Surface fractal dimension, water adsorption efficiency, and cloud nucleation activity of insoluble aerosol.

Ari Laaksonen,Jen-Ping Chen,Hui-Ming Hung,Athanasios Nenes,Jussi Malila

doi:10.1038/srep25504

Abstract

Surface porosity affects the ability of a substance to adsorb gases. The surface fractal dimension D is a measure that indicates the amount that a surface fills a space, and can thereby be used to characterize the surface porosity. Here we propose a new method for determining D, based on measuring both the water vapour adsorption isotherm of a given substance, and its ability to act as a cloud condensation nucleus when introduced to humidified air in aerosol form. We show that our method agrees well with previous methods based on measurement of nitrogen adsorption. Besides proving the usefulness of the new method for general surface characterization of materials, our results show that the surface fractal dimension is an important determinant in cloud drop formation on water insoluble particles. We suggest that a closure can be obtained between experimental critical supersaturation for cloud drop activation and that calculated based on water adsorption data, if the latter is corrected using the surface fractal dimension of the insoluble cloud nucleus.

Highlights

Temperature, and R is the radius of the droplet encompassing the insoluble particle
The surface area of an object within a radius r is proportional to rD, where D is the surface fractal dimension
The surface fractal dimension is an important quantity in determining the critical supersaturation of cloud droplets forming on insoluble nuclei

Summary

Introduction

Temperature, and R is the radius of the droplet encompassing the insoluble particle. This equation can be used to produce a curve of S vs. R (or N) that has a maximum, marking the critical supersaturation at which the cloud drop is formed. Kumar et al.[4] determined the FHH parameters of different particle types based on measured critical supersaturations, whereas Hatch et al.[5] and Hung et al.[6] determined A and B based on adsorption measurements, and used the FHH-adsorption–activation theory to predict the critical supersaturations. These two approaches do not produce satisfactory closure. Motivated by this discrepancy, we show how surface structure, expressed by the fractal dimension, and its interaction with water vapour induce the observed critical supersaturations. This allows for an unprecedented degree of understanding of how surface porosity and the associated capillary condensation affect hygroscopic growth and cloud drop activation of water insoluble aerosols in the atmosphere

Objectives

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Conclusion