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The oceans represent a significant global source of atmospheric aerosols. Sea spray aerosol (SSA) particles comprise sea salts and organic species in varying proportions. In addition to size, the overall composition of SSA particles determines how effectively they can form cloud droplets and ice crystals. Thus, understanding the factors controlling SSA composition is critical to predicting aerosol impacts on clouds and climate. It is often assumed that submicrometer SSAs are mainly formed by film drops produced from bursting bubble-cap films, which become enriched with hydrophobic organic species contained within the sea surface microlayer. In contrast, jet drops formed from the base of bursting bubbles are postulated to mainly produce larger supermicrometer particles from bulk seawater, which comprises largely salts and water-soluble organic species. However, here we demonstrate that jet drops produce up to 43% of total submicrometer SSA number concentrations, and that the fraction of SSA produced by jet drops can be modulated by marine biological activity. We show that the chemical composition, organic volume fraction, and ice nucleating ability of submicrometer particles from jet drops differ from those formed from film drops. Thus, the chemical composition of a substantial fraction of submicrometer particles will not be controlled by the composition of the sea surface microlayer, a major assumption in previous studies. This finding has significant ramifications for understanding the factors controlling the mixing state of submicrometer SSA particles and must be taken into consideration when predicting SSA impacts on clouds and climate., Competing Interests: The authors declare no conflict of interest.

The oceans, atmosphere, and clouds are all interconnected through the release and deposition of chemical species, which provide critical feedback in controlling the composition of our atmosphere and climate. To better understand the couplings between the ocean and atmosphere, it is critical to improve our understanding of the processes that control sea spray aerosol (SSA) composition and which ones plays the dominate role in regulating atmospheric chemistry and climate.

Previous laboratory measurements and field observations have suggested that the reactive uptake of N2O5 to sea spray aerosol particles is a complex function of particle chemical composition and phase, where surface active organics can suppress the reactive uptake by up to a factor of 60. To date, there are no direct studies of the reactive uptake of N2O5 to nascent sea spray aerosol that permit assessment of the role that organic molecules present in sea spray aerosol (SSA) may play in suppressing or enhancing N2O5 uptake kinetics. In this study, SSA was generated from ambient seawater and artificial seawater matrices using a Marine Aerosol Reference Tank (MART), capable of producing nascent SSA representative of ambient conditions. The reactive uptake coefficient of N2O5 (γ(N2O5)) on nascent SSA was determined using an entrained aerosol flow reactor coupled to a chemical ionization mass spectrometer for measurement of surface area dependent heterogeneous loss rates. Population averaged measurements of γ(N2O5) for SSA generated from salt water sequentially doped with representative organic molecular mimics, or from ambient seawater, do not deviate statistically from that observed for sodium chloride (γ(N2O5)NaCl = 0.01-0.03) for relative humidity (RH) ranging between 50 and 65%. The results are consistent with measurements made under clean marine conditions at the Scripps Institution of Oceanography Pier and those conducted on nascent SSA generated in the marine aerosol reference tank. The results presented here suggest that organic films present on nascent SSA (at RH greater than 50%) likely do not significantly limit N2O5 reactive uptake.

We report measurements of the product yield for nitryl chloride (ClNO2) production following the reactive uptake of dinitrogen pentoxide (N2O5) to a wide variety of ambient seawater samples as well as seawater mimics. The ClNO2 yield, as measured for ambient seawater collected from both coastal and open ocean waters, was found to be both insensitive to chlorophyll-a, a marker for biological activity, and significantly lower (0.16-0.30) than that expected for equivalent salt-containing solutions (0.82 ± 0.05). Suppression in the ClNO2 yield can be induced by the addition of aromatic organic compounds (e.g., phenol and humic acid) to synthetic seawater matrices. In the case of phenol, surface tension measurements reveal that the surface phenol:chloride ratio can be enhanced by more than a factor of 100 as compared to bulk ratios for subtle changes in surface tension (<1.5 mN m(-1)), providing a mechanism to suppress ClNO2 production at low bulk phenol concentrations. We interpret measurements of the dependence of the ClNO2 yield on phenol using a kinetic model, where we confine the surface enhancement in phenol to the top 1 nm of the interface. Our results are most consistent with a model where N2O5 is ionized within the first three water monolayers (<1 nm), where the product nitronium ions react rapidly with interfacial phenol molecules. These results suggest that ClNO2 may not be formed at the air-sea interface at the yield expected for NaCl, and that the reactive uptake of N2O5 and the subsequent product yield of ClNO2 may serve as a unique probe for the composition of the interfacial region of the sea surface microlayer.

