Tiny water drops produced from bubble bursting play a critical role in forming clouds, scattering sunlight, and transporting pathogens from water to the air. Bubbles burst by nucleating a hole at their cap foot and may produce jets or film drops. The latter originate from the fragmentation of liquid ligaments formed by the centripetal destabilization of the opening hole rim. They constitute a major fraction of the aerosols produced from bubbles with cap radius of curvature (R) > ∼0.4 × capillary length (a). However, our present understanding of the corresponding mechanisms does not explain the production of most submicron film drops, which represent the main number fraction of sea spray aerosols. In this study, we report observations showing that bursting bubbles with R < ∼0.4a are actually mainly responsible for submicron film drop production, through a mechanism involving the flapping shear instability of the cap with the outer environment. With this proposed pathway, the complex relations between bubble size and number of drops produced per bubble can be better explained, providing a fundamental framework for understanding the production flux of aerosols and the transfer of substances mediated by bubble bursting through the air–water interface and the sensitivity of the process to the nature of the environment.

Marine aerosols play a critical role in impacting our climate by seeding clouds over the oceans. Despite decades of research, key questions remain regarding how ocean biological activity changes the composition and cloud-forming ability of marine aerosols. This uncertainty largely stems from an inability to independently determine the cloud-forming potential of primary versus secondary marine aerosols in complex marine environments. Here, we present results from a unique 6-day mesocosm experiment where we isolated and studied the cloud-forming potential of primary and secondary marine aerosols over the course of a phytoplankton bloom. The results from this controlled laboratory approach can finally explain the long-observed changes in the hygroscopic properties of marine aerosols observed in previous field studies. We find that secondary marine aerosols, consisting of sulfate, ammonium, and organic species, correlate with phytoplankton biomass (i.e., chlorophyll-a concentrations), whereas primary sea spray aerosol does not. Importantly, the measured CCN activity (κ app = 0.59 ± 0.04) of the resulting secondary marine aerosol matches the values observed in previous field studies, suggesting secondary marine aerosols play the dominant role in affecting marine cloud properties. Given these findings, future studies must address the physical, chemical, and biological factors controlling the emissions of volatile organic compounds that form secondary marine aerosol, with the goal of improving model predictions of ocean biology on atmospheric chemistry, clouds, and climate.

With oceans covering 71% of the Earth's surface, sea spray aerosol (SSA) particles play an important role in the global radiative budget by acting as cloud condensation nuclei and ice nucleating particles (INPs). By acting as INPs, SSA particles affect the structure and properties of mixed‐phase clouds by inducing freezing at warmer temperatures than the homogeneous freezing temperature. Climate models that incorporate marine INPs use the emission of submicron SSA in INP parameterizations because these particles contain a higher fraction of organic mass. Here we show supermicron SSA particles, produced using a natural breaking wave analogue, are the major source of INPs throughout the lifecycle of a phytoplankton bloom. Additionally, supermicron SSA particles are shown to be more efficient INPs than submicron SSA particles, because they carry a greater number of ice active components. Thus, supermicron SSA needs to be incorporated in INP parameterizations for future climate models.

In this study, we have investigated the effect of hydroxyl radical (OH) oxidation reactions on the formation and chemical composition of marine-derived aerosols. Marine aerosols can be classified into two categories: primary sea spray aerosol (SSA) produced upon the breaking of waves, and secondary marine aerosol (SMA) produced upon the oxidation of gas phase species. Here we simultaneously investigated the impact of heterogeneous OH oxidation reactions on chemically complex supermicron SSA as well as the formation of SMA in the submicron regime through the oxidation of volatile organic compounds (VOCs). A marine aerosol reference tank (MART) filled with water from a labgrown phytoplankton bloom was used to produce SSA particles and VOCs representative of those found over the ocean, which were then sent through a potential aerosol mass (PAM) reactor where they were exposed to OH radicals. Online and offline methods were used to compare unreacted nascent primary SSA to the marine aerosols that resulted from sending the MART headspace, which includes any gases and existing primary SSA, through the PAM. Several single particle methods of analysis, including micro-Raman spectroscopy and atomic force microscopy−photothermal infrared (AFM-PTIR) spectroscopy, were used to investigate composition and size of substrate deposited particles. In situ composition measurements of PM1 particles were made using an aerosol mass spectrometer (AMS) to understand submicron marine aerosol chemistry. Raman spectra of SSA showed that heterogeneous OH oxidation reactions significantly lower the amount of organic matter found in supermicron SSA particles, which are dominated, in part, by nitrogen containing species (e.g., amino sugars/amino acids) during periods of high biological productivity. Furthermore, AFM and AMS analyses showed the formation of secondary marine aerosols in the submicron size regime due to oxidation of biologically produced VOCs. To our knowledge, this is the first study in which lab-produced authentic marine aerosols produced during a phytoplankton bloom have been exposed to OH radicals. The results provide important insights to how the combined effects of ocean biological activity and OH oxidation reactions both ultimately play roles in determining the chemical composition of marine aerosols (SMA and SSA) across multiple size regimes and formation mechanisms.