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Moving the Ocean-Atmosphere to the Laboratory

In CAICE, we are focused on understanding how aerosols, small particles suspended in air, impact our atmosphere, clouds, and climate. In order to do this, we are developing state-of-the art approaches for measuring the chemical components in aerosol particles, as a first step towards understanding how particles interact with water vapor, ice, and other constituents of the atmosphere.

Overall, CAICE scientists are interested how the surfaces of tropospheric aerosols transform the composition of the atmosphere. Aerosols are dynamic highly complex structures, which can be comprised of thousands of different chemical species, and evolve over time as they undergo chemical reactions in the atmosphere. Probing the many factors affecting the evolution of aerosol composition over time has remained an elusive challenge.

We currently are focusing on sea spray aerosols formed by breaking waves in the ocean. These sea spray aerosols are formed through bubble bursting processes at the air-sea interface. Sea spray aerosol particles are rich in chemical complexity and constitute a major global source of aerosol particles. Thus, in early studies CAICE scientists, ranging from chemists to oceanographers to marine biologists, developed a novel approach for transferring the ocean-atmosphere system into a laboratory setting in order to create realistic sea spray aerosol particles that can be used for fundamental studies of their reactivity and physicochemical properties. This is being accomplished through the development and integration of state-of-the-art experimental, computational, and theoretical approaches.

Experimental Approaches

CAICE researchers are developing and using new innovative tools such as the ocean-atmosphere wave facility, Marine Aerosol Reference Tank (MART), and Mini-MART to mimic natural wave breaking and generate realistic sea spray aerosols with identical composition to those over the real ocean.  In CAICE, aerosol particles are analyzed individually using mass spectrometry, microscopy, and spectroscopy.  Through these studies, we are developing additional detailed experiments that can probe fundamental chemical interactions occurring at the molecular level, with a focus on complex chemical interfaces.  In order to bring real-world chemical complexity into the laboratory, CAICE has also been using marine phytoplankton and bacteria to synthesize the complex array of chemicals used in our experiments — this allows us to study richly complex chemical systems that are representative of the open ocean.

Figure 3. Marine Aerosol Reference Tank (MART). The 210 L Plexiglass tank (A) is approximately 1 m in the longest dimension. (B) The internal water distribution assembly is o-ring sealed with a spillway slot. (C) Flow meter. (D) Tank water sampling spigot. (E) Centrifugal pump. (F) Flow shunt control valve. (G) Timing relay control box. (H) Tank drain and purge valves. (I) Solenoid valve. Image and caption adapted from Stokes et al., AMT 2013
Figure 4. Miniature Marine Aerosol Reference Tank (mini-MART). The tank (25 × 25 × 30 cm, 19 L total volume) is made from Plexiglas with an O-ring sealed, Plexiglas lid. The intermittent plunging jet is formed by water escaping from alternating chambers in a rotating water wheel labeled (a) and powered by an external motor (c). An exit port on the wheel (indicated by the white star) allows the water to fall from the wheel to the water surface. The tank is filled with approximately 6 L of water to the water fill line indicated by the arrowhead (<). A vertical stainless steel aerosol sampling tube (b) penetrates the tank lid for sampling near the water surface. Image and caption adapted from Stokes et al., AMT 2016.

Theoretical Approaches

Our innovative experimental approaches provide us unique insights into the chemical composition of sea spray aerosols, allowing us to then recreate, simulate, and visualize the dynamic nature of these suspended particles. In CAICE, we are exploring interactions at the surface of sea spray aerosols through a unique computational microscope. Based on findings from theoretical simulations, new experiments can be developed and tested to both verify simulations and unravel new insights into the chemical nature of aerosols. These investigations are critical for understanding how sea spray aerosols affect our climate and environment, because the arrangement of molecules at the surface of sea spray aerosols influences the chemical reactivity of these particles in the atmosphere.

Figure 5. Illustration of experimental and computational approaches used for studying sea spray aerosol. Scanning transmission X-ray microscopy image (Step 2) illustrates the diversity in spatial distribution of organic molecules containing COO- functional groups [adapted from Ault et al., ES&T, 2013].

Overview

In CAICE, our goal is to study, understand and ultimately predict chemical reaction processes occurring in/on complex sea spray aerosol particles. To accomplish this, we must first replicate the mechanisms through which aerosols are produced at the ocean-atmosphere interface.

