Ice nuclei are rare particles in the atmosphere that can cause cloud droplets to freeze and form ice crystals. Each individual droplet of water in a cloud can exist as liquid or ice, and liquid droplets can remain unfrozen down to extremely low temperatures—as low as -40°C if the water is very pure. However, if a supercooled liquid droplet encounters an ice nucleating particle, it freezes. Instantly.
But how on earth can fires freeze clouds?
It turns out that some smoke particles from burning wood and grasses—biomass burning aerosols—can act as ice nuclei in the atmosphere. When these smoke particles collide with the surfaces of supercooled water droplets, a small fraction of the particles that are ice nuclei cause the droplets to instantaneously freeze and form ice crystals in the sky. These ice crystals grow quickly by sucking up water from any remaining liquid droplets. This is why frozen clouds are the main source of precipitation over land—glaciating a cloud is the best way to cause the cloud’s particles to grow so large that they fall back to Earth as rain and snow.
“Biomass burning aerosol, or wood smoke, is a complex mixture of soot particles, organic carbon, and inorganic salt components,” explains Ryan Sullivan, assistant professor of Mechanical Engineering and Chemistry. “We know that when you burn some fuels very hot, like certain tall grasses, they emit a large number of ice nuclei. We also know that biomass burning aerosol undergoes a lot of chemical reactions as the dense smoke is mixing and diluting with background air. So the biomass smoke particles’ chemistry is rapidly evolving, but we don’t know how that chemical evolution of the particles in the wood smoke changes their ice nucleation properties.”
Figuring out on the molecular scale how individual ice crystals form is a tricky process, however—especially when you’re flying through the clouds, trying to take accurate particle property measurements while your turbulent airplane is being violently bombarded with ice (as Sullivan has, of course, done). Now, though, Sullivan’s research is more convenient—his research lab has designed several instruments and controlled environments in which to study ice nucleating mechanisms on individual particles.
Gaining Some Control Over Climate Change
“We have developed ‘aerosol optical tweezers,’” says Sullivan, “a device that allows us to trap a single droplet in a bright green laser beam and hold it stably for hours, allowing us to watch how the droplet evolves or responds to changes in its gas phase environment or while being bombarded with potential ice nuclei. The trapping laser also induces the droplet’s Raman vibrational spectrum which tells us, every second, the size and composition of the droplet, and when a particle collides with it, if it then freezes.”
Sullivan observes changes in the particle like, for example, what happens if the humidity around the droplet is changed, or what products form when the particle is exposed to air pollutants such as ozone. Sullivan and his group have even successfully engineered the device to trap supercooled droplets at subzero temperatures—a first in his field.
But why would measuring this change in a particle’s properties be important? Well, the short answer is that as clouds shift in their ratio of liquid particles to ice particles, they have a surprisingly large influence on the global energy balance and temperature of our planet.
“Liquid clouds reflect incoming solar radiation or sunlight, so they have a net cooling effect on the planet, but ice crystals actually trap outgoing infrared radiation so they have a net warming effect,” Sullivan explains.
So, if smoke contains ice nuclei, then one can imagine the impacts of industrial air pollutants or wildfires on the proportion of ice in the clouds—if the proportion of ice crystals in the clouds increases, so does global warming.
But that understanding goes both ways. If we can learn how to change the abundance of ice nuclei in the atmosphere, some say we could influence the ratio of ice to liquid in clouds, thereby gaining some control over climate change.