ABSTRACT
BACKGROUND: The potential of diesel soot aerosol particles to break up into smaller units under mechanical stress was investigated by a direct impaction technique which measures the degree of fragmentation of individual agglomerates vs. impact energy. Diesel aerosol was generated by an idling diesel engine used for passenger vehicles. Both the aerosol emitted directly and aerosol that had undergone additional growth by Brownian coagulation ("aging") was investigated. Optionally a thermo-desoption technique at 280 degrees C was used to remove all high-volatility and the majority of low-volatility HC adsorbates from the aerosol before aging. RESULTS: It was found that the primary soot agglomerates emitted directly from the engine could not be fragmented at all. Soot agglomerates permitted to grow additionally by Brownian coagulation of the primary emitted particles could be fragmented to a maximum of 75% and 60% respectively, depending on whether adsorbates were removed from their surface prior to aging or not. At most, these aged agglomerates could be broken down to roughly the size of the agglomerates from the primary emission. The energy required for a 50% fragmentation probability of all bonds within an agglomerate was reduced by roughly a factor of 2 when aging "dry" agglomerates. Average bond energies derived from the data were 0.52*10-16 and 1.2*10-16 J, respectively. This is about 2 orders of magnitude higher than estimates for pure van-der-Waals agglomerates, but agrees quite well with other observations. CONCLUSION: Although direct conclusions regarding the behavior of inhaled diesel aerosol in contact with body fluids cannot be drawn from such measurements, the results imply that highly agglomerated soot aerosol particles are unlikely to break up into units smaller than roughly the size distribution emitted as tail pipe soot.
ABSTRACT
Raman microspectroscopy has been applied to follow structural changes in spark discharge (GfG) soot and light-duty diesel vehicle (LDV) soot upon oxidation and gasification by nitrogen oxides and oxygen in a diesel exhaust aftertreatment model system at 523 and 573 K. Raman spectra have been recorded before and during the oxidation process, and spectral parameters have been determined by curve fitting with five bands (G, D1-D4). For GfG soot, a steep initial decrease of the relative intensity of the D3 band suggested rapid preferential oxidation of a highly reactive amorphous carbon fraction, while a less steep but also substantial decrease of band widths (in particular, the D1 band) indicated a slower overall increase of chemical homogeneity and structural order in the partially oxidized material. The spectroscopic changes are in agreementwith a strong decrease of chemical reactivity at increasing mass conversion of GfG soot. In contrast, the spectral parameters and reactivity of partially oxidized LDV soot remained largely unchanged throughout the oxidation process. Overall, the spectroscopic and kinetic findings suggest that Raman spectroscopic parameters provide information about the relative abundance and structural order of graphitelike and amorphous carbon and can be used as proxies for the chemical reactivity of soot undergoing oxidation and gasification. Thus, Raman spectroscopy promisesto become an efficient tool forfurther investigation and optimization of diesel exhaust aftertreatment in continuously regenerating traps and particle filters.
