Cavity ring-down spectroscopy (CRDS) is a high-sensitivity technique used to measure the absolute concentrations of absorbing species. We have used CRDS to measure spatially dependent concentration profiles near a physical surface that obstructs the light path. A decrease in detection sensitivity resulting from interaction between the circulating cavity ring-down beam and the surface was observed. Simple calculations based on geometrical obstruction by the solid surface significantly underestimate the sensitivity loss, suggesting that diffraction is important. Calculations that include diffraction effects more closely agree with experimental data.