Scientists working on North Carolina's Outer Banks have noticed a change in the air in recent years - one that could also have implications for the Chesapeake.

In the past two decades, the amount of ammonia wafting across the coastal bays to their air monitor site has nearly doubled.

In the late 1970s, only about 20 percent to 30 percent of airborne nitrogen collected in their rainfall samples was in the form of ammonium. Now, that percentage has climbed to "the high 40s," says Hans Paerl, a researcher with the University of North Carolina's Institute Of Marine Sciences in Morehead City.

The most likely suspect, Paerl said, is the rapid growth of industrial hog farms in the state's coastal counties. "We have a lot of animal operations upwind from us," Paerl said.

Paerl is not alone in his concern. The amount of ammonia in the air - and the amount falling onto water and soil - is on the rise. Like Paerl, most suspect the nation's growing demand for animal products is driving the increase: Livestock is thought to be the largest single source of ammonia emissions.

"If animal populations are increasing, then ammonia emissions are increasing," explained James Galloway, professor and chair of the University of Virginia's Environmental Sciences Department. Worries about the impact of airborne nitrogen on the Bay and other coastal waters isn't new - it has been growing for years. But almost all of the emphasis in controlling this "atmospheric deposition" has focused on nitrogen oxides (NOx) emissions from power plants, cars and other sources.

A spate of recent regulatory actions are aimed at reducing NOx emissions, which are the largest source of nitrogen deposition. But nationwide emissions of ammonia, which are mostly unregulated, are increasing. And it's possible, some officials and scientists say, that ammonia could become the largest source of airborne nitrogen entering coastal waters.

"The suspicious side of me says that all of the NOx controls could very easily be overwhelmed by the increases in ammonia emissions as we get into more intensive animal agriculture," said Joel Baker, a scientist at the University of Maryland's Chesapeake Biological Laboratory who has been studying the atmospheric deposition on water for years.

Still, ammonia concerns are based largely on what might be considered circumstantial evidence. Scientists routinely tick off a list of "data gaps" about ammonia: monitoring data is sparse, little is known about how far it moves through the air and how that varies under different conditions, inventories of ammonia "sources" are thought to be incomplete - just to name a few problems.

"We've focused on NOx and nitrate because that's what people know how to measure best," said Joe Scudlark, a scientist in the University of Delaware's College of Marine Studies. The university operates an air-monitoring site at at Lewes, Del. - one of only three sites close to the Bay with long-term records of ammonium wet deposition. It has shown an ammonia increase of about 10 percent since the late 1970s, though the monitoring site is not in the heart of an intense livestock area.

"There is a suspected atmospheric ammonia problem in the Bay, but it has not been demonstrated," concluded Rich Valigura, a scientist with the National Oceanic and Atmospheric Administration's Air Resources Laboratory.

But that may change soon. This year, the Bay Program has approved an extensive air sampling program to better quantify the ammonia issue within the watershed and ultimately help develop an ammonia "airshed" that would identify regions sending ammonia to the Chesapeake and its drainage basin.

Ammonia is particularly worrisome for the Chesapeake and other coastal waters because of its potential to fuel algae growth. [See article at top of page.] While any form of nitrogen can contribute to algae blooms, ammonia is like high-octane nitrogen: It can cause even greater growth.

Said Paerl: "It's sort of like throwing meat to the lions."

Ammonia causes much of the odor that wafts out of barn yards, wastewater treatment plants and even bad breath in humans. But several studies indicate that the biggest share of ammonia emissions is from animal wastes and fertilizers.

About 90 percent of the emissions within the Bay watershed stem from agricultural activities, according to a recent report from the Bay Program's Scientific and Technical Advisory Committee, which analyzed information from a 1990 "emissions inventory" by the EPA. Areas with highest emissions were Pennsylvania's Lancaster and Lebanon counties, other parts of southcentral Pennsylvania, the Shenandoah Valley in Virginia and West Virginia, and parts of Maryland.

But the accuracy of the EPA's emissions inventory - like almost all issues related to ammonia - is suspect, scientists say. Although the 1990 figures were considered an improvement over previous estimates, the agency has not placed as much emphasis on collecting reliable emissions inventory information about ammonia - which is largely unregulated - as it has NOx.

"In comparison, they [the inventories] are terrible," said Robin Dennis, a scientist with NOAA's Air Resources Laboratory who has worked with EPA's Regional Atmospheric Deposition Model to calculate air deposition to the Bay and its watershed.

