Detailed simulations of global climate in the past, present, and future require precise knowledge of the role of aerosols within the radiation balance of the atmosphere. How well do they scatter light? How well do they make both liquid and ice clouds? CAICE is working to understand how chemistry can help climate scientists predict these properties.
In regions of the atmosphere is super-saturated with respect to water vapor, which particles will be seeds for liquid cloud droplets? If the composition of the particle changes, how does the cloud activity respond?
Instrument Focus: Cloud Condensation Nuclei Counter
Aerosol particles, if they are large enough and soluble enough in water, can act as the seeds for cloud droplets. These particles, called cloud condensation nuclei (or CCN), can be counted by an instrument that keeps air super-saturated with respect to water vapor (relative humidity > 100%). Once particles pass through this region of super-saturation, the ones that grew into cloud drops can be counted, while the rest pass through undetected. Comparing with an instrument that counts all particles while the composition of the aerosol is changed reveals the influence of particle chemistry on its cloud activity.
The ability for water to condense on aerosol particles (a.k.a. – aerosol hygroscopicity) when the air has a relative humidity < 100% is based on the particle’s chemical composition. How does a change in relative humidity alter an aerosol particle’s ability to scatter sunlight back to space?
Instrument Focus: Cavity Ring-down Spectrometer
The main climate influence of aerosol hygroscopicity is the ability for particles to more efficiently scatter sunlight away from the Earth’s surface when particles grow due to water uptake, reducing the amount of energy that the planet can absorb. The cavity ring-down spectrometer measures light ‘extinction’ directly by firing visible laser light through a cavity (tube) that contains aerosol particles — on each end are 99% reflective mirrors, so a little bit of light leaks out to a detector every time it bounces back and forth in the cavity. The time it takes for the light to stop leaking out is called the ‘ring-down time’, and the faster the ring-down time, the more effective the particles inside the cavity were at scattering or absorbing that light. The instrument used by CAICE, built by the Cappa Research Group, also includes a photoacoustic spectrometer, which directly measures the absorption of light by particles. Using both in tandem, we can find out exactly how much light the particles scatter and absorb, which are key parameters in accurately calculating the atmospheric radiation balance.
Ice Nucleation Activity
Ice crystals will form ‘homogeneously’, that is, molecules of water vapor will come together and form ice crystals, only when the temperature in the atmosphere dips down to about -40 degrees Celsius. Ice clouds that form in temperatures warmer than that formed through ‘heterogeneous’ ice nucleation, where an aerosol particle provides a surface for an ice crystal germ to form.
Instrument Focus: Continuous Flow Diffusion Chamber
Perhaps the most chemically selective atmospheric process, ice crystal nucleation by particles is one of the least understood atmospheric processes in which aerosols participate. In collaboration with scientists at Colorado State University, CAICE researchers can measure the chemical contents of aerosols in parallel with the continuous flow diffusion chamber (CFDC), which subjects particles to air which is supersaturated with water vapor (like the CCN counter above), but at temperatures down to -40 degrees Celsius. We can also perform single droplet freezing studies on these particles to learn more about the location of ice nucleation on aerosols to direct our further studies of the impact of aerosol particle composition and morphology on the formation of ice in clouds. The goal is to derive a fundamental understanding of the chemical and/or morphological controls on heterogeneous ice nucleation.