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 atmosphericHONO concentrations.