Abstract
This study presents a modeling approach to the formation and growth of nucleation mode particles when sampling aerosol from the exhaust of a diesel engine. The simulation assumes primary particle formation due to sulphuric acid nucleation and subsequent particle growth as hydrocarbons condense on the primary nuclei. The modeling results are validated against experimental data of particle number concentrations at various dilution ratios and aging times after raw exhaust sampling. In addition, exhaust aerosol of different characteristics is produced by fitting in the engine exhaust line a diesel oxidation catalyst, a diesel particle filter, and a combination of the two. The model is able to satisfactorily reproduce the particle number and mass concentrations of exhaust particles in the nucleation mode for a variety of sampling and after-treatment conditions, with a small delay (50-200 ms) in the initiation of nucleation compared to the experiments. The measured and simulated particle concentrations are in the same order of magnitude with the exact simulated values, depending on the fuel-sulphur conversion rate and the profile of organics considered in the exhaust. This study indicates that a detailed chemical analysis of the exhaust gas, including organic speciation and sulphuric acid concentration, combined with an aerosol dynamics model, would result in a satisfactory prediction of the effect of after-treatment devices and sampling conditions on the exhaust aerosol characteristics. However, it should be stressed that modeling of diesel-exhaust processes is still in an early phase and further work is merited to better understand the processes and mechanisms involved.
Highlights
Ambient ultrafine (< 100 nm) particles are commonly measured in a broad range of geographical locations and atmospheric conditions (e.g., Hussein et al, 2004, Kittelson et al, 2004; Zhang et al, 2004; 2005)
With respect to particle number concentration in the NM (Fig. 2(b)) the model predicts that this largely depends on the fuel sulphur conversion fraction (FSCF) considered
The measurements showed that a distinct nucleation mode appears as fast as 60 ms after exhaust emission and continues to grow both in number and in mass for several hundreds of milliseconds in the sampling line
Summary
Ambient ultrafine (< 100 nm) particles are commonly measured in a broad range of geographical locations and atmospheric conditions (e.g., Hussein et al, 2004, Kittelson et al, 2004; Zhang et al, 2004; 2005). Diesel powered vehicles are primary emitters of such particles New technologies, such as diesel particulate filters (DPF), may eliminate soot particles (solid fraction– accumulation mode (AM), mean diameter > 50 nm); but nanoparticles (volatile and semivolatile fraction - nucleation mode (NM), mean diameter < 50 nm) may still be present in high numbers due to the nucleation of species after dilution and cooling of the exhaust (AbdulKhalek et al, 1998; Ntziachristos et al, 2004a; Vaaraslahti et al, 2004; Burtscher, 2005; Rönkko et al, 2006). Other suggestions include chemiions generated during combustion (ion–induced nucleation, Yu et al, 2003) and particle fragmentation (Gramotnev and Gramotnev, 2005)
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