Organic Enrichment, Physical Phase State, and Surface Tension Depression of Nascent Core–Shell Sea Spray Aerosols during Two Phytoplankton Blooms

Sea spray aerosols (SSAs) affect the Earth’s climate directly by scattering solar radiation and indirectly by acting as ice and cloud condensation nuclei. The relative magnitude of these effects remains uncertain, in part, from substantial compositional and morphological variability between individual particles. Here, the evolving heterogeneity within populations of primary SSAs produced from wave breaking of natural seawater within a wave flume is investigated. Over the course of the study, two successive phytoplankton blooms were induced in the seawater. The morphology, organic volume fraction, hygroscopicity, phase state, and surface tension of individual SSAs collected via deposition on a substrate were characterized using atomic force microscopy. Particles between ca. 0.3 and 1 μm in volume equivalent diameter displayed a distinctive morphology revealing an inorganic core coated with an organic shell. The inferred organic volume fraction was the largest at the peak of the first bloom. The corresponding shell thicknesses ranged from 21 to 40 nm at 20% relative humidity (RH). The organic shell phase state of the majority of the particles during both blooms was semisolid at 20% and 60% RH. At 20% RH, a minor fraction of the organic shells behaved as a solid, while at 60% RH some behaved as liquids during the first bloom. Similar results were evident at 20% RH for the second bloom but with no observed liquid particles at 60% RH. The thick, semisolid organic coatings could potentially reduce atmospheric water and gas uptake efficiencies onto SSAs at lower RH, along with the potential for ice nucleating activity. However, at 80% RH, the SSAs deliquesced and exhibited liquid-like behavior with surface tension values measured over individual particles of 41–87 mN m^–1, demonstrating high particle-to-particle variability. The suppressed surface tension at 80% RH relative to pure water is attributed to the high concentrations of surface-active organic compounds, potentially further limiting the diffusion rate of gas molecules through the interface.