People across the Bay watershed have, over the last decade, slashed the amount of nitrogen pollution they generate, mostly without realizing it.

For that matter, so have people in Ohio, North Carolina, parts of Michigan and even Toronto.

Every time they flick on a light, drive their car, or even mow their lawn, they are producing dramatically less nitrogen pollution than was the case little more than a decade ago.

A steady stream of air regulations has had the side effect of making — somewhat ironically — air pollution controls one of the most effective means of controlling water pollution.

Recent computer model estimates from the Bay Program show that air pollution controls have reduced the amount of nitrogen entering the Chesapeake by 10 percent since 1985, a 35 million pound reduction that accounts for about a third of all estimated nitrogen reductions achieved to date.

Such a result would have been unimaginable just two decades ago when nitrogen pollutants in the air were deemed an "uncontrollable" source of pollution. Today, the results are not only tangible, but were counted in the Bay pollution diet — or total maximum daily load. It was the first time air pollution reductions were factored into a TMDL dealing with coastal nitrogen pollution.

"That's a recognition that national laws are driving the air reductions, and those national laws have knock-on benefits," said Lewis Linker who, as modeling coordinator for the EPA's Bay Program Office, has been working for two decades to improve estimates of how air pollution affects water quality. "And, those benefits are to estuaries everywhere."

Ironically, neither the Bay nor other estuaries were the intended beneficiaries of those controls. They stemmed from various clean air regulations aimed at protecting human health or reducing acid rain.

"It really is something of a secondary effect," Linker said. "The primary driver is human health concerns. But that drives down nitrogen loads in the Chesapeake Bay, and we benefit."

Nitrogen gas is the most common element in the atmosphere, but it is not generally available to plants. But reactive forms of nitrogen generated as air pollutants are readily taken up by terrestrial plants when deposited on land or by algae if it falls directly on water, such as the Bay.

There are two main forms:

  • Nitrogen oxides, or NOx, which are created as a byproduct of fossil fuel combustions and land as nitrate, a form of nitrogen. NOx also contributes to the creation of ground level ozone, or smog; particulate pollution; acid rain; and regional haze problems.
  • Ammonia emissions, largely from agriculture, which can contribute to some human health problems as well as water quality issues. Ammonia is largely unregulated.

While NOx emissions have decreased significantly, ammonia emissions have slightly risen in the watershed, according to Bay Program estimates.

Identifying a problem

The recognition that air pollution is part of the problem — and part of the solution — facing the Bay and other coastal waterways marks a dramatic change over the last quarter century.

When an environmental group, the Environmental Defense Fund, released a report in 1988 saying that air pollution was contributing a quarter of the nitrogen entering the Bay, many officials reacted in disbelief. But a study by the federal EPA shortly thereafter came back with a higher figure — 40 percent.

Over time, scientists refined that figure, ultimately estimating that roughly a third of the Bay's nitrogen came from air pollution. But doing anything about it seemed farfetched because there is no easy way to use the Clean Air Act to control water pollution.

In the early 1990s, it seemed unlikely that air pollution would ever result in significant reductions to Bay nitrogen levels, and the Bay Program wrote it off as "uncontrollable."

And it did seem unmanageable. Scientists determined that only about half of the NOx pollution affecting the Chesapeake originated from within the Bay states.

Most of the rest drifted in from a larger 570,000-square-mile "airshed" that reached into Canada to the north, Indiana and Kentucky to the west, and included most of South Carolina. A portion of the emissions blew in from even farther away.

"How do you go to Kentucky or West Virginia and say, 'Your power plants and cars are causing us problems here in the Bay. What are you going to do about it?" one state air official told the Bay Journal in 1995. "It's a big political problem to overcome."

But things began to change as evidence mounted that showed existing air standards for ozone and particulates did not adequately protect public health.

The EPA began strengthening those standards in the late 1990s. That forced a series of new regulations to control pollutants that contributed to the problem, including NOx: tighter tailpipe emission standards for cars and trucks; changes in gasoline formulations; cleaner engines on boats and lawn mowers; less pollution from ships; and almost everything else that burned oil, gas or coal. New rules began to rein in emissions that crossed state borders.

