Neutralizing the rain: After much success in the battle against acid rain, challenges remain
Copyright Mwanner, Creative Commons Attribution-ShareAlike 3.0 Unported
National Atmospheric Deposition Program
National Atmospheric Deposition Program
Copyright Nino Barbieri, Creative Commons Attribution-ShareAlike 3.0 Unported
Every Tuesday at 9 a.m., Dave Warner collects water from a white plastic 3.5-gallon tub that sits on a strip of tall grass between two cornfields at the University of Nebraska Agricultural Research and Development Center near Mead, Neb. For more than 30 years, the bucket has collected all forms of precipitation — from hail to rain to snow — to be analyzed for nitric and sulfur oxides, the main components of acid rain.
Across the country, that same Tuesday morning collection happens at more than 250 stations that make up the National Atmospheric Deposition Program National Trends Network, which is devoted to tracking changes in acid deposition from rain and dry particulates. In operation since 1978, the National Trends Network has tracked the impacts of human actions, such as coal burning and the combustion of fossil fuels for transportation, on altering the pH of rainfall.
The data gathered by the National Trends Network was just one facet of the information presented to Congress in 1990 that led to amendments to the Clean Air Act, which was originally passed in 1970. Among other provisions, one amendment called for government regulation of sulfur dioxide emissions, a known cause of acid rain. The aim of the amendment was to reduce sulfur dioxide emissions, created primarily by fossil fuel-burning facilities, to about 50 percent of the 18.9 million tons produced annually in 1980, and in turn, to alter the trend of ever-increasing problems tied to acid rain throughout the Northeast and Midwest. The goal was to reach that 50 percent reduction mark by 2010.
When the numbers were tallied in 2010, it was clear that the Clean Air Act amendments had accomplished their goal. With the regulations imposed, sulfur dioxide emissions were down to 8.9 million tons per year. Ecosystems that had been all but wiped out by overly acidic precipitation were beginning to recover, such as Brooktrout Lake in New York’s Adirondack Mountains. In its annual report on acid rain, the Environmental Protection Agency (EPA) suggested that the decrease in emissions was directly related to reduced levels of heart attacks and respiratory conditions, saving an estimated 20,000 to 50,000 lives annually. In fact, by 2010, the measures had been so successful that there did not seem to be an acid rain problem at all, says Doug Burns, a researcher at the U.S. Geological Survey and director of the National Acid Precipitation Assessment Program based in Troy, N.Y.
In fact, Burns says, “if you were to ask someone on the street today what they thought about the acid rain problem, they would likely say, ‘What acid rain problem?’” he says. People heard a lot about the problem from the 1970s through the 1990s, but not a lot has been said about it in more than a decade, so, he says, people tend to think the problem must have been solved.
But that is not the case. “It’s not a problem that’s gone away,” says David Gay, program coordinator at the National Atmospheric Deposition Program, which is headquartered at the University of Illinois at Urbana-Champaign. “It’s just a problem that’s gotten better.”
Since the Clean Air Act amendments, maps of sulfur dioxide deposition in the U.S. have shown remarkable changes. Places like the Ohio River Valley were covered in glaring red patches that indicated more than 30 kilograms per hectare of soil in the annual measurement reported in 1985. Today, the maps have almost no red at all, just various shades of green that indicate levels closer to 20 kilograms per hectare.
However, as scientists have been learning over the last decade or so, sulfur dioxide is not the only pollutant contributing to acid rain. In fact, its sudden reduction has allowed other players to emerge, changing scientists’ understanding of the nature of the problem.
The Acid Rain Issue
The definition of acid rain is not easy to pin down. First, “acid rain” is a terrible misnomer, says Christopher Lehmann, lab director at the National Atmospheric Deposition Program. It is more accurately called acid deposition, as both wet precipitation and dry particulates from emissions can contribute to acidification. The process begins with the emission of chemicals, including sulfur dioxide, nitric oxide (and nitrogen dioxide) and ammonia, into the atmosphere. Once there, they can react with common atmospheric components like oxygen and water vapor and can be redeposited on the surface, either dissolved in rain or as dry particulates. (Ammonia can be tricky, Lehmann notes. Ammonia actually forms a base when it interacts in the environment, which can shift the pH back toward neutral, but it is still harmful to ecosystems.)
On the pH spectrum, any value below a neutral 7 is considered acidic. For example, fresh lemon juice, a highly acidic substance, has a pH of 2. Rainwater that is considered pure is closer to 5.6. Precipitation with a pH lower than 5 is called acid rain, Lehmann says. Long-term exposure to rainwater at that pH will impact environments that are susceptible to acid toxicity, especially freshwater ecosystems such as lakes.
Sulfur dioxide, nitric oxide and ammonia all have natural sources, including volcanoes and decaying plants, but the dominant contribution comes from human-made sources, such as coal-burning power plants and stockyards full of livestock whose wastes produce large amounts of ammonia, says Todd Schimelfenig, a lead research technologist at the Mead National Trends Network station. Acid rain is a problem that began well before we decided to measure or legislate it.
