This study identifies a previously unrepresented process, Recondensation-Induced Nucleation (RIN), linking combustion microphysics to ambient ultrafine particle formation. Laboratory engine experiments, parcel modeling, and Community Multiscale Air Quality (CMAQ) simulations show that including RIN can significantly reduce number concentration underestimation from 75 % to 22 % while maintaining PM_2.5 accuracy, providing a mechanistic basis for improving urban aerosol modeling.