In addition, provisions of the 1990 Clean Air Act amendments aimed at curbing acid rain began forcing sharp reductions in utility NOx emissions starting in the late 1990s. NOx emissions, which had been steady for decades, began to drop nationally, and in the region.

Cleaner air cleans the Bay

Estimates from the Bay Program's computer models show that about 450 million pounds of nitrogen landed on the Bay's 64,000-square-mile watershed in 1985 as the result of NOx emissions. That was slashed by 55 percent, to about 200 million pounds, in 2011.

The greatest reductions took place in the mountains of western Maryland and central Pennsylvania which are downwind of major coal-fired power plants in the Midwest, and in urban areas surrounding the District of Columbia and stretching to Baltimore, which benefitted from both tailpipe and upwind utility controls.

But the sharp cuts in deposition on the watershed don't result in equally significant reductions in the amount of nitrogen that ultimately washes off the land and into the Chesapeake. On the region's vast forests and agricultural lands, plants absorb much of the nitrogen before it can make it to streams.

As a result, the massive air reductions only resulted in about a 26-million-pound reduction in the amount of airborne nitrogen ultimately washing into the Chesapeake from its watershed, according to Bay Program model estimates. (In addition, the amount of nitrogen landing directly on the Bay dropped from 26 million pounds a year to 17.4 million pounds a year. That brings the total reduction attributed to air pollution to about 35 million pounds a year.)

"The benefits of the reductions in deposition rates is really being buffered by the forests, which are going to take up 80–90 percent of the nitrogen," said Jeff Sweeney, a modeler with the Bay Program. "The outputs are really dependent on where the deposition lands."

Indeed, while forests effectively buffer nitrogen inputs, impervious surfaces can have the opposite effect. In heavily developed areas with lots of roofs and pavement, airborne pollution is quickly collected in stormwater and washed into streams. Much of the deposition reductions in those areas can be quickly translated into reductions in waterways.

New insights from forests

But it is possible that forests may not be so stingy about leaking nitrogen as the model assumes.

Keith Eshleman, a scientist with the University of Maryland Center for Environmental Science Appalachian Laboratory, who has been studying the impact of atmospheric deposition on mountain streams for more than two decades, said he has seen "surprising" reductions in nitrogen concentrations over the last 15 years.

"It looks like, when we look at the data, that things really began to go down in the late '90s and early 2000," Eshleman said. "That is just about the time that acid rain reductions went into effect."

Eshleman said he has seen a significant drop in nitrogen concentrations in streams in nine forested watersheds of various sizes he has examined. Part of the reason, he suggested, is that some forests have been bombarded with nitrogen deposition for so long that they no longer retain the nutrient as effectively as they once did.

"It looks like that in the watersheds that were most saturated, we are seeing nitrogen concentrations (in streams) coming down extremely rapidly," Eshleman said. "We didn't find any of these nine systems that isn't responding at all."

Long-term monitoring conducted by the U.S. Geological Survey on Young Womans Creek, a forested watershed in northcentral Pennsylvania, where air deposition reductions have been significant, shows that nitrogen concentrations in streams have been reduced by more than 50 percent over the last two decades.

Linker said those findings may cause modelers to revisit some of their model assumptions. "The observed is the gold standard," he said. "If you go out and you measure on a fine scale, then you've got the real deal."

Economic benefits of air controls

Regardless of how much is ultimately seeping through to the Bay, some scientists argue that — pound for pound — controlling air pollution results in the greatest environmental benefits for the region.

That's because a molecule of nitrogen that starts as an air pollutant has more potential to cause harm than one that starts as a fertilizer. The air pollutant harms human health in the form of particulate or ozone pollution. Ozone pollution also damages crops and forests. When nitrogen lands, it can contribute to problems such as the acidification of soils and streams (acid rain), nitrogen saturation of forests and loss of biodiversity.

