1-7: Acid Deposition

The National Environmental Policy Act of 1969 (NEPA)

The chemistry of the atmosphere is changing in many ways. The National Environmental Policy Act of 1969 (NEPA) established the basic US national charter for protection of the environment. The act establishes policy, sets goals, and provides means for carrying out the policy. It states that it is the obligation of the Federal government to:

1. fulfill the responsibilities of each generation as trustee of the environment for succeeding generations;
2. assure for all Americans safe, healthful, productive, and aesthetically and culturally pleasing surroundings;
3. attain the widest range of beneficial uses of the environment without degradation, risk to health or safety, or other undesirable and unintended consequences;
4. preserve important historic, cultural, and natural aspects of our national heritage, and maintain, wherever possible, an environment which supports diversity, and variety of individual choice;
5. achieve a balance between population and resource use which will permit high standards of living and a wide sharing of life's amenities; and
6. enhance the quality of renewable resources and approach the maximum attainable recycling of depletable resources.

Acid Deposition

Acid deposition is a general term that includes more than simply acid rain. Acid rain suggests liquid precipitation that has a more acidic level than normal. However, dry acidic particles also can settle out of the atmosphere, and acidic vapors can interact directly with plants and structures at the earth's surface. So to broaden the concept beyond acid rain, we use the term acid deposition to include both wet and dry deposition of particles and capture of acidic vapors at the earth's surface.

Anthropogenic air pollutants can be created as gases or vapors, solids, or liquids and may be transformed to other states while in the atmosphere. For instance, SO₂ is a gas emitted as a byproduct from burning coal and leads to the formation of sulfate (SO₄) particles in the atmosphere, which combine with H₂O to produce H₂SO₄, which is sulfuric acid. Oxides of nitrogen (e.g., NO, NO₂) are gases produced by high-temperature combustion from, for instance, automobiles and power plants follow a similar pathway to become nitric acid in the atmosphere. These substances may be removed from the atmosphere either as dry or moist particles or as vapors or they may remain in the atmosphere and attach to naturally occurring precipitation particles then be rained out or snowed out.

Figure 1 sketches the stationary (e.g., smokestacks) sources, but we also have mobile (e.g., trucks, cars) sources that produce substances leading to acid deposition and how they are transported and transformed to become part of dry deposition or wet deposition after combining with atmospheric H₂0 either in cloud processes or scavenged by precipitation.

When these acidic materials reach the earth's surface they may lodge on plants or in the soil. Many eventually enter waterways and lead to a rise in the acidic level of streams, rivers, marshes and lakes. In some cases, such as Iowa, soils are slightly deficient in nutrients such as sulfur (for optimum agricultural production) and so a little additional sulfur being deposited on Iowa soils is not a detriment to agriculture production, in fact it may even be considered to have a positive effect. However, it may have a negative effect if it leaves the atmosphere as rain on the plants or structures. The point to be made is that there may be both positive and negative effects of acid deposition, depending on the subsystem of atmosphere/biosphere/lithosphere being considered in isolation.

Lakes and ponds tend to become repositories for some of these acids that fall as rain and are transported by surface run-off to standing water bodies. Sometimes these acid surges enter ponds at vulnerable times such as in spawning periods in the spring when a snow melt releases acidic material from accumulated deposition on snowpack over the winter.

Natural sources of acidic material, such as volcanoes and sea spray, produce a natural acidity to normal precipitation.

Atmospheric Transport

Atmospheric transport of pollutants can occur over long or short distances depending on atmospheric structure and dynamics. So in fact in some cases the near environment (1 to 2 km) of a power plant or a factory may be adversely affected, such as yellowing of needles on pine trees or diminished yield of crops. On the other hand, long-range transport can carry acidic material ten to 100 km or much further from the source before they settle out.

Figure 2 shows different meteorological conditions near the earth's surface and the different accompanying atmospheric dispersion conditions. At the left of each plume sketch is a small plot of temperature as a function of height in the lower atmosphere. The dashed line in each case gives the temperature condition for the atmosphere that we call "neutral" , in that such a temperature condition neither promotes or suppresses convective turbulence. If the actual temperature line is more vertical than this dashed line, we say the atmosphere is "stable" and suppresses convection. This condition typically occurs on clear, calm evenings. If, on the other hand, the line is more horizontal than the dashed line, we say the atmosphere is "unstable" and promotes convection. Sunny summer days typically lead to this condition. Combinations of stable conditions near the surface and unstable aloft or vice versa also are shown.

Typical behavior of a plume from an elevated source is shown for each temperature condition. Notice the marked difference in the location nearest the stack at which the plume likely would reach the ground. In some cases a plume may travel hundreds of kilometers under stable conditions at night without coming near the ground, but be brought down to the surface at some remote location the next morning when it is caught by the looping conditions as surface heating creates unstable conditions near the surface.

The more complicated structures, such as an unstable lower atmosphere capped by a stable upper layer (called a temperature inversion, or just inversion), is typical in certain geographical locations such as Los Angeles, Denver, and Houston. These conditions confine pollutants to a shallow layer near the surface creating smog and unhealthy conditions.

