From cylinder to city: How the recondensation-induced nucleation shapes urban aerosol number

Abstract. Air-quality models frequently underestimate fine particle number concentration (PNC), particularly in the nucleation/Aitken range—while reproducing PM_2.5 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.

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 H₂SO₄–H₂O 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_2.5 by 21 % at the Xitun urban site; incorporating RIN reduced the PNC bias to 14 % with negligible change in PM_2.5. 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.