Ion specific Interactions and Surface Enrichment on Mixed Aerosol Surfaces

Aerosols have a net cooling effect in our atmosphere, mainly due to their ability to act as cloud condensation nuclei (CCN) and to reflect incoming solar radiation [1]. In aerosols the surface is particularly important as it is the site for many processes which are central to cloud formation, in particular evaporation and condensation. Aerosol surfaces contain a wide variety of organic and inorganic chemical compounds. Understanding the resulting surface structure and composition is a necessary first step in gaining a better understanding of the role aerosols play in the atmosphere.

In particular, the surface composition affects the hygroscopicity of the particle and thereby its ability to act as a CCN [2]. While the surface propensity of individual compounds has been studied widely, much less is known about cooperative effects in more complex mixed systems [3]. Here we use sum-frequency-generation (SFG) spectroscopy in conjunction with surface tension data to study whether surface competition or cooperative enhancement can be observed in mixed solutions of halides and acids or alcohols of different chain length.

We find that in certain cases the presence of different ions can affect the surface structure in a unique way. However, these effects are not systematic and appear to depend on the specific nature of the interaction between the ions and the respective organic compound. This further highlights that the surface structure of mixed systems cannot be extrapolated from studies of simple solutions.

 

[1] IPCC. Climate Change 2013: The Physical Science Basis. [2] P. Zieger et al., Nat. Commun. 2017, 8, 15883. [3] M.T. Lee et al., Phys. Chem. Chem. Phys. 2019, 21, 8418