Cloud condensation experiments probe nanoscale effects on water vapor adsorption by insoluble particles

Understanding the interactions of atoms and molecules with the surfaces of nanoparticles is important not only for technological processes such as heterogeneous catalysis, but also in atmospheric science. The adsorption of water molecules onto insoluble aerosols enables them to become nuclei for cloud droplets. Hitherto, predictions of the cloud condensation efficiency of insoluble particles based on water adsorption measurements have not agreed with laboratory experiments on cloud drop formation. Here, we show that agreement is obtained for a wide range of particle types by accounting for two nanoscale factors that affect the adsorption energies of water molecules: the fractal heterogeneities of particle surfaces and the particle size. The size effect becomes pronounced at adsorption layer thicknesses of hundreds of water layers, typical during the activation of cloud nuclei but difficult to detect in standard adsorption experiments. Our discovery improves understanding of gas and vapor adsorption on nanomaterials in general. The nucleation of cloud droplets by insoluble cloud condensation nuclei (CCN) has been studied extensively during the past two decades, but predictions of the cloud condensation efficiency of insoluble particles based on water adsorption measurements do not always agree with laboratory experiments on cloud drop formation. Here, the authors show that accounting for the fractal heterogeneities of particle surfaces and the particle size improves the agreement between theoretical predictions and CCN activation experiments.