Köhler theory for a polydisperse droplet population in the presence of a soluble trace gas, and an application to stratospheric STS droplet growth

[摘要] We consider the equilibrium behavior of a polydisperse aqueous dropletpopulation as a function of relative humidity (RH) when a soluble trace gas, such as nitric acid, is present in the system. The dropletpopulationexperiences a splitting when the RH is increased sufficiently. This splitting is not related to the traditionalKöhler activation of cloud droplets, as it may occur at relative humidities below100%. Remarkably, the splitting always takes place insuch a way that the largest size class of the(discretized) droplet population starts taking up the soluble trace gas efficiently,growing steeply as a function of RH, and forcing the smaller droplets to shrink. We consider this behavior in terms of open and closedsystem Köhler curves (open system referring to one in which the trace gas concentration remains constant and closed system to one inwhich the gas concentration decreases as a result of uptake of the trace gas). We show how the open and closed systemKöhler curves are related, and that the splitting of the population can be explained interms of closed system curves crossing the Köhler maxima of the open system curves. We then go on to consider time-dependent situations,and show that due to gas-phase mass transfer limitations, the splitting of the size distributions moves toward smaller sizes as therate of RH increase becomes more rapid. Finally, we consider stratospheric supercooled ternary solution droplet populations, andshow that the splitting described using the new theory may lead to formation ofbimodal size distributions in the stratosphere.

[学科分类] 大气科学