FIGURE 5-1 Four-season average of PM2.5 composition for 15 U.S. cities, 2008.
SOURCE: EPA, 2010.


ratio, free radical concentration, surface reactivity, optical properties, functional group distribution, porosity, surface area, pore size distribution, surface-averaged energy distribution, and particle density. “The whole point of this modeling approach is that the transition from gas phase to particles is very fast, and there are very few techniques—I think almost none—that can deal with the 1-to-50-nanometers range,” she said. “So modeling can basically help to fill the gap in these regimes that we’re interested in.”

Interactions with Biology

The AMPI calculations generate a list of possible chemical structures that would be present in a carbonaceous material in the environment, but the question then becomes, how do these chemicals and particles interact with biological systems? The answer, explained Violi, is the province of nanotoxicology at the level of the individual cell, the smallest unit in a living organism, and the point of interaction between a carbonaceous nanoparticle and a cell is the cell membrane.

The cell membrane is a lipid bilayer composed of lipids, cholesterol, and proteins surrounded by water. It is a fluid system, one in which the lipids are always moving and through which diffusion can occur. Using a molecular dynamics approach, Violi’s team has been able to model how carbonaceous particles of varying morphologies impact the natural diffusion of lipids that is constantly taking place in the cell membrane. Indeed, model calculations show that carbonaceous nanoparticles immediately alter lipid diffusion and that the alterations depend on the morphology of the particles (Fiedler and Violi, 2010). Because the proper functioning of the lipid membrane depends on lipid diffusion, this model suggests at least one mechanism by which these particles could cause toxicity.

Violi and her team have since used this model to test whether the decision by an international consortium of regulatory agencies, including EPA, to use C60 fullerenes as the only standard for toxicological testing protocols of nanoparticles in the range of 0.5-1.5 nanometers is scientifically sound. The team ran the model using the C60 nanoparticle to see if the results reproduced all of the characteristics seen in model runs that used carbonaceous material from combustion. The answer was no; the reason appears to be that an important parameter is surface area, and the surface area of C60 is small compared to that of many other carbonaceous nanoparticles.

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