The homogeneous nucleation of three gases, NO2, H2O, and NH3, to form NH4NO3 aerosols and HONO gas is proposed to be a new significant source of atmospheric HONO. Four reaction pathways are examined by density functional theory calculations of various complexes of reactants and products involved. NH3 is shown to play a critical role in lowering the Gibbs free energy barrier and stabilizing the HNO3 product by forming particulate ammonium nitrate. In conclusion, the system of gaseous NO2, H2O, and NH3 is expected to undergo a spontaneous homogeneous nucleation process, which is likely an important source of nighttime atmospheric HONO concentrations. (C) 2010 Elsevier B. V. All rights reserved.