Chemical consequences of mixing atmospheric droplets of varied pH

The mixing of droplets of different pH and acid-neutralizing capacity that occurs when bulk samples of cloud, fog, and rainwater are collected yields solutions that are not in equilibrium with the atmosphere in which the droplets originally equilibrated. In most instances, bulk solutions are expected to outgas volatile solutes to reestablish equilibrium with the atmosphere, with concomitant changes in solution pH. This mixing-induced gas exchange complicates equilibrium and kinetic modeling of pH-dependent atmospheric processes and limits the use of bulk samples for estimation of atmospheric fluxes of volatile weak acids and bases (SO₂(g), NH₃(g), etc.). Calculations that attempt to interrelate bulk solution pH, bulk solution S(IV) concentration, and atmospheric SO₂(g) partial pressure are shown to be fundamentally incorrect, unless the bulk sample has completely reequilibrated with the same atmosphere in which its component droplets were formed. A theoretical proof and several numerical examples are presented to show that droplet mixing at constant atmospheric partial pressures of soluble gases cannot occur without gas exchange.