Characterization of transient CO₂ transport in two convecting aerosol droplets in Tandem

A study of carbon dioxide absorbed by atmospheric aerosol droplets can account for the carbon cycle in the atmosphere in part. To figure out the microphysics of carbon dioxide transport in the atmosphere, a numerical method is developed to predict the transient absorption processes of CO₂ by a pair of convecting droplets in tandem. Particular attention is paid to the mass transport influenced by the droplet-droplet interaction. The governing equations include the continuity and momentum equations in gas phase, and the continuity, momentum, and species equations in the liquid phase. Two important parameters of droplet spacing (0.5 and 3 droplet diameter) and Reynolds number (0.1, 1, and 10) are considered. The results show that radial diffusion dominates the H₂CO₃ transport in the droplets at Re = 0.1, whereas internal circulation plays an important role in moving the solute at Re = 10. As a result, the absorption rates of the two droplets are close to each other, regardless of what the droplet spacing is. For the case of Re = 1, the diffusion and the internal circulation are in a comparable state. The absorption rates of the droplets are affected by the droplet spacing to a small extent. The absorption rate of the downstream droplet is always lower than that of the upstream one, resulting from the disturbance of flow field around the former.