Ultimately, it reaches the Bay where it contributes to water quality problems.

That's a concept known as the nitrogen cascade — nitrogen can take many forms and have many different impacts as it cascades through the environment.

In a 2011 paper, Melissa Birch, a scientist at Tufts University, showed that while agricultural releases of nitrogen in the Bay region were twice those of air pollution, controlling air pollution was 2.5 times more economically beneficial. Birch and her co-authors estimated that agricultural nitrogen caused $1.7 billion in damages, whereas air pollution caused nearly $4.4 billion in harm within the watershed — $3.9 billion of that stemmed from human health impacts. The rest was damages to the Bay, forests and crops.

Pound-for-pound, the paper found, the cost of controlling air pollution was similar to controls on agricultural sources and wastewater discharges, and less than stormwater.

The EPA's Science Advisory Board, in a 2011 report on the impacts of reactive nitrogen, cited that paper in suggesting further efforts should be made to control air pollution.

"The cascading economic costs of damage highlights the importance of regulating air emissions because of their impacts on human health as well as their large contribution to the degradation of Chesapeake Bay water quality," the report stated. "Hence, if one is interested in reducing water impacts of [reactive nitrogen], the total reduction of damage may rely nearly as much on stricter enforcement of the Clean Air Act as the Clean Water Act."

Clouds on the horizon?

But a report from the U.S. Governmental Accountability Office earlier this year noted that the EPA faces many challenges in trying to control air pollution to benefit waterways. It said air pollutants — including sulfur dioxides, NOx and mercury — were degrading waters in many areas of the United States.

The EPA primarily sets air quality standards — which drive most air pollution reductions — to protect human health. But it also has the authority to set "secondary" standards to protect ecosystems. As recently as 2011, the agency said current air regulations were not sufficient to protect aquatic ecosystems from acid rain and nutrient over-enrichment from air pollution, but it later concluded it did not have sufficient scientific information to establish a secondary standard, the GAO report said.

Not all of the challenges to further air reductions are scientific. Congress has grown increasingly antagonist toward more air pollution reductions over the years, and members have launched numerous — so far unsuccessful — efforts to block various regulations.

In the face of such opposition, the Obama administration in 2011 delayed until this year a planned EPA review of its ozone standard, which if tightened could spur further emission reductions.

In August 2011, the EPA finalized the Cross-State Air Pollution Rule which would have required further reductions of emissions, including NOx, in 28 states. Some of the reductions stemming from the rule are assumed in the EPA to help the watershed meet its nitrogen goals.

But last year, a three-judge panel of the U.S. Court of Appeals for the DC Circuit issued an opinion that would strike down the rule. The EPA is appealing the decision.

Also, not all forms of nitrogen emissions are being regulated. As NOx emissions are decreasing, emissions of ammonia are on the rise. Most ammonia emissions, which are largely unregulated, stem from animal manure, which has been increasing in the Bay watershed for the last quarter century.

According to Bay Program estimates, ammonia deposition on the watershed has increased from about 145 million pounds a year to about 160 million pounds annually, or about 10 percent. That offsets some of the impact of the NOx reductions, especially in areas with large amounts of animal agriculture.

While NOx emissions used to account for nearly three quarters of all nitrogen deposition in the watershed, deposition today is divided nearly evenly between the two types.

Controlling ammonia emissions from agriculture has long been controversial, and it's unclear whether those emissions would be regulated in the future. But some management practices proposed by the states to help meet Bay cleanup goals would reduce ammonia emissions.

At the same time, other regulations are in the works for gasoline formulations and tailpipe emissions which would likely further reduce NOx deposition.

Whatever lies ahead, the pace of nitrogen reductions from air pollution will almost certainly slow, simply because emissions have already been reduced by more than half.

"It's like the investment saying, past performance does not guarantee future returns," Linker said. In addition, population growth and the increased energy demand that goes with it will offset some of the gains made through new regulations.

"We should not expect the future to look like the past, with those very rapid and sustained reductions," Linker said. "We are entering more of a maintenance phase."