The heterogeneous processing of atmospheric aerosols by reaction with nitrogen oxides results in the formation of particulate and adsorbed nitrates. The water content of these hygroscopic nitrate aerosols and consequently the nitrate ion concentration depend on relative humidity, which can impact the physicochemical properties of these aerosols. This report focuses on the 310 nm photolysis of aqueous sodium and calcium nitrate solutions at pH 4 over a wide concentration range of nitrate ion concentrations representative of atmospheric aerosols. In particular, the quantum yield (phi) of nitrite formation was measured and found to significantly decrease at high concentrations of nitrate for Ca(NO3)(2). In particular, phi for Ca(NO3)(2) was found to have a maximum value of (7.8 +/- 0.1) x 10(-3) for nitrate ion solution concentrations near one molal, with the smallest quantum yield for the highest concentration solution above 14 in nitrate ion, phi = (2.3 +/-2.0) x I V. The effect of the addition of the radical scavenger, formate, on the 310 nm photolysis of these solutions was also investigated and found to increase phi by a factor of 2 or more for both sodium and calcium nitrate solutions. In the presence of formate, Ca(NO3)(2) solutions again showed a significant decrease in phi with increasing NO3- concentration: phi = (1.4 +/- 0.1) x 10(-2) at (1.0 +/-0.1) x 10(-2) in NO3- compared to phi = (4.2 +/- 0.3) x 10(-3) at 14.9 +/- 0.1 in NO3-. This decrease in phi was not observed in NaNO3 solutions. The change in electronic structure, as evident by the more pronounced shift of the n-pi* absorption band away from actinic wavelengths with increasing concentration for Ca(NO3)(2) compared to NaNO3, is most likely the origin of the greater decrease in phi for Ca(NO3)(2) compared to NaNO3 at elevated NO3- concentrations. The role of nitrate photochemistry in atmospheric aerosols and the atmospheric implications of these concentration dependent quantum yields are discussed.