AQUEOUS CHEMISTRY. “Super-cooled liquid water is rarely pure water, foremost because it has itself nucleated heterogeneously on a cloud condensation nuclei. Hence, depending on their size, supercooled water droplets in the atmosphere tend to be solutions with differing degrees of concentration. The effect of the solute is to reduce the water activity.”
http://www.mpimet.mpg.de/fileadmin/staff/stevensbjorn/teaching/skript-5.pdf