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2 What Are Small Particles and Why Are They Important? Small particles—ranging in size from about one nanometer Finlayson-Pitts explained that a major research objective to tens of microns—are ubiquitous in the natural and engi- is to determine the three-dimensional structure of particles neered worlds. In the atmosphere, small particles impact both in the atmosphere. Based on a large body of chemical data warming and cooling of the climate. In Earth’s subsurface, accumulated over many years, it is known that atmospheric small particles impact soil and water quality. In living sys- nanoparticles contain a large number of polar groups, includ- tems, small particles impact organism health and viability. In ing carboxylic acids, amines, and alcohols. However, little is catalysis and reaction engineering, small particles enhance known about how the particles assemble. Other unanswered reaction specificity and rates. In materials design and syn- questions include the following: Do particles self-assemble thesis, small particles provide new and enhanced properties. in air? Do the polar groups end up inside a hydrophobic shell, However, in all of these scientific and engineering domains, or do they end up on the outside? a lack of understanding about the properties and chemical M any factors contribute to understanding climate, composition of small particles limits our ability to understand, according to Finlayson-Pitts. For example, understand- predict, and control their applications and impacts. Speakers ing the three-dimensional arrangement of the particles is in this session discussed the crucial types of information that extremely important. If polar groups are on the outside of need to be determined about small particles in different media. the particles, then they will be expected to take up water and act as cloud condensation nuclei more efficiently than if they are buried inside the particles. Some of the studies INTRODUCTION performed in Finlayson-Pitts’s laboratory revealed that The workshop began with an introduction by co-chair it is not unusual to find polar groups on the inside at the Barbara Finlayson-Pitts, Professor of Chemical Sciences nanoscale. From a bulk chemical composition point of at the University of California, Irvine. She illustrated the view, significant water uptake might be expected; however, importance of small particles by looking at the example of that does not happen because the hydrophobic shell forms a nanoparticle in the atmosphere, as shown in Figure 2-1. at the nanoscale. Research has determined that gaseous precursors form low-volatility products that nucleate to form new particles, ATMOSPHERIC AEROSOLS AND CLIMATE but little is known about how this occurs, even after decades Steven Schwartz of Brookhaven National Laboratory of research. Furthermore, little is known about how the par- ticles then grow into nanoparticles. Because of their size, discussed the influences of aerosols on climate and climate nanoparticles have a high surface-to-volume ratio, which change, and the challenges associated with representing means that the properties of the particles, in terms of their those influences using models. Understanding how atmo- chemistry and photochemistry, will not necessarily be the spheric aerosols affect Earth’s energy balance and sensitivity same as those of the bulk material (because size is one of to climate change is a critical piece to understanding how the unique and desirable characteristics of nanoparticles). the greenhouse effect might limit the future development of Little is known about that chemistry as well. society’s energy economy. 5
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6 CHALLENGES IN CHARACTERIZING SMALL PARTICLES FIGURE 2-1 An airborne nanoparticle can have many fates. SOURCE: Armandroff, 2011. R02144 Aerosol Influences on Climate and Climate Change Figure 2-1 influence can be observed when aerosols absorb warming rather than reflect sunlight. Schwartz stated that the widely Schwartz explained that aerosols are particlesuneditable bitmapped image suspended varying impacts of aerosols on climate change “must be in air and generated from a variety of sources including understood and characterized, and ultimately represented in organic vapors from vegetation, dust, industrial sulfur diox- climate models.” ide emissions, biomass burning, ocean sea salt, and others, as shown in Figure 2-2. All of these materials can undergo Earth’s Energy Balance and Perturbations chemical reactions in the atmosphere to produce particles. Aerosols are characterized by how they scatter light, pro- Figure 2-3 shows the relationship of aerosols in the global ducing effects such as urban haze and photochemical smog. energy balance. Of the 343 Watts per square meter (W/m2) of For example, the haze that hangs over beaches results from solar power provided from the sun, approximately 70 percent aerosolized sea salt reflecting light. Atmospheric aerosols is absorbed (237 W/m2). The balance of that amount of solar affect Earth’s climate in two ways: energy with the amount of thermal infrared emitted from the planet (also 237 W/m2) is essential for maintaining a constant • They reflect sunlight upward, decreasing the amount temperature. of sunlight that reaches Earth’s surface, which subse- The 390 W/m2 coming from Earth’s surface represents the quently cools the planet. This is known as the aerosol greenhouse effect, which results from the infrared-absorbing direct effect. gases in the atmosphere, as well as clouds, that radiate • They serve as seed particles for the formation of cloud energy back toward Earth. Increases in greenhouse gases droplets. such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and the chlorofluorocarbons (CFCs) contribute to the Without aerosols, there would be no clouds, and Earth’s greenhouse effect. These gases together account for about climate would be very different. 2.6 W/m2 of the energy radiating back to Earth’s surface, or As the atmospheric aerosol concentration, measured in less than 1 percent of the atmosphere’s greenhouse effect. particles per cubic meter (particle/m3), increases, and if all Schwartz said that a major challenge for the climate research other parameters are equal, more cloud droplets form. Drop- community is in representing climate change resulting from let formation increases the scattering in clouds and therefore those gases in climate models and in confidently predicting the likelihood that sunlight will be reflected from the top of what the consequence of perturbation would be as the con- the cloud, which has a cooling influence on the climate. A centrations of those gases change.
