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4 Analyzing Nanoparticles in Complex Mixtures In real-world applications, synthetic nanoparticles are Although particulate delivery systems are used widely in rarely if ever encountered in their native form. Rather, they many industries to solve specific delivery problems, they are occur mixed with various inert ingredients to create pills, tab- not without their own issues. Dry powders create dust and lets, and pellets. In the environment, however, nanoparticles can be hard to handle. They can flood out of hoppers or not interact with other materials, becoming coated with organic flow at all. If inhaled, they can cause health problems. “We matter or clumping together in agglomerates. Research has want to take advantage of their properties, but we want to shown that these and other transformations produce materials handle them in delivery forms that are suitable for us,” said with chemical, toxicological, and environmental behavior Litster. To illustrate the versatility of powder processing, he that differ, sometimes in profound ways, from that of the listed many of the reasons for packaging small particles in native nanomaterial. As a result, it can be challenging to dry delivery form (Table 4-1). predict the consequences of releasing a nanomaterial into The processes used to make these delivery forms are many the environment. Speakers in this session addressed some and varied, Litster explained (Figure 4-1). Materials can be of the challenges to measuring and predicting the properties compacted by tableting, dry granulation, or compaction. and behavior of complex nanoparticle formulations and dis- They can be mixed with a liquid binder in a fluidized system cussed the often surprising findings that come from studying to form granules or spray dried from slurries or solutions. nanomaterials as they occur in the real world. With this variety of methods available for creating small particles, a major area of research involves determining how the choice of method, or a change from one method to DESIGN AND MANUFACTURE OF DELIVERY FORMS another, impacts the structure of the delivery form and its FOR SMALL PARTICLES distribution properties. The real goal of research in this field, James Litster of Purdue University spoke about par- explained Litster, is to understand both those processes and ticles in the micron and submicron size range that are the properties they impact to facilitate design of a granule used in industrial applications, with a specific focus on the that disintegrates and disperses in a particular way, that has delivery forms for those particles. For example, catalysts a certain resistance to attrition, or that has a certain chemi- and absorbents are often used in pellet form, detergents in cal stability and shelf life. “This is a non-trivial and only granular form, and drugs in micronized form. Many of the partially solved problem,” he said. new drug molecules being developed are poorly soluble, so Many barriers to progress in this area exist. Researchers drug formulators are using small particle delivery vehicles lack a quantitative understanding of how microstructure to increase the solubility and uptake of these molecules. Dry develops during processing of complex, multiphase delivery powder aerosols based on relatively large lactose particles forms. In addition, predictive and quantitative scaling rules, are used to deliver drugs into the lungs, although dispersion and process design models that track multidimensional of the powder aerosol is often poor. Whatever the route of distributions of properties are relatively rare. Finally, robust administration, the goal is to deliver a drug payload in a way on-line techniques for measuring the microstructure and that does not change the drug molecule and that does not distributions of important properties also are rare. allow it to aggregate. 37

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38 CHALLENGES IN CHARACTERIZING SMALL PARTICLES TABLE 4-1 Reasons for Packaging Small Particles in Dry Delivery Form Reason Typical Application To produce useful structural forms powder metallurgy To provide a defined quantity for dispensing and metering agricultural chemical granules, pharmaceutical tablets To eliminate dust handling hazards or losses briquetting of waste fines To improve product appearance food products To reduce caking and lump formation fertilizers To improve flow properties for further processing pharmaceuticals, ceramics To increase bulk density for storage detergents To control dispersion and solubility instant food products To control porosity and surface-to-volume ratio catalyst supports To improve permeability for further processing ore smelting To create nonsegregating blends of powder ingredients ore smelting, agricultural chemicals, pharmaceuticals SOURCE: Litster, 2010. FIGURE 4-1 Approaches to creating delivery forms. SOURCE: Litster, 2010. Although these comments apply to powder processing inR02144 