Spatiotemporally Resolved Measurements of CO2 Distribution at the Air-Water Interface Using Tunable Diode Laser Spectroscopy

Abstract

The transport of CO2 across the air-water interface is central to physical oceanography and carbon sequestration. A comprehensive understanding of this process requires high-resolution diagnostics of diffusion, absorption, and reaction across a wide range of spatial and temporal scales. The current study presents a novel measurement method to quantify the CO2 distribution at the air-water interface. This method combines the advantages of tunable diode laser spectroscopy and rapid spatial beam scanning for in situ, nonintrusive, and spatiotemporally resolved measurement of the CO2 concentration distribution above the interface. The performance of this method was examined in a series of quasi-1D experiments in a miniature gas chamber, where the diffusion and absorption of CO2 into pure water and alkaline solutions of different pH values were continuously monitored. An effective time resolution of 5 ms and an effective spatial resolution of 1 mm were achieved. The observed gas-phase CO2 distribution evolution agreed with the classic one-dimensional diffusion model, which validated the accuracy of the current method. PH-dependent dynamics of interfacial CO2 concentration were also observed. The CO2 depletion rate is highly pH-sensitive at low pH and saturates at pH = 10, revealing complex competition between the gas-phase and the liquid-phase transport processes. The current method's high spatial and temporal resolution holds promise for studying cross-interface gas transport under more complex flow conditions, in both field measurements and laboratory studies.