The hysteresis cycle of concentration in a solution droplet under changing humidity

¹ CNR ISAC Istituto di Scienze dell’Atmosfera e del Clima, Via Fosso del Cavaliere 100, 00133 Roma, Italy
² CNR ISAC Istituto di Scienze dell’Atmosfera e del Clima and INFN, Sez. Lecce, Str. Prov. Lecce-Monteroni, 73100 Lecce, Italy

^a e-mail: a.lanotte@isac.cnr.it

Received: 23 May 2013
Revised: 7 November 2013
Accepted: 2 June 2014
Published online: 11 July 2014

Abstract

For a solution droplet in equilibrium with the atmospheric environment, a relationship exists between radius and concentration, which allows to express the saturation ratio of the droplet as a function of either one of these two parameters. The curves showing the complete behaviour of saturation ratio as a function of radius, for various sizes of NaCl nuclei, were previously presented for both wholly and partially dissolved salt. Here, the dependence of saturation ratio on droplet concentration, rather than on its radius, is examined and plotted for various NaCl nuclei. The occurrence of an analogous, but X-shaped, hysteresis phenomenon, characterizing the behaviour of the solution concentration in a growing-shrinking cycle of a solution droplet under changing humidity, is evidenced and discussed. An insoluble spherical core is assumed to be always present inside the condensation nucleus, so that the onset of the sudden salt re-crystallization is triggered at a well defined concentration value.