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By Victoria Parsons

Chicken Little was partially right. The sky isn't falling - but contaminants coming from the sky have become a critical concern in Tampa Bay.

Once considered to be a small part of the pollution pie, atmospheric deposition is now known to be a major source of contaminants, contributing from 40 to 50% of the total nitrogen to Tampa Bay. Emissions from power plants, automobiles and industrial smokestacks all contain nitrogen oxides - or NOx - along with a mŽlange of other contaminants including mercury, lead, cadmium and PAHs.

"It's difficult to grasp how driving a car in the watershed can affect water quality, but we're learning more and more about the relationship between air pollution and the overall health of the bay," says Holly Greening, senior scientist at the Tampa Bay Estuary Program.

Over the past 20 years, atmospheric deposition has become increasingly important as other nitrogen sources come under control, she adds. "Wastewater treatment plants and industrial point sources used to contribute more than 50% of the load, but they're pretty small sources now," she said. "And as one source is diminished, the percentage from others becomes that much more important."

Nitrogen, while necessary for life, has become the single most damaging contaminant in Tampa Bay. In excess amounts, it fuels the growth of algae that cloud the water and prevent sunlight from reaching seagrasses. As the algae die and decay, they consume oxygen that other sealife needs to survive.

"A Giant Chemistry Experiment"

While scientists know far more about atmospheric deposition in Tampa Bay than they did even five years ago, many questions remain about the complex interactions between air and water. For instance, not all the contaminants from the atmosphere that end up in the bay actually fall on it - they also fall on land and are carried to the bay in stormwater. Different types of nitrogen come from different sources and may have different effects in the bay. And sometimes nitrogen that falls into the water is actually reabsorbed into the air in a constant flux that works toward equilibrium.

A multi-year, multi-million-dollar initiative called BRACE - or Bay Regional Atmospheric Chemistry Experiment - is helping to resolve some of those questions. Funded partially by a Tampa Electric Company fine, BRACE will document the changes in air and water quality that occur as TECO switches its Gannon station from coal to clean-burning natural gas, as well as improve estimates of atmospheric nitrogen deposition to the bay and determine if sources are local, regional or national.

"It's a giant chemistry experiment," says Leroy Shelton, assistant director of the air division at the Environmental Protection Commission of Hillsborough County. "It's building on data collected in the past, and bringing together a national panel of experts."

Scientists from state and local agencies, the National Oceanic and Atmospheric Administration, universities from across the country and the federal Environmental Protection Agency gathered at a site in Valrico in 2002 to run hundreds of tests with state-of-the-art equipment. An airplane flew patterns to characterize urban pollution and multiple pieces of redundant monitors measured upward of 100 gases and particles from five air quality and four meteorological sites on the bay and in its watershed.

The first question - how much nitrogen in Tampa Bay comes from the sky - has been answered: about 40 or 50%, depending upon rainfall. Roughly half of the atmospherically derived nitrogen falls directly on Tampa Bay, and the other half lands first in the watershed and then flows into the bay through stormwater, surface water or groundwater.

Getting to the Source

The 2002 study indicates a 60-40 split in NOx in Hillsborough County with the larger percentage coming from stationary sources including electrical power plants and 40% coming from motor vehicles. That's likely to change with a "lite" version of the intensive monitoring effort scheduled for May 2005, one year after TECO's Gannon plant is repowered.

"We think repowering Gannon is going to cut out about 25,000 tons of nitrogen per year, which will mean that emissions from motor vehicles and other internal combustion engines are going to play a more important role in total nitrogen loadings," said Noreen Poor, the University of South Florida professor charged with pulling the massive amount of data together.

While many answers were predictable, some surprises were found, Poor adds. For instance, the initial model focused on one form of nitrogen - nitrogen oxide from fossil fuel combustion - but BRACE helped identify a second form of nitrogen that may affect Tampa Bay.

"We started out looking at the oxidized forms of nitrogen, but found that ammonia has a major role to play," she said. "The largest source is likely to be intensive agriculture, but fertilizer plants and landfills may be involved."

Nitrogen levels near highways were lower than expected, although samples may have been diluted because monitors were at ground level and the gaseous fumes dispersed quickly. Tampa Bay's marine environment also required that researchers modify national assumptions about atmospheric deposition, because salt influences the nature of nitrogen compounds. And results from the airplane research changed scientific theories by demonstrating very distinct layers of nitrogen rather than the anticipated flow from ground up.

Results from the Tampa Bay monitoring initiative are being studied around the country, with a special section on BRACE to be published in a professional journal in early 2005, she adds.

BRACE will continue through at least 2005, with modeling in progress to determine the proportions of mobile and point sources, as well as where the nitrogen comes from - a vital question since the Tampa Bay airshed stretches from New Orleans to Atlanta and across the entire state.

"Now that we know that atmospheric deposition of nitrogen has a significant impact on Tampa Bay, it's critical that we learn more about sources, both local and regional," she said. "With populations growing rapidly along the bay, the sheer number of people in the area will directly impact the health and productivity of Tampa Bay."

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