Most of the nanoparticle number emissions from diesel engines are found in the nucleation mode (Dp < ∼ 30nm). These nanoparticles are mainly formed by nucleation as diesel engine exhaust gas cools and dilutes in the atmosphere. Diesel nanoparticles have raised concerns because of their suspected human health effects. There are two main theories describing diesel nanoparticle nucleation: homogeneous nucleation, most likely binary of sulfuric acid and water, and ion-induced nucleation. In this study we assess the likelihood of the ionic mechanism. Previous studies have shown that diesel nucleation mode particles carry little or no electrical charge (Jung and Kittelson 2005). This could be due to the fact that those particles were never charged, or it could be due to the fact that they were neutralized quickly during dilution and sampling. In the first part of this study, we estimated the extent of neutralization of charged nuclei during the dilution process and found it too slow compared to the residence time in our system to account for the absence of charge observed in previous work. In the second part of this study, we compared nuclei mode formation with and without chemi-ions during dilution and sampling by using an ion trap at the upstream of dilution. Removing upstream ions had no significant influence on the nucleation mode, suggesting that ionic nucleation did not play an important role. We also calculated ion concentration histories, accounting for recombination and attachment during expansion stroke in after combustion. This calculation indicates that ion concentration in the exhaust is too low to account for nucleation mode formation.
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