CAICE research of complex and environmentally relevant systems has shown that chemical species are selectively transferred into the aerosol phase. In particular, fatty acids and divalent cations show high enrichment in sea spray aerosols. CAICE researchers are focused on better understanding the fundamental chemical structures and properties driving these observed macroscopic trends. This project integrates experimental spectroscopic studies of organic species and ions at interfaces and within nanoscopic clusters with an array of theoretical approaches.  The highly controlled experiments yield benchmarks for the theoretical simulations and calculations. Also theory aids in the interpretation of the experiments and provides a way to extrapolate microscopic results to macroscopic behavior.

This integrated approach has produced a number of exciting conclusions. Recently, CAICE experimentalists discovered sodium promoted fatty acids to the surface of water, an unexpected outcome. Molecular dynamics simulations from a resulting CAICE collaboration, showed that this macroscopic behavior is the result of contact ion pairing between the sodium ion and fatty acid head group. Better understanding the physical chemistry that drives the behavior of organic species and ions at the water-air interface not only improves our understanding of sea spray aerosol chemistry, but also has implications for an array of fields such as biophysics, biochemistry, and materials chemistry.

Featured Result

Patterson et al. (2016) developed a unique approach to preserve and image hydrated sea spray aerosol (SSA) particles By flash freezing particles soon after collection and utilizing cryogenic transmission electron microscopy, Patterson et al. (2016) were able to examine the native physical structure and chemical organization of hydrated droplets which are usually modified by the high vacuum requirements of electron microscopy. Even fragile biological structures contained in SSA such as bacteria (a), diatoms (b), viruses (c), and vesicles (d) were detected.

Collaborating PIs
Heather Allen
Mark Johnson
Christopher Mundy
Francisco Paesani
Wei Xiong

Featured Publications

As particle size decreases, enrichment of organic material within sea spray aerosols (SSA), especially of relatively hydrophobic compounds, generally increases. However, even at a specific size SSA particles can still exhibit a range of chemical compositions. CAICE seeks to generate a quantitative understanding of how the bubble bursting production mechanism controls the size-dependent composition of sea spray aerosols.

When a bubble reaches the water’s surface, the bubble film cap bursts and produces film drops.  The collapse of the remaining cavity can then produce jet drops. Typically, it is assumed that submicrometer SSA particles mainly originate from film drops. However, paradigm shifting CAICE findings published in PNAS indicate a substantial fraction of submicrometer SSA particles are also produced from jet drops! These two production pathways result in two chemically distinct submicrometer SSA populations with significantly different ice nucleating activities. Ongoing collaborations are focused on the development of a mathematical model that can be used to quantitatively predict the transport of dissolved organic species, particulate material, and salts into SSA. The ability to predict the size-dependent organic transfer into nascent SSA particles between ~30 nm to > 10 μm will represent a significant advancement in our understanding of SSA composition and morphology.

Featured Result

The first direct composition measurements of isolated submicron jet and film drops, by Wang et al. (2017), show that organic material transferred in film (top) and jet (bottom) drops, are remarkably distinct. These results indicated that the sampled film drops contained a higher fraction of aliphatic or less water-soluble organic species (green bars) while organic material in jet drops was dominated by oxygen-containing or more water soluble species (pink bars). This result definitively illustrates that the chemically distinct submicron SSA particle types can be generated by different bubble bursting production mechanisms.

Collaborating PIs
Dale Stokes
Kimberley A. Prather
Grant Deane

Featured Publications

• The Role of Jet and Film Drops in Controlling the Mixing State of Submicron Sea Spray Aerosol Particles.

The bubble bursting mechanism, which generates sea spray aerosols (SSA), is an inherently chemically selective process. Surface active compounds are enriched at air-water interfaces, which includes both the surface of the ocean as well as the bubble as it rises through the water column. Because of this selectivity, the chemical composition of SSA particles cannot be easily extrapolated from the composition of seawater. One of the central objectives of CAICE is to develop a greater understanding of the chemical composition of nascent SSA. CAICE studies of complex and environmentally relevant systems have quantitatively shown that some organic molecules, such as lipids and saccharides, and inorganic ions, such as calcium, are enriched in SSA relative to seawater.

CAICE work is currently focused on determining the key physicochemical properties that cause the selective transfer of organic compounds across the air-sea interface and into SSA. Recently, fundamental properties such as surface adsorption and water solubility were suggested to play a major role in governing the transfer of organic molecules into SSA.

Featured Result

Cochran et al. (2016) combined experimental results with fundamental physical chemistry principles to better understand the selective transfer of molecules into SSA. They estimated the concentration of organic species at the liquid-air interface as a function of solution concentration as described by van der Waals adsorption isotherms (left). Utilizing this technique they modeled the surface composition of a specific alkanoic acid solution which closely agreed with real experimental measurements of aerosol particles (right).

Collaborating PIs
Elizabeth Stone
Vicki H. Grassian
Richard Cochran
Kimberly A. Prather

Featured Publications

Although traditional studies of aerosol composition have focused on bulk measurements, the average composition cannot be used to accurately predict atmospheric processes when individual particle compositions vary greatly within a population. SSA particle chemical composition varies as a function of size and can range from pure organic to nearly pure salt. In addition, there is a diversity of organic compounds and particulate biological matter, whose transfer is into individual SSA particles depends upon factors such as particle size, production mechanism, and biochemistry of the water. CAICE addresses this real-world complexity by emphasizing innovative measurements of the composition and physical properties of single particles from the nanometer to supermicron size range. Ongoing work in CAICE is focused on designing new techniques to not only study the composition of single particles but to examine the physical structure and chemical organization within individual SSA particles under a range of environmentally relevant conditions.

Featured Result

Morris et al. (2015) invented a new technique to determine the surface tension of individual submicrometer liquid droplets, by measuring the force on special atomic force microscopy probes as they approach and then retract from the droplets (left). This innovative method has allowed direct single particle surface tension measurements of fundamental model systems at a broad range of relative humidities, with Lee et al. (2017) extending the approach on other SSA-relevant systems and showing good agreement to bulk solution surface measurements for relatively low viscosity systems (middle, right).

Collaborating PIs
Alexei Tivanski
Robert Continetti
Vicki H. Grassian
Kimberley A. Prather
Rommie Amara

Featured Publications

CAICE is focused on developing methods using biological processes to replicate and control the chemical complexity of SSA on both small and large scales. CAICE research has shown that SSA composition is determined by a balance between the rates of phytoplankton production and microbial degradation processes. This important finding contrasts previous paradigms that predicted major changes in SSA composition would occur at the peak of phytoplankton blooms. Ongoing CAICE work is now focused on understanding the physical, chemical, and biological properties that govern the transfer of microorganisms and large biomolecules to SSA. Before CAICE, the transfer of microbes or complex macromolecules to SSA, as a function of biological activity, had never been explored. This endeavor has exciting implications, as the transfer of intact microbes and active enzymes into SSA could transform these particles into tiny atmospheric bioreactors with evolving chemical compositions.

Featured Result

Patterson et al. (2016) developed a unique approach to preserve and image hydrated sea spray aerosol (SSA) particles By flash freezing particles soon after collection and utilizing cryogenic transmission electron microscopy, Patterson et al. (2016) were able to examine the native physical structure and chemical organization of hydrated droplets which are usually modified by the high vacuum requirements of electron microscopy. Even fragile biological structures contained in SSA such as bacteria (a), diatoms (b), viruses (c), and vesicles (d) were detected.

Collaborating PIs
Rommie Amaro
Vicki H. Grassian
Kimberley A. Prather