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7 WHAT ARE SMALL PARTICLES AND WHY ARE THEY IMPORTANT? FIGURE 2-2 The role and sources of atmospheric aerosols in the environment. SOURCE: Schwartz, 2010. R02144 Figure 2-2 bitmapped, unalterable FIGURE 2-3 Schematic of major global, annual average radiant energy fluxes of the Earth-atmosphere system, given in Watts per square meter (W/m2). Blue numbers represent the short-wavelength radiant energy of the solar spectrum entering the planet, and red numbers represent the long-wavelength infrared radiation being emitted by the planet. SOURCE: Schwartz, 2010 (modified from Schwartz, 1996 and Ramanathan, 1987). Climate Sensitivity-Definition, Importance, The range of sensitivities in the current models roughly coin- Past and Current Estimates cides with the Intergovernmental Panel on Climate Change (IPCC) “likely” temperature change uncertainty range of 1.5- Climate sensitivity is a measure of how responsive the 4.0°C (for 2090-2099 relative to 1980-1999; IPCC, 2007a). temperature of the climate system is to a change in the Given the increases in atmospheric CO2, CH4, N2O, and radiative forcing; it is expressed in degrees Kelvin per Watt CFC concentrations over the industrial period, and using per meter squared (K/(Wm–2)) (Schwartz, 2007). There have the IPCC’s 2007 best estimate for climate sensitivity of 3 K, been many estimates of climate sensitivity dating back to the the expected rise in Earth’s temperature due to the increase late 1970s (NRC, 1979). Schwartz described the attempts of greenhouse gases, CO2, CH4, N2O, and CFCs over the that have been made to calculate climate sensitivity from industrial period (1780 to present) is calculated to be 2.1 K. various climate models (Figure 2-4). Large-scale computer However, the observed increase since 1860 is only 0.8 K. models have been used to represent all of the processes that Schwartz explained (based on a published report, Schwartz take place in the atmosphere that govern our climate and et al., 2010) that Earth has not warmed as much as expected climate change. In terms of the greenhouse effect, one way is from forcing by long-living greenhouse gases for several to look at sensitivity and to ask: How much would the global reasons. temperature increase if the CO2 concentration were doubled?
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8 CHALLENGES IN CHARACTERIZING SMALL PARTICLES Sensi vity to 2 x CO2 ( T2x, K) Sensi vity to 2 x CO2 ( T2x, K) Sensi vity , K/(Wm-2) Sensi vity , K/(Wm-2) FIGURE 2-4 A summary of major national and international assessments and current climate models of climate sensitivity. (left) Value of climate sensitivity for past assessments by year assessment was conducted, (right) current (2010) values of 19 different IPCC AR4 models. SOURCE: Schwartz, 2010. A couple of these possible reasons will be discussed R02144odels and in empirical determination of sensitivity. As a in m greater detail below. Figure 2-4 if the climate community is going to make the neces- result, Schwartz elaborated on climate forcing by aerosols and sary advances in understanding climate sensitivity, then it graphs are uneditable raster bitmap how estimates of aerosol direct forcing are made by linear must determine the parameters that drive aerosol forcing modeling and radiation transfer modeling. These calculations arewith much greater accuracy than at present. Required going y-axis labels vector type use aerosol depth data collected daily over north central forward, said Schwartz, are multiple approaches that include Oklahoma by the U.S. Department of Energy (Michalsky et laboratory studies of aerosol processes, field and satellite al., 2010), and cloud albedo and radiative forcing data from measurements of aerosol properties and processes, and daily measurements of effective radius and liquid water path consideration of aerosol processes in atmospheric chemistry (Kim et al., 2003). He reviewed an illustration of climate transport models. The community must evaluate the vari- forcing over the industrial period (Figure 2-5) from the ous models by comparing model results with observations IPCC’s 2007 report (IPCC, 2007), which shows that negative and then using the best models to calculate the forcings aerosol forcing substantially offsets greenhouse gas forcing for inclusion in global climate change models. In addition, and that the uncertainty in aerosol forcing dominates the the community must better understand many of the aerosol uncertainty in total forcing. processes in order to represent them in models (Figure 2-6). Although Schwartz would have preferred to identify one Schwartz stressed the need for multiple types of measure- reason for the difference between the predicted and observed ments at the same place and time and for laboratory experi- temperature changes, the current state of research leads to ments that provide details that cannot be extracted from field two possible explanations: that aerosol forcing counteracts measurements. greenhouse gas forcing, or that climate sensitivity is lower Schwartz stated that radiative forcing by incremental than the consensus value. Estimates of climate sensitivity greenhouse gases already in the atmosphere could poten - might be wrong because they largely ignore aerosol forcing. tially lead to dangerous interference with the climate sys - Aerosol forcing has been overlooked because individual tem. Given the present uncertainty of climate sensitivity, the aerosol particles are short-lived, as was demonstrated when point in time at which greenhouse gas emissions reach an the Chernobyl reactors released a pulse of cesium-137 allowable level, range from about –30 years to +30 years. aerosol particles that resided in the atmosphere for only Furthermore, climate sensitivity must be known with much approximately 1 week. In contrast, greenhouse gases reside greater accuracy for effective development of energy strate- in the atmosphere for decades to centuries. However, what gies, and atmospheric aerosols offset an unknown fraction this scenario overlooks is that atmospheric aerosols are con- of the warming forcing of incremental greenhouse gases. tinually replenished, producing a steady-state level that can The present uncertainty in aerosol forcing greatly limits influence radiative forcing. accuracy in determining climate sensitivity, making the In summary, Schwartz said that climate sensitivity and need for fundamental aerosol research both essential and aerosol forcing are intrinsically coupled, both in climate urgent.
