Nitrate Anions and Ion Pairing at the Air−Aqueous Interface

Nitrate ions are ubiquitous in aqueous-phase atmospheric aerosols from the polluted to the remote troposphere and are involved in a variety of atmospheric reactions. Thus, a fundamental understanding of nitrate ions at the air−aqueous interface is of prime importance with respect to understanding atmospheric aerosol chemistry. In the present study, investigations of the air−aqueous interface of a series of divalent metal-nitrate solutions, Mg(NO 3 ) 2, Ca(NO 3 ) 2, and Sr(NO 3 ) 2, were carried out using vibrational sum frequency generation (VSFG) spectroscopy. The vibrational symmetric stretch mode of nitrate ions at the air−aqueous interface (1047 cm −1 and 1063 cm −1 ) was directly probed. Analysis of the VSFG spectra reveals the perturbation from cation−anion interactions on interfacial nitrate ions. Ion pairing between interfacial nitrate anions and divalent metal cations follows the same trend as bulk ion pairing: Sr 2+ > Ca 2+ > Mg 2+ . Moreover, nitrate anions in the air−aqueous interfacial region are found to be relatively free from Coulombic effects of Mg 2+ cations for Mg(NO 3 ) 2 concentrations with at least seven hydrating water molecules on average per ion.