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Natural Air Pollution: Sulfur Released by the Ocean

Ocean Image - The satellite image to the right shows the biological productivity of the earth's oceans. The purple and pink hues mark areas of low concentrations of plant life in the ocean, while the yellow and orange hues show highly productive areas. Northern oceans, which are among the most productive on earth, may be responsible for releasing to the atmosphere large amounts of natural sulfur. Sulfur particles can change the properties of clouds, and affect global change.

A sensitive instrument used by wineries, breweries and the petroleum industry to detect sulfur recently helped institute researchers formulate new theories about sources of chemicals in the atmosphere.

The sulfur-detecting instrument was designed by Geophysical Institute Assistant Professor of Chemistry Richard Benner while he was finishing his doctoral thesis at the University of Denver, and patented shortly thereafter.

The instrument, which can detect sulfur in air and in liquid, has been sold commercially to breweries and wineries, where it is used to measure sulfur compounds that create distinctive tastes in beverages, and to petrochemical companies, where it is used to track sulfur released during the cracking process involved in turning crude oil into gasoline.

"I developed the instrument solely because I knew it would be useful for atmospheric studies and, by serendipity, it turned out also to be good for commercial purposes," Benner said.

Measuring Sulfur---The diagram at left illustrates how Geophysical Institute scientists measured sulfur released from the ocean aboard a research vessel in the Atlantic Ocean. As wind blew past the sulfur-detecting instrument on deck, a sensitive sensor measured, each second, sulfur concentrations that had been mixed by eddies of air swirling above the water. Sulfur is released by phytoplankton in the ocean.

Sulfur, one of the basic building blocks of life, can be harmful when released into the environment. Sulfur particles can cause a haze of air pollution dense enough to hinder visibility; they are a major component of acid rain, and, when they are propelled into the upper atmosphere, they are partially responsible for the destruction of the ozone layer. "When you are standing on the rim of the Grand Canyon, or looking across the Alaska Range on a clear day, but you can't see a thing through the haze, you are looking at particles mostly made up of sulfur," Benner said.

Sulfur particles also can change the properties of clouds, making them more numerous and more reflective of sunlight. Global climate change theorists suspect that reflective clouds may stop some of the sun's heat from reaching the earth and affect the dynamics of global warming or cooling.

Recently, Benner's sulfur-detecting instrument has been used to challenge theories about the source of sulfur in the atmosphere. Current theories hold that about half of the earth's airborne sulfur is derived from man-made sources, such as fossil fuel combustion or power-plant emissions, and that about half is derived from natural sources.

Benner's data seems to indicate that a much greater amount of sulfur in the atmosphere is released naturally than previously believed. His research indicates that up to two-thirds of the sulfur in the earth's atmosphere may be released through natural sources, and that a major portion of that sulfur may come from the ocean. Natural sulfur also can be released through land bogs and volcanic eruptions.

His theory centers around a study performed for six weeks in the summer of 1992, when the instrument was taken aboard a research vessel in the Atlantic Ocean, near the Azore Islands off the coast of Portugal.

Scientists determined long ago that phytoplankton, microscopic plants in the ocean, release dimethylsulfide into ocean water and release sulfur into the atmosphere. For years, the measurement of sulfur flux (the amount of sulfur released from the ocean to the atmosphere) has been accomplished indirectly.

Scientists who have been unable to directly measure sulfur flux from the ocean, extrapolated this calculation from measurements of dimethylsulfide in ocean water and of the wind speed above the water. These indirect measurements form the basis of most currently held theories on the amount of sulfur released to the atmosphere.

Using his sulfur detector, Benner was able to make the first ever direct real-time measurements of sulfur in the air above the ocean surface. The measurements, which were made once every second during the six-week study, allowed Benner to gain new insights into the behavior of sulfur gases in the marine environment that were unavailable prior to his experiment.

Collecting data each second also enabled Benner to measure turbulence, which is the main mechanism for mixing sulfur in the air.

Generally, high concentrations of sulfur can be found in air close to the ocean surface, while lower concentrations can be found at higher elevations. Eddies of air that blow in circular patterns above the ocean surface mix these concentrations, making flux measurements quite complicated.

Benner was able to use these eddies to his advantage. His sulfur detector is the first instrument capable of measuring the concentration of flux created by the mixing action of the eddies.

"We determined the flux by measuring how much sulfur was going up in the eddies and how much was going down," Benner said.

Sifting through these different concentrations to find out how much airborne sulfur is being released from the ocean involves a series of complex calculations, which were performed by Clara Jodwalis, a doctoral graduate student in atmospheric chemistry. Using a mathematical process known as the Fourier Transform, Jodwalis determined sulfur flux from the Atlantic Ocean near the Azores.

The process took nearly two years to complete. Jodwalis spent the first six months preparing the data. Only data obtained while the instrument was pointed straight into the wind and when the boat was not rocking too much was acceptable. For correlation purposes, Jodwalis also needed information about wind speed, and air and sea temperatures during periods of sample collection.

From the sample, Jodwalis determined that the amount of sulfur escaping from the ocean off the coast of Portugal appears to be significantly higher than previously believed.

The results also seem to indicate that there may be significantly more sulfur in the atmosphere released from natural sources than previously believed, and that northern oceans, which are among the most productive on earth, may be major sources of the planet's natural sulfur.

Satellite pictures and biological studies indicate that the Gulf of Alaska contains some of the highest dimethylsulfide ever measured, which is why Benner hopes to use the new instrument to measure sulfur flux there soon.

The Fourier Transform---Clara Jodwalis, a doctoral graduate student in atmospheric chemistry, used a mathematical process knows as the Fourier Transform to determine sulfur flux from the Atlantic Ocean. The results of the intensive process, which took nearly two years to complete, seem to indicate that there may be significantly more natural sulfur in the atmosphere than previously believed, and that the Gulf of Alaska may be among the planet's major contributors.
Chromatograph---Jodwalis uses a gas chromatograph, pictured in the lower right corner of the photo above, to separate sulfur compounds, and a sulfur-detecting sensor, upper left, to measure the varying amounts of different sulfur compounds she obtained from a sample gained during a recent research project. The adjoining computer graphically displays the peak amounts of sulfur released from the ocean. Photo by Evelyn Trabant.