Byron E. Pedler

The production, size, and chemical composition of sea spray aerosol (SSA) particles strongly depend on seawater chemistry, which is controlled by physical, chemical, and biological processes. Despite decades of studies in marine environments, a direct relationship has yet to be established between ocean biology and the physicochemical properties of SSA. The ability to establish such relationships is hindered by the fact that SSA measurements are typically dominated by overwhelming background aerosol concentrations even in remote marine environments. Herein, we describe a newly developed approach for reproducing the chemical complexity of SSA in a laboratory setting, comprising a unique ocean-atmosphere facility equipped with actual breaking waves. A mesocosm experiment was performed in natural seawater, using controlled phytoplankton and heterotrophic bacteria concentrations, which showed SSA size and chemical mixing state are acutely sensitive to the aerosol production mechanism, as well as to the type of biological species present. The largest reduction in the hygroscopicity of SSA occurred as heterotrophic bacteria concentrations increased, whereas phytoplankton and chlorophyll-a concentrations decreased, directly corresponding to a change in mixing state in the smallest (60-180 nm) size range. Using this newly developed approach to generate realistic SSA, systematic studies can now be performed to advance our fundamental understanding of the impact of ocean biology on SSA chemical mixing state, heterogeneous reactivity, and the resulting climate-relevant properties.

Significance Primary production generates a reservoir of dissolved organic carbon (DOC) in the ocean as large as the global inventory of atmospheric CO 2 . Once formed, DOC accumulates on timescales from less than 1 hour to millennia. Bacteria are important contributors to the respiration of DOC to CO 2 and the conversion of DOC to refractory biopolymers. Yet, the quantitative contribution of individual species within diverse consortia to DOC cycling remains unknown. We report that a single bacterial strain can consume as much DOC as diverse free-living microbial communities. This taxon is commonly observed in seawater when labile carbon is available, and may serve a key ecosystem function by rapidly recycling and regulating the level of DOC while also supporting ocean food webs.

The composition and properties of sea spray aerosol, a major component of the atmosphere, are often controlled by marine biological activity; however, the scope of impacts that ocean chemistry has on the ability for sea spray aerosol to act as cloud condensation nuclei (CCN) is not well understood. In this study, we utilize a mesocosm experiment to investigate the impact of marine biogeochemical processes on the composition and mixing state of sea spray aerosol particles with diameters < 0.2 µm produced by controlled breaking waves in a unique ocean‐atmosphere facility. An increase in relative abundance of a distinct, insoluble organic particle type was observed after concentrations of heterotrophic bacteria increased in the seawater, leading to an 86 ± 5% reduction in the hygroscopicity parameter ( κ ) at 0.2% supersaturation. Aerosol size distributions showed very little change and the submicron organic mass fraction increased by less than 15% throughout the experiment; as such, neither of these typical metrics can explain the observed reduction in hygroscopicity. Predictions of the hygroscopicity parameter that make the common assumption that all particles have the same bulk organic volume fractions lead to overpredictions of CCN concentrations by 25% in these experiments. Importantly, key changes in sea spray aerosol mixing state that ultimately influenced CCN activity were driven by bacteria‐mediated alterations to the organic composition of seawater.

In the ocean, breaking waves generate air bubbles which burst at the surface and eject sea spray aerosol (SSA), consisting of sea salt, biogenic organic species, and primary biological aerosol particles (PBAP). Our overall understanding of atmospheric biological particles of marine origin remains poor. Here, we perform a control experiment, using an aerosol time-of-flight mass spectrometer to measure the mass spectral signatures of individual particles generated by bubbling a salt solution before and after addition of heterotrophic marine bacteria. Upon addition of bacteria, an immediate increase occurs in the fraction of individual particle mass spectra containing magnesium, organic nitrogen, and phosphate marker ions. These biological signatures are consistent with 21% of the supermicrometer SSA particles generated in a previous study using breaking waves in an ocean-atmosphere wave channel. Interestingly, the wave flume mass spectral signatures also contain metal ions including silver, iron, and chromium. The nascent SSA bioparticles produced in the wave channel are hypothesized to be as follows: (1) whole or fragmented bacterial cells which bioaccumulated metals and/or (2) bacteria-derived colloids or biofilms which adhered to the metals. This study highlights the potential for transition metals, in combination with specific biomarkers, to serve as unique indicators for the presence of marine PBAP, especially in metal-impacted coastal regions.