Hereafter, we explore the effect of the different solvation environments on the light absorption of nitrate. From each MD trajectory analyzed above, we calculated the UV-visible absorption spectrum at a finite temperature using the ensemble approach described in the methods section. The full spectra are displayed in Figure , while the most environmentally relevant lowest-energy band is shown in Figure ). The spectra in Figure exhibit a profound absorption band due to due to a π → π * transition at 205 nm, followed by a minor absorption band related to a n → π * transition at ∼ 300 nm. The molar absorption coefficient (ε) of the n → π * transition is several orders of magnitude smaller than ε from the π → π * transition, as the former is forbidden by symmetry. However, finite-temperature distortion and the presence of cations and polar solvent molecules break the symmetry and result in a finite absorption band at the edge of the solar spectrum. The wavelength at the maximum absorptivity (λ max ) is between 305 and 307 nm, regardless of the solvation environment. In experiments, λ max is at 301 and 303 nm for nitrate in aqueous solution originating from HNO 3 and NH 4 NO 3 , respectively. For deprotonated HNO 3 , we observe a slight blue shift of λ max of ∼ 2 nm when nitrate is at the air-ice interface, whereas no shift in the peak position occurs for the NH 4 NO 3 system. Such a small shift in λ max , corresponding to excitation energy changes of about 30 meV, is far below the accuracy of our computational approach. Hence, we conclude that our calculations predict no significant solvatochromic shift for nitrate on ice. This result is consistent with UVvisible absorption measurements showing no shift for nitrate in liquid water and in a frozen solution. While we see no significant solvatochromic shift, the coupling with ammonium or ammonia affects the molar absorption coefficient of nitrate. The presence of ammonia leads to an enhancement of ε, in contrast with a reduction of ε induced by coupling with ammonium in Figure . In contrast, this change in ε was not seen experimentally between NO - 3 /NH + 4 and HNO 3 in solution. Figure shows the energy diagrams for two representative configurations of the NO - 3 /NH 3 (a) and the NO - 3 /NH + 4 complexes on ice. In the first case, the ammonia provides an occupied state with energy quasi-degenerate with the HOMO of NO - 3 . This state provides a finite contribution to the S 0 → S 1 excitation of the complex, thus enhancing the overall molar absorption coefficient. Conversely, NH + 4 does not provide additional excitation channels, and its presence partly disrupts the first solvation shell of the nitrate anion, leading to a small decrease in the molar absorption coefficient.