Partitioning of atmospherically relevant ions between bulk water and the water/vapor interface

Recently, surface-sensitive spectroscopy data and molecular dynamics simulations have generated intense interest in the distribution of electrolyte ions between bulk water and the air/water interface. A partitioning model for cations and anions developed for biopolymer surface is extended here to interpret the effects of selected acids, bases, and salts on the surface tension of water. Data for electrolytes were analyzed by using a lower-bound value for the number of water molecules in the surface region [0.2 H2O angstrom(-2) (approximately two layers of water)], obtained by assuming that both Na+ and SO42- (i.e., Na2SO4) are fully excluded from this region. Surface-bulk partition coefficients of atmospherically relevant anions and the proton are determined. Notably, we find that H+ is most strongly surface-accumulated, I- is modestly accumulated, NO3- is evenly distributed, and OH- is weakly excluded.