Sea spray aerosols (SSA) originate from bubbles bursting at the air-sea interface. The sea surface microlayer (SSML) exists as a thin skin on the surface of the ocean and represents an organic rich, biologically active region where bubbles burst after waves break. The chemical composition, physical phase state, and internal structure of sea spray aerosol particles are controlled by the chemistry, physics, and biochemistry that occur within this microlayer, as well as the bulk seawater which feeds into the SSML. In CAICE, we are unraveling the chemical linkages between bulk seawater, the SSML, and the distribution of SSA particle compositions (see Figure 1).

We have learned that sea spray aerosol particles are far more complex than just small droplets of seawater. In fact, the bubble bursting process that produces these small particles at the ocean surface leads to several interesting phenomena, including:

  • SSA particles can have a greater concentration of organic material compared to the seawater from which they are derived (oftentimes many orders of magnitude greater).
  • The chemical composition of sea spray aerosol particles depends strongly on their size (small particles are much different than larger particles in term of composition).
  • SSA chemical composition spans a wide range, ranging from particles that are mainly salt (NaCl – sodium chloride) to others that are almost devoid of salt!! Those that are devoid of salt consist of organic compounds such as carbohydrates, proteins, and fatty acids!

SSA particles, especially those smaller than 1 micrometer in size, are highly enriched in organic species as noted above. Atmospheric chemistry and climate models must treat sea spray as an array of particle types that vary from pure salts to pure organics and mixtures between the two. By producing realistic SSA, CAICE studies have clearly demonstrated that freshly produced aerosol particles from the ocean are governed by physical production processes as well as biochemical processes in seawater (Fig.1).

Figure 1. Effects of biochemistry processes in seawater and bubble bursting mechanisms at the air-sea interface on the organic compositions of SSA

We are increasing our overall understanding of sea spray aerosol chemical composition and the link with production mechanisms using an integrated “top-down” (investigating the chemistry of the complex system) and “bottom-up” (testing fundamental chemical principles by using simple systems) approach. This groundbreaking approach allows us to replicate a highly complex aerosol, sea spray, in a laboratory setting and has required an interdisciplinary approach to properly replicate all physical, biological, and chemical processes present in the real world.  In addition, we are developing and utilizing a variety of different experimental and theoretical approaches to study the chemical complexity of nascent sea spray aerosol.

Overall, selectivity studies represent an important area of research as the quantity and type of organic species that get transferred from the ocean to the atmosphere can influence the reactive chemistry of these particles and their climate-relevant properties.

An example: Using integrated top-down and bottom-up techniques, we have discovered that the physicochemical behavior of molecules and ions at the sea-air interface plays a major role in defining their transfer into SSA during the bubble-bursting process. Additionally, recent work has shown that charged species (such as inorganic ions/trace metals) located at the aerosol surface may play a key role in dictating sea spray aerosol particle chemical reactivity and climate properties!

Figure 2. Cartoon representation of linear carboxlyate (LC-) and dicarboxylate (DLC-) molecules along with the inorganic salts within the bubble film located at the ocean surface. LCs at the air−water interface are more efficiently transferred into the aerosol phase relative to the DLC molecules residing in the bulk solution away from the interface. Strong interactions between LCs and calcium at the air− water interface lead to the selective enrichment of calcium in the resulting aerosol. [Cochran et al., JPC Lett. 2016]

Overview

In CAICE, we are currently focused on understanding the chemical transformations that take place both at the air-sea interface as well as in sea spray aerosol after being ejected from the ocean.  These chemical reactions alter the chemical and physical properties of sea spray aerosol. Current efforts are divided into three major thrust areas:

  1. Heterogeneous/multiphase reactions
  2. Biologically-mediated control of chemistry
  3. Photochemical reactions the sea surface microlayer and sea spray aerosol.

Overview

CAICE is actively involved in investigating the impact that individual particle chemistry has on climate properties- namely the ability to take up water, serving as cloud seeds, and the ability to form ice crystals.

It has been hypothesized that climate-relevant properties of sea spray aerosol, including water uptake and ice nucleating ability, depend on the chemical composition of each particle and phase state. The coverage and ordering of organic molecules on a particle surface is also hypothesized to be a significant factor in affecting climate properties. The chemical complexity of the atmosphere provides an additional and important dimension to aerosol studies, leading to a variety of molecular transformations in particles. Therefore, a range of surface tensions, water solubilities and ice nucleation activities are anticipated for sea spray aerosol particles in the natural environment. CAICE provides the tools and expertise to effectively probe these chemical interactions and pathways both through experiments and computational simulations.