(1)Department of Chemistry

(2)Department of Mechanical Engineering

(3)Applied Science University of California, Davis, California 95616

The presence of the aromatic compounds in the particulate matter represents a potential hazard to human health. Several PAHs and nitro-PAHs have been identified as carcinogenic compounds. The extent of human exposure to PAHs and their derivatives depends on the partitioning between the gas and particle phases as well as the size distribution of the particle fraction containing the PAHs. There is a greater potential health impact if the PAHs are components of particles that penetrate and deposit into the bronchia and alveoli of the lungs. Since the toxicity of particles from different sources varies widely, such studies would be much more valuable if the particles from a source were characterized with respect to size and composition.

In our study we report the mass spectra of particles that were sized using an impactor. The mass spectra are the summation of 240 laser shots at different points on the sample surface. Each laser spot was 90 µm diameter, which would ablate material from many 1 µm diameter particles. The signal was derived from many particles, thus obtaining reproducible results that are representative of a particular particle size range. The spectra to be presented are from a badly tuned engine. The most intensive low mass peaks correspond to PAHs commonly found in diesel exhaust. The first series corresponds to graphite-like PAHs isomers containing only 6 member rings. The second series, with every other peak overlapping with the first series, corresponds to PAHs containing five member rings and methyl PAH.

Soot particles emitted from a diesel engine are usually observed as chain aggregates composed of several tens to hundreds of primary spherical particles. The LDITOF/MS results yield insight to soot formation by examination of the smallest particles. The observation of K+ and HSO4- suggests that the small particles may have nucleated around an inorganic core. The extreme high mass range of PAHs observed for nano-particles would not be observable by traditional GC/MS.

The ability of LDITOF/MS to characterize the chemical composition of diesel soot particles that were sized using an impactor has been demonstrated. Results indicate a variation of the chemical composition with particle size. In general finer particles have a high concentration of extremely large PAHs, potassium, and sulfate. The larger particles contain lower mass PAHs. Our data show the presence of many PAHs in diesel exhaust that have not been fully characterized as compounds found in diesel soot, including extremely large PAHs not amenable to GC/MS analysis.

Our study reveals that there are differences in the chemical composition of diesel particles. Our technique can be used to characterize different sources according to the size of the particles. Accurate assessment of human health hazards associated with particulate matter requires chemical analysis associated with particle size and source. Such information would be very valuable for addressing the question of bioactivity associate with different classes of particles.