Aerosol-cloud interactions are one of the largest sources of uncertainty in our understanding of the Earth’s climate system. In order to develop better predictive models and understand how the climate will respond to future changes in atmospheric composition, we must determine the sources and nature of aerosols which serve as cloud condensation nuclei (CCN), thus influencing the properties of clouds. Oceans cover 70% of the Earth’s surface and represent a major source of atmospheric aerosols. Sea spray aerosol (SSA) is formed by the action of breaking waves, whereas secondary marine aerosols (SMA) are formed from the oxidation products of gases emitted from the oceans. Biological activity in seawater (i.e. the life, death, and interactions of marine phytoplankton, bacteria, and viruses) can significantly affect the chemical composition of SSA through processing of dissolved organic matter and SMA through the emission of volatile gases. This dissertation investigates the cloud-relevant properties of SSA and SMA generated using ocean-atmosphere simulators in the laboratory, with a specific emphasis on the influence of biological activity in seawater on the properties of these aerosols. For the first time, SMA was produced from the oxidation of the headspace gases of a phytoplankton bloom grown in natural seawater, enabling measurements of its chemical composition and CCN activity. Overall, these studies show that the formation and properties of SMA are much more sensitive to biological activity in seawater than SSA. In addition, the chemical composition of SMA is highly dependent on the extent of photochemical oxidation, with a distinct shift from organic-rich to sulfate-rich composition in response to increased atmospheric aging. This change in SMA composition leads to a significant change in its hygroscopicity. These results suggest that the properties of SMA evolve temporally in the atmosphere, which has implications for CCN concentrations and cloud properties over the oceans.