Ryan Sullivan: Studying Atmospheric Particles Using Aerosol Optical Tweezers


In the aerosol optical tweezers, we send a
bright laser beam through the back of a microscope objective, and when it comes out, it’s a
very tightly-focused laser beam that forms a three-dimensional radiant force trap. And
we trap a droplet in air, in that laser beam. So, we’re levitating a droplet in air in
light, and the great thing about the optical tweezers is if the droplet were to drift to
one side or the other, it will be pushed back by the laser beam. So it’s a restoring trap,
and that allows us to trap droplets for hours or days if we want. When the droplet is trapped
in the laser beam in the optical tweezers, that same laser beam also induces vibrational
spectrum of the droplets called a Raman spectrum. We collect the Raman scattered photons,
the light, from the droplets through the same microscope objective we use to form the trap.
So, we’re measuring the Raman spectrum every second of the droplet, and that allows us
to probe how the droplet’s size and composition and other properties are changing. We use
this experiment to simulate the evolution of real particles in the atmosphere. Secondary
organic aerosol is a really important component of particles in the atmosphere, particulate
matter. Particulate matter is toxic, and so we’re concerned with understanding how particulate
matter forms so we can prevent or reduce its production. Particles in the atmosphere also
play really important roles in modulating climate. They can reflect incoming solar radiation
or absorb it, depending on what they are made out of. Particles are also what create clouds.
So, every cloud droplet or ice crystal in the atmosphere nucleated on an original particle
seed, but the ability of particles to make clouds and change properties of clouds and
climate depends on what those particles are made out of. Secondary organic aerosol is
a major component of particles in the atmosphere. It forms in the gas phase through reactions
of organic vapors, volatile organic compounds, usually with oxidants. So, what we’ve done
in the optical tweezers is we’ve simulated this reaction directly in the trapping chamber,
which hasn’t been done before. We float in alpha-Pinene vapor, which is emitted by
trees. Alpha-Pinene has a double bond that’s very juicy to being attacked by ozone. So
we then float ozone into the chamber, and let the alpha-Pinene and ozone mix. The alpha-Pinene
becomes oxidized and starts condensing onto the droplet that we already have trapped in
the laser beam, but also brand-new particles from alpha-Pinene oxidation nucleate and grow,
and those start to coagulate and add to the particle that is tweezed in the laser beam.
We can watch all of this occur in real time. So, once we add the secondary organic aerosol
using the very realistic atmospheric chemical reaction, now we can start to probe the properties of
secondary organic aerosols. This is why we’re doing this experiment, creating genuine secondary
organic aerosols in the optical tweezers experiment because there are a lot of key properties
of this secondary organic aerosol that are not very well understood, which means we don’t
understand how this material that is ubiquitous in the atmosphere is really affecting atmospheric
chemistry, clouds, and climate change. And the optical tweezers provides us a unique
ability to probe many of these important properties in a direct manner. And we are using this
new information to improve our understanding of the reactivity of particles in the atmosphere:
how they change the chemistry of the atmosphere, how they nucleate clouds and ice crystals,
change the properties of clouds, and play a key but uncertain role in changing the planet’s
climate.