Spray-mediated air-sea gas exchange of carbon dioxide in high winds

Understanding the processes that control the exchange of gases between the ocean and atmosphere is crucial to understanding the role of the ocean in the global carbon cycle and predicting the response of the ocean to global change. Gas exchange is more complicated for carbon dioxide than for many other gases because carbon dioxide reacts with water to form carbonic acid, bicarbonate ions, and carbonate ions.

This project seeks to incorporate the full carbonate system into modeling a particular process affecting ocean-atmosphere gas exchange: the exchange of gases with droplets of sea spray produced by breaking waves in high winds. This work builds on previous NSF-funded projects that developed a model for sea spray gas exchange for non-reactive gases. The project will generate a new model of sea spray-mediated gas transfer that will contribute greatly to understanding of air-sea gas exchange in high winds and is needed to improve climate models and predictions.

The results from the project will be made available as open source code. The project will support graduate and undergraduate student research, incorporate mentoring of underrepresented and first-generation students, and involve public outreach.

The objectives of this project build upon the foundational Andreas Gas Exchange Spray model (AGES) developed for non-reactive gases, by adding the components of a reacting gas system and coupling these to a scalable physical model. By constraining the CO2 system as a spray droplet evolves, the investigators will also constrain the alkalinity and pH profile and therefore the chemical environment that controls other reactions and transformations within the droplet.

The investigators expect that, relative to previously modelled non-reactive gases, sea spray will play a much more significant role for CO2 gas exchange. CO2, unlike the previously modeled gases, will be produced during sea spray evaporation as the carbonate and bicarbonate equilibria shift in an evaporating and acidifying spray droplet. This process is significant for a better understanding of gas exchange at high wind speeds to improve current parameterizations.

This project also constrains the chemistry of sea spray drops before reaching the aerosol stage and therefore creates a significant link between surface-ocean and atmospheric chemistry. The team will use the model to generate an estimate of the role sea spray serves in CO2 air-sea exchange in the Southern Ocean. This will present the results in a convenient and relevant global context which may prove useful for global modeling and climate assessments.

Finally, the model generates novel gas-exchange parameterizations expressed in terms of a net transfer coefficient that can be divided into bubble, interfacial, and sea spray transfer coefficients. These parameterizations will be proposed at different levels of complexity with and without explicitly accounting for the wave variables in case a wave model is not available.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.