powder does not have a uniform packing structure, Coarse general, they are especially true when the primary particle and therefore the mechanism by which the liquid interacts Figurew4-1 powder is completely different. As a result, a disk- size is less than 10 microns. In that size domain, Litster ith the explained, “the surface forces thatcollection of 5 to stick want the particles uneditable raster bitmapaimages granule forms. shaped rather than spherical together dominate over gravity and inertia of the particle, resulting in particles that are cohesive, flow poorly, and make Models for Engineering Design complex structures with different levels of aggregation at different length scales.” Creating an engineering design requires understanding at Fine powders behave very differently because they many different levels, including of the physics and accompa- don’t fluidize properly. Litster demonstrated this fact with nying models at different length scales. It is also necessary to images showing the dramatically different behavior of a understand the physical interactions that occur at the single- drop of liquid on a bed of fine powder versus coarse powder granule or single-particle level as well as at the bulk flow (Figure 4-2). The granule structure of fine powder is round, level. Also needed are macroscopic models that provide the and the drop burrows into the bed and absorbs loose aggre- structure of the delivery form or the distribution of properties gates of the particles to make relatively round granules. and product models and account for how the delivery form

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39 ANALYZING NANOPARTICLES IN COMPLEX MIXTURES Fine powder 10 ms 50 ms 0.1 s 0.5 s 1s Coarse powder 4 ms 8 ms 0.2 s 0.75 s 1 ms 20 ms FIGURE 4-2 Differences in behavior for (top) fine powder (5 mm), and (bottom) coarse powder (70 mm). SOURCE: Litster, 2010. behaves when used as intended, that is, what happens to a powder versus bulk density. The resulting measurements medication tablet when it is swallowed or to a detergent gran- revealed that the coating increased the bulk density of the ule when it is added to the washing machine. The properties 02144 particles, which had the beneficial effect of increasing R API Figure flowability. 4-2 of the particles of interest and the processing equipment used collection of uneditable sing atomic force microscopy, the SOPS researchers U raster photos to make the delivery system serve as the input for the models, which span from the molecular to the bulk scale. are currently studying a simpler system—one in which with vector labels Together with his colleagues at the National Science aluminum particles replace the API particles—to study how Foundation-funded Engineering Research Center for Struc- the roughness and morphology of the core particle affect tured Organic Particulate Systems (SOPS), Litster is follow- adhesion forces and the properties of the final coated particle. ing a multiscale approach to compaction modeling, with They use a combination of modeling techniques to predict a the goal of developing function-structure relationships distribution of adhesive forces across the core particle’s sur- for the design and optimization of delivery systems. As face. The experiments have shown that the adhesion force is an example of what this approach entails, he discussed a substantially reduced when the particles are coated, primar- process wherein particles are used to surface coat a larger ily because the coating creates a larger separation between micron-sized particle, which then becomes part of a cluster the API particles, which reduces the overall van der Waal’s of particles or granules and ultimately a tablet. The functions interaction. The experiments, said Litster, demonstrate that and properties of each of these particles must be character- it is now possible to measure single particle properties for ized at different length scales. Also needed are modeling real particles that are nonspherical and have rough surfaces. approaches that address length scales from the single-particle Moreover, the measurements can be used to predict the level through the macroscopic level, ultimately providing the behavior of the particles and to estimate how much coating tablet’s density and chemical distribution. is needed to produce the necessary level of disaggregation. Litster discussed two research projects that are being car- The second project involves making an agglomerate, ried out at the Center. The first project involves micron-sized as opposed to single particles, the delivery form. An inter- particles of an active pharmaceutical ingredient (API) that mediate step to making a tablet involves creating and then are meant to be delivered by an aerosol. The project’s goal compacting a ribbon of a formulation, the key properties of is to determine if drugs in this size range can be modified by which are its bulk density and its distribution. Near-infrared coating them with smaller particles to turn cohesive powders images of the ribbon revealed that variation in the density of into free-flowing powders that can be more easily metered the ribbon depends on the formulation used and the condi- and dosed in different delivery forms. tions under which the ribbon is produced. The ribbon’s den- In a set of experiments, SOPS researchers used a mecha- sity affects the granule and ultimately the tablet properties. nofusion approach to distribute a silica nanoparticle coating The researchers question whether they can predict ribbon onto API particles and measured the bulk flowability of the density on the basis of particle properties and process condi -