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9 WHAT ARE SMALL PARTICLES AND WHY ARE THEY IMPORTANT? FIGURE 2-5 Atmospheric contributors to positive and negative climate forcing over the industrial period (1780 to present). SOURCE: Adapted from IPCC, 2007b. R02144 Figure 2-5 uneditable raster bitmap FIGURE 2-6 Aerosol processes that must be understood and represented in models. SOURCE: Ghan and Schwartz, 2007. © American Meteorological Society. Reprinted with permission. chemistry. He likened that situation to regulating all airborne HEALTH IMPACTS OF AMBIENT AIR PARTICLES R02144gaseous pollutants in sum. He noted, however, that over time, Figure 2-6 Mort Lippmann of New York University (NYU) dis- reductions in the total mass of PM correlate with improve- cussed the state of science of particles and health research, raster bitmap uneditable ments in human health. Figure 2-7 depicts particle volume characterizing our understanding of the correlation between distributions by particle size for various PM sources. particle chemistry and health as primitive. Airborne particles, Initial PM regulations addressed particles between 2.5 and also known as particulate matter (PM), are regulated by 10 microns. They were modified, however, when research the weight per unit volume of total PM without regard for showed that fine particles, defined as those smaller than
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10 CHALLENGES IN CHARACTERIZING SMALL PARTICLES FIGURE 2-7 Particle volume distributions by particle size for various PM sources. SOURCE: Lippmann, 2010. R02144 Figure 2-7 uneditable raster bitmap 2.5 microns (PM2.5) in aerodynamic diameter, are capable following the messenger,” said Lippmann. “You have to start of reaching deep into the lungs and are associated with the getting at the active components.” public health statistics for mortality and morbidity, due to However, PM10 mass, which is often dominated by con- causes such as cardiovascular diseases, liver disease, and to tributions from PM2.5, is a poor indicator of the respiratory a small extent, lung cancer (Pope et al., 2004). PM10 refers system risks associated with the fraction of particles that to particle diameter size less than 10 microns (and includes deposits within the tracheobronchial airways. In addition, PM2.5). Data reveal a relationship between particle composi- the effect of PM10 or PM2.5 composition on human health tion and adverse health effects (Figure 2-8), but the relation- is unknown. Neither PM2.5 nor PM10 mass is useful as an ship is not well understood. index of risk associated with ultrafine PM (particle diameter Air pollution enters the body through the nose and mouth. less than 0.1 micron). Lippmann noted that, at the time of The size of a particle determines where it ultimately ends up the workshop, the Environmental Protection Agency (EPA) in the lungs. Fine particles penetrate and are deposited deep was planning to lower the standard for fine-particulate mass. into the lungs. The soluble components, said Lippmann, are The real focus, in his opinion, should be on characterizing extracted and enter the blood stream, traveling throughout the chemical components of fine particulate matter, under- the body, which explains the occurrence of adverse effects standing their impact on human health, and then developing in the liver, brain, heart, and other tissues. Research is start - regulations that reduce levels of the most toxic components. ing to find health effects associated with particles in the 2.5-10 micron range, particularly the exacerbation of asthma. Research in Identifying Health Impacts of PM Exposure I n summarizing the current knowledge about PM, Lippmann said that measuring PM2.5 mass has been a Research to study PM chemistry is under way. At NYU, useful surrogate index of adverse health risks. It correlates Lippmann leads the National Particle Component Toxicity better with cardiovascular effects than other monitored air Initiative that aims to compile data on elemental tracers from pollutants. However, its risk coefficient varies, presumably different sources to enable source apportion analysis that resulting from differences in PM composition, and he called can be used to determine what sources generate PM mass. for studies aimed at better understanding the relationship Particles in New York City, for example, are dominated by between PM composition and adverse health effects. “The oil combustion. As a result, there are more than an order of epidemiology indicates we reduced the public health impact magnitude higher levels of nickel and vanadium in New York just by going after the messenger, but you can only go so far City air than in the rest of the United States. In comparison,
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11 WHAT ARE SMALL PARTICLES AND WHY ARE THEY IMPORTANT? FIGURE 2-8 Chemical composition and relative mortality risk coefficients averaged across 60 metropolitan areas for which fine particulate matter (PM2.5; particles smaller than 2.5 microns) speciation data are available. SOURCE: Reproduced with permission from Environmental Health Perspectives. Lippmann et al., 2006. R02144 tracers associated with steel industry furnaces dominate asFigureetermined that nickel and vanadium vary “much more sig- d 2-8 uneditable raster bitmap mortality than any other measured elemental the primary PM source in Birmingham, coal sources domi- nificantly with nate in western Pennsylvania, and wood burning dominates component” (Lipfert et al., 2006, Lippmann et al., 2006). in Seattle. Lippmann said that, in a sense, people are simply aging Lippmann’s group has looked at the distribution of fine faster from the chronic exposure. Large cohort studies are particle components within different parts of New York City now finding significant health effects from exposure to fine and found interesting results. Daily samples collected over PM. For example, the Women’s Health Initiative Study, con- the course of