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Chemistry 217

Second Exam 2008

Time Limit 4 hours

 

1)      Please read the attached article on methane and answer the following questions.

a)    Why it so surprising that plants emit methane under well oxygenated conditions in the environment?

b)   What makes methane such a significant greenhouse gas?

c)    Why is the production of significant amounts of methane from normal vegetation sending climate modelers “back to the drawing board”?

 

GREENHOUSE PLANTS? VEGETATION MAY PRODUCE METHANE

By Sid Perkins                                                         January 11th, 2006

BREATHING OUT. Trees and other plants may emit substantial quantities of the potent greenhouse gas methane, according to a recent battery of lab tests.

 

Lab tests suggest that a wide variety of plants may routinely do something that scientists had previously thought impossible—produce methane in significant quantities.

Methane, like carbon dioxide, traps heat in Earth's atmosphere. Scientists have been studying natural sources of methane for decades but hadn't pegged plants as a producer, notes Frank Keppler, a geochemist at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. Previously recognized sources of methane include bacterial action in the digestive systems of ruminants such as cows and in the saturated soils of swamps and rice paddies.

Now, Keppler and his colleagues find that plants, from grasses to trees, may also be sources of the greenhouse gas. "This is really surprising," Keppler says, because most scientists assumed that methane production requires an oxygen free environment.

In its experiments, Keppler's team scrutinized the gaseous emissions of a variety of plants and their debris at normal atmospheric oxygen concentrations. A gram of dried plant material, such as fallen leaves, released up to 3 nanograms of methane per hour when the temperature was about 30°C. Each 10°C rise above that temperature, up to 70°C, caused the emission rate to approximately double.

Living plants growing at their normal temperatures generated even larger quantities of methane, as much as 370 ng per gram of plant tissue per hour. Methane emission more than tripled when the plants, either living or dead, were exposed to sunlight.

The team's experiments took place in sealed chambers with a well-oxygenated atmosphere, so it's unlikely that bacteria that thrive without oxygen generated the methane, says Keppler. Experiments on plants that were grown in water rather than in soil also resulted in methane emissions, another strong sign that the gas came from the plants and not soil microbes.

From their data, the researchers estimate that the world's plants generate more than 150 million metric tons of methane each year, or about 20 percent of what typically enters the atmosphere. They report their findings in the Jan. 12 Nature .

"This is some pretty strange chemistry," says David C. Lowe, an atmospheric chemist with the National Institute of Water and Atmospheric Research in Wellington, New Zealand. One reason that scientists hadn't considered plants as a source of the gas is that the laws of thermodynamics don't favor methane production in an oxygen-rich environment. However, Lowe notes, many plants produce volatile hydrocarbons that contribute to haze and smog (SN: 12/7/02, p. 360: Available to subscribers at http://www.sciencenews.org/articles/20021207/bob8.asp).

The new finding is an "interesting observation," says Jennifer Y. King, a biogeochemist at the University of Minnesota in St. Paul. Because some types of soil microbes consume methane, they may prevent plant-produced methane from reaching the atmosphere. Field tests will be needed to assess the plants' influence, she notes.

The Keppler team's results may partially explain the large methane plumes recently observed over some tropical forests by Earth-orbiting satellites, says John B. Miller, an atmospheric scientist at the Earth System Research Laboratory in Boulder, Colo. Although such plumes are unsurprising during the rainy season, when methane-producing soil microbes are most active, they also appeared during the dry season.

The new findings will probably spur researchers to revise their models of where and how methane is generated as well as their interpretations of the gas' concentrations measured in ancient ice cores. "This is a big deal if it's real," says Stanley C. Tyler, an atmospheric chemist at the University of California, Irvine.

 

2)  The hydroxyl radical has often been described as the detergent for the atmosphere.  The figure below shows the global distribution of OH at the base of the atmosphere and a profile taken above at rain forest in Africa.   Explain global distribution of OH and the shape of the vertical profile.

 

3)       Describe the major physical and chemical processes responsible for the production of ground level ozone pollution (smog).  Using the data shown in Appendix A, comment on the recent claim that current pollution levels in Maine are caused by emissions in Southern New England.

 

4)       Colby is a major consumer of fossil fuel used for heating and electrical generation for the campus.  Using the data in Appendix B, predict the relative concentration and chemical form of sulfur and nitrogen deposition to central Maine over the next six hours.  Please be as specific as possible.

 

5)       The addition of sulfate to the stratosphere by volcanic eruptions has been demonstrated to create global cooling by increasing the global albedo.  How much would the average global albedo have to change to decrease the average temperature of the earth by one degree Kelvin?

 

Appendix A:  The map below is generated from the NOAA HYSPLIT web page using real atmospheric data for the last 96 hours.

 

The HYSPLIT trajectory map shows an aerial (plan) view of the path(s) an air parcel(s) took, and a vertical view of its movement at different altitudes. Symbols are used along each trajectory to indicate the position of the air parcel over the calculational period, the interval of which can be defined by the user.

 

The vertical view at the bottom of the map shows the height of the air parcel measured at these corresponding tick marks. The height of the air parcel is measured in meters above model ground level (AGL).

 

 

Appendix B. Waterville is in the center of the map.  Dispersion map for a plume emitted from Waterville.