By contrast, ammonia releases in agriculture have been fairly well studied because farmers who apply manure or fertilizer need to take potential losses into account to make sure their crops are getting enough nitrogen.

Though most agree that agriculture is the main source of ammonia emissions, many suspect that other sources have been undercounted. Ammonia also comes from wastewater treatment plants, industrial refrigeration units, tailpipes and smokestacks. Some industries add ammonia to reduce NOx emissions.

Ammonia concentrations in Baltimore are twice that in rural Kent County, MD, said Baker, of the Chesapeake Biological Laboratory, though he noted that Kent County is not in a heavy agricultural area. "The prevailing misconception is that urban areas would be lower because of the agricultural uses of ammonia," Baker said. "But frankly, I think it's a population density kind of deal."

Nor is anyone certain where ammonia goes once it gets into the air. "I don't have a very good feel for whether I'm looking at Midwestern ammonia or western Sussex County ammonia," said Scudlark, who has been looking at Delaware rainfall samples for nearly two decades. For years, the conventional wisdom has been that ammonia does not travel far from its source - perhaps only a few hundred yards or a few miles. But recent studies have suggested that as ammonia (a gas) converts to ammonium (a particle) in the atmosphere, it can travel hundreds of miles depending on weather conditions.

"The likely range of ammonia transport is probably less than NOx, but it depends on the degree that the ammonia and ammonium are lifted above the precipitation zone," Galloway said. "Global models show that ammonia can be transported long distances, greater than 1,000 kilometers [620 miles]."

That means that even though overall livestock populations within the Bay watershed have remained steady in recent years, according to Bay Program data, the Chesapeake is likely receiving ammonium deposition from upwind areas with intensive agricultural operations such as North Carolina and Ohio.

"The known sources of ammonia are increasing rather quickly, things like the turkey farms and hog farms with accumulated buildups of manure and manure piles and lagoons," said Jack Cosby, a scientist at the University of Virginia, who worked on the ammonia report for the Scientific and Technical Advisory Committee.

"There clearly has been a huge increase in the number of animals being raised in livestock operations in the 'airshed,'" he said. "The implications from the agricultural activity, of course, is that those concentrations should be going up, along with the transport and deposition."

About a quarter of all the nitrogen that enters the Chesapeake each year results from atmospheric deposition. But how much of the Bay's nitrogen problem is caused by ammonia?

"The answer is, we don't know," Valigura said.

Dennis, whose modeling work helped to define a 350,000-square-mile "NOx airshed" that impacts the Bay, estimated that about 30 percent of the inorganic nitrogen - the portion that is generated mostly from human activities - is ammonia. The rest is nitrate stemming mostly from NOx emissions.

But, Dennis cautioned, the ammonia figure is "kind of an uncertain number because we don't have measurements where we need them to check the model out."

Generally, data from a handful of sites around the Bay watershed suggest that ammonium accounts for about 40 percent of the inorganic nitrogen in rainfall, with nitrate - mostly stemming from NOx emissions - accounting for about 60 percent.

There's a greater amount of uncertainty about the ammonia measurements though, because samples at most sites are collected weekly, not daily. That gives the ammonium a chance to convert back to ammonia gas and volatilize into the air, or to be consumed by microorganisms while it sits in the collection bucket.

Also, not all the ammonium comes down in rainfall. Some falls to the ground as dry deposition. But dry deposition estimates vary widely. "There is basically a huge lack of information to go after good estimates for ammonia in the Bay watershed," said Rick Artz, of NOAA's Air Resources Laboratory.

To help address some of those questions, the Bay Program is funding a $77,000 study in 1998 that will help gather more information about ammonia concentrations in the air throughout the watershed. That, scientists say, is a start in filling in the data gaps about ammonia sources, distribution and transport.

The information is important to help define an "ammonia airshed" for the Chesapeake that would be distinct from the "NOx airshed" that has already been developed. That may still be about two years away, Dennis said. Initial estimates show that the ammonia airshed will be larger than the Bay watershed, but smaller than the NOx airshed. But, Dennis noted, the ammonia airshed will be large enough to raise some big questions.

"The question will be whether the hog farms in North Carolina are affecting Chesapeake Bay," he said. "I think the Chesapeake Bay folks are going to want to know about that. Is it just the Neuse, the Albemarle-Pamlico Sounds [in North Carolina] that are affected, or is it also the Bay? And right now the suggestions are that yes, it is the Bay. Which also means the chicken farms from the Bay may be important to the Albemarle-Pamlico as well."