150 Journal of Chemical Education  •  Vol. 86 No. 2 February 2009  • www.JCE.DivCHED.org  • © Division of Chemical Education

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Nature: Our Atmosphere in the Year of Planet Earthby Sabine Heinhorst and Gordon C. Cannon

The International Year of Planet Earth

Since the Inter-national Union of Geological Sciences (IUGS) and UNES-CO have declared 2007–2009 the Inter-national Year of Planet Earth (Figure 1), we would be remiss if we did not alert our read-ers to the Year of Planet Earth supplement to Nature [2008, 451, January 17, 257–303] that covers several aspects of climate change and its effect on society. We want to emphasize that the entire series of excellent articles and essays is worth reading and lends itself to classroom discussions and reflections on anthro-pogenic effects on global climate, challenges for society, and the need for sustainability measures in the years to come. Educators might also want to browse http://www.yearofplanetearth.org/ (accessed Nov 2008) for downloadable resource material related to various Earth science topics.

Particularly relevant to the topic of this column is the article by L. R. Kump from Pennsylvania State University on the rise of oxygen in Earth’s atmosphere over geological time (pp 277–278). The author summarizes the existing evidence for “the great oxidation event” some 2.45 billion years ago, discusses likely geological and biological causes for the rise in atmospheric oxygen at the end of the Archaean, and points out questions that remain unanswered. One of these is the timing of this change in atmospheric composition (Figure 2). The oxygenic photosynthesis activity of cyanobacteria is widely believed to have been a major contributing factor. Prior research, however, had found molecular proxies for the presence of these microbes in rocks whose age predates the rise in atmospheric oxygen by several hundred million years and had therefore suggested a much earlier evolutionary origin of cyanobacteria.

Recently, researchers from three Australian universities re-examined the same rocks that had been studied earlier. B. Rasmussen and colleagues report [2008, 455, October 23, 1101–1104] that the results they obtained using NanoSIMS (secondary ion probe mass spectrometry) (Figure 3), a relatively new analytical technique that detects trace elements with very high sensitivity (<200 atoms) and superb spatial resolution (50 nm). The technique can reveal minute differences in isotope or element distributions between tiny textures in rocks or between subcellular structures in biological samples. Based on their measurements of carbon isotope ratios, the researchers con-cluded that the formation of the 2.7 billion years old Western Australian shales they examined pre-dates the acquisition of 2α-methylhopanes, the geological hydrocarbon biomarkers for

Figure 2. Important geological and biological events in Earth’s history. The appearance of oxygen in the atmosphere and fossil evidence for the presence of cyanobacteria and eukaryotes in the Proterozoic are indicated by solid arrows. A dashed arrow marks the earlier appearance of cyanobacteria at the end of the Archaean that had been predicted based on biomarker evidence. Adapted from W. F. Fischer, Nature 2008, 455, 1051–1052.

Figure 1. The official logo of the International Year of Planet Earth. The red inner circle represents the solid Earth; green indicates its biosphere, dark blue its hydrosphere. The outer light blue circle represents its atmosphere. Courtesy IYPE Secretariat.

cyanobacteria. Their findings again place the origin of these or-ganic compounds, and that of cyanobacteria, to a time closer to “the great oxidation event”. See also News and Views commen-tary by W. W. Fisher [2008, 455, October 23, 1051–1052].

Our Forests—Notorious Air Polluters?

J. Lelieveld and colleagues from the Max Planck Institute for Chemistry in Mainz, Germany, have revisited the effects the vast amounts of volatile organic compounds (VOCs) that are emitted by terrestrial vegetation have on our atmosphere [2008, 452, April 10, 737–740]. The VOCs released by plants amount to a staggering 1015 g (gT) of carbon annually and are particularly rich in 2-methyl-1,3-butadiene, better known as isoprene, as well as larger mono- and sesquiterpenes (Figure 4). To the plants, these compounds serve as signals that attract pollinators or deter pests; isoprene is thought to increase their thermotolerance and their resistance to reactive oxygen species, such as ozone. But what is the link of these biogenic emissions to global warming? Since isoprene readily reacts with hydroxyl

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© Division of Chemical Education  • www.JCE.DivCHED.org  •  Vol. 86 No. 2 February 2009  •  Journal of Chemical Education 151

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radicals, the current model for atmospheric chemistry had predicted that plant-derived hydrocarbon emissions would contribute significantly to a depletion of hydroxyl radicals, the “detergents” of the atmosphere and, therefore, to the accumula-tion of harmful trace gases. Produced in the troposphere by photolysis of ozone in the presence of water, the highly reactive hydroxyl radical is responsible for the oxidation of the green-house gas methane and the conversion of CO and NO to CO2 and NO2, respectively.

Lelieveld et al. directly measured hydroxyl radicals and other troposphere constituents during aircraft flights over the tropical rain forests that cover large areas of Guyane, Suriname, and Guy-ana and are essentially free of anthropogenic emissions. They de-tected significantly higher hydroxyl concentrations than predicted by the current troposphere model and proposed the existence of a hydroxyl recycling pathway through reaction of oxidized VOCs with HO2. Lab experiments confirmed that a significant amount of OH can be recycled by such reactions. Assuming at 40–80% hydroxyl recycling efficiency, incorporation of this pathway into a modified atmospheric chemistry model led to a much better fit with the measured hydroxyl radical levels and their variations over a 24 h period (high in sunlight, low in the dark). Clearly, the results of this study show that pristine tropical forests are able to sustain the oxidation capacity of their troposphere and prevent ac-cumulation of toxic gases despite huge emissions of VOCs. In his News and Views commentary [2008, 452, April 10, 701–702], A. Guenther points to the need for similar field studies in areas that are affected by anthopogenic activities to determine how the resulting higher levels of key air pollutants (e.g. NO2) influence troposphere chemistry and air quality.

