| Abstract
| - Diesel engines are known to emit high number concentrationsof nanoparticles (diameter < 50 nm), but the physicaland chemical mechanisms by which they form are notunderstood. Information on chemical composition is lackingbecause the small size, low mass concentration, andpotential for contamination of samples obtained by standardtechniques make nanoparticles difficult to analyze. A nano-differential mobility analyzer was used to size-selectnanoparticles (mass median diameter ∼25−60 nm) fromdiesel engine exhaust for subsequent chemical analysis bythermal desorption particle beam mass spectrometry.Mass spectra were used to identify and quantify nanoparticlecomponents, and compound molecular weights andvapor pressures were estimated from calibrated desorptiontemperatures. Branched alkanes and alkyl-substitutedcycloalkanes from unburned fuel and/or lubricating oilappear to contribute most of the diesel nanoparticle mass.The volatility of the organic fraction of the aerosolincreases as the engine load decreases and as particlesize increases. Sulfuric acid was also detected at estimatedconcentrations of a few percent of the total nanoparticlemass. The results are consistent with a mechanism ofnanoparticle formation involving nucleation of sulfuric acidand water, followed by particle growth by condensationof organic species.
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