In the Lab

• By Hannah R. Kolni
• Sep 01, 2006

Tiny Airborne Particles May be Climate Change Culprits
A few yearsback, a scientist at the Weizmann Institute of Science and his colleagues caused a storm in the atmospheric community when they suggested that tiny airborne particles, known as aerosols, may be one of the main culprits causing climate change -- having, on a local scale, an even greater impact than the greenhouse gas effect.

Attempts at understanding how these particles influence clouds have raised many questions. A new paper by Dr. Ilan Koren of the Weizmann Institute Environmental Studies and Energy Research Department and Dr. Yoram Kauffman of the NASA/Goddard Space Flight Center, published in Science Express online, weaves together two opposing effects of atmospheric aerosols to provide a comprehensive picture of how they may be affecting our climate.

Cloud formation is dependent upon the presence of small amounts of aerosols, such as sea salt and desert dust. These tiny particles serve as the seeds around which water vapor in the air condenses, forming tiny water droplets that rise as they release heat. As the small droplets rise, they collide and merge with larger droplets. When the droplets reach a critical size, gravity takes over, causing them to fall from the cloud in the form of rain.

One of the controversies surrounding the extent of aerosol impact on climate change is the duality of their influence. On the one hand, Koren and his colleagues previously found evidence to suggest that the extra seeds planted in the atmosphere by the emission of manmade aerosols (pollution, forest fires, and fuel combustion) lead to more, but smaller-sized, water droplets. The formation of larger water droplets by the collision process is less efficient and, therefore, rainfall is suppressed. The smaller droplets are lifted higher up into the atmosphere, creating larger and taller clouds that will persist longer. Not only does this alter the whole water cycle, but the increased cloud cover reflects more of the sun's radiation back into space, creating a local cooling effect on Earth.

But to complicate matters, Koren, in another study, showed that certain types of aerosols -- those containing black carbon -- can also decrease cloud cover, ultimately leading to a warming effect. This occurs as black carbon absorbs part of the sun's radiation, warming the surrounding atmosphere and reducing the difference in temperature between the Earth's surface and the upper atmosphere. This combination prevents atmospheric instability -- the condition needed to form clouds and rain. A stable atmosphere means fewer clouds; fewer clouds mean less reflection of sunlight; less reflection of sunlight and absorption of radiation lead to warming.

Policymakers have argued that, in the bottom line, the warming effect of the greenhouse gases and the (mainly cooling) aerosol effect may balance each other out so that the net global climate change will be small. Koren argues that it is the local climate change that is problematic: Clouds may persist without releasing their rain over regions where they would normally precipitate, such as rainforests, and move to precipitate over regions where rain is not needed, such as oceans. Or the effect could lead to the warming up of cold and the cooling down of hot regions. These additional effects to the already problematic warming by greenhouse gases could have disastrous repercussions in the long run.

Also controversial is the question of how such tiny localized particles affect weather systems thousands of miles away from their sources. There is no doubt that aerosols do play a role, but the skeptics believe it is negligible compared to meteorological key players such as temperature, pressure, the amount of water vapor in the air, and wind strength.

What Koren needed was a way to separate meteorological from aerosol influences -- something which was lacking in his previous studies. Together with Kauffman, he used a network of ground sensors (AERONET) to measure the effect of aerosol concentration on cloud cover. Radiation absorption is less affected by meteorology, so if the skeptics are right and meteorology is the main influence, then the correlation between aerosol absorption and cloud cover should have been seen in only a few circumstances, but this was not the case. They observed the duality effect on clouds: As total aerosols increase, cloud cover increases; and as radiation absorption by aerosols increases, cloud cover decreases -- for all locations, and for all seasons.
Backed up with a mathematical analysis, it becomes harder to deny that it is, in fact, aerosols that have the major influence.

"We hope that this study has finally provided closure," Koren said. "Hopefully policymakers will start to tackle the issue of climate change from a different perspective, taking into account not only the global impact of aerosols and greenhouse gases, but local effects too."

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Power Plants Influence Regional Mercury Emissions
The amount of mercury emitted into the atmosphere in the northeastern United States fluctuates annually depending on activity in the electric power industry, according to researchers at the Yale School of Forestry & Environmental Studies.

Xuhui Lee, professor of meteorology, and Jeffrey Sigler, a recent Yale PhD and now a postdoctoral researcher at the University of New Hampshire, co-authored the Yale study"Recent Trends in Anthropogenic Mercury Emission in the Northeast United States." They found that, between 2000 and 2002, the emission rate of mercury decreased by 50 percent, but between 2002 and 2004 the rate increased between 50 and 75 percent. During that five-year period, overall emissions declined by 20 percent.

