When a raindrop with fixed radius falls downward in association with terminal velocity, a recirculation bubble always accompanies the gas-phase flow field in the aft region of the drop. With regard to drop internal flow structure, it is sensitive to the drop's size. When the drop radius is small (e.g. rs=200μm), only a single vortex motion is seen inside the drop. Under such a situation, sulfur dioxide absorbed from the interface is mainly transported from the area in front of the aft stagnation point. In contrast, as the drop is relatively large, say, rs=500μm, it is of interest to find that a two-vortex motion, composed of a primary and a secondary vortexes, appears. As a result, the onset of SO2 transport process occurs at the drop's surface near where the two vortexes meet. Accordingly, it is recognized that transient SO2 transport processes inside a raindrop is essentially affected by drop size.