Fluorescent Visualization of Chemical Profiles across the Air-Water Interface

Chemical transfer across the air-water interface is one of the most important geochemical processes of global significance. Quantifying such a process has remained extremely challenging due to the lack of suitable technologies to measure chemical diffusion across the air-water microlayer. Herein, we present a fluorescence optical system capable of visualizing the formation of the air-water microlayer with a spatial resolution of 10 mu m and quantifying air-water diffusion fluxes using pyrene as a target chemical. We show for the first time that the air-water microlayer is composed of the surface microlayer in water (similar to 290 +/- 40 mu m) and a diffusion layer in air (similar to 350 +/- 40 mu m) with 1 mu g L-1 of pyrene. The diffusion flux of pyrene across the air-water interface is derived from its high-resolution concentration profile without any pre-emptive assumption, which is 2 orders of magnitude lower than those from the conventional method. This system can be expanded to visualize diffusion dynamics of other fluorescent chemicals across the air-water interface and provides a powerful tool for furthering our understanding of air-water mass transfer of organic chemicals related to their global cycling.