Experimental investigation of Aerosol effects on Haze-Cloud interactions.

Clouds are crucial to Earth's climate system, influencing radiation and water cycles. Their formation and properties are shaped by aerosol characteristics, including concentration, size, and composition. Especially when high aerosol concentrations and weak forcing (i.e., small supersaturation) are present, interactions between haze and cloud droplets can be important to the overall microphysical evolution in a cloud. The Michigan Tech convection cloud chamber, known as the Pi chamber, can sustain clouds in a steady state for several hours, allowing precise control over aerosol size, concentration, and temperature gradients to study their effects on cloud formation. The focus of this work is to use the Pi chamber to explore aerosol effects on haze-cloud interactions.

Experiments in the Pi chamber show that varying the aerosol injection rate, aerosol size, and temperature gradient can cause a transition from single-mode to bimodal droplet size distributions, where the smaller mode corresponds to unactivated haze droplets. Additionally, these experiments help explore how changes in these parameters affect the total liquid water content and identify conditions under which haze liquid water becomes significant compared to cloud droplet liquid water. Haze particles and cloud droplets are defined based on the critical size of the injected dry aerosols. A time scale analysis is used to explore the relative importance of kinetic limitations for aerosol activation compared to cloud droplet growth and droplet removal by sedimentation. For most aerosol sizes, injection raters, and supersaturation forcing (via temperature gradient), two peaks and one minimum between them can be observed. We explore the impact of aerosol properties on the shapes of haze and cloud droplet modes, and the condition when the two modes are merged.