Fluorescence Imaging of Air-Sea Gas Exchange using Methoxybenzenes

The exchange of low to moderately soluble gases and volatile chemical species between air and water is controlled by a sublayer within the aqueous viscous boundary layer which has a thickness of just 10 to 350 µm at the water surface. This is why knowledge about the mechanisms of the transport process is still not sufficient. Only high resolution imaging techniques have the potential to measure concentration fields in this thin layer. All techniques used so far, required to set up complex chemical systems or to use expensive fluorescent dyes so that the usage in large wind-waves facilities normally is not possible. Here we show that methoxybenzenes dissolved in water exhibit a high fluorescence intensity. An InnoLas SpitLight Compact OPO-355 with UV extension was used to tune the excitation wavelength between 220 and 280 nm and to find the optimum excitation wavelength. The simplest methoxybenzene, anisole, is a very sensitive tracer, but other methoxybenzenes can also be used. It is sufficient to use ppm-levels in the air space to obtain a high enough fluorescence intensity to measure the concentration profile across the thin aqueous boundary layer at the water surface.