Current climate and atmospheric chemistry models assume that all sea spray particles react as if they are pure NaCl. However, recent studies of sea spray aerosol particles have shown that distinct particle types exist (including sea salt, organic carbon, and biological particles) as well as mixtures of these and, within each particle type, there is a range of single-particle chemical compositions. Because of these differences, individual particles should display a range of reactivities with trace atmospheric gases. Herein, to address this, we study the composition of individual sea spray aerosol particles after heterogeneous reaction with nitric acid. As expected, a replacement reaction of chloride with nitrate is observed; however, there is a large range of reactivities spanning from no reaction to complete reaction between and within individual sea spray aerosol particles. These data clearly support the need for laboratory studies of individual, environmentally relevant particles to improve our fundamental understanding as to the properties that determine reactivity.

The rates of heterogeneous reactions of trace gases with aerosol particles are complex functions of particle chemical composition, morphology, and phase state. Currently, the majority of model parametrizations of heterogeneous reaction kinetics focus on the population average of aerosol particle mass, assuming that individual particles have the same chemical composition as the average state. Here we assess the impact of particle mixing state on heterogeneous reaction kinetics using the N2O5 reactive uptake coefficient, γ(N2O5), and dependence on the particulate chloride-to-nitrate ratio (nCl(-)/nNO3(-)). We describe the first simultaneous ambient observations of single particle chemical composition and in situ determinations of γ(N2O5). When accounting for particulate nCl(-)/nNO3(-) mixing state, model parametrizations of γ(N2O5) continue to overpredict γ(N2O5) by more than a factor of 2 in polluted coastal regions, suggesting that chemical composition and physical phase state of particulate organics likely control γ(N2O5) in these air masses. In contrast, direct measurement of γ(N2O5) in air masses of marine origin are well captured by model parametrizations and reveal limited suppression of γ(N2O5), indicating that the organic mass fraction of fresh sea spray aerosol at this location does not suppress γ(N2O5). We provide an observation-based framework for assessing the impact of particle mixing state on gas-particle interactions.

Single particle analysis of individual sea spray aerosol particles shows that cations (Na(+), K(+), Mg(2+), and Ca(2+)) within individual particles undergo a spatial redistribution after heterogeneous reaction with nitric acid, along with the development of a more concentrated layer of organic matter at the surface of the particle. These data suggest that specific ion and aerosol pH effects play an important role in aerosol particle structure in ways that have not been previously recognized.

We present vibrational sum frequency generation (SFG) spectra of the external surfaces and the internal interfaces of size-selected sea spray aerosol (SSA) particles generated at the wave flume of the Scripps Hydraulics Laboratory. Our findings support SSA particle models that invoke the presence of surfactants in the topmost particle layer and indicate that the alkyl chains of surfactant-rich SSA particles are likely to be disordered. Specifically, the SFG spectra suggest that across the range of sizes studied, surfactant-rich SSA particles contain CH oscillators that are subject to molecular orientation distributions that are broader than the narrow molecular distribution functions associated with well-ordered and well-aligned alkyl chains. This result is consistent with the interpretation that the permeability of organic layers at SSA particle surfaces to small reactive and nonreactive molecules may be substantial, allowing for much more exchange between reactive and nonreactive species in the gas or the condensed phase than previously thought. The SFG data also suggest that a one-component model is likely to be insufficient for describing the SFG responses of the SSA particles. Finally, the similarity of the SFG spectra obtained from the wave flume microlayer and 150 nm-sized SSA particles suggests that the SFG active CH oscillators in the topmost layer of the wave flume and the particle accumulation mode may be in similar chemical environments. Needs for additional research activities are discussed in the context of the results presented.

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.