HR: 1340h
AN: A13B-0913    [Abstracts]
TI: Cloud Droplet Size in Low Saturation Ratio Causing Incompletely-Deliquesced CCN
AU: * Shiba, S
EM: shiba@cheng.es.osaka-u.ac.jp
AF: Osaka University, 1-3, Machikaneyama-cho, Toyonaka, Osa 560-8531 Japan
AU: Hirata, Y
EM: hirata@cheng.es.osaka-u.ac.jp
AF: Osaka University, 1-3, Machikaneyama-cho, Toyonaka, Osa 560-8531 Japan
AU: Yagi, S
EM: yagi@ise.setsunan.ac.jp
AF: Setsunan University, 17-8, Ikedanaka-machi, Neyagawa, Osa 572-8508 Japan
AB: Introduction It is customarily understood that condensational growth curve of hygroscopic CCN exhibits discontinuity at a so-called deliquescence point. According to classical experiments, with increase in relative humidity (i.e., saturation ratio), hygroscopic solid particle dissolves sharply at a critical vapor pressure, deliquescence pressure, and increases its volume abruptly. However, recent experiments reveal that deliquescence curve increases not abruptly at a point but smoothly over a narrow band of humidity (deliquescence band). This suggests that CCN gradually dissolves in the water coating CCN with increase in saturation ratio. To examine equilibrium size of cloud droplets in deliquescence band, a mathematical model based on a thermodynamic consideration has been constructed and model calculations have been done. Mathematical Model When hygroscopic CCN is completely dissolved into cloud droplet, Köhler model is used to estimate the equilibrium cloud droplet size. However, in the case that there is no sufficient water vapor (i.e., low saturation ratio) to dissolve CCN completely and that hygroscopic solid CCN remains in the droplet, we cannot use Köhler model, because it treats the equilibration between gas (ambient vapor) and liquid (complete solution). Deliquescence process contains three phases of solid (CCN undissolved), liquid (coat over CCN) and gas. Two phase-transitions from solid to liquid and from vapor to liquid control the process. Then, the model must fulfill the equilibrium conditions at solid-liquid and gas-liquid interfaces simultaneously. The deliquescence curve, which is similar to Köhler one, due to this idea is obtained as a simple relation between saturation ratio and cloud droplet radius. Also, coat-kernel equation which gives the relation among cloud droplet size, residual solid CCN size and solubility of CCN is obtained. Model Calculations Cloud droplet size in deliquescence band is given numerically as the intersection of saturation curve, which is determined by water mass conservation in the air parcel, and deliquescence curve. Once droplet size is known, we can obtain residual solid CCN size from coat-kernel equation. Deliquescence band in terms of cloud droplet (or residual solid CCN) radius is determined from coat-kernel equation. Conclusions Model calculations are sensitive to dry CCN size and show that: Normalized cloud droplet size in deliquescence band ranges from the lower limit of unity to the upper limit of the peculiar value to CCN species; The larger dry CCN size is, the lower deliquescence saturation ratio becomes; and The larger dry CCN size is, the narrower deliquescence band becomes.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0320 Cloud physics and chemistry
DE: 3311 Clouds and aerosols
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005