Concentrated aqueous nitrate aerosols are present in the Earth's atmosphere as a result of heterogeneous reactions of sea salt and mineral dust aerosol with nitrogen oxides ( e. g., NO2, NO3, HNO3 and N2O5). Because the water content of these aerosols depends on relative humidity ( RH), the composition and nitrate ion concentration will also depend on RH. Unlike the original aerosols, aqueous nitrate aerosols are photochemically active at solar wavelengths. To gain a better understanding of the nitrate ion chromophore in concentrated aqueous nitrate aerosols, we have measured the ATR-FTIR and UV/vis spectra of concentrated nitrate solutions over a large concentration range. Both ATR-FTIR and UV/vis spectroscopy show changes in the nitrate ion spectra with increasing concentration. Ab initio calculations are used to aid in the assignment and interpretation of these spectra. From these data, we predict that the photoreactivity of aqueous nitrate aerosols will strongly depend on relative humidity as the molecular and electronic structure of the nitrate ion becomes increasingly perturbed from that of the isolated ion in highly concentrated atmospherically relevant solutions.