the year showed that the relative amounts of ducted by the University of Washington, has shown that pre- nickel and vanadium varied between winter and summer viously healthy women exposed to fine PM develop elevated months and between the northern and southern parts of the cardiac disease over time (Zhang et al., 2009). Another study city. This finding led the researchers to identify residual oil focusing on exposure to particles smaller than 2.5 microns in burned as the source of these metals. The differences were nine heavily polluted California counties found that overall explained by the fact that there are two primary sources of mortality rose with increasing exposure to fine particles, as residual oil pollution—ocean-going ships coming into the did mortality from respiratory disease, cardiovascular dis- Port of New York and New Jersey and boilers used to pro- ease, and diabetes (Ostro et al., 2006). vide heat and hot water to New York City buildings. These Lippmann’s group also has been conducting animal findings are important because Lippmann and others have studies focused on the effects of chronic PM exposure. In
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12 CHALLENGES IN CHARACTERIZING SMALL PARTICLES one study, he and his collaborators exposed a strain of mice is comparing the effects of breathing CAPs prepared from bred to be susceptible to exposure-induced cardiovascular air collected at Sterling Forest, New York, and the Mount disease to concentrated Sterling Forest State Park ambi- Sinai School of Medicine in Manhattan with those produced ent aerosol (which is an index of the regional eastern U.S. by comparable exposures to freshly generated sidestream aerosol, much of which is secondary from the Midwest and cigarette smoke, whole diesel engine exhaust, and the gas- Southwest). Exposure lasted 6 hours, 5 days a week, for eous component of whole diesel exhaust. 6 months. The bottom line from these experiments is that the In this study, CAPs exposure was about 105 micrograms/ m3. Sidestream smoke averaged 480 micrograms/m3, but it mice experienced a wide range of health effects including cardiac dysfunction, atherosclerosis, obesity, and metabolic also contained all the other products of tobacco combustion syndrome (Sun et al., 2009, Ying et al., 2009). including carbon monoxide, cyanide, and nitrous oxide. Recently, Lippmann’s group has been collecting and ana- Whole diesel exhaust had particulate levels that averaged 4 36 micrograms/m 3 p lus combustion gases. When the lyzing daily fine PM filter measurements made by state agen- cies. Because funds for this work are limited, his group has researchers examined plaque accumulation, all three particle- restricted its study to data from Detroit and Seattle. Detroit exposed groups had significant plaque excesses compared to was chosen as typical of the eastern United States where mice that were exposed only to the gaseous component of states are struggling to meet PM2.5 of 15 micrograms/m3, diesel exhaust. The biggest increase was seen in the group and Seattle as representative of a city that easily meets this exposed to CAPs. It is likely, said Lippmann, that the metal standard, with an annual average PM2.5 of 9 micrograms/m3. content of the particles drives aortic plaque buildup. Seattle’s nickel levels are much higher than Detroit’s, largely In summary, said Lippmann, epidemiological studies because it is a big seaport handling ships using cheap heavy using speciation data show stronger associations of cardio- oil. As expected, fine particle mass levels are much higher pulmonary effects with transition metals than with PM2.5 in Detroit. mass, and toxicological studies in animals provide support In terms of cardiovascular disease in the two cities, for the influence of transition metals. Data from the Chemical Seattle’s rates are higher, most likely because of elevated Speciation Network (CSN) on PM2.5 components have been nickel, vanadium, and sulfur levels associated with heavy essential to the progress to date in demonstrating stronger oil burning. Lippmann and his colleagues observed similarly associations for metals than for PM2.5 mass, but data on elevated levels of cardiovascular disease near the largest PM2.5 components have been too limited in frequency to Asian nickel refinery located in China, and 1,000 miles adequately support definitive time-series studies and too downwind from a nickel smelter in Ontario. Nickel smelters, limited in spatial coverage to adequately identify the effects Lippmann noted, do not emit vanadium, although they do of PM components that are not uniformly distributed. He emit chromium and iron in addition to nickel. recommended that an expanded CSN network could support In another study, Lippmann’s group measured a variety of epidemiological research that could provide a sound basis for biological markers in two populations of women in China. the development of National Ambient Air Quality Standards The two groups were similar except that one group lived near for toxic PM components that contribute only small fractions a nickel smelter while the other lived on the other side of a of PM mass, permitting more targeted controls to benefit mountain from a smelter. Particulate levels in the two cities public health at lower overall cost and societal disruption. were comparable, at about the average of a typical eastern It is important to keep in mind, Lippmann added, that the U.S. city, but nickel levels were 76-fold higher in the city effects in mortality and hospital admissions do not affect near the smelter. Copper, arsenic, and selenium were higher, everybody. Sensitive segments of the population are driving too, but not by much. Markers of cardiovascular disease these statistics. Therefore, the impact of the particles will be were significantly elevated in the women who lived in the felt mainly by elderly people, not by healthy young people. smelter city compared to the matched cohort. From the data, Lippmann