Revealing the Impacts of Chemical Complexity on Submicrometer Sea Spray Aerosol Morphology

Sea spray aerosol (SSA) ejected through bursting bubbles at the ocean surface is a complex mixture of salts and organic species. Submicrometer SSA particles have long atmospheric lifetimes and play a critical role in the climate system. Composition impacts their ability to form marine clouds, yet their cloud-forming potential is difficult to study due to their small size. Here, we use large-scale molecular dynamics (MD) simulations as a "computational microscope" to provide never-before-seen views of 40 nm model aerosol particles and their molecular morphologies. We investigate how increasing chemical complexity impacts the distribution of organic material throughout individual particles for a range of organic constituents with varying chemical properties. Our simulations show that common organic marine surfactants readily partition between both the surface and interior of the aerosol, indicating that nascent SSA may be more heterogeneous than traditional morphological models suggest. We support our computational observations of SSA surface heterogeneity with Brewster angle microscopy on model interfaces. These observations indicate that increased chemical complexity in submicrometer SSA leads to a reduced surface coverage by marine organics, which may facilitate water uptake in the atmosphere. Our work thus establishes large-scale MD simulations as a novel technique for interrogating aerosols at the single-particle level.

Evolution of Hydrogen-Bond Interactions within Single Levitated Metastable Aerosols Studied by In Situ Raman Spectroscopy

Atmospheric aerosols can exist as supersaturated (metastable) liquid or glassy states, with physical and chemical properties that are distinct from the solid or liquid phases. These unique properties of aerosols have substantial implications on climate and health effects. Direct investigations on metastable aerosols remain a challenge because any interfacial contact can cause heterogeneous nucleation. In this study, in situ Raman spectroscopic and Mie scattering imaging analysis is applied to metastable aerosols in the absence of physical contact using an environment-controlled electrodynamic balance (EDB). This has allowed a detailed study of the O-H stretching regions of the Raman spectrum, revealing evidence for the rearrangement of hydrogen-bonding structures of levitated aqueous citric acid (CA) and aqueous sucrose droplets at metastable liquid states. We found that carboxyl groups in a CA droplet yield distinctive dynamics of strong and weak hydrogen bonds, whereas hydroxyl groups in a sucrose droplet show correlated strong and weak interactions. Such effects are particularly important in a supersaturated solution. These results indicate that metastable liquid aerosols from different sources may exhibit distinct physical and chemical behavior.

Evolution of aqSOA from the Air-Liquid Interfacial Photochemistry of Glyoxal and Hydroxyl Radicals

The effect of photochemical reaction time on glyoxal and hydrogen peroxide at the air-liquid (a-l) interface is investigated using in situ time-of-flight secondary ion mass spectrometry (ToF-SIMS) enabled by a system for analysis at the liquid vacuum interface (SALVI) microreactor. Carboxylic acids are formed mainly by reaction with hydroxyl radicals in the initial reactions. Oligomers, cluster ions, and water clusters formed due to longer photochemistry. Our results provide direct molecular evidence that water clusters are associated with proton transfer and the formation of oligomers and cluster ions at the a-l interface. The oligomer formation is facilitated by water cluster and cluster ion formation over time. Formation of higher m/z oligomers and cluster ions indicates the possibility of highly oxygenated organic components formation at the a-l interface. Furthermore, new chemical reaction pathways, such as surface organic cluster, hydration shell, and water cluster formation, are proposed based on SIMS spectral observations, and the existing understanding of glyoxal photochemistry is expanded. Our in situ findings verify that the a-l interfacial reactions are important pathways for aqueous secondary organic aerosol (aqSOA) formation.

Molecular dynamics simulation of the local concentration and structure in multicomponent aerosol nanoparticles under atmospheric conditions

MD simulations predicted core–shell or partially engulfed morphologies (depending on the type of the organic compound present) in multicomponent aerosol nanoparticles.