The transport of ${\mathrm{CO}}_2$ 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 measurement method to quantify the ${\mathrm{CO}}_2$ 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 ${\mathrm{CO}}_2$ 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 ${\mathrm{CO}}_2$ into pure water and alkaline solutions of different pH values were continuously monitored. A time resolution of 5 ms and a spatial resolution of 1 mm were achieved. The observed gas-phase ${\mathrm{CO}}_2$ distribution evolution agreed with the classic one-dimensional diffusion model, which validated the accuracy of the current method. PH-dependent dynamics of interfacial ${\mathrm{CO}}_2$ concentration was also observed: the ${\mathrm{CO}}_2$ depletion rate is highly pH-sensitive at low pH and saturates at pH $\approx$ 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.

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• Carbon sequestration
• Laser spectroscopy
• Multiphase flows
• Oceanography
• Transport phenomena
• Gas-liquid interfaces