From cylinder to city: how recondensation-induced nucleation in vehicle exhaust shapes urban aerosol number

Abstract

Air-quality models frequently underestimate fine particle number concentration (PNC), particularly in the nucleation/Aitken range-while reproducing PM2.5 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 further examine particle evaporation under in-cylinder condition and the self-limiting nature of nucleation. A parameterized RIN module was implemented in the Community Multiscale Air Quality (CMAQ) model and evaluate under Taiwan/West-Pacific urban conditions. Without RIN, CMAQ underpredicted PNC by 75 % and overpredicted PM2.5 by 21 % at the Xitun urban site; incorporating RIN reduced the PNC bias to 22 % with negligible change in PM2.5. The RIN mechanism thus transfers accumulation-mode mass to Aitken-mode number, not only partially alleviates the low-PNC bias but also the low Aitken- to accumulation mode number ratio bias found at the Xitun site. While RIN improves PNC estimates, it also leads to overestimation at the smallest sizes, likely reflecting inherent limitations in the modal parameterization.