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Harbor pollution may produce ozone

Air pollution--once thought to originate only from land-based emissions--may also be produced in Boston Harbor. This finding, from a study by MIT researchers, indicates a need for closer monitoring of the effects of treated wastewater, runoff and combined sewage overflows that find their way into the harbor.

Air pollution is produced in part from natural gases known as nonmethane hydrocarbons (NMHCs). Trees and burning vegetation emit large amounts of these gases. Humans also produce these hydrocarbons with their wastes and by burning wood and vegetation at low temperatures. Both sewage and the burning of fossil fuels have increased NHMC levels significantly in the last 150 years.

NMHCs contribute to the manufacture of ozone. While ozone in the stratosphere provides a layer of protection from the sun's radiation, ozone in the lower atmospheric levels contaminates the air we breathe.

NMHCs also find their way into marine environments. "Any additional source of NMHCs is something you want to know about," said Professor Ronald Prinn, director of MIT's Center for Global Change. To ascertain whether or not NMHCs were migrating from Boston Harbor into the air, or from the air into the Harbor, Professor Prinn and research scientist Xianqian Shi undertook a pilot study in and around the Harbor. The project, a collaboration with the Massachusetts Water Resources Authority (MWRA), is what Professor Prinn believes to be the first detailed study of NMHCs in both air and water in a coastal metropolitan area. The study was funded by the MIT Sea Grant College Program.

"When NMHCs get into the ocean they can be broken down by microorganisms," Professor Prinn said. NMHCs help produce ozone when they enter the atmosphere, in a process analogous to the oxidation of carbon monoxide to carbon dioxide. When carbon monoxide grabs one oxygen atom from an oxygen molecule to become carbon dioxide, a lone oxygen atom is left. This atom attaches itself to an oxygen molecule (O2), forming ozone (O3).

"NMHCs do the same thing because they get oxidized. and then they produce carbon monoxide and then ozone," Professor Prinn explained. Aided by two MIT undergraduates in chemical engineering, Oliver Schneider and Stephanie Shaw, the researchers collected samples of water and the air directly above it from eight sites around Boston Harbor. Levels of NMHCs in the samples were then measured with a gas chromatograph, which separates various components of a gas. The researchers found higher levels of NMHCs in the water samples than in the corresponding air samples, indicating a flux of the gases from the inland harbor water into the surrounding urban air.

These findings were somewhat surprising. "Many people would tend to assume that a harbor in a metropolitan area is a passive receiver of pollution," Professor Prinn said. The findings suggest that the Harbor plays an active role with regard to these pollutants.

"I have a strong suspicion that these gases are produced by microorganisms using sewage effluent as a nutrient source, and that the microorganisms are producing these lighter hydrocarbons as byproducts," he said.

The big unknown is whether the atmospheric samples are indicative of the actual NMHC levels in the air. Continuous sample collection during a 24-hour period would permit averaged measurements, taking into account variations in traffic flow and resulting pollution.

While exact levels may not be known, the effects of the gases are. They are most obvious on calm, hazy days when some people find that breathing is more difficult and their eyes become red and irritated.

To fully assess the environmental impact of the new outfall pipe that will send sewage effluent to the outer harbor, Professor Prinn has suggested that baseline measurements of NMHCs be taken now. Local policy bodies, municipal engineers and environmental scientists must all collaborate in devising controls of NMHC emissions from the harbor to improve metropolitan Boston's air quality.

A version of this article appeared in MIT Tech Talk on March 8, 1995.

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