concluded that nickel is likely to be the agent most PARTICLES IN SOIL AND WATER responsible for PM2.5-induced cardiovascular effects, and Michael Hochella from Virginia Polytechnic and State that copper, arsenic, and selenium may play a role. A reduced capacity for endothelial repair, as measured by changes in University started his talk by giving his major take-home mes- several biomarkers, may partially explain the critical role of sage: “Nanoparticles are everywhere. We’re breathing them nickel in PM2.5-associated cardiovascular disease. right now. They’ve been around the planet since its origin.” Lippmann also discussed a study that he and his col- There are vastly more nanoparticles present every day, in laborators are conducting on cardiovascular plaque progres- biological systems and in the atmosphere, than humans can sion produced by subchronic exposures to concentrated ever manufacture. “What’s going to be most important as we ambient PM2.5 (CAPs), which is prepared by taking fresh move into the age of nanotechnology is the overprint that samples and concentrating the particles without filtration. humans can already put on what already exists,” he explained. Using a mouse model of cardiovascular disease, his team
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13 WHAT ARE SMALL PARTICLES AND WHY ARE THEY IMPORTANT? Humanity has previous experience with a hazardous nano- visible (Rosso et al., 1999). However, now, many groups, material, asbestos. A complex set of minerals, asbestos is a including his, are stepping back to look more at changes in nanomaterial in Hochella’s view because, even though it is a particles due to environmental influences, such as lead sul- long fiber, two of its dimensions are in the nanometer range, fide and dissolution of the lead content. It is now possible to and it is these dimensions that enable it to cause pulmonary examine individual sites such as defects on surfaces as they fibrosis, lung carcinoma, and mesothelioma. Asbestos was dissolve, an important process to understand because, as first recognized as a human carcinogen in the 1950s. Two these particles dissolve, they release dangerous species into decades later the EPA put in place the first workforce regula- solution that become bioavailable. tions. These were followed in 1986 by regulations pursuant The whole particles can also be highly bioavailable, to the Asbestos Hazard Emergency Response Act. Hochella particularly as they become smaller and more soluble them- characterized both sets of regulations as responsible reac- selves. However, contrary to thermodynamic prediction, tions to a true hazard, but he also opined that the public and sometimes nanoparticles become less soluble when they politicians have misused these regulations, with the resulting become very small, and sometimes their solubility remains waste of billions of dollars in misguided asbestos abatement unchanged. Hochella noted that these types of studies can efforts. He stated that society can learn from the mistakes it provide information on radiative forces that interest atmo- made in dealing with asbestos and avoid a similar waste of spheric scientists. resources in dealing with nanomaterials. Hochella explained that nanoparticles existed at the begin- The industrial production of nanomaterials has become nings of the universe. Nanodiamonds, about 2 nanometers a huge business, producing several hundreds of billions of or 150 atoms in diameter, were recovered from meteors left dollars worth of products in the biotech, energy, electronics, from the beginning of the solar system. Conditions during and aerospace industries. Hochella predicted that, over the Earth’s early history were also conducive to the formation next couple of years, the economic value of nanomaterials of nanoparticles. As a result, biological systems have been will exceed $1 trillion. Although industrial output of nano- exposed to nanoparticles since life first arose on the planet. materials is significant, it is dwarfed by Earth’s generation Hochella and his colleagues are studying how biological of nanoparticles, which is in the hundreds of teragrams/year, systems interact with and even make use of nanoparticles. or about 1 million metric tons (Figure 2-9). Nanoparticles There are bacteria, for example, that respire using iron nano- that Earth produces include nanosilver, fullerenes (C60), and particles as a source of electrons rather than oxygen. These carbon nanotubes. bacteria alter their respiration rate in response to changes in Even more than 10 years ago, Hochella said, it was nanoparticle size (Hochella et al., 2008). already possible to look at nanomaterials at atomic levels in incredible detail, including measuring the electronic spectra Nanoparticles Enter the Environment in Many Ways of individual atoms. He showed a scanning tunneling micros- copy image of a pyrite surface with individual iron atoms “What do nanoparticles in the oceans have to do with climate change?” asked Hochella. A great deal, it turns out, is due to iron oxide nanoparticles being deposited into rivers and from glaciers, which eventually end up into the world’s oceans (Raiswell et al., 2006). These nanoparticles provide most of the iron that ocean-dwelling phytoplankton require for photosynthesis, which is the most important biological CO2 sink in the ocean. At the largest non-weapons-related Superfund site in the United States, a contaminated mining site in Western Montana the size of Germany, researchers have found pre- viously unknown nanoparticles in the rivers draining into the site from Butte, Montana, including titanium dioxide and lead nanoparticles. These findings prompted Hochella and his group to attempt to better understand how metals as toxic as lead, copper, zinc, arsenic, and cadmium move in the river system. Fifteen years ago, when the group started this effort, nobody understood how these metals were being solubilized, that is, whether they were being carried in river water on organic molecules or bacteria or as nanoparticles. FIGURE 2-9 An inventory of human versus natural production Using flow field flow fractionation techniques, Hochella’s of nanoparticles. R02144 team found that nanoparticles were present in the water. Figure 2-9 SOURCE: Hochella, 2010. uneditable raster bitmap
