Chemical Education Today
Reports from Other Journals
Research Advances by Angela G. King
Children on School Buses May Face Increased Exposure to Diesel Pollution
sured using an AeroVironment CTA-1000 continuous analyzer. iF can be broken down into two components: intake by passengers (self-population intake fraction, iFSP) and intake by all others (intake fraction excluding self-population, iFnon-SP). For a school bus self-population, the self-population intake fraction can be calculated by using equation 1:
Diesel particles are extremely small and can deposit deep in the lungs, whereas larger particles are filtered out by the nose, mouth and throat. A number of studies have linked diesel particles to adverse health effects. For example, a large assessment of air pollution in the Los Angeles area found that diesel particles are responsible for most of the cancer risk from outdoor air pollution. Children are especially susceptible to air pollution because they have high inhalation rates and large lung surface area per body weight, as well as narrow airways and immature immune systems. Diesel particle pollution inside urban school buses may be worse than levels found in the surrounding roadway air, according to the work of Julian Marshall, Eduardo Behrentz, and others at University of California. It has generally been assumed that other vehicles on the road are the source of elevated particle levels. But the recent study of school buses in the Los Angeles area shows that much of the pollution inside a school bus comes from the bus itself, and children on board may be inhaling more diesel particles than previously believed. Intake fraction (iF ) is important when studying the health effects of air pollution. It is the ratio of total attributable intake to total emissions, or the fraction of emissions inhaled by the exposed population. The average individual intake fraction (iFi) can also be determined. To determine the iF and iFi values researchers employed tracer-gas experiments. SF6, a tracer gas, was metered into bus engine exhaust and concentrations of that gas inside the buses were mea-
QB is the average volumetric breathing rate (L person᎑1 min᎑1), P is the average number of people riding the bus, C is the average on-board SF6 concentration (g L᎑1) and E is the experimental SF6 mass emission rate into the bus’s exhaust (g min᎑1). Marshall and Behrentz analyzed results from a UCLA school bus experiment where the researchers took out the seats and essentially turned the buses into mobile chemistry labs, driving them along actual school bus routes in the greater Los Angeles area. Six buses were involved in the study: two older high-emitting diesel buses from 1975 and 1985; two diesel buses that are more representative of current fleets; one diesel bus outfitted with a particle trap; and one bus powered by compressed natural gas. Self-pollution levels turned out to be substantial for all six buses, but older buses and close-windowed buses were higher, according to the researchers. The average value for intake fraction across all bus runs was 27 grams inhaled per million grams emitted, with the highest value at around 100 per million.
Figure 1. Intake fraction for an average vehicle and for school buses in the South Coast Air Basin. X-axis category labels refer to the model year of the bus. Reprinted with permission from Environ. Sci. Technol. 2005, 39, 2559–2563. Copyright 2005 American Chemical Society.
Figure 2. Individual intake fraction. The first value is for a typical person’s inhalation of emissions from an average vehicle in the South Coast Air Basin. The remaining values are for a student’s inhalation of emissions from the school bus on which they commute. The scale is logarithmic. Reprinted with permission from Environ. Sci. Technol. 2005, 39, 2559–2563. Copyright 2005 American Chemical Society.
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Q B PC E
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Chemical Education Today
Reports from Other Journals “This may not sound like a lot,” says Julian Marshall, lead author of the study. “But intake fraction values for vehicle emissions are 5–15 per million in a typical U.S. urban area, and about 50 per million in a large urban area like Los Angeles. This means that for every ton of pollution emitted by a school bus, the cumulative mass of pollution inhaled by the 40 or so kids on that bus is comparable to—or in many cases larger than—the cumulative mass inhaled by all the other people in an urban area.” The California Air Resources Board, which sponsored the original study by UCLA, recommends minimizing commute times, using the cleanest buses for the longest commutes, accelerating the retirement of older buses, and decreasing bus caravanning and idling time to reduce children’s exposure to bus-related air pollutants. But it is important to note when all safety factors, such as collisions and traffic safety, are considered, school buses are still the safest way to get children to school, despite the self-pollution. “Based on our work, if a policymaker wants to reduce health effects from diesel for the population as a whole, then school buses are a good source to target,” Marshall says.
More Information 1. Marshall, Julian D; Behrentz, Eduardo. Vehicle Self-Pollution Intake Fraction: Children’s Exposure to School Bus Emissions. Environ. Sci. Technol. 2005, 39, 2559–2563. 2. More information on the school bus study can be found online at http://www.arb.ca.gov/research/schoolbus/schoolbus.htm (accessed Jul 2005). 3. This Journal has published resources available to facilitate class coverage of air pollution and related problems. See Hollenberg, J. Leland; Stephens, Edgar R.; Pitts, James N., Jr. Demonstrating the Chemistry of Air Pollution. J. Chem. Educ. 1987, 64, 893–894 and Rockwell, Dean M.; Hansen, Tony. Inventory Control: Sampling and Analyzing Air Pollution: An Apparatus Suitable for Use in Schools. J. Chem. Educ. 1994, 71, 318–322.