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40 CHALLENGES IN CHARACTERIZING SMALL PARTICLES tions. They have created a macroscopic finite element model great deal of interest in supplying drugs in a stabilized amor- that predicts the ribbon density distribution using data on phous state. The downside of such an approach is likely to the macroscopic bulk properties for the powder going into be reduced stability and shorter shelf life. the ribbon-making machine. Now, the researchers want to be able to predict how the bulk properties of the ribbon PARTICLES IN THE ENVIRONMENT would be altered by changes in the properties of the original Pedro Alvarez of Rice University said there is a general mixture resulting from, for example, a change in the surface properties of the API. consensus in the environmental engineering community that To explore this idea, the researchers have created a multi- engineered nanoparticles are being used and introduced into particle finite element model that represents a ribbon under commercial products at a rate that is much faster than the stress. With this model, they can predict the behavior of a rate at which we are acquiring the information needed to particle assembly under shearing or compaction. The surface ensure that these materials are compatible with the environ- energy, adhesive properties, roughness, and shape of the con- ment, that we can handle them responsibly, and that we can stituent particles are model inputs. The model also requires dispose of them properly. It is his belief “we are at that point data on the mechanical properties of the particles, such as in history where we can actually steward nanotechnology as their elastic moduli and plastic properties. a tool for sustainability as opposed to it becoming a future In conclusion, said Litster, the end performance of small environmental liability, but this requires taking a proactive particles depends critically on how they are packaged. How- approach to risk assessment, and I would like to argue that ever, many problems still must be solved to predict product we are not doing enough in that regard.” structure and performance from formulation properties and To make his point, he noted that the number of research process variables. However, he added, multiscale approaches publications on nanotechnology are doubling every 3 years, to both characterization and modeling hold promise for better but the number of publications relating to environmental engineered products. nanotechnology account for only 5 percent of that total. And in fact, the papers that actually make important contri- butions to understanding the fate, transport, reactivity, and Discussion bioavailability of nanoparticles in the environment account In response to a question about combination therapy and for only 0.25 percent of the total nanotechnology literature. multiple drug absorption sites, Litster said that research has Alvarez’s group’s contribution to the field is in the area yet to address the issue of differential drug delivery of this of how nanoparticles interact with microorganisms in the sort, where one drug would be absorbed in the stomach and environment. Bacteria are the foundation of all ecosystems, the other in the intestines. One possibility is to use some of but they also are very convenient models for studying cyto- the techniques for dealing with low-solubility drugs. In that toxicity and therefore can serve as a useful model system for case, making the drug particles tiny and delivering them in studying possible environmental impacts of nanotechnology. amorphous form can produce locally high drug concentra- In other words, if a microbe is adversely affected by exposure tions in the stomach, where they will be absorbed faster to a substance, it indicates a need to worry about what the than expected. It might then be possible to create larger, substance may do to higher order organisms. metastable particles that flow through the stomach and reach After 7 years in the field, Alvarez has learned that it the intestines, where they will be absorbed. is difficult to generalize about the potential impact of Research is being conducted in these areas, and one nanomaterials because of the many ways in which they can approach to tackling this problem would be to measure interact with the