Figure 3. Secondary Ion Mass Spectrometry (SIMS). The sample whose composition is to be analyzed is bombarded with a beam of high-energy primary ions. These interact with the surface and lead to the ejection of atoms, small molecules, and charged particles (secondary ions). A beam of secondary ions of the same polarity is accelerated in an electrical field. The ions are sorted according to their mass and energy in the mass spectrometer and counted in the ion detector. Adapted from a similar figure at http://presolar.wustl.edu/work/what_is_sims.html (accessed Nov 2008).

Our Oceans—Destroyers of Tropospheric Ozone!

The importance of stratospheric ozone in protecting Earth’s biosphere from the harmful effects of ultraviolet rays is unde-niable. In the troposphere, however, this form of oxygen is a greenhouse gas that wreaks havoc with the biosphere in general and with human health in particular, while at the same time gen-erating beneficial, air-cleansing hydroxyl radicals. Tropospheric ozone formation takes place mainly through photochemical reactions in the NOx-rich air over continental landmasses. The open oceans, particularly those in tropical latitudes, contribute to ozone’s destruction through processes that have largely been ascribed to efficient photolysis in these regions of high light intensity and water vapor levels. However, the kinds of chemi-cal interactions that take place in the atmosphere immediately above the water level (marine boundary layer) are clearly more complex, and the extent to which other molecular species con-tribute to ozone destruction above the open ocean are not well understood.

An example of such potential ozone destroyers are ocean-derived halogen compounds. These can be of biological origin, such as the organic iodide compounds released by marine algae, or they can be natural seawater constituents that are continually released into the air in sea spray, such as bromide ions. They are converted into reactive bromine and iodine atoms by photolysis and contribute to the removal of ozone from the marine atmo-spheric boundary layer by reactions with ozone, hydroxyperoxy radicals, or NO2. The picture of their contribution to ozone removal is complicated by the fact that the halogen oxides that arise from reaction with ozone actually regenerate O3.

Figure 4. Examples of volatile organic compounds (VOCs) emitted by forests. Shown are 2-methyl-1,3-butadiene (isoprene), (R)-(+)-α-pinene (turpentine, a monoterpene), and bergamotene (a sesquiterpene). Terpenes are synthesized from two or more isoprene building blocks and vary greatly in structure. Many of them are responsible for the fragrance of plants and plant extracts and are the key ingredients of essential oils (e.g. menthol from peppermint, limonene from citrus fruit, zingiberene from ginger, and caryophyllene from cloves). Monoterpenes are built from two isoprene units, and sesquiterpenes from three, diterpenes from four, and so on. Structure by Bing Yu.

Sample

Beam of accelerated secondary ions to mass spectrometer and ion detector

for analysis

Extraction lens

Secondary ions ejected from sample

Beam of high energy primary ions

2-methyl-1,3-butadiene

(R)-(+)- -pinene bergamotene

152 Journal of Chemical Education  •  Vol. 86 No. 2 February 2009  • www.JCE.DivCHED.org  • © Division of Chemical Education

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A team of researchers from the Universities of York and Leeds in the UK, the Instituto Nacional de Meteorologia Geo-fisica, Cape Verde, and Caltech’s Jet Propulsion Laboratory now report [2008, 453, June 26, 1232–1235] the results of a year’s worth of measurements off the Cape Verde coast that assessed the levels of ozone and of the molecular species known to af-fect its formation and destruction (e.g. water, CO, NO, NO2, CH4, VOCs and their oxidation products, as well as BrO, IO, and OIO). K. A. Read and colleagues found that the measured ozone levels and their diurnal variations agreed with theoreti-cal models only if the halogen oxide species were included in the model. Their measurements and simulations showed that by not considering the halogen contribution to ozone reduc-tion in the marine boundary layer the original model had overestimated ozone levels by 12% and underestimated ozone loss by 47%. However, as R. von Glasow points out in the ac-companying News and Views commentary [2008, 453, June 26, 1195–1196], more long-term studies at different geographical sites, preferably employing ships to ensure that measurements truly reflect open ocean conditions, are needed to solidify the proposed role of halogens in ozone destruction in the marine boundary layer and deliver a complete picture of the role of Earth’s oceans in this process.

More on the International Year of Planet Earth all sites accessed Nov 2008http://www.unesco.org/science/earth/iype.shtml

How the NanSIMS works: http://presolar.wustl.edu/work/what_is_sims.html

More on geobiology and biomarkers from MIT’s Roger Summons’ Web site: http://eaps.mit.edu/geobiology/biomarkers.html

More on reactions of hydroxyl radicals in the troposphere: http://www.niwa.cri.nz/pubs/wa/ma/16-1/detergent and http://www.atmosphere.mpg.de/enid/24y.html

NASA’s ozone hole watch site: http://ozonewatch.gsfc.nasa.gov/

NASA feature article on tropospheric ozone: http://earthobservatory.nasa.gov/Features/OzoneWeBreathe/

Supporting JCE Online Materialhttp://www.jce.divched.org/Journal/Issues/2009/Feb/abs150.htmlAbstract and keywords. Full text (PDF) with links to cited URLs. JCE Featured Molecules for February 2009 (see p 256 for details)

Sabine Heinhorst and Gordon C. Cannon are members of the Department of Chemistry and Biochemistry, University of Southern Mississippi, Hattiesburg, MS 39406-0543; sabine.heinhorst@usm.edu; gordon.cannon@usm.edu.

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