Atmospheric aerosol particles, i.e. liquid or solid particles floating in air, are an important component determining the quality of the air we breathe and influencing the Earth’s radiative balance. These particles are in constant interaction with the gas phase surrounding them, and a large fraction of them has originated from gas-to-particle conversion processes that take place in the atmosphere. The sizes of atmospheric particles range several orders of magnitude from clusters of only a few nanometers to e.g. dust particles or cloud droplets of several tens of microns, and they consist of complex mixtures of inorganic and organic molecules. Furthermore, concentrations of atmospheric aerosol particles typically vary in scales that are considerably smaller than the temporal and spatial scales captured by e.g. climate or air quality models. This complexity and variability presents a challenge for accurately describing the effects of these tiny particles within the Earth system. In my presentation I will introduce some of our present work dealing with molecular interactions of atmospheric aerosol particles with the ambient gas phase, discuss the potential relevance of these small-scale processes for regional air quality and global climate, and outline some of the important open questions.