microbial ecosystem. For example, some primary properties and rate constants under well-defined engineered nanomaterials, such as the fullerenes or ceria, conditions. Computer simulations could then predict what require direct contact with the cell to exert a toxic effect. For combinations of particle size, dissolution rate, and addi - those materials, anything that happens in the environment that tional nucleation-inhibiting polymer would produce the best hinders their bioavailability—becoming coated with organic system. matter, for example—could mitigate any potential toxicities. In response to a question from Schwartz, Litster stated Other materials, such as quantum dots, can release toxic ions that the roles that thermodynamics and kinetics play in that kill microorganisms. Titanium dioxide nanoparticles and determining how particle size affects solubility remain aminofullerenes can generate reactive oxygen species that controversial. Kinetics likely plays a role, but so, too, will kill microbes. thermodynamics because the concentration of drug in solu- Understanding how these materials impact the micro- tion is significantly higher than the solubility of the most bial ecosystem is challenging because much of the work stable crystalline phase. Litster agreed with Schwartz’s com - conducted today uses pure and very well-characterized ment that a metastable phase will yield a higher solubility, nanoparticles, which Alvarez referred to as “virgin nano- and he added that the pharmaceutical industry is showing a particles.” However, nanoparticles can undergo a wide vari-

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41 ANALYZING NANOPARTICLES IN COMPLEX MIXTURES Commonalities ety of modifications and transformations when they are in the environment (Figure 4-3). “They are going to agglomerate, Alvarez made two points about commonalities. First, as they are going to be chemically or biologically transformed,” the concentration of nanoparticles increases to the point that he said. “They may aggregate and precipitate, and at the very it exceeds the absorption capacity of the soil, the nanoparticle least, they are going to lose coatings or acquire coatings.” no longer exists in its original structure. Instead, the nano- Each of these modifications will affect a nanoparticle’s particle forms large aggregates and precipitates that are more mobility, reactivity, bioavailability, and toxicity. As a result, difficult to bioaccumulate. To illustrate why this is important, said Alvarez, the field has a real need for more analytical Alvarez compared the bioaccumulation of C60 to that of the methods that allow for dynamic characterization of nano- polynuclear aromatic hydrocarbon phenanthrene. In theory, particles as they interact with bacteria in conditions that phenanthrene is less hydrophobic than C60 and, therefore, resemble their environment. For example, the idea of dose should be less bioavailable. However, bioaccumulation of is not well understood because it is unclear how much of a C60 is 100 times lower than that of phenanthrene because material actually gets into a bacteria and whether it agglom - its molecular size makes it more difficult to transport across erates or partitions inside a cell. the cell membrane. “The take-home message here is that the What is known, Alvarez said, is that nanoparticles tend traditional risk assessment protocol using thermodynamically to aggregate. Salts in the environment promote coagulation based partitioning coefficients to predict fate in the environ- and precipitation, which reduce the potential toxicity of a ment does not work,” said Alvarez. nanomaterial. Aggregation reduces the specific surface area The second commonality is found in the interaction of of a particle, which, in turn, decreases toxicity. Changes in nanoparticles with natural organic matter (NOM). In the case surface chemistry and reactivity can also reduce the toxic- of C60, NOM can serve as a sponge that traps this compound, ity of aggregated particles compared to individual particles. hindering direct contact and bioavailability, and therefore It is also clear that nanomaterials almost certainly end up toxicity. In one set of experiments, Alvarez’s team showed in wastewater streams and ultimately in sludge. Therefore, that adding just a few of milligrams of C60 to a bacterial cul- it is important to consider the impact that nanoparticles ture caused the organisms to stop producing carbon dioxide, in sludge might have on the terrestrial food chain. Using a sign of metabolic activity. Adding clean sand to the culture carbon-14 labeling, Alvarez’s group has shown, for example, along with the C60 had no protective effect, but adding even that fullerenes accumulate in earthworms (Li et al., 2010). FIGURE 4-3 Processes that can possibly transform manufactured nanoparticles or modify their surfaces and aggregation states in the environment. SOURCE: Alvarez et al., 2009. R02144 Figure 4-3