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14 CHALLENGES IN CHARACTERIZING SMALL PARTICLES Upon further analysis using mass spectrometry and transmis- from wastewater treatment plants that is applied to farm sion electron microscopy, they found that the nanoparticles fields. Atomic-scale electron diffraction Fourier transform were covered with metals. Nanoparticles of the iron mineral microscopy revealed that these are silver sulfide nano- known as goethite, for example, were carrying arsenic hun- particles (Kim et al., 2010). These particles are likely enter- dreds of kilometers from its source, and in fact, into drinking ing the ecosystem when they are shed from clothing treated water. Nanoparticles of ferrihydrite, a different iron oxide with silver nanoparticles that serve as antibacterial agents. mineral, carried arsenic, zinc, and copper into the system, He presented an inventory of nanoparticle occurrence while nanoparticles of the titanium dioxide mineral brookite (Figure 2-10), characterizing them as either manufactured, carried lead into the system. Now that these nanoparticles incidental as a byproduct of diesel emission or industrial have been identified, said Hochella, their role in environ- activity, or naturally occurring. In discussing this inventory, mental and biological processes can be studied in detail. he said, “We haven’t found the way nature makes a quantum As an example, Hochella discussed Schwertmannite, dot yet, but we’re working on that.” a rare but important iron oxyhydroxide sulfate mineral. Hochella concluded his talk by noting that there are many Schwertmannite nanoparticles have long, thin whiskers sources of natural nanoparticles: dust in the atmosphere, sea that are visible at 1.8 million-fold magnification and can spray, and even hydrothermal events in the deep ocean that carry arsenic. Such knowledge enables research focused on bleed seawater as a supercritical fluid heated up to 350°C, understanding how the nanoscale structure of these particles ripping all kinds of elements out of the ocean crust and influences their interaction with biological systems. thereby creating nanoparticles in the deep ocean. Melting icebergs, rivers, and volcanoes also add nanoparticles to the environment. Figure 2-11 shows a global budget of naturally Ubiquitous Nanoparticles occurring and organic nanoparticles. Still unknown is what Recently, Hochella’s team obtained EPA samples taken the additional contribution—and impact—of manufactured from wastewater treatment plants. They discovered that particles will be given that the nanotechnology revolution is manufactured silver nanoparticles are common in the sludge just starting. FIGURE 2-10 An inventory of nanoparticle occurrence. CNT = carbon nanotube; QD = quantum dot. SOURCE: Hochella, 2010. R02144 Figure 2-10
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15 WHAT ARE SMALL PARTICLES AND WHY ARE THEY IMPORTANT? FIGURE 2-11 The global budget for naturally occurring inorganic nanoparticles. SOURCE: Hochella, 2010. R02144 PARTICLES IN BIOLOGICAL SYSTEMS localization of small, electron-dense nanoparticles (LeGros et Figure 2-11 al., 2005). Cryogenic light microscopy allows the location of Gerry McDermott of the University of California, San raster bitmap uneditable fluorescently tagged small particles or cellular structure inside Francisco, talked about visualizing where particles end up the cell to be identified (LeGros et al., 2009). Images from the inside cells and quantifying the number that have entered the two techniques correlate well, which permits combining data cell. Many new imaging tools have been developed over the on cellular imaging with molecular localization. past few years, including soft x-ray tomography and cryo- genic light microscopy (McDermott et al., 2009), that have Soft X-ray Tomography not yet been used to study nanoparticles in cells but have a huge potential for that purpose. Computed tomography (CT) is an iconic instrument in The rationale for imaging small particles in cells, said clinical diagnosis that takes and computationally assembles a McDermott, is to know the location of the small particle, series of x-ray snapshots to create a three-dimensional image. particularly if the goal is to develop a therapeutic delivery Compared to the two-dimensional projection afforded with system. It is important to know, for example, if a particle a conventional x-ray film, CT imaging provides exquisite delivers a drug to the intended target inside the cell. “Does insight into the internal structure of the human body. it go inside the nucleus or does it get stuck in a membrane Soft x-ray tomography miniaturizes this concept to pro- somewhere?” he asked. vide the same exquisite detail about the internal structure of For environmental nanoparticles, intracellular destination a cell. With this technique it is feasible to visualize in great is an important aspect of whether the particle can cause a detail how a cell responds to environmental factors such as change in cell phenotype. It is also important to understand drug molecules or small particles. The structural detail can whether small particles alter the subcellular architecture of also yield new insights into fundamental processes of cell the cell or the locations and concentrations of specific mol- biology, such as the cell cycle. ecules or molecular complexes, and whether such alterations Soft x-ray tomography offers many advantages over cause changes in the fundamental biochemistry that takes conventional imaging such as light microscopy and electron place inside the cell. microscopy. Tomography provides spatial resolution as Two new imaging techniques provide important insight low as 50 nanometers or as high as 10 nanometers, and it about these issues. Soft x-ray tomography for high-resolution does so in three dimensions. It is fast, requiring only 2 to 3 three-dimensional imaging of single cells allows for the direct minutes to collect a full tomographic data set, which makes
