The impact of sea spray aerosols (SSAs) on Earth's climate remains uncertain in part due to size-dependent particle-to-particle variability in SSA physicochemical properties such as morphology, composition, phase state, and water uptake that can be further modulated by the environment relative humidity (RH). The current study investigates these properties as a function of particle size and RH, while focusing on submicrometer nascent SSA (0.1−0.6 μm) collected throughout a phytoplankton bloom. Filter-based thermal optical analysis, atomic force microscopy (AFM), and AFM photothermal infrared spectroscopy (AFM−PTIR) were utilized in this regard. AFM imaging at 20% RH identified five main SSA morphologies: prism-like, core−shell, rounded, rod, and aggregate. The majority of smaller SSAs throughout a bloom were rounded, while larger SSAs were core−shell. Filter-based measurements revealed an increasing organic mass fraction with decreasing SSA size. The organic matter is shown to primarily reside in a rounded and core−shell SSA, while the prism-like and rod SSA are predominantly inorganic salts (i.e., sodium chloride, nitrates, and sulfates) with relatively low organic content, as determined by AFM−PTIR spectroscopy. AFM phase state measurements at 20% RH revealed an increasing abundance of core−shell SSA with semisolid shells and rounded SSA with a solid phase state, as the particle size decreases. At 60% RH, shells of core−shell and rounded SSA uptake water, become less viscous, and their phase states change into either semisolid or liquid. Collectively, findings reveal the dynamic and size-dependent nature of SSA's morphology, composition, phase states, and water uptake, which should be considered to accurately predict their climate-related effects.

Abstract. Understanding the impact of sea spray aerosol (SSA) on the climate and atmosphere requires quantitative knowledge of their chemical composition and mixing states. Furthermore, single-particle measurements are needed to accurately represent large particle-to-particle variability. To quantify the mixing state, the organic volume fraction (OVF), defined as the relative organic volume with respect to the total particle volume, is measured after generating and collecting aerosol particles, often using deposition impactors. In this process, the aerosol streams are either dried or kept wet prior to impacting on solid substrates. However, the atmospheric community has yet to establish how dry versus wet aerosol deposition influences the impacted particle morphologies and mixing states. Here, we apply complementary offline single-particle atomic force microscopy (AFM) and bulk ensemble high-performance liquid chromatography (HPLC) techniques to assess the effects of dry and wet deposition modes on the substrate-deposited aerosol particles' mixing states. Glucose and NaCl binary mixtures that form core–shell particle morphologies were studied as model systems, and the mixing states were quantified by measuring the OVF of individual particles using AFM and compared to the ensemble measured by HPLC. Dry-deposited single-particle OVF data positively deviated from the bulk HPLC data by up to 60 %, which was attributed to significant spreading of the NaCl core upon impaction with the solid substrate. This led to underestimation of the core volume. This problem was circumvented by (a) performing wet deposition and thus bypassing the effects of the solid core spreading upon impaction and (b) performing a hydration–dehydration cycle on dry-deposited particles to restructure the deformed NaCl core. Both approaches produced single-particle OVF values that converge well with the bulk and expected OVF values, validating the methodology. These findings illustrate the importance of awareness in how conventional particle deposition methods may significantly alter the impacted particle morphologies and their mixing states.

Sea spray aerosol (SSA) represents the largest source of natural primary aerosol with climate relevance in cloud formation. The aerosol-cloud activation process is influenced by saccharides, which comprise a large SSA organic mass fraction. Saccharides are enriched relative to sodium in SSA by several orders of magnitude but the mechanisms of that enrichment remain poorly understood. Here, saccharide enrichment in laboratory-generated SSA was quantified via bubble bursting experiments using marine-relevant model systems. The resulting particles exhibited core−shell morphology previously observed in SSA, as identified by single particle atomic force microscopy (AFM). Measured enrichment factors (EFs) from filters indicated significant enrichment in aerosol <250 nm in diameter (EF = 1.68 ± 0.19) for the anionic polysaccharide (alginate) and no enrichment (EF = 1) for neutral short-chain saccharides (glucose, sucrose, raffinose, and cyclodextrin). Concurrent surface tension depression was observed for the surface microlayer (SML) with alginate (−Δ12.2 mN m −1 relative to seawater matrix) but not for the short-chain saccharides. Together, results indicate that surface activity of these systems result in saccharide enrichment. Moreover, model system complexity was increased through calcium addition which significantly increased alginate enrichment in aerosol <250 nm in diameter (EF = 2.44 ± 0.26). Separately, protein addition caused the greatest alginate enrichment increase in 500−1000 nm diameter aerosol (EF = 5.77 ± 0.61). These results indicate saccharide surface activity and cooperative interactions with protein and calcium that enhance saccharide enrichment. However, the model systems have not reproduced EFs of natural SSAs and the role of complex ocean biology still needs to be evaluated.

The SeaSCAPE campaign replicated the marine atmosphere in the laboratory to investigate the links between biological activity in the ocean and the properties of primary sea spray aerosols, volatile organic compounds, and secondary marine aerosols.

Marine aerosols strongly influence climate through their interactions with solar radiation and clouds. However, significant questions remain regarding the influences of biological activity and seawater chemistry on the flux, chemical composition, and climate-relevant properties of marine aerosols and gases. Wave channels, a traditional tool of physical oceanography, have been adapted for large-scale ocean-atmosphere mesocosm experiments in the laboratory. These experiments enable the study of aerosols under controlled conditions which isolate the marine system from atmospheric anthropogenic and terrestrial influences. Here, we present an overview of the 2019 Sea Spray Chemistry and Particle Evolution (SeaSCAPE) study, which was conducted in an 11,800 L wave channel which was modified to facilitate atmospheric measurements. The SeaSCAPE campaign sought to determine the influence of biological activity in seawater on the production of primary sea spray aerosols, volatile organic compounds (VOCs), and secondary marine aerosols. Notably, the SeaSCAPE experiment also focused on understanding how photooxidative aging processes transform the composition of marine aerosols. In addition to a broad range of aerosol, gas, and seawater measurements, we present key results which highlight the experimental capabilities during the campaign, including the phytoplankton bloom dynamics, VOC production, and the effects of photochemical aging on aerosol production, morphology, and chemical composition. Additionally, we discuss the modifications made to the wave channel to improve aerosol production and reduce background contamination, as well as subsequent characterization experiments. The SeaSCAPE experiment provides unique insight into the connections between marine biology, atmospheric chemistry, and climate-relevant aerosol properties, and demonstrates how an ocean-atmosphere-interaction facility can be used to isolate and study reactions in the marine atmosphere in the laboratory under more controlled conditions.