The mass transfer mechanism across the air-water interface due to the impingement of a single liquid drop was investigated through laboratory experiments using particle imaging velocimetry (PIV) and planar laser-induced fluorescence technique (PLIF). Velocity and CO2 concentration fields in the liquid after the impingement were visualized. The results show that the impingement of a single liquid drop on the water surface generates several vortex rings near the water surface. The vortex rings renew the water surface and also convect the CO2 gas dissolved near the water surface downward. The vortical motion clearly shows that the vortex rings work as surface-renewal eddies. The radius, center velocity and presence time of surface-renewal eddies increase with increasing momentum of the impinging drop. This suggests that surface-renewal eddies with larger radius and faster center velocity are induced by the impingement of a single drop with larger vertical momentum, and air-water mass transfer is promoted by such eddies. Based on the surface-renewal concept including the area and time fractions, a model for the air-water mass transfer due to multiple impingements of drops is also proposed. (C) 2013 Elsevier Ltd. All rights reserved.