Regulating the Rain
Chemist Robert Angus Smith coined the term “acid rain” in 1852, about 20 years before he published work detailing a connection between pollution in the air and elevated acidity levels that were appearing in Manchester, England, following the start of the industrial revolution there in the 1700s.
It is likely that acid rain was making an impact long before Robert Angus Smith made his observations. The industrial revolution began in the United States in the middle 1800s, and with it came all of the byproducts that create acid rain. As the population grew, so did the impact on the atmosphere. Eventually, so much pollution was being released into the atmosphere that rain itself began wreaking havoc on ecosystems. At the time, trees in parts of the eastern United States were dropping their leaves en masse, leaving large patches of forest looking as though they were stuck permanently in winter.
The situation only grew worse until 1955, when Congress passed the Air Pollution Control Act, an investigatory act that funded research into air quality issues. Though a federal law, it was left to individual states to investigate their own air pollution concerns. An early example was a 1959 epidemiology report from the California Department of Public Health that directly linked 1,200 deaths over a 10-day period in August of 1955 to a fluke weather pattern that trapped air pollution over Los Angeles. After similar observations were collected by other states, the first Clean Air Act was passed in 1963 with the mandate to develop strategies for monitoring and controlling air pollution. A further step was taken in 1967, when another act was passed to study pollution that was being transported across state lines by wind and weather.
However, it wasn’t until 1970 that the federal government became an enforcer of air pollution policy. Congress passed an updated Clean Air Act, and, in the same year, the EPA was created in part to help enforce the federal and state regulations that were born of the act.
In 1990, Congress passed an amendment to the act that included specific provisions for monitoring acid deposition in the 48 contiguous states and the District of Columbia. The acid rain amendment laid the groundwork for a cap-and-trade type of emissions trading program for sulfur dioxide, Lehmann says. Factories could purchase an allotment of allowable sulfur dioxide emission units, which could subsequently be traded among factories, ensuring a steady cap on the total amount of sulfur dioxide produced by each state. In addition to purchasing emission units of sulfur dioxide, factories and power plants were given the option of using sulfur scrubbers or switching to low-sulfur fuels to further lower their emissions footprint.
The enforcement of the 1990 amendment led to sulfur dioxide emissions being halved by 2010. In regions particularly prone to acid rain, such as the Ohio River Valley, the pH of rainfall has steadily climbed back to normal from below 4, Gay says. But, he points out, the Clean Air Act amendments only put regulations on one piece of the puzzle; issues with nitric oxide and ammonia remain unresolved and may indeed prove more complex to solve. When the scientific community was advising regulators and policymakers, Burns says, “[we weren’t] as aware of the biological effect of nitrogen as [we are] now.” And that lack of understanding is reflected in the initial legislative measures, which focused almost entirely on the sulfur problem.
Over the past three decades, the precipitation bucket at Mead and 250 or so stations nationwide have sent weekly aliquots of water to the National Atmospheric Deposition Program headquarters for analysis. The program is actually a coalition of five tracking networks that track everything from mercury deposition to statistics that help design atmospheric models. For acidity tests on precipitation, results are collected by one of the subgroups, the National Trends Network.
Each station has to meet certain qualifications to ensure samples are not biased. There can be no feedlots within 500 meters of the collector and no public roads within 100 meters. Vegetation around the collector has to be lower than 0.6 meters. The stations are also placed well clear of urban areas, so that daily activities such as construction projects don’t interfere with readings, Schimelfenig adds. Such restrictions make for some hard-to-reach collection stations.
“Some of these sites are in very remote locations,” he says, pointing out the Loch Vale station in Colorado which is a 5-kilometer trek up a mountain. Another site in Puerto Rico can see more than 2 meters of rain each year, causing floods near the gathering location. Stations are located in every state except Rhode Island, plus Puerto Rico, Argentina and Canada.
Started in 1978, the National Trends Network focuses on the wet deposition, the easier to track of the two types of deposition. The source of wet deposition can be identified down to a county of origin using back-trajectory modeling of weather patterns in the upper atmosphere, Lehmann says. Back-trajectory modeling is like pushing rewind on the weather, watching how recorded wind currents and storm patterns would have affected the distance particles would have traveled.
These data produce maps that paint a pictorial story of wet deposition: In the West, roughly beyond the dividing line of the Mississippi River, states rely more on hydropower and natural gas, Gay says; in addition, the coal west of the Rockies is considerably lower in sulfur than eastern coal. So, he says, these regions saw less of an impact both from acid rain and the subsequent reductions. The story changes east of the Mississippi, however. The largest source of sulfur dioxide is the Ohio River Valley through Pennsylvania and New York, where there is considerable mining of relatively high-sulfur coal, and coal-fired power plants are the dominant power source. The particulates from these regions are carried by wind and weather, Gay says, leading to acid rain issues that stretch from Maine to Alabama. Tracking the flow of acid rain has impacted how the legislation has worked over the past few decades. The mobile nature of the problem is reflected in the Clean Air Act’s trading policy among states.
