How volatile components catalyze vapor nucleation.

Gas phase nucleation is a ubiquitous phenomenon in planetary atmospheres and technical processes, yet our understanding of it is far from complete. In particular, the enhancement of nucleation by the addition of a more volatile, weakly interacting gaseous species to a nucleating vapor has escaped molecular-level experimental investigation. Here, we use a specially designed experiment to directly measure the chemical composition and the concentration of nucleating clusters in various binary CO-containing vapors. Our analysis suggests that CO essentially catalyzes nucleation of the low vapor pressure component through the formation of transient, hetero-molecular clusters and thus provides alternative pathways for nucleation to proceed more efficiently. This work opens up new avenues for the quantitative assessment of nucleation mechanisms involving transient species in multicomponent vapors.