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42 CHALLENGES IN CHARACTERIZING SMALL PARTICLES a little bit of soil containing only 0.2 percent organic matter He said the matter of inventorying nanoparticle produc- had a marked effect on bioavailability and restored metabolic tion, use, and release within well-defined spatial domains activity (Li et al., 2008). They found that they could produce must be settled. There is still debate as to whether inventory- this protective effect by adding as little as 0.1 milligrams/ ing should be mandatory or voluntary, and whether products liter (mg/L) of humic acid to the culture medium. Because should have labels informing consumers about the presence of these results, and the fact that NOM levels in water in of certain nanomaterials. Some in the scientific community the environment are typically on the order of 3-10 mg/L or have suggested that nanoparticles be labeled (for example, to higher, Alvarez said he is much less worried about the acute be fluorescent, or with a radioisotope) so they can be traced ecotoxicity of C60. in the environment. Particle coating can also influence nanomaterial toxicity, Alvarez noted that in March 2009 Rice University held explained Alvarez. His group has shown, for example, that an international workshop on the eco-responsible design and iron nanoparticles coated with polyaspartate do not stick disposal of engineered nanomaterials at which about 50 envi- to bacteria, eliminating toxicity. The caveat to this obser- ronmental engineers and scientists were asked to address the vation is that coated nanoparticles are also more stable in question, “What critical knowledge gaps and opportunities the environment and more likely to bind to NOM in water, exist to inform and advance the design of environmentally which may increase transport in the environment. Therefore, benign engineered nanomaterials and the management of there may be a tradeoff in terms of reducing the possibility wastes containing them?” Figure 4-4 summarizes their con- of acute toxicity versus increasing transport and long-term sensus (Alvarez et al., 2009). accumulation. Alvarez concluded by saying that there is much to learn The bottom line, said Alvarez, is that no matter how toxic about nanomaterial bioavailability, potential toxicity, and a material might be, if there is no exposure, then there is mechanisms of action by looking at how these materials no risk. Preventing exposure by intercepting nanomaterials interact with bacteria and other microorganisms, with the before they enter the environment might be the most effec- implication that materials that impact microbial processes tive means of limiting potential environmental damage. If can seriously damage an ecosystem’s health. The good eliminating exposure is not feasible, then another approach news, he added, is that many things in the natural environ- is to engineer away the properties that make a nanomaterial ment interact with nanomaterials in a way that reduces their hazardous without compromising the properties that make it bioavailability and therefore their potential to cause harm. useful. This approach was taken when hydrofluorocarbons A lthough nanotechnology-triggered microbial toxicity were engineered to replace chlorofluorocarbons when the represents a worry, nanotechnology also offers many new latter were found to be environmental hazards. approaches to addressing environmental problems in a more benign way. For example, nanomaterials may enable new approaches to disinfecting water that does not require the Nanoparticles Are Abundant in Nature use of chlorine. Alvarez reiterated the ideas that the environment is filled with nanoparticles and that, given the large number of Discussion naturally occurring nanoparticles compared to the number of engineered nanoparticles, it may be difficult to discern Barbara Finlayson-Pitts commented that one message the risk from exposure to the engineered nanoparticles. He she got from Alvarez’s presentation was that equilibrium believes that the size-dependent properties of engineered modeling is not going to work, just as it is not working with nanoparticles are more accentuated than those of many of the atmospheric nanoparticles. Alvarez agreed with this assess- naturally occurring nanoparticles that have been in contact ment, noting that nanoparticles constantly change in the with nature for millions of years. environment as they acquire and lose surface features and To truly understand exposure and risk, there is a need to undergo reactions with materials in the environment. There- obtain more information about the sources of nanoparticles. fore, modeling their fate in the environment is very challeng- To do so, three questions need to be answered: ing; models for predicting the behavior of nanomaterials are not going to be as accurate as are models for dealing with • What are the main entry points of engineered nanopar- pure chemicals. ticles and the scale of discharge into the various envi- Finlayson-Pitts also asked Alvarez to discuss how the ronmental compartments? shape and dimension of a nanoparticle affects its health • What forms of an engineered nanoparticle are being impacts. Alvarez replied that it is important to consider that discharged? there are two categories of properties that matter for deter- • What are the realistic, environmentally relevant expo- mining health impacts. The first is how the particle moves sures to those forms? to a receptor, and the second is the properties of the original particle (such as shape or dimension). In terms of impact