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16 CHALLENGES IN CHARACTERIZING SMALL PARTICLES high specimen throughput possible. Cells are imaged whole, McDermott showed tomographic reconstructions of a hydrated, unfixed, and unstained in a near-native state. The yeast cell (Figure 2-13), and of malaria parasites and gold instrument’s field of view is large enough to image one nanoparticles inside a red blood cell (Figure 2-14). The latter eukaryotic cell or about 200 bacterial cells at a time. And, images were used to track how the malaria parasite takes up unlike conventional microscopy that uses fluorescent tags nutrients once it has invaded a red blood cell (Hanssen et to image specific cellular structures, soft x-ray tomography al., 2011). images all of a cell’s internal structures at once. McDermott also showed images detailing internal struc- The instrument concept is simple (Figure 2-12), although tural changes that occur as the pathogenic species of it only became possible to build the necessary focusing yeast known as Candida albicans grows and undergoes a devices in the past 3 years because of the revolution in transformation from nonpathogenic to highly pathogenic. nanofabrication. Because no material has an appreciable These images show that the yeast expands the number of refractive index for x-rays, focusing is done using nano- mitochondria-filled tubes during this transformation, sug - fabricated Fresnel lenses. The outer rings of the zone plate, gesting that tube formation could be a fruitful target for shown in Figure 2-12, determine the spatial resolution of drug disruption. Collaborating with a group at Stanford the microscope. University, McDermott’s group developed and tested a Although simple in concept, in real life the instrument series of protease-resistant peptide-like drugs known as is large and expensive with many pipes and high vacuum peptoids as antifungal agents that would be capable of block- chambers. The x-rays are generated by the Advanced Light ing tube formation (Chongsiriwatana et al., 2008). In tests Source at Lawrence Berkeley National Laboratory and are of two different peptoids, the researchers found that both delivered to a separate room that houses the microscope. peptoids were effective at greatly reducing the growth of the McDermott hopes that, in the future, the microscope will be mitochondria-filled tubes. However, when fungi treated with able to use less expensive plasma x-ray sources. Imaging of the two peptoids were imaged using soft x-ray tomography, whole cells is done in what is known as the water window, a they found that one of the two agents produced large changes region of the spectrum in which water does not absorb x-rays in the fungal nucleus that could be problematic if that agent but biomolecules do. ever proceeded to human clinical use (Figure 2-15) (Uchida FIGURE 2-12 Full-field transmission soft x-ray microscope. SOURCE: McDermott et al., 2009. R02144 Figure 2-12 uneditable raster bitmap
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17 WHAT ARE SMALL PARTICLES AND WHY ARE THEY IMPORTANT? FIGURE 2-13 One slice from a tomographic reconstruction (left) and the three-dimensional reconstruction of a yeast cell clearly shows the individual organelles inside the cell. SOURCE: McDermott, 2010. R02144 Figure 2-13 uneditable raster bitmap FIGURE 2-14 Tomographic image of red blood cell containing malaria parasites and gold nanoparticles. Parasites were allowed to invade red blood cells containing gold particles. The image is a rendered model overlaid onto a virtual section. The parasite surface is depicted in translucent gray and the digestive vacuole in blue. The model reveals the morphology of the gold particle-containing structure in the parasite cytoplasm. Scale bar, 3.5 μm. R02144 SOURCE: Hanssen et al., 2011. Figure 2-14 uneditable raster bitmap
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18 CHALLENGES IN CHARACTERIZING SMALL PARTICLES FIGURE 2-16 Correlated cryo-fluorescence and x-ray images of Schizosaccharomyces pombe, with the fluorescence image showing labeled vacuoles on the left and x-ray images of unlabeled vacuoles in the middle two images. The tomographic reconstruction on the right shows the unlabeled vacuoles. SOURCE: McDermott, 2010.R02144 Figure 2-16 uneditable raster bitmap OPEN DISCUSSION In response to a question from Pedro Alvarez of Rice University about the size distribution of natural versus manufactured nanoparticles, Hochella said, “These natural nanomaterials show the same fascinating size-dependent properties as manufactured or synthetic ones do.” He added that nanoparticles with the same chemical composition and molecular structure from the two sources have similar distributions in terms of physical, chemical, electrical, and magnetic properties. However, another participant noted a key difference between natural and synthetic nanoparticles is that synthetic nanoparticles can be much more monodisperse in terms of size and shape and homogeneous in terms of chemical composition compared to naturally occurring col- loidal particles. Barbara Karn of the EPA (now at the National Science FIGURE 2-15 Soft x-ray tomography of C. albicans cells after Foundation) commented that manufactured nanomaterials treatment with two peptoids. The top seven images (A-F) show often have elements such as indium that are not found in nat- internal changes seen with the first of two peptoids, while the urally produced nanoparticles. She asked if that was a con- bottom image (G) shows the larger nuclear changes produced by cern, particularly with regard to water treatment plants and R02144 the second