Sea spray aerosol (SSA) is the aerosol produced by breaking ocean waves.

Aerosol is a general term for the liquid or particles suspended in gas. Under this umbrella term, sea spray aerosol (SSA) is the aerosol produced by breaking ocean waves. Among many other aerosols such as fog and dust, SSA is a main target of current atmospheric chemistry research because it is a major component of natural aerosol along with mineral dust. Notably, the headquarter of CAICE (Center for Aerosol Impacts on Chemistry of the Environment) locates right at University of California, San Diego. CAICE is a collaborative aerosol research team which assembles scientists from different universities, and it is led by UC San Diego professor Kimberly Prather. Professor Prather is dedicated to studying human impact on atmosphere and climate. In her early research, she invented aerosol time-of-flight mass spectrometry (ATOFMS) to measure the size and composition of small aerosols. The current aerosol studies at CAICE include its reactivity, size distribution and biological composition under different atmospheric conditions. 

Aerosol interacts closely with weather and climate: In addition to aerosol’s direct cooling effect through light scattering, aerosol also has an indirect cooling effect by serving as seeds that form cloud droplets. Without aerosols, cloud droplets will form at a much higher supersaturation. (Supersaturation: Partial pressure of water is higher than its equilibrium pressure) Nevertheless, how aerosol interacts with climate is still one of the largest uncertainties. It is in the hope that if we could understand the property and composition of natural aerosols we could further investigate the impact of anthropogenic aerosols on climate. 

There are 4 modes of aerosol categorized by size. The smallest one is nucleation and the approximate size range is 1-10nm. Nucleation dominates in number, and it is mainly produced from photochemical reaction and combustion. By definition, the initial nucleation mode aerosol can form from vapor, and when aerosol continues to grow through the condensation of vapor on its surface, it will form a larger mode called Aitken mode with size range 10-100nm. Researchers found that the formation of this mode is exponentially correlated with wind speed, quantity wise. The third mode is accumulation mode, approximate size range 100-1000μm. After coagulation, which is the process of aerosol particle colliding with each other, the aerosol can grow into coarse mode-size ranger greater than 1μm, this mode dominates in surface area and volume. In particular to sea spray aerosol, a size growth is usually observed in the transition of nascent SSA (forms immediately from breaking wave within the 30s of production) to secondary SSA (aerosol form after the process of chemical and atmospheric reaction).

Figure 1. Schematic multi-modal particle size distribution with typical transformations and example particle types within each mode. Adapted from “Particle Size-Distribution” by Deutscher Wetterdienst, online. Copyright by DWD. 

Sea spray aerosols are composed primarily of sea salt, organic and inorganic matter. One interesting study is to analyze the change in SSA composition as they undergo biological activity such as phytoplankton blooms. In a previous study, researchers at CAICE regulated chlorophyll a (a source of organic nutrient) concentrations and found in most cases that increasing its concentration increases the organic fraction of SSA and decreased the hygroscopicity of SSA (Cochran, 2017).

The issue with studying SSA in natural settings is that the background aerosol co-present in the environment could affect the accuracy of the analysis. Thus, to minimize the background impact, the novel approach CAICE developed is to replicate ocean waves in an indoor laboratory setting. A 33 meters long wave flume containing seawater was monitored to generate aerosol by controlling the frequency of breaking wave. Its validity had been tested: In a published paper by Professor Prather, it was said “The measured bubble spectrum for the breaking waves used in this study matches the shape and Hinze scale of bubble spectra measured previously for open ocean breaking waves.” (Prather, 2013)

The measurements of aerosols include number concentration, size distribution and composition, and the measurements are conducted using specific instruments. Aerodynamic Particle Sizer (APS) measures the size of relatively large aerosol (0.6-10um). The velocity of a particle is first measured based on the time it takes to pass through two laser beams (time of flight TOF) at a given amount of kinetic energy. (What other information do you need to calculate the diameter of a particle?)

With smaller particles, scanning electric mobility sizer (SEMS) or scanning mobility particle sizer (SMPS) charges the particle. As particles go through an electric field, different size particle will experience different velocities and thus varied trajectories. Depended on the voltage applied, selected particles within certain range of diameter can pass through the electric field and get counted. Aerosol Time-of-Flight Mass Spectrometry (ATOFMS) measures the size and composition of an individual aerosol particle and was invented by Professor Prather. Similar to APS, ATOFMS measure the diameter of the particle by the time it takes to pass through two lasers, then the composition is decided by the time-of-flight mass spectrometry. The commercial instrument Aerosol Mass Spectrometer measures similar things except it provides information about size segregated particles instead of a single particle. 

Figure 2.  ATOFMS By Benjamin Haywood – Own work, CC BY-SA 4.0. Retrieved from https://commons.wikimedia.org/w/index.php?curid=39073594

The study of sea spray aerosol still requires massive investigations to better understand the substances aerosols carry out from the ocean, the reactions aerosols undergo in atmosphere and ultimately aerosol’s interaction with cloud formation and weathering. 

Glossary of Terms

• Aerosol: Liquid or particle suspended in gas
• Supersaturation: Partial pressure of water is higher than its equilibrium pressure
• Heterogeneous reaction: An reaction involve reactants in different phrases (Solid, gas, liquid) 
• Nascent aerosol: Aerosol immediately form from the source of production
• Secondary aerosol: Aerosol forms after reactions take place
• Hygroscopicity: Reactivity to water molecule

References

1. File:ATOFMS.png. (2017, November 26). Wikimedia Commons, the free media repository. Retrieved 17:58, October 5, 2019 from https://commons.wikimedia.org/w/index.php?title=File:ATOFMS.png&oldid=269402388.

2. Prather, K. A., Bertram, T. H., Grassian, V. H., Deane, G. B., Stokes, M. D., DeMott, P. J., Zhao, D. (2013, May 7). Bringing the ocean into the laboratory to probe the chemical complexity of sea spray aerosol. Retrieved from https://www.pnas.org/content/110/19/7550.

3. Richard, V.Trueblood1, J., D.Estillore1, A., S.Morris2, H., M.Sultana1, C., A.Dowling2, J., Ssa. (2017, May 11). Molecular Diversity of Sea Spray Aerosol Particles: Impact of Ocean Biology on Particle Composition and Hygroscopicity. Retrieved from https://www.sciencedirect.com/science/article/pii/S2451929417301201.

4. Schematic multi-modal particle size distribution with typical transformations and example particle types within each mode. (n.d.). Photograph. Deutscher Wetterdienst. Retrieved from https://www.dwd.de/EN/research/observing_atmosphere/composition_atmosphere/aerosol/cont_nav/particle_size_distribution_node.html

5. File:ATOFMS.png. (2017, November 26). Wikimedia Commons, the free media repository. Retrieved 17:58, October 5, 2019 from https://commons.wikimedia.org/w/index.php?title=File:ATOFMS.png&oldid=269402388.