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43 ANALYZING NANOPARTICLES IN COMPLEX MIXTURES FIGURE 4-4 Toward eco-responsible nanotechnology. SOURCE: Alvarez et al., 2009. or reactivity, it may be that the properties of the original R02144 Mineral dust is composed of oxides and carbonates, and, nanoparticle will end up being the most important, but that as the catalyst community has demonstrated, particulate Figure 4-4 is still unknown. oxides are reactive materials. As a result, a large amount of uneditable raster bitmap Andrea Violi of the University of Michigan asked if there rich chemistry occurs on the surface of mineral dust particles is more exposure to engineered C60 or to C60 particles emitted in the atmosphere. Grassian described six different types of during combustion or volcanic eruption. Alvarez did not know surface reactions and mechanisms that might be operating. the answer but noted that C60 reportedly can be produced “What we try to do is design laboratory studies to better by volcanic eruptions and that it might exist at trace levels understand this surface chemistry,” she said, with the goal of in nature. He added, however, that when it comes to human understanding the global impacts of small particles. health, the leading source of risk is likely to be associated Grassian’s research approach involves examining the indi- with respiratory uptake of particles in the work environment. vidual components of mineral dust to determine how chemis- try differs among the different clays, oxides, and carbonates that occur in mineral dust. For example, her group has been SURFACE CHEMISTRY, TRANSFORMATIONS, studying the effect of relative humidity on the chemistry AND GLOBAL IMPACTS that takes place on the surface of various mineral particles. Vicki Grassian of the University of Iowa focused her Although a great deal of research has been conducted on remarks on transformations and surface chemistry of mineral particle surface chemistry , she said little is known about dust, which makes up a large fraction of the aerosol mass. what happens in the environment that lies between the two Mineral dust particles measure from about 0.1 microns (or extremes of dry and wet, a domain in which surface-absorbed 100 nanometers) to much larger particles. Particles at the water is likely to play an important role. smaller end of this range stay aloft for 1 to 2 weeks and are In one set of experiments, for example, Grassian’s group transported great distances by the atmosphere. During that looked at the reactions of nitric acid on calcium carbonate 1- to 2-week time period, a lot of interesting chemistry takes particles at various levels of relative humidity (RH), all place on the particle’s surface, and the particle transforms as greater than 10 percent RH. The study showed that nitrate it undergoes surface reactions with other chemicals that enter forms on the surface of the particles at the same time that the environment. water is absorbed on the particles, but nitrate forms only in

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44 CHALLENGES IN CHARACTERIZING SMALL PARTICLES the presence of water vapor. Microscopic images clearly that eventually leads to the production of sulfate (Laskin et show that, during the process, the particles undergo a al., 2005, Sullivan et al., 2006), which atmospheric chemists marked change in morphology (Figure 4-5) and transform had not anticipated. from solid to liquid. This transformation likely occurs in the atmosphere, Grassian said. Complex Mineral Dusts When these transformations take place in the atmosphere, they can alter the particles’ impact on climate. Grassian’s Grassian is working to forge even stronger links between team has shown that as calcium carbonate particles react with her group’s experimental work and atmospheric chemistry. nitric acid and transform into calcium nitrate particles, their Recently, she and her collaborators have begun studying absorption of water increases dramatically and the particles complex authentic mineral dust particles from different nearly double in size. These transformed particles are not sources. As a first step, her team conducted elemental analy- only 100 times better at serving as cloud condensation nuclei sis on bulk samples collected from China and Saudi Arabia. than were the original particles, but also they become photo- All of the dusts contain silicon, aluminum, calcium, iron, chemically active. This