Comparison of Nitrate and Chloride Anions at the Air-Water Interface by Second Harmonic Generation and Surface Tension

Some of the most common ions in the atmosphere are nitrate (NO3-) and chloride (Cl-) which play key roles in many atmospheric reactions, including ozone depletion. Additionally, they influence the iron equilibria that dictate reaction kinetics. Heterogeneous reactions are heavily influenced by the presence of ions at the air-water interface but there is still not a definitive distinction between the surface propensity of NO3- and Cl-. The impact of NO3- and Cl- on the hydrogen-bonding structure at the air-water interface has been examined using second harmonic generation (SHG) and surface tension measurements. The impact of ferric iron on the propensity of NO3- has also been examined with surface tension by comparison to sodium nitrate. The SHG electric field shows that the hydrogen-bonding structure of NaNO3 and NaCl solutions is significantly different from neat water but shows a slightly different dependence on concentration. Surface tension measurements support this change in structure by displaying significantly different concentration dependence. The introduction of iron has a large impact on the surface as shown by the increased surface tension slope as well. To confirm the presence of iron-anion dipoles, geometry optimization of ferric chloride complexes was performed to obtain dipole moments.