The monitoring stations cannot account for every aspect of acid deposition, however. Though the National Trend Network stations can collect dry particulates, they do not currently do so. Tracking the point of origin of dry particulates is much more difficult, because where they fall is not dependent on precipitation patterns, and how they move in the lower atmosphere is chaotic, Lehmann says. The networks also don’t add much to the question of just what sort of impact reducing sulfur dioxide has had, Gay says. “The atmosphere is a big chemical reactor,” he says. “You stop putting one chemical in and the balance switches to some other chemical.”
Nitric oxide and ammonia emissions are still being produced with almost no oversight, whereas sulfur dioxide emissions continue to drop every year. “We’re not exactly sure how reducing the sulfur input to the atmosphere has changed the cycling of the other components in the atmosphere,” Lehmann says, citing mercury compounds and ammonia as examples of pollutants that could be affected by the reduction of sulfur.
The Need for Continued Monitoring: The Unknowns
That is why continued monitoring of acid deposition is important, Lehmann says. At the moment, acid deposition sits on the edge of what Lehmann calls “the risks of environmental monitoring.” There are two big issues. First, with levels of sulfur dioxide falling and recoveries happening in previously acidified ecosystems, people might think the acid rain problem is fixed forever. But, he says, “If you don’t monitor, how do you know if the problem returns?” Second, there are the unknowns. With ongoing uncertainty over how the atmosphere may be affected by the shift away from sulfur dioxide as a main cause of acid deposition, it is more important than ever to continue monitoring in order to understand how things are changing, he says.
Through most of the last half-century it was thought that acidic precipitation had a primarily topical impact — water that struck leaves caused them to wither, or sleet that filled ponds might cause fish to die off. As time has gone on, however, scientists have learned that long-term buildup of nitrogen and sulfur oxides in the soil is actually the biggest issue, Burns says. Most soils have the ability to neutralize some amount of acid entering the ecosystem, but once the buffering capabilities are overwhelmed, acid deposition causes the degradation of important nutrients, stripping the soil and damaging trees and other vegetation, he says.
The impact of acidity on plant life can vary regionally and among species. In the Midwest, studies using National Trends Network data along with other data have shown that corn and soybean crops actually prefer soil with elevated levels of sulfur, although when too much is introduced the acidity can become too high for the crops to thrive, Schimelfenig says. In North Carolina, nitrogen buildup strips calcium from the soil, which red spruce trees need to protect against injury brought on by the freeze-thaw cycle, Burns says.
In addition, high soil acidity can cause toxic amounts of other elements, such as aluminum, to be released from compounds such as aluminosilicate minerals in the soil. “Most researchers now believe that [releasing bound elements] is what leads to the toxic impact of acid rain,” Lehmann notes. Plants take up those unbound elements, causing damage, such as stunted root growth.
According to a 2011 report to Congress by the National Acid Precipitation Assessment Program, scientists are currently studying the critical load — or threshold — of nitrogen and sulfur that ecosystems can handle. Each ecosystem will have a unique critical load value for sulfur and nitrogen. By investigating critical loads, the report states, scientists can assess the effectiveness of current policies and help shape future legislation.
In 2011, the EPA finalized the Cross-State Air Pollution Rule, a new set of regulations aimed at lowering atmospheric concentrations of sulfur dioxide by another 70 percent and nitric oxide by more than 50 percent by 2014. For sulfur dioxide, that would reduce the annual load to about 3 million tons a year — quite a change from the 18.9 million tons produced annually in 1980. For nitric oxide, reducing emissions to 1 million tons per year is not only important for reducing acid deposition, but also for slowing the deleterious effect nitric oxide has on ozone.
Ammonia is an entirely different issue. Because it comes from ranching and stockyard sources, regulations would likely have to involve the Department of Agriculture as well as the EPA and other government branches, Schimelfenig says. Getting all of those groups to cooperate on a piece of legislation may take some time. In the meantime, researchers at the National Atmospheric Deposition Program have just added the Ammonia Monitoring Network, so that when the time comes to work on legislation, the data will already be available, he says.
Even with the ongoing challenges, legislation like the Clean Air Act has changed the landscape of acid deposition, Burns says. For example, Schimelfenig notes, since the large drop-off in sulfur dioxide emissions began, farmers have begun to adjust the amount of sulfur in their fertilizers to reflect the change. And recent reports suggest that after New York’s Brooktrout Lake was restocked with brook trout in 2006 — for the first time since it was found to be devoid of fish in 1984 — the fish are surviving now.
Overall, Lehmann says, the steps taken to curb acid deposition have done their job. “Research has shown … reduced acidic impacts to the ecosystems can be directly tied back to reduced emissions from the power plants.” However, he adds, there’s a lot more to be done, such as assessing the changes that have happened in the atmosphere that scientists didn’t anticipate.
The coalition of government bodies following the changing landscape of acid deposition is sizeable. The National Acid Precipitation Assessment Program, which publishes annual reports using the National Trends Network data, is affiliated with the EPA, NASA, NOAA, Department of Energy and more. Although each agency has its own mandate, all have a vested interest in continued monitoring. So for now, the 9 a.m. Tuesday collections will continue in Mead and across North America as scientists try to tease out the ever-changing landscape of acid rain and its impact on environment and human health.