increased photochemical activity magnesium, and a few other elements, but the relative levels increases the possibility that additional chemistry will occur of these elements vary across samples. at the surface of the transformed particles, leading to further This finding raised the question of how best to think about changes that might influence climate in ways that are not those particles. “Maybe we want to measure elemental com- yet understood. positions of single particles and then look at the particles that Another type of chemistry that can occur on the surface contain 15 percent calcium and compare them to particles of calcium carbonate particles starts with the absorption of with less than 15 percent calcium,” Grassian said. “But what sulfur dioxide. In the absence of water vapor, little happens if all the calcium is on the inside of a particle that’s coated by when calcium carbonate particles mix with sulfur dioxide, a silica shell? Then perhaps what we really want to measure even in the presence of oxygen. The surface absorbs sulfur is single particle surface chemistry.” dioxide, but little chemistry occurs. In the presence of water In a series of experiments, Grassian took calcium- vapor, however, calcium sulfite crystals start to grow on containing dust particles from several dust sources and the particles. Water vapor clearly triggers chemistry to take mixed them with gaseous nitric acid. As expected from her place on the particle surface. Grassian noted that molecu- team’s earlier work, the particles reacted with nitric acid, lar dynamics simulations of ion mobility over the surface but the reaction was not uniform when viewed using scan- of these particles would provide useful insights into these ning electron microscopy (SEM). SEM images showed that processes. the reaction occurred only at places on the particle surface The results of these types of studies are now being used that associated with local composition of the particle, and to help interpret measurements being made in the field. For hydroscopic growth was complex as a result. example, field measurements in Israel and China have shown Natural particles are more complex not only chemically, that calcium carbonate particles react with sulfur dioxide in but also morphologically. They are not all spheres, which will the environment and that sulfite forms in an intermediate step impact remote sensing data from which aerosol concentra- FIGURE 4-5 Calcium carbonate particles react with nitric acid in the presence of water vapor. SOURCE: Grassian, 2010. R02144 Figure 4-5 uneditable raster bitmap

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45 ANALYZING NANOPARTICLES IN COMPLEX MIXTURES tions are calculated. Research is needed, Grassian said, to rather they can undergo chemical and physical transforma- better understand how the complex shape distribution of tions that will impact their “climate” properties. Laboratory natural dust particles influences optical and chemical behav- studies can provide insights into these transformations, ior in the atmosphere. but the complexity of real atmospheric dusts means that Particle size is also heterogeneous in natural dusts, and the the insights may not reflect what really happens in the behaviors of particles in the atmosphere vary with their size. environment. Particles can aggregate in the atmosphere, which impacts the particles’ size, shape, and density and their available surface Discussion area, which in turn impacts their surface chemistry and their dissolution in water droplets. Dissolution will then affect In response to a comment from Abhaya Datye about the aggregation. possibility that metals from catalytic converters may be get- Grassian’s team has started to examine aggregation and ting into road dust, Grassian responded that there is evidence dissolution of differently shaped and sized particles. In one of those metals, including nanosized platinum particles, in study of iron nanorods and microrods, they found that iso- road dust. She added that research is starting to look at the lated nanorods displayed enhanced dissolution compared to health effects of those metal dusts. the microrods. However, in the aggregated state, nanorods When asked what characterization tools she would like to were stable against dissolution, such that microrod disso- have to advance her work, Grassian said she would like to per- lution was significantly suppressed (Rubasinghege et al., form single particle surface chemistry routinely, particularly 2010). Aggregation affected not only the size of the particle, of single particles that are components in a complex mixture. but also its chemistry. Alvarez seconded this wish, adding that he would like access To conclude, Grassian stated that it is important to recog- to a technology capable of discerning surface properties or nize that particles are not stable entities in the atmosphere; surface chemistries in complex matrices.

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