Sulfur dioxide absorption in a cloud droplet under the influence of convective flows

Unsteady absorption characteristics of sulfur dioxide by a cloud droplet in convective flows are predicted numerically to recognize the physical mass transport processes. Three different Reynolds numbers, Re_g=0.643, 1.287, and 12.87 are studied and compared with each other by considering the droplet with various velocities. The results indicate that for the Reynolds number of 0.643, sulfur dioxide always penetrates toward the droplet centerline throughout the entire absorption period. In contrast, when the Reynolds number is 12.87, most of the time the concentration contours parallel the streamlines and the lowest sulfur dioxide concentration is located at the vortex center. As a consequence, the diffusion distance is reduced by a factor of three and the absorption time for the droplet reaching the saturated state is shortened in a significant way. With regard to an intermediate Reynolds number such as 1.287, a two-stage mass transfer process can be clearly identified, and the contour core is inconsistent with the vortex center. The present study elucidates that the strength of a droplet's internal motion plays a vital role in determining sulfur dioxide absorption process.