the same laser light.

“With other techniques, you kind of get a static snapshot of the particle,” Sullivan explained. But with AOT, researchers can watch the same particle for hours as it changes in response to different stimuli, which is a much more realistic way to observe how they might behave in the real atmosphere. 

“Particles float around in the atmosphere for at least a week on average,” Sullivan said. “They’re so dynamic — their composition and other properties are constantly evolving.”

That evolution can result not only in the changing of particles emitted into the atmosphere from Earth, but in entirely new ones that are being formed. Secondary organic aerosols (SOAs) are molecules formed directly in the atmosphere from the oxidation of organic molecules, such as those emitted by trees, vehicles and consumer products. These particles are an important but highly variable component of the atmosphere and can have effects on pollution, air quality, clouds and climate, and human health.

In a 2017 study in the journal Environmental Science & Technology, Sullivan’s lab captured and analyzed secondary organic aerosol for the first time with AOT. He was assisted by Neil Donahue, a professor of chemistry and chemical engineering, and Kyle Gorkowski, a postdoctoral researcher at McGill University who worked on his Ph.D. under Sullivan and Donahue.

“It’s very complex material,” Sullivan said of working with SOA, which they generated directly in the AOT chamber from ozone reacting with the organic vapor α-pinene, a terpene molecule released by trees. “You will get dozens or hundreds of different chemical products as a result — it’s like a runaway chain reaction with all sorts of branching.” This SOA is a major component of atmospheric particulate matter and the AOT approach provides a unique way to directly study its properties and chemistry.

Using their tweezed SOA particles, Sullivan and his collaborators published a study the following year in the journal Environmental Science: Processes & Impacts reporting their new method to analyze the properties and morphology of particles that separate into two separate chemical phases based on the Raman spectra collected from the AOT. In most cases the SOA formed a separate shell phase around another core phase, and their new analysis allowed them to determine the properties of both phases as they change through continued chemical reactions.

The results were the first direct confirmation of what researchers had suspected about SOA droplets —that they would “phase separate” in the atmosphere, forming a core of aqueous or hydrophobic organic material surrounded by a shell of oxidized secondary organic material.

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