Where Did All the Nitrogen Go? Algae and cyanobacteria consume massive amounts of dissolved inorganic carbon, fixed inorganic nitrogen, and phosphate as they generate organic products through photosynthesis. The C:N:P ratio in these products, known as the Redfield ratio, is relatively constant throughout the marine realm, with a value of 106:16:1. Upon remineralization of the organic products, carbon dioxide, ammonium, and phosphate are released in the same proportions. If ample oxygen is present, chemoautotrophic bacteria oxidize the released ammonium to nitrate, a process known as aerobic ammonium oxidation. The nitrate is then ready to be consumed by phytoplankton and the cycle repeats itself. Conversely, heterotrophic denitrification occurs in oxygen-poor waters. In this process, anaerobic prokaryotes oxidize organic compounds as nitrate is reduced to nitrogen gas. In this process some ammonium is also generated. But scientists have known that anoxic waters do not accumulate as much ammonium www.JCE.DivCHED.org
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as would be stoichiometrically predicted. Classically, this ammonium deficit was attributed to the bacterial combination of ammonium and nitrate to release N2. An international team led by Marcel Kuypers from the Max Planck Institute for Marine Microbiology recently presented evidence that anammox, the anaerobic oxidation of ammonium to N2 with nitrite serving as an electron acceptor, plays an important role in the loss of nitrogen from the world’s oxygen-minimum zones (OMZs). OMZs account for 30– 50% of total nitrogen loss; understanding the process by which the nitrogen is removed is a breakthrough for scientists studying the marine environment. The Benguela current runs along the southwest African continental shelf. In the current, upwelling waters have high concentrations of oxygen and nutrients while deeper waters are oxygen-depleted due to the consumption of oxygen by decomposing settling algal biomass. The bottom waters also have a nitrogen deficit as indicated by a reduced N:P ratio. Investigators studied the Benguela upwelling to see whether the OMZ supported Planctomyces spp., anaerobic bacteria that oxidize ammonium under anaerobic conditions with the help of nitrite. Planctomycetales, commonly known as anammox bacteria, had previously been found in wastewater bioreactors but there was no published evidence of their presence in OMZs. Kuypers considered the ammonium deficit in OMZs as an indication that that they could be contributing to the loss of nitrogen. Scientists measured salinity, temperature, density, turbidity, and oxygen levels with a conductivity–temperature– depth (CTD) system. Collected water samples were also analyzed on board a collection vessel for nitrate, nitrite, phosphate and ammonium concentrations. The Redfield ratio was then used to determine the fixed-inorganic nitrogen deficit for samples. Water samples were collected at specific depths and analyzed using a carbon and nitrogen analyzer for particulate carbon and by GC–MS for fatty acid content. Samples from specific depths were also subjected to epifluorescence microscopy to determine the average number of anammox bacteria present. Collected water samples were incubated with combinations of Na15NO3, 15NH4Cl and Na15NO2. GC isotope ratio MS was then employed to determine the ratios of 15N14N:14N14N and 15N15N:14N14N produced by microbial action. DNA was also extracted, isolated and cloned by standard procedures. What did the researchers learn from all their hard work? Each day anammox removes 1–5 mmol/m2 of fixed inorganic nitrogen from the oxygen-poor Benguela upwelling. Given the size of the suboxic shelf, this means 1.4 ⫾ 1 Tg of fixed nitrogen might be lost from Benguela each year due to anammox. An estimated 80–150 Tg of fixed nitrogen is lost from OMZ waters worldwide each year. In the past, this was all attributed to heterotrophic denitrification since there was no other known process that could transform inorganic fixed nitrogen to N2 until the discovery of anammox in 1995. Armed with the evidence and experience Kuyper’s team gained studying the Benguela upwelling, they are now prepared to determine the extent that anammox contributes to nitrogen loss from OMZs worldwide.
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More Information 1. Kuypers, Marcel M. M.; Lavik, Gaute; Woebken, Dagmar; Schmid, Markus; Fuchs, Bernhard M.; Amann, Rudolf; Jorgensen, Barker, Bo; Jetten, Mike S. M. Massive Nitrogen Loss from the Benguela Upwelling System through Anaerobic Ammonium Oxidation. Proc. Nat. Acad. Sci. 2005, 102, 6478–6483. 2. More information on the Benguela upwelling can be found at http://oceancurrents.rsmas.miami.edu/atlantic/benguela_2.html (accessed Aug 2005). 3. Marcel Kuyper’s research is described online at http://www.mpibremen.de/en/Marcel_Kuypers.html (accessed Jul 2005). 4. Microbial reduction of nitrates is discussed in a public understanding article in this Journal. Barnum, Dennis W. Some History of Nitrates. J. Chem. Educ. 2003, 80, 1393.
