Chemical Education Today
Reports from Other Journals
Nature: Earth’s Atmosphere and Beyond by Sabine Heinhorst and Gordon Cannon
Since the theme for this issue of the Journal of Chemical Education is “Earth’s Atmosphere and Beyond”, we have first chosen articles from Nature that show connections between changes in the composition of our planet’s atmosphere and either anthropogenic activities or natural phenomena. Our final choices fit into the “and beyond” category with news features and articles about the planets Mars and Pluto. Earth’s Atmosphere
photo provided by Jillian Gregg, © Cornell Univ.
If asked to predict whether plants grow faster in the city or the country, most people would probably answer: “In the country, of course!” After all, city air and soil are known to contain high levels of pollutants, such as oxides of nitrogen and sulfur that are likely to be detrimental to plant growth. A carefully controlled study by Jillian Gregg and colleagues from Cornell University and the Institute of Ecosystem Studies in New York (2003, 424, July 10, 183–187) shows that, surprisingly, cottonwood trees grew significantly faster in New York City than in surrounding rural areas of the Hudson Valley and Long Island. To distinguish among the multiple anthropogenic factors that might be responsible for the observed growth enhancement in the urban environment, the researchers monitored air and soil pollutants, atmospheric CO2 levels, light intensity, precipitation, and temperature throughout their three-year experiment. Through a combination of field experiments, soil transplants, nutrient budgets, trials using special growth chambers, and statistical data evaluation, they were able to exclude any soil effects and to pinpoint the detrimental effect of higher cumulative ozone exposure outside the urban core. Although individual one-hour peak concentrations can be higher in urban centers, the reactions of ozone formation are cyclical, such that ozone is continually created and destroyed within the urban center. Once the air mass has
Figure 1. Researcher Jillian Gregg standing between her experimental cottonwood trees that were grown in New York City (to her right, left in the photograph) and their counterparts grown in the surrounding countryside (to her left). The city trees are significantly taller than the country trees.
moved to rural environments, where the nitric oxide precursor that scrubs ozone from the urban atmosphere is no longer abundant, the high ozone concentrations remain in the atmosphere longer. Hence, the cumulative exposure period is much longer in rural environments. The study challenges the commonly held belief that the impact of urban air pollution on rural areas is negligible and points to the need for a more thorough assessment of the extent to which anthropogenic factors change Earth’s atmosphere. Human activities clearly have been identified as the cause for a variety of air pollution problems, but who would have thought that the humble pine tree is an air polluter as well? A group of scientists from the University of Helsinki, Harvard University, and The Risø Institute in Denmark (2003, 422, March 13, 134) report that the coniferous forests of Earth’s northern hemisphere emit enough nitrogen oxides to potentially affect the atmospheric nitrogen balance. The researchers found that at less than 1 ppb ambient concentration of NOx , the needles of pine trees release these gases upon exposure to UV light. It is not known whether the nitrogen oxides arise through metabolic activity of the plants or through UV-induced chemical reactions on the leaf surface. Methyl chloroform (MCF) is an air pollutant that has been implicated in contributing to the depletion of ozone from the stratosphere. Although most MCF released into the atmosphere is oxidized by hydroxyl radicals, its lifetime in the troposphere is sufficiently long (5–6 years) for a portion of the MCF molecules to reach the stratosphere. In the 1980s, MCF was used extensively as an industrial solvent for dry cleaning, in coatings, degreasing, and other applications; according to the 1987 Montreal Protocol,1 it was to be phased out by 1996 in the developed countries. Indeed, MCF emissions have decreased considerably in the 1990s. However, recently reported inconsistencies in trends for tropospheric hydroxyl radical levels, which are calculated based on measured MCF levels, have led Krol and colleagues (2003, 421, January 9, 131–135) to the reassessment of atmospheric MCF levels over central Europe. The team of scientists from laboratories in The Netherlands, Germany, and the United Kingdom collected air samples at ground stations and during flights over Europe, and determined MCF levels by gas chromatography/mass spectrometry. Their results, combined with data obtained from other European MCF assessment initiatives, were fed into a simulation program that models air transport and was used to calculate more reliable tropospheric hydroxyl radical concentrations. The study concludes that MCF emissions from continental European sources are considerably higher than had previously been measured at a ground station off the west coast of Ireland. The authors speculate that continued active use of MCF, particularly in southern and eastern Europe, diffusion from disposal sites, or release from consumer goods
JChemEd.chem.wisc.edu • Vol. 80 No. 10 October 2003 • Journal of Chemical Education
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Chemical Education Today
Reports from Other Journals photo by NASA, JPL Mars Exploration Rover Mission
manufactured with MCF that had been banked before 1996, could be potential sources of this air pollutant. … and Beyond Because of the possibility that Mars may have harbored some form of life in the past, the planet continues to fascinate us, and several missions have been launched in past years to explore the red planet. A series of news features in the May 29, 2003 issue (2003, 423, 473–477) summarizes initiatives by NASA and by the European Space Agency (ESA) that are currently underway to provide further support for the presence of life on Mars by gathering geological evidence for the presence of Martian water in the past and by measuring ratios of carbon isotopes in the soil of the planet. Equally interesting are articles in this collection about the Mars landers used in these missions, and the people who work behind the scenes. The interested reader who wants to stay abreast of the latest developments in Mars exploration may want to look up the relevant NASA and ESA sites: http://mars.jpl.nasa.gov/mer/; http://www.esa.int/export/ SPECIALS/Mars_Express/index.html. Whereas the close alignment of Mars and Earth every 26 months facilitates the planning of space missions that require a minimum of fuel, scientists wishing to study Pluto are not so lucky. A space craft has yet to visit that most distant planet in our solar system, which travels around the sun in an elliptical 248-year orbit. An investigation of Pluto’s atmosphere has proven extremely difficult, since reliable measurements can only be taken when the planet occults (passes in front of ) a suitable star—a rather infrequent event. Stellar occultation in 2002 allowed more refined measurements of Pluto’s thin nitrogen atmosphere than were originally possible during a similar event in 1988. Teams from the U.S. led by J. L. Elliott and a multi-national group headed by B. Sicardy report that, surprisingly, Pluto’s atmosphere has expanded during the past 14 years (2003, 424, July 10, 165–168 and 168–170; News and Views commentary by W. Hubbard on pp 137–138). Observations from telescopes at locations throughout the Americas and Hawaii have led to the conclusion that the two-fold increase in at-
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Figure 2. The rock abrasion tool on the robotic arm of the Mars Exploration Rover grinds away the rock’s surface, allowing scientific instruments to analyze the rock’s interior.
mospheric pressure between 1988 and 2002 is likely due to changes in the planet’s surface temperature and the accompanying re-equilibration of gaseous N2 with nitrogen surface ice, a phenomenon not unlike that observed for Neptune’s satellite Triton. It is not clear yet whether the increase in Pluto’s surface temperature can be explained by an increase in absorbance of solar radiation caused by the observed darkening of Pluto. For more information about Pluto, visit http://pluto.jhuapl.edu. Note 1. Information about the Montreal Protocol may be found at http://www.unep.ch/ozone/montreal.shtml (accessed Aug 2003).
Sabine Heinhorst and Gordon Cannon are in the Department of Chemistry and Biochemistry, University of Southern Mississippi, Hattiesburg, MS 39406-5043; email:
[email protected] and
[email protected].
Journal of Chemical Education • Vol. 80 No. 10 October 2003 • JChemEd.chem.wisc.edu
