Conversely, we observed that when ammonium nitrate is deposited on ice, NO - 3 can pair with NH + 4 , or even form a hydrogen-bonded complex with NH 3 . In the case of NO - 3 /NH + 4 pairing, our calculations predict a slightly attenuated molar absorption coefficient, whereas when a NO - 3 /NH 3 complex is formed the molar absorption coefficient of nitrate is enhanced roughly by a factor of 1.5. At the concentration of our simulations, corresponding to 0.3/nm 2 , the formation of aggregates of nitrate have a relatively low probability, but at higher concentrations and different pH conditions pairing of nitrate with ammonium or ammonia would become more relevant and affect light absorption. Nevertheless, while our results suggest that the formation of surface aggregates influences the photon uptake of nitrate, the computed variations of the molar absorption coefficient are relatively small compared to the enhancement of photolysis at the air-ice interface observed in experiments. It is then sensible to assume that such enhancement is mostly due to augmented quantum yields for either or both nitrate photodissociation channels. To achieve a full picture of nitrate photolysis, further modeling, and experimental work is needed to determine the quantum yield of the two nitrate photolysis channels at the air-ice interface and in bulk aqueous solutions.