Designer Wires Semiconductor nanowires have great potential for use in applications ranging from biosensors to silicon planar technology replacements, providing researchers learn to control the nanowires’ composition, diameter, length and morphology. In the past, vapor–liquid–solid (VLS) and solution–liquid–solid (SLS) phase catalytic growth methods have afforded the best approach to growing wires that are uniform in diameter throughout their entire length. Producing uniform nanowires
is a requirement for applications and a challenge for scientists. Now researchers from IBM and the Advanced Materials Research Institute at the University of New Orleans have developed a rapid new method of generating crystalline PbSe wires. Led by Dmitri Tapalin and Christopher Murray of IBM’s T. J. Watson Research Center, researchers working on this project developed a solution-based synthesis of PbSe wires through the oriented attachment of nanocrystals that form chains, attach and fuse along identical crystal faces into single crystalline nanowires. This process is similar to polymerization reactions well known in organic chemistry. The small blocks of crystalline solid consisting of ~5000 atoms each (socalled nanocrystals) “polymerize” into the long nanowire. The nanowire length is typically tens of micrometers while the diameter can be varied from 4 nm to 20 nm. Surfactant mixtures were employed to produce nanowires uniform in diameter. The wires assembled in less than a minute in this one-pot reaction and formed stable colloidal suspensions which can be easily processed. By controlling the shape of the nanocrystal building blocks and reaction conditions, the team of investigators controlled nanowire morphology and obtained straight, zigzag, helical, and branched nanowires. PbSe served as the model for this new approach to oriented attachment leading to technologically important nanowires. PbSe is a direct gap semiconductor with high electron and hole mobilities. Lead selenide nanowires were formed by injecting lead oleate and trioctylphosphine selenide mixed with
Figure 3. High resolution (a) SEM and (b) TEM images of PbSe nanowires grown in solution in the presence of oleic acid. (c) Overview and (D–F) high resolution TEM images of PbSe nanowires grown in the presence of oleic acid and n-tetradecylphosphonic acid. Insets show selected area electron diffraction from a film of PbSe nanowires (in c) and single nanowires (in d). The nanowire diameters can be tuned from (e) ~4nm to (f) ~18nm. Reprinted with permission from J. Am. Chem. Soc. 2005, 127, 7140–7147. Copyright 2005 American Chemical Society.
Figure 4. Shape evolution of PbSe nanowires grown for (a) 30 s and (b) 10 min at 170 ⬚C using a 1:3 molar ratio of Pb:Se precursors and oleic acid as a stabilizing agent. Arrows point to the (111) face defects. The average wire morphology and structure of the hypothetical nanocrystals repeat unit are depicted to the right of the TEM images. Reprinted with permission from J. Am. Chem. Soc. 2005, 127, 7140– 7147. Copyright 2005 American Chemical Society.
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Reports from Other Journals Reports from Other Journals Figure 5. (a) Star-shaped PbSe nanocrystals and (b–e) radially branched nanowires. (d) TEM image of the (100) view of the branched nanowire and the corresponding selected area diffraction pattern. (e) TEM image of the (110) view of the branched nanowire and the corresponding selected area electron diffraction pattern. The cartoon (b) shows a four-armed branched nanowire. (f and h) Branched nanowires where the length of the sidearm varies along the nanowire, as depicted in cartoon (g). Reprinted with permission from J. Am. Chem. Soc. 2005, 127, 7140–7147. Copyright 2005 American Chemical Society.
Figure 6. An overview of PbSe nanowire morphologies. Reprinted with permission from J. Am. Chem. Soc. 2005, 127, 7140–7147. Copyright 2005 American Chemical Society.
stabilizing oleic acid into 250 ⬚C phenyl ether. Key factors in determining the product’s characteristics were temperature, reactant concentration, and the presence of stabilizing agents. For instance, temperatures >170 ⬚C produce nanowires while lower temperatures produce nanocrystals that do not polymerize into nanowires. Nanowire shape, diameter, and morphology varied with the cosurfactants present during synthesis. For instance, the addition of n-tetradecylphosphonic acid (TDPA) promoted the growth of long, straight, and skinny wires while primary amines produced wires with a zigzag morphology due to octahedral building blocks. Researchers were also able to prepare rectangular nanorings with both width and length smaller than the Bohr excitonic radius for PbSe. New efforts are currently characterizing the semiconductor properties in these confined geometries.
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More Information 1. Cho, Kyung-Sang; Talapin, Dmitri V.; Gaschler, Wolfgang; Murray, Christopher B. Designing PbSe Nanowires and Nanorings through Oriented Attachment of Nanoparticles. J. Am. Chem. Soc. 2005, 127, 7140–7147. 2. More information on the Advanced Materials Research Institute at the University of New Orleans, where this research was conducted, can be found at http://www.amri.uno.edu/ (accessed Jul 2005).
Angela G. King is Senior Lecturer in Chemistry at Wake Forest University, P.O. Box 7486, Winston-Salem, NC 27109;
[email protected].
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