Microphysics of atmospheric carbon dioxide uptake by a cloud droplet containing a solid nucleus

Microphysics of atmospheric carbon dioxide absorption by a cloud aerosol containing a spherical solid nucleus at low Reynolds number (Re-g = 1) is investigated through numerical simulation. Particular emphasis is placed on the impact of the solid particle on the solute transport process in the aerosol. Simulations indicate that carbon dioxide transported by internal circulation in a pure water droplet is significant. Therefore, during the absorption period the lowest CO2 concentration moves from the droplet center, through the centerline, and then toward the vortex center. When a solid nucleus with r(s)/r(1) = 0.5 exists in the droplet, the vortex strength is reduced markedly. Nevertheless, the role played by the internal circulation on mass transport is still obvious. This results in the lowest CO2 concentration being located at the front stagnation point of the nucleus as the exposure time is sufficiently long. Once the nucleus is large to a certain extent, such as r(s)/r(1) = 0.8, the internal flow retarded by the solid particle is more remarkable, whereby the radial diffusion becomes very important. Consequently, the entire mass transfer is characterized by one-dimensional transport. The above investigations clearly reveal that the nature of the solute movement in the droplet is highly relevant to the nucleus size. As a whole, with increasing size of the solid particle, diffusional mass transfer progressively dominates convective mass transfer; however, it simultaneously increases the surface-to-volume ratio of the liquid phase. Because the importance of the ratio is beyond that of the diffusion, the absorption rate of the aerosol rises as the nucleus size increases.