Particle deliquescence in a turbulent humidity field

Abstract

The atmosphere contains aerosol particles, some of which are hygroscopic in nature. These particles have direct and indirect effects on weather and climate. Furthermore, turbulence causes fluctuations in temperature, water vapor content, and relative humidity (RH). Turbulent humidity fluctuations may influence, among others, the phase state of specific hygroscopic particles. One process of particular interest in that context is particle deliquescence, which is the phase transition of solid particles to solution droplets. It occurs at a certain RH, the so-called deliquescence relative humidity (DRH), which in turn depends on, e.g., the particle substance. This study investigates the deliquescence behavior of sodium chloride particles in a turbulent humidity field, in particular addressing the questions of whether and how turbulent relative humidity fluctuations affect the number and number fraction of deliquesced particles. The turbulent moist-air wind tunnel LACIS-T (Turbulent Leipzig Aerosol Cloud Interaction Simulator) is used for this study. The results show that the number of deliquesced particles is influenced by turbulent RH fluctuations. On the one hand, particle deliquescence can be observed although the mean RH is smaller than the DRH. On the other hand, there are cases for which non-deliquesced particles are present even though the mean RH is larger than the DRH. In general, the number fraction of deliquesced particles depends on a combination of mean relative humidity, strength of humidity fluctuations, and residence time of the particles in the turbulent humidity field. The study concludes that relying solely on the mean relative humidity is inadequate for determining the phase state of deliquescent particle species in the atmosphere. It is necessary to additionally consider both the humidity fluctuations and the particle history.