peptoid. These larger changes could prove problematic their discharges into waterways. Addressing this comment, in human use. Figure 2-15 Hochella said that for some elements, such as lead or gold, SOURCE: Uchida et al., 2009. uneditable raster bitmap nature has done a good job concentrating them in locations that humans then mine. Those materials are probably found in naturally occurring nanoparticles. For others, however, as et al., 2009). McDermott also showed images obtained using Karn noted, humans are now mining materials from much cryo-fluorescence microscopy in combination with x-ray deeper in Earth, elements that would not be exposed on the tomography (Figure 2-16). McDermott concluded by noting planet’s surface. These elements are being concentrated and that these changes would not have been seen with standard put into products, and the result is that humans are “dramati- microscopy, which highlights the potential benefits of using cally changing the distribution of that element on the planet’s these new techniques in cell biology studies. surface.” For example, in some cases it can cause a material
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19 WHAT ARE SMALL PARTICLES AND WHY ARE THEY IMPORTANT? to become more bioavailable and possibly toxic, which is is an overriding control strategy that will help mitigate the why there is a need to develop a better understanding of the potential impacts on climate and health. life-cycle impacts of synthetic nanoparticles on the envi- Lippmann responded, “There has been an effective and ronment. In fact, elaborated Hochella, that is why Virginia continuing effort to reduce the emissions of sulfur dioxide Tech and other institutions around the world are starting into the atmosphere, with the Clean Air Act leading to a sustainable nanotechnology centers. He and his colleagues 50 percent reduction [in airborne sulfur dioxide levels].” at Virginia Tech, for example, are studying how cadmium, a Reducing the sulfur content of diesel fuel has had an addi- critical component of quantum dot nanoparticles, gets into tional substantial impact on reducing sulfate aerosols that the environment and what its fate will be. By understand- he predicted “will change the reflectivity of the atmosphere, ing such processes, it may be possible to either move away because sulfate is the best light scatter of all of them.” from using cadmium, or other potentially toxic materials, or Schwartz noted that sulfur emissions in general, and par- design ways to produce these materials in a more environ- ticle emission specifically, are increasing in the developing mentally friendly way. world. He added that a reduction in aerosol emissions may Lippmann followed up by asking Hochella about nano- increase the greenhouse effect, which may or not be a good particulate silver and whether there is any idea of what the outcome. Hochella suggested that increasing the output of environmental consequences will be. Hochella replied that iron-containing nanoparticles, which could boost phyto- the answer is no, which is exactly why research is needed plankton productivity, could help ameliorate global warming. now. He noted that nanoparticulate silver is included in a This possibility, he said, points to the difficulty in developing large number of consumer items because of its antimicrobial an overriding control strategy for nanomaterials. properties, but there is concern about what will happen to A participant asked the speakers to address any outstand- the planet’s microbial ecosystems when large quantities of ing issues in characterizing small particles. Lippmann replied nanoparticle silver enter and become concentrated in the that he would like to see more work on characterizing the environment. composition of particles in the air and the health implica- Lippmann inquired about what asbestos has to do with tions of the composition given that the tools to do so, such as nanoparticles. Hochella responded that he defines a nanopar- x-ray fluorescence, are on the threshold of making the needed ticle as one with at least one of its three dimensions in the measurements. He said that such data would enable control nanometer range, and asbestos meets that criteria. Clay efforts to focus on emissions of specific, toxic materials minerals fit this definition in one dimension, and it is that rather than particles in general, which could potentially save dimension that in part imparts the special properties of clay a great deal of money. and many other minerals as well. He added that catalyst Schwartz said that he would like to see research develop researchers, who worry about stopping agglomeration, can a better understanding of the processes that are responsible learn from studying the behavior of natural fibers such as for the formation and growth of atmospheric aerosols. “I asbestos. Lippmann added that even when nanoparticles do think we are really on the cusp of a revolution in terms of agglomerate, they still have nanoscale features on their sur- characterization of the composition of these newly formed faces that must be considered when designing and studying particles,” he said. “I think we’ll that find many of the aerosol such materials. chemistry and growth models that are being used right now Finlayson-Pitts noted, “Ultimately what we’re interested to try to estimate aerosol impacts on climate are going to turn in are the impacts of particles, the good and the bad, and in out to be all wrong.” An audience member added that such the case of a bad, how do we mitigate? What should we con- studies should also include surface properties because, as trol?” She remarked that Schwartz, Lippmann, and Hochella the catalyst community knows well, surface properties and addressed different aspects of trace metals and organics car- composition are both important for determining a particle’s ried on the surface of nanoparticles, and she asked if there behavior.
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