In response to a question from Barbara Finlayson-Pitts about whether the soot models account for the complex organic functionality found on the surface of soot particles, Violi said that the modeling code she described tracks all of the surface chemistry and all of the functional groups and reactive sites on a particle’s surface. The models also reproduce the finding that radical species exist on the surfaces of soot particles.

Mort Lippmann commented that he was encouraged that modeling efforts are being directed at aqueous droplets such as hygroscopic sea salt because liquid microdroplets are very important from a health perspective. He added, however, that he would like to see more work on droplets as well as on particles other than carbon because metal-containing nanoparticles are probably a bigger issue for human health. Rhonda Stroud remarked that looking at liquid particles is a challenge because many of the tools available today are geared toward studying solid materials. However, efforts are under way to design microfluidic cells for electron microscopy instruments that may be useful for studying soft materials such as droplets.

Steve Schwartz suggested that the modeling community might want to pursue knowledge of the ice crystal habitat in clouds given the importance of ice crystals to climate. Violi agreed with Schwartz’s subsequent comment that it could prove fruitful to examine the kinetics of ice crystal growth under varying conditions, and Tobias noted that the atomistic simulation community has undertaken a large effort to model ice crystal formation. Stroud added that NASA Ames has a transmission electron microscope specifically designed to take in situ measurements of ice that may provide the kind of data needed to inform and validate modeling efforts.

Schwartz reiterated his earlier comment about the importance of validating model results with experimental data, particularly those on the biological impacts of nanomaterials. Violi remarked that her team always tries to conduct its modeling work in collaboration with experimentalists. The work on lipid bilayer modeling, however, is a relatively new endeavor, and efforts are under way to develop experimental benchmarks for these models. Tobias agreed with both sets of comments and suggested some experiments that could prove useful. For example, x-ray diffraction studies on stacks of lipid bilayers could look for changes in the density distributions with and without particles. If the particles were deuterium labeled, then it would be possible to use neutron diffraction to determine the exact location of the particles and how they are distributed in the bilayer.

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