A combined intensity and lifetime-based laser-induced fluorescence technique (i-$$\tau$$LIF) with dye-doped nanobeads tracer for dissolved oxygen imaging below the water surface

Abstract The interfacial transfer process of atmospheric gases, such as oxygen, in a turbulent flow environment is characterized by a very thin concentration boundary layer with a highly dynamic concentration distribution further below the water surface. In order to capture such small-scale details at high time and spatial resolutions, a combined intensity and lifetime-based laser-induced fluorescence technique (i- $$\tau$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>τ</mml:mi> </mml:math> LIF) is presented. The combined method allows to use the iLIF method to obtain concentration images, while concurrently, one or more in situ benchmark data points are obtained non-intrusively using the $$\tau$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>τ</mml:mi> </mml:math> LIF method, which enables the reconstruction of the absolute (intensity to concentration) iLIF calibration curve from the generally valid normalized iLIF calibration curve. An accurate prediction of the benchmark points is possible, provided their values lie in the lower range of the oxygen concentration. The performance of the combined i- $$\tau$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>τ</mml:mi> </mml:math> LIF system was assessed by conducting experiments of interfacial gas transfer promoted by evaporative surface cooling. It was shown that the system was able to reveal the temporal and spatial variations of the concentration boundary layer as well as the structural developments of the deep-sinking plumes and the small-scale plumes that tend to linger near the surface, which are typically found in evaporative-cooling-induced gas transfer. Quantitative validation tests, including a one-to-one comparison of the i- $$\tau$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>τ</mml:mi> </mml:math> LIF results with microsensor measurements, yielded very good agreement. The high spatial resolution of the system allowed direct quantification of instantaneous concentration profiles and, hence, the estimation of the instantaneous gas transfer velocity. Graphic abstract