Air-quality models frequently underestimate fine particle number concentration (PNC), particularly in the nucleation/Aitken range—while reproducing PM<sub>2.5</sub> mass more accurately, suggesting that key number-forming processes are missing from current frameworks. We propose and investigate a physically motivated pathway, Recondensation-Induced Nucleation (RIN), in which pre-existing ambient aerosols are vaporized during combustion and subsequently re-nucleate as the exhaust cools, selectively boosting particle number with negligible impact on mass.</p> <p>Controlled four-stroke engine experiments demonstrate that a distinct nucleation mode (<30 nm) appears only when ambient aerosols are present in the intake air, providing direct laboratory evidence of RIN. Parcel-model simulations of H<sub>2</sub>SO<sub>4</sub>–H<sub>2</sub>O systems were then used to probe particle evaporation under in-cylinder condition and the observed self-limiting behavior of nucleation. A parameterized RIN module was then implemented in the Community Multiscale Air Quality (CMAQ) model and tested over Taiwan. Without RIN, CMAQ underpredicted PNC by 75 % and overpredicted PM<sub>2.5</sub> by 21 % at the Xitun urban site; incorporating RIN reduced the PNC bias to 14 % with negligible change in PM<sub>2.5</sub>. The RIN mechanism thus transfers accumulation-mode mass to Aitken-mode number, not only improve the low-PNC bias but also the low Aitken- to accumulation mode number ratio bias found at the Xitun site. Therefore, the inclusion of the RIN mechanism provides a mechanistic basis for improving health-relevant urban aerosol simulations.