Sullivan uses Nobel Prize winning technology to study airborne particles

The earth’s atmosphere is full of airborne particles called aerosols. As these particles travel in the air, they encounter and react with other chemicals such as secondary organic aerosol, evolving into a chemically complex goop. For example, a gas given off by pine trees could be oxidized and then encounter other gases and particles from car exhaust, fossil fuel burning or even a forest fire. In the air, these particles play a significant role in climate and, when inhaled, can cause serious health problems.

The effects of particulate matter on health and the environment are dictated by the composition and properties of these complex particles, which makes understanding how they chemically evolve critical.

Atmospheric chemist Ryan Sullivan is the first in North America to use optical tweezers—a technique for which Arthur Ashkin was awarded the 2018 Nobel Prize in Physics at a ceremony this week—to better understand secondary organic aerosols.

Optical tweezers take advantage of the small forces exerted by light to trap and gently manipulate small particles or droplets. In the technique, a laser beam is focused using a microscope lens. The trapping laser induces a Raman vibrational spectrum from the droplet, which is collected using the same lens. This spectrum provides direct real-time measurements of how the droplet’s size, composition and structure evolve.

Particles last in the atmosphere for days or even weeks. They are constantly exposed to water, sunlight and a wide range of chemicals.

Ryan Sullivan, Associate professor of chemistry and mechanical engineering, Carnegie Mellon University

In his lab, Sullivan traps a single aerosol droplet, a few microns in size, in the tweezers’ focused laser beam. He then exposes the droplet to different types of gases and particles and measures data each second as the droplet’s composition evolves over the course of a day, or even days.

“It allows us to study particles in a more realistic way. Particles last in the atmosphere for days or even weeks. They are constantly exposed to water, sunlight and a wide range of chemicals,” said Sullivan, associate professor of chemistry and mechanical engineering at Carnegie Mellon. “Optical tweezers allow us to simulate the chemical evolution of particles much closer to what happens in the actual atmosphere.”

Aerosol optical tweezers represent a complimentary tool to single-particle mass spectrometry, which Sullivan also uses to study individual atmospheric particles. With mass spectrometry, Sullivan could only look at the particle at one given point in time, giving him a mere snapshot. Optical tweezers provide a movie that shows how the particle evolves and changes in response to what it encounters, simulating atmospheric chemistry.

In a recently published study using aerosol optical tweezers, Sullivan discovered that the secondary organic aerosol droplet would phase separate and create a protective shell around the particle’s reactive, aqueous core. Within this tiny core was an emulsion that contained more secondary organic aerosol particles.

“This emulsified state of secondary organic aerosols has implications for the particles’ behavior that we hadn’t even considered,” said Sullivan. “It means that the organic aerosol won’t necessarily be only in the shell surrounding the droplet, but can instead be hiding from the atmosphere or reacting in the aqueous core. As no one had previously studied secondary organic aerosols using optical tweezers, we weren’t aware that it could form a colloid of tiny liquid-like particles suspended in a droplet.”

The optical tweezers allow Sullivan’s students to determine key properties of these particles that are not possible using other techniques. Sullivan’s research group has recently developed a new method to directly measure pH and thus acidity during his aerosol optical tweezer experiments.

“pH is important to many chemical processes that we can study directly using optical tweezers, including aerosol particle structure, its chemical reactivity and evolution, and even protein folding,” said Sullivan. “Being able to measure the pH of levitated microdroplets in real time will be invaluable.”