On the mechanism of laser-induced annealing of soot

Abstract Ultrafine soot particles emitted from combustion devices and biomass burning are a major particulate pollutant for human health and a major climate forcer. Unprecedented efforts have been made to understand the mechanism of soot formation and the physical, chemical, and optical properties of soot particles at different stages of maturity. Pulsed laser-induced incandescence (pLII) has become a powerful tool for in-situ measurements of soot volume fraction and primary particle size and to investigate the effects of pulsed laser irradiation on soot absorption properties. Experimental studies have confirmed that a high-power laser pulse can enhance the absorption of young soot particles through laser-induced annealing. Previous studies have ascribed the observed changes in soot absorption by pulsed laser irradiation to thermal annealing. In this study, a numerical study was conducted to model the effect of pulsed laser irradiation on the absorption efficiency of soot of different maturities to reproduce the results of a recent double-pulse pLII experiment. The numerical results based on thermal annealing models proposed in the literature failed to capture the enhanced peak LII signals of laser-heated young soot compared to those of un-preheated soot. By assuming the laser-induced annealing of soot particle is attributed to both thermal and photon mechanisms, the modified LII model can reproduce the experimentally observed enhancement in the peak LII signal of laser irradiated soot of different maturities. The findings of this study serve as indirect evidence to support the conjecture that the photon mechanism plays an important role in laser-induced annealing of young soot.