The dramatic annual changes in mercury emissions, the study's authors say, cannot be explained climatologically by air-flow patterns that would bring either clean or polluted air into the region.

Mild winters and a correspondent decrease in the need for regional power plants to burn coal could partially explain the decline in mercury emissions, according to the authors. The study, published this summer in the Journal of Geophysical Research-Atmospheres, estimates that power plants account for up to 40 percent of total emissions in New Jersey, New York, and Pennsylvania, and in New England.

"The study highlights just how important power plants are in influencing regional mercury emission," Sigler said. "We should not forget other source categories when formulating abatement policies, since they also contribute significant amounts to the total emissions," Lee added.

Mercury, which converts to highly toxic methyl mercury in groundwater, is found in fish and can cause neurological problems in developing human fetuses and dementia and organ failure in human adults who eat fish in large amounts and over long periods.

The Yale study was conducted at Great Mountain Forest in northwestern Connecticut. The measurements were restricted to wintertime so data on carbon dioxide that comes from the same combustion sources as mercury would not be distorted by photosynthesis. The researchers used carbon dioxide to trace mercury back to its sources with a unique method called "tracer analysis."

"To our knowledge, using the carbon dioxide to trace mercury over a long time period hasn't been done before," the authors said. "We started with actual mercury that's in the atmosphere, worked back to sources that emit it, then calculated the emission rate."

The U.S. Environmental Protection Agency (EPA), which does not regulate mercury emissions, determines the mercury emission rate by taking an inventory of existing sources.

"Although the EPA 's approach is highly useful, it requires accurate measurements of mercury emitted from the smokestack per ton of fuel burned," Sigler said. "These data are hard to come by. Our top-down technique circumvents those rather cumbersome problems and allows for much more timely estimates of mercury emission. It's difficult to get annual changes in the emission rate with the inventory approach."

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Ocean Gas Blowouts May Drive Global Warming
Gas escaping from the ocean floor may provide some answers to understanding historical global warming cycles and provide information on current climate changes, according to a team of scientists at the University of California, Santa Barbara. The findings are reported in the July 20 online version of the scientific journal, Global Biogeochemical Cycles.

Remarkable and unexpected support for this idea occurred when divers and scientists from UC Santa Barbara observed and videotaped a massive blowout of methane from the ocean floor. It happened in an area of gas and oil seepage coming out of small volcanoes in the ocean floor of the Santa Barbara channel -- called Shane Seep -- near an area known as the Coal Oil Point seep field. The blowout sounded like a freight train, according to the divers.

Atmospheric methane is at least 20 times more potent than carbon dioxide and is the most abundant organic compound in the atmosphere, according to the study's authors, all from UC Santa Barbara.
"Other people have reported this type of methane blowout, but no one has ever checked the numbers until now," said Ira Leifer, lead author and an associate researcher with UCSB's Marine Science Institute. "Ours is the first set of numbers associated with a seep blowout."

Leifer was in a research boat on the surface at the time of the blowouts. Aside from underwater measurements, a nearby meteorological station measured the methane "cloud" that emerged as being approximately 5,000 cubic feet, or equal to the volume of the entire first floor of a two-bedroom house. The research team also had a small plane in place, flown by the California Department of Conservation, shooting video of the event from the air.

Leifer explained that, when this type of blowout event occurs, virtually all the gas from the seeps escapes into the atmosphere, unlike the emission of small bubbles from the ocean floor, which partially, or mostly, dissolve in the ocean water. Transporting this methane to the atmosphere affects climate, according to the researchers. The methane blowout that the UCSB team witnessed reached the sea surface 60 feet above in just seven seconds. This was clear because the divers injected green food dye into the rising bubble plume.

Co-author Bruce Luyendyk, professor of marine geophysics and geological sciences, explained that, to understand the significance of this event (which occurred in 2002), the UCSB research team turned to a numerical, bubble-propagation model. With the model, they estimated methane loss to the ocean during the upward travel of the bubble plume.

The results showed that for this shallow seep, loss would have been approximately one percent. Virtually all the methane -- 99 percent of it -- was transported to the atmosphere from this shallow seep during the blowout. Next, the scientists used the model to estimate methane loss for a similar-sized blowout at much greater depth -- 250 meters. Again, the model results showed that almost all the methane would be transported up to the atmosphere.

Over geologic time scales, global climate has cycled between warmer, interglacial periods and cooler, glacial periods. Many aspects of the forces underlying these dramatic changes remain unknown. Looking at past changes is highly relevant to understanding future climate changes, particularly given the large increase in atmospheric greenhouse gas concentrations in the atmosphere due to historically recent human activities, such as burning fossil fuels.

