Size-Resolved Sea Spray Aerosol Particles Studied by Vibrational Sum Frequency Generation

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

Inside versus Outside: Ion Redistribution in Nitric Acid Reacted Sea Spray Aerosol Particles as Determined by Single Particle Analysis

Single particle analysis of individual sea spray aerosol particles shows that cations (Na⁺, K⁺, Mg²⁺, and Ca²⁺) 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

Heterogeneous Reactivity of Nitric Acid with Nascent Sea Spray Aerosol: Large Differences Observed between and within Individual Particles

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

Size-Dependent Changes in Sea Spray Aerosol Composition and Properties with Different Seawater Conditions

A great deal of uncertainty exists regarding the chemical diversity of particles in sea spray aerosol (SSA), as well as the degree of mixing between inorganic and organic species in individual SSA particles. Therefore, in this study, single particle analysis was performed on SSA particles, integrating transmission electron microscopy with energy dispersive X-ray analysis and scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy, with a focus on quantifying the relative fractions of different particle types from 30 nm to 1 μm. SSA particles were produced from seawater in a unique ocean-atmosphere facility equipped with breaking waves. Changes to the SSA composition and properties after the addition of biological (bacteria and phytoplankton) and organic material (ZoBell growth media) were probed. Submicrometer SSA particles could be separated into two distinct populations: one with a characteristic sea salt core composed primarily of NaCl and an organic carbon and Mg²⁺ coating (SS-OC), and a second type consisting of organic carbon (OC) species which are more homogeneously mixed with cations and anions, but not chloride. SS-OC particles exhibit a wide range of sizes, compositions, morphologies, and distributions of elements within each particle. After addition of biological and organic material to the seawater, a change occurs in particle morphology and crystallization behavior associated with increasing organic content for SS-OC particles. The fraction of OC-type particles, which are mainly present below 180 nm, becomes dramatically enhanced with increased biological activity. These changes with size and seawater composition have important implications for atmospheric processes such as cloud droplet activation and heterogeneous reactivity