Understanding the partitioning of surface-active material in marine aerosols

Kimberly Prather, University of California, San Diego

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Marine atmospheric aerosols make up a significant portion of the planetary aerosol budget but represent one of the largest sources of uncertainty in our current climate models. Aerosols directly impact the global energy balance by interacting with incoming solar radiation. Indirectly, they serve as nuclei for cloud formation and provide surfaces for heterogeneous chemistry influencing the gas composition of our atmosphere; such atmospheric chemistry impacts greenhouse gases like methane, ozone and water vapor, and key pollutants such as nitrates and sulfates.

Recent developments in the collection and characterization of nascent sea spray aerosols (SSA) have led to a greater understanding of their chemical components and size distributions, as well as the physical mechanics of bubble production and bubble bursting that lead to the transfer of organic and biological material from the ocean to the atmosphere. However, due to significant limitations in single particle analysis instrumentation, there is still an incomplete understanding of individual SSA chemical composition and morphology and how this directly influences the climate-relevant properties of SSA. It has been shown experimentally that SSA are coated with organic surfactants, and that these surfactants significantly impact the transfer of water and reactive uptake of gases through the SSA surface. Additionally, recent evidence has shown that enzymes are present and likely active in SSA. Thus a major outstanding goal is to understand the complexity of the SSA and especially, how the complexity of SSA impacts the chemical reactions that take place at the air-water interface that formed by the surfaces of the SSA.

We had already proposed to run large-scale all-atom molecular dynamics (MD) simulations of SSAs in their intact whole particle form. Thus, using many types of experimental data, we have used tools in the Amaro lab to develop 3D molecular-level whole SSA particles. These modeled SSAs mimic in composition ‘real world’ SSAs and provide a radical change in approach to most (all) current studies at the molecular scale, which utilize linear slabs of lipid and fatty acid monolayers to explore structural dynamics of SSA constituents.

By utilizing our data-centric methods to modeling these SSAs, we will provide the first glimpses into the detailed time-dependent dynamics of nascent SSA, to address questions such as: (i) what are the mechanisms and potentials of mean force for water escape from SSAs, and how do their thermodynamics and kinetics of transport across the monolayer change upon the addition of lipases to the SSA, and further, with the combined addition of lipases and other lipid or fatty acid molecules, (ii) with what frequency do enzymes in the interior of the SSA partition to the monolayer surface, and what are the mechanisms of that process, including controlling factors, (iii) how does curvature of the SSA particle affect these processes (noting that here we will compare to our ongoing and published work on smaller model systems that utilize the linear monolayer slab approach), and (iv) how does varying surface tension or surface area per lipid alter and affect these properties?

Notably, the whole SSA particles we have built contain roughly 50 million atoms, including hydrogens and all interior solvent and other molecules. Leadership class facilities are the only possible computational architectures where systems with this appreciable size and complexity can be effectively simulated using these physics based approaches (MD simulations).  We propose to run a series of all-atom MD simulations on the intact SSAs in order to systematically explore the structure, dynamics, and function of the SSAs and their constituent elements.