One hypothesis, called the "Clathrate Gun" hypothesis, developed by James Kennett, professor of geological sciences at UCSB, proposes that past shifts from glacial to interglacial periods were caused by a massive decomposition of the marine methane hydrate deposits.

Methane hydrate is a form of water ice that contains a large amount of methane within its crystal structure, called a clathrate hydrate. According to Kennett's hypothesis, climatic destabilization would cause a sharp increase in atmospheric methane -- thereby initiating a feedback cycle of abrupt atmospheric warming. This process may threaten the current climate, according to the researchers. Warmer oceans caused by current global climate change are likely to release methane currently trapped in vast hydrate deposits on the continental shelves. However, consumption of methane by microbes in the deep sea prevents methane gas released from hydrates from reaching the ocean surface and affecting the atmosphere.

Bubbles provide a highly efficient mechanism for transporting methane and have been observed rising from many different hydrate deposits around the world. If these bubbles escape singly, most or all of their methane would dissolve into the deep-sea and never reach the atmosphere. If instead, they escape in a dense bubble plume, or in catastrophic blowout plumes, such as the one studied by UCSB researchers, then much of the methane could reach the atmosphere. Blowout seepage could explain how methane from hydrates could reach the atmosphere, abruptly triggering global warming.

Thus, these first-ever quantitative measurements of a seep blowout and the results from the numerical model demonstrate a mechanism by which methane released from hydrates can reach the atmosphere. Studies of seabed seep features suggest such events are common in the area of the Coal Oil Point seep field and very likely occur elsewhere.

The authors explain that these results show that an important piece of the global climate puzzle may be explained by understanding bubble-plume processes during blowout events. The next important step is to measure the frequency and magnitude of these events. The UCSB seep group is working toward this goal through the development of a long-term seep observatory in active seep areas.

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Corn Waste Potentially More than Ethanol
After harvesting corn, whether for cattle feed or corn on the cob, farmers usually leave the stalks and stems (stover) in the field. But now, a team of Penn State researchers think corn stover can be used not only to manufacture ethanol, but to generate electricity directly.

"People are looking at using cellulose to make ethanol," said Dr. Bruce E. Logan, the Kappe Professor of Environmental Engineering at Penn State. "You can make ethanol from exploded corn stover, but once you have the sugars, you can make electricity directly."

Logan's process uses a microbial fuel cell to convert organic material into electricity. Previous work has shown that these fuel cells can generate electricity from glucose and from municipal wastewater, and that these cells can also directly generate hydrogen gas.

Corn stalks and leaves -- about 250 million tons a year -- make up a third of the total solid waste produced in the United States. Currently, 90 percent of corn stover is left unused in the field. Corn stover is about 70 percent cellulose or hemicellulose, complex carbohydrates that are locked in chains. A steam-explosion process releases the organic sugars and other compounds in the corn waste, and these compounds can be fed to microbial fuel cells.

The microbial fuel cells contain two electrodes and anaerobic bacteria -- bacteria that do not need oxygen -- that consume the sugars and other organic material and release electrons. These electrons travel to the anode and flow in a wire to the cathode, producing electrical current. The water in the fuel cell donates positive hydrogen atoms that combine with the electrons and oxygen to form water.

The microbial fuel cells were inoculated with domestic wastewater and a nutrient medium containing glucose, the researchers reported in the journal Energy and Fuels. Once established, the bacteria colonies were fed the sugary organic liquid obtained from steam exploding corn stover.

The researchers: Logan, Yi Zuo, Penn State graduate student in environmental engineering, and Pin-Ching Maness, senior scientist, National Renewable Energy Laboratory, report that "the conversion of organic matter to electricity, on the basis of biological oxygen demand removal, was relatively high with greater than 93 percent of the biological oxygen demand removed."

In essence, there is no organic matter left to cause problems when disposing of the remaining liquid because there is nothing left to oxidize. The process converts all the available energy to electricity. The electrical production is about one watt for every square meter of surface area at about 0.5 volts. A typical light bulb uses 60 watts. To increase wattage, the surface area needs to increase. To increase voltage, fuel cells can be linked in series.

"Producing electricity from steam exploded corn stover adds to the energy diversity of our portfolio," Logan said. "Electricity can be used to pump water uphill for later use, directly run light, heat, and equipment, or electrolyze water to create hydrogen."

The Penn State researcher and colleagues have also used microbial fuel cells and wastewater to produce hydrogen gas directly.

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This article originally appeared in the 09/01/2006 issue of Environmental Protection.