The flow conditions for generating periodically oscillatory flows and thus entraining a large amount of air bubbles into the water mass in a vertical dropping pool, which can be used potentially as a green aerator, were investigated experimentally. The approaching flows passing through upstream reach of a vertical dropping pool were studied under sub-critical condition. A wave gauge was used to measure the free surface fluctuations in the pool, and flow visualization technique was employed to reveal the flow structure of the dropping flows qualitatively. Under certain conditions, the falling flow over a vertical dropping pool forms a switching jet that oscillates up and down periodically and impinges on the bottom and the downstream corner of the pool alternately. The switching jet switches between an impinging jet (or napped flow) and a sliding jet (or skimming flow), causing it to oscillate periodically with a unique period and to entrain a large number of air bubbles into the switching jet, thus enhancing the dissolved oxygen quantity and turbulent mixing. The primary frequency of the periodic oscillation was determined by applying spectral analysis to the time history of wave-gauge measurements for the free surface elevation of the dropping flows. Variables influencing the fundamental oscillation frequency were carefully checked, and an empirical relation between a weighted Strouhal number and a grouped non-dimensional parameter was proposed to predict the primary frequency of the periodically oscillatory flow.