Specifically, not only NO - 3 /NH + 4 is used in several reference experiments, 9 but it is relevant beyond polar atmospheric chemistry, as HNO 3 and ammonia react in the gas phase to form solid particulate which is a major contributor to urban smog. Furthermore, we work under the assumption that the structure of the solvation environment of NO - 3 in LLR is similar to that in bulk water at room temperature, whereas it is unlikely to have NO - 3 incorporated in crystalline bulk ice. We find that the lowest energy band in the absorption spectra of all four systems does not exhibit any shift in the peak wavelength of absorption. However, the proximity of ammonium species at ice surfaces influences the molar absorption coefficient of nitrate, suggesting that clustering at the air-ice interface may affect light absorption and the photolysis of nitrate.