Exploring the Vapour-Liquid Equilibrium and Mass Transport Dynamics of Water in Aerosol

Abstract:

In order to fully interpret the chemistry occuring in aqueous aerosol, one must first explore how water influences the physical state of the condensed particle phase. Both the chemical dynamics and physical properties of aqueous aerosol are significantly influenced by the vapour-liquid partitioning of water. Furthermore, the rate of evaporation and condensation of water in atmospheric aerosol plays an important role in governing the size distribution and number concentration of aqueous particles and cloud droplets.

We present a study on single aerosol particles undergoing evaporation while confined in an electrodynamic balance. Precise time-resolved measurements of the particle radius allow us to elucidate the microphysical factors influencing water partitioning over a range of temperatures (248 - 330 K) and relative humidities (0 - 95 %).

Specifically, we explore the vapour pressure of pure water in supercooled conditions, establishing values over the widest range of supercooled temperatures assessed experimentally. We show that the mass accommodation and evaporation coefficients, analogous parameters describing the influence of the gas-liquid interface on mass transport, must be greater than 0.5 for uncontaminated water surfaces over the temperature range analysed. We go on to report the impact of insoluble organic monolayers at the gas / liquid interface in retarding the rate of evaporation of water, reducing the evaporation coefficient by up to five orders of magnitude. Such monolayers may also influence the uptake of reactive species from the gas phase, affecting the chemistry occuring at the droplet surface and in the bulk. Finally, we discuss how the water activity of aqueous organic solution droplets may be determined using the measured evaporation dynamics, allowing the hygroscopicity of aerosol at water activities approaching unity to be established with unprecedented accuracy. Compositional measurements under these conditions allow the chemistry occuring in dilute aerosol droplets to be interpreted more fully.