Pyrophoricity of nascent and passivated nano-aluminum particles in air is studied theoretically using energy balance analyses. The work incorporates size-dependence of physicochemical properties of particles at nano-scales, and considers free-molecular and radiation heat exchange with the surrounding environment. The heterogeneous oxidation process is modeled using the Mott-Cabrera mechanism. Nascent aluminum particles with diameters lower than 32 nm are predicted to be pyrophoric. The critical diameter for particles passivated with 0.3-nm thick oxide layer is calculated as 3.8 nm. Particles with oxide layers thicker than 0.3 nm are found to be non-pyrophoric. The sensitivity analysis suggests that the model results are significantly affected by the choice of physicochemical properties, polymorphic state of the oxide layer, parameters of the Mott-Cabrera oxidation kinetics, and heat-transfer correlation. The critical particle size increases by 40%, when bulk material properties calculated at room temperature are used and the oxide layer is assumed to be in a crystalline form. It decreases by 43%, when free-molecular effects are neglected. (c) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.