Chemical analysis of diesel engine nanoparticles using a nano-DMA/thermal desorption particle beam mass spectrometer

Diesel engines are known to emit high number concentrations of nanoparticle s (diameter < 50 nm), but the physical and chemical mechanisms by which the y form are not understood. Information on chemical composition is lacking b ecause the small size, low mass concentration, and potential for contaminat ion of samples obtained by standard techniques make nanoparticles difficult to analyze. A nano-differential mobility analyzer was used to size-select nanoparticles (mass median diameter similar to 25-60 nm) from diesel engine exhaust for subsequent chemical analysis by thermal desorption particle be am mass spectrometry. Mass spectra were used to identify and quantify nano particle components, and compound molecular weights and vapor pressures wer e estimated from calibrated desorption temperatures. Branched alkanes and a lkyl-substituted cycloalkanes from unburned fuel and/or lubricating oil app ear to contribute most of the diesel nanoparticle mass. The volatility of t he organic fraction of the aerosol increases as the engine load decreases a nd as particle size increases. Sulfuric acid was also detected at estimated concentrations of a few percent of the total nanoparticle mass. The result s are consistent with a mechanism of nanoparticle formation involving nucle ation of sulfuric acid and water, followed by particle growth by condensati on of organic species.