Engineering Development of the Free Ocean CO2Enrichment (FOCE) Experiment

Globally, the burning of fossil fuels for energy production produces over 25 gigatons of CO
₂
per year and this material is released directly into the atmosphere. While approximately half of the CO
₂
has remained in the atmosphere long-term, most of the rest has been absorbed by the surface ocean. This has resulted in a lowering of the surface ocean pH by about 0.1 units since the beginning of the industrial revolution and if society is able to stabilize atmospheric CO
₂
levels at twice their pre-industrial concentrations will result in a lowering of surface ocean pH by 0.25 units. While many are asking the question of whether we should pursue direct ocean CO
₂
sequestration, the FOCE experiment is asking what will be the impact of the pH change on the ocean. In order to address this question, MBARI science and engineering have designed a small-scale in situ CO
₂
enrichment experiment to assess the chemical and biological impacts in a manner analogous to the land-based Free Air CO
₂
Enrichment (FACE) experiments. This prototype design is testing the ability to control pH within a fixed but freely exchanging volume of sea water. The technology concept for the experiment is based on a small ring structure using a central valve to direct the flow of pH altering fluid. The initial phase of the project uses concentrated HCl mixed with sea water and includes directional and volume control to achieve a desired pH offset. Control feedback is obtained by using pH sensors in the center of the control volume. Other aspects of the design that address the inherent time delays and noise of the associated pH signal are also discussed. Test results will show the capability of the system to maintain close loop control of pH in a given volume. Sea trials then demonstrate the ability of this initial system at a selected site to control pH to specified average level over a given amount of time. Further discussion includes systems in-situ results analysis, corrective actions, upgrades, and the anticipated next phase for FOCE including the use of CO
₂
addition to change the local chemistry