Book excerpt: The change in planetary albedo due to aerosol-cloud interactions (aci) during the industrial era is the leading source of uncertainty in inferring Earth's climate sensitivity to increased greenhouse gases from the historical record. Examining pristine environments such as the Southern Ocean (SO) helps us to understand the pre-industrial (PI) state and constrain radiative forcing associated with aci (RFaci). Cloud droplet number concentration (Nd) is a key aci indicator variable. Using global climate models (GCMs), this study finds that the hemispheric contrast in Nd of liquid clouds between the pristine SO and the polluted Northern Hemisphere observed in the present-day (PD) can be used as a proxy for the increase in Nd from the PI. The hemispheric difference constraint and MODIS satellite observations suggest that PI Nd may have been higher than previously thought and provide an estimate of RFaci between -1.2 and -0.6 Wm-2. Southern Ocean liquid clouds can reach Nd levels comparable to the polluted outflows of East Asia and the United States despite persistent precipitation depletion associated with mid-latitude storm systems. This high Nd in one of the most pristine regions on Earth motivates further investigation of the mechanisms driving Nd in the real world and better inclusion of the mechanisms in models. In this study, aerosol and cloud microphysical data from the 2018 Southern Ocean Cloud Radiation Aerosol Transport Experimental Study (SOCRATES) aircraft campaign are used to identify a novel and potentially important mechanism missing or poorly represented in models: production of new particles through synoptic uplift. The small, Aitken mode particles produced in this process dominate the free tropospheric atmosphere in the summertime SO. We find it is likely that entrainment of free tropospheric Aitken aerosols is a leading contributor to sub-cloud cloud condensation nuclei and thus may be a key control on Nd. The free tropospheric Aitken reservoir may maintain the persistently high Nd observed across the SO against precipitation depletion. Finally, our observational comparisons with nudged Community Atmosphere Model (CAM6) hindcasts highlight large aerosol number and composition discrepancies that may significantly and negatively impact the ability of current climate models to capture aci in pristine, PI environments.