Other geographical constraints, like a mountain valley, can reduce natural ventilation and trap pollutant near the surface. In Vail, Colorado most condominiums have fireplaces which, when operating simultaneously under low wind and stable atmospheric conditions, emit enough smoke to accumulate and cause significant air pollution problems.

Coastal areas, sites of most of the world's largest cities, have unusual atmospheric circulation patterns that may lead to air pollution problems. A typical example is the recirculation of air pollutants from the Chicago area due to the Lake Michigan sea breeze. During the day in summer when no large-scale systems are dominating the weather, the sea breeze blows from the lake toward land and carries urban air pollutants further inland. Here they rise and return at high elevations to locations over the lake. The sea breeze on the following day may then recirculate these pollutants into the Chicago area, or, if they have gradually drifted to the northwest, they could (and occasionally do) degrade air quality for smaller cities along the western shore of Lake Michigan.

Sulfur and Nitrogen Compounds

For our consideration of acidic deposition, we will limit discussion to sulfur and nitrogen compounds. The major sulfur-based pollutant is sulfur dioxide (SO₂). Sulfur is a constituent of most coal, but the sulfur content varies with the location of the mine. Coal from the western United States is very low in sulfur, whereas eastern-US coal has higher sulfur content. At one time it was a state law that Iowa State University was required to burn coal mined in Iowa. This coal had a quite high sulfur content and produced large amounts of sulfur dioxide as a byproduct of combustion.

SO₂ gas oxidizes to a sulfate (SO₄) at a rate of 4% per hour. Airborne sulfate particles are therefore deposited back to the earth's surface before they travel very far from their sources. Later in the semester we will discuss the rapidly growing economies of China and other Asian countries. For many of these developing countries, including India, their growth likely will be based on expanding power production that depends on coal of quite high sulfur. The consequences of this are more serious than simply increased acid deposition, as we will learn when we talk about global warming and the effects of sulfates.

Other sulfur compounds are given in Figure 3.

Hydrogen sulfide, H₂S, sometimes known as rotten-egg gas, is produced naturally in the soils and decaying vegetation, but also is produced by combustion. Dimethyl sulfide also has natural sources and over the ocean is believed to be an agent in enhancing natural precipitation.

Nitrogen compounds in the lower atmosphere include nitric oxide, nitrogen dioxide, nitrous oxide, nitrogen trioxide, and ammonia.

Of these, nitrous oxide does not really belong with acid-producing chemicals because it is a very stable compound and does not decompose rapidly enough to participate in chemical reactions in the troposphere.

pH

pH is the quantity that we use to measure acidity, as you will recall from high-school chemistry. pH is the negative logarithm of the number of hydrogen ions. pH ranges from 0 to 14, with 7 being neutral, less than 7 being acidic, and greater than 7 being basic. Therefore a liquid with a pH of 4 is ten times as acidic as one of pH 5 and 100 times as acidic as one of pH of 6.

Natural precipitation is not perfectly neutral, because of dissolved CO₂ which gives it a pH of about 5.65. Therefore, precipitation is considered acidic if it has pH less than about 5.6. Airborne soil particles in the western United States are somewhat more basic and give precipitation in that region a higher natural pH, whereas in the eastern United States natural particulates tend to make precipitation (even in the absence of anthropogenic contributions) slightly more acidic.

Effects of Acid Deposition

There has been considerable controversy on the effects of acid deposition on vegetation. It is reasonably well agreed that over large areas there has been relatively little agricultural or horticulture damage due to acid precipitation. But I emphasize that this is on the large scale. There are numerous examples of severe damage to crops or forests near sources of acidic emission. But widespread damage is much more difficult to quantify. Forests, for example, may be affected both by direct deposition on vegetation and through chemical processes in the soil. Agricultural soils are generally well buffered, but forest soils may not be as resistant to effects of acid deposition.

Waterways present another area that can be very vulnerable to consequences of acid deposition. We can have input of acidic substances directly from the atmosphere or through runoff or through generation of H+ ions within the watersheds. A lake has a natural buffering capacity particularly if it is in a region that has limestone that combines with the H ions to form H₂O and CO₂. The alkalinity of the lake decreases with only a small change in pH as acid rain and runoff enter. However if the bicarbonate ions become depleted, then the pH will drop and acidic conditions prevail. Surges of acidic material may occur in the spring with snowmelt or during the summer with heavy rains. These episodes may be particularly detrimental to marine organisms if they occur at vulnerable spawning times.

The pH may ultimately stabilize around 4.0 as humus and aluminum react to absorb the free hydrogen ions. Increase in concentration of aluminum ions can lead to aluminum toxicity which may make the water become clear. In this regard, you might say that a clear lake is a nice place to visit but you might not want to live there, particularly if you were a sensitive marine organism. Acid tolerance of various fish is shown in an EPA educational website. Concepts of sensitivity and critical load provide quantitative estimates of impacts of acid deposition on various ecosystems.

Figure 4 shows the pH levels in rainfall over the U.S. from the 1950s, 60s, and 70s. The area of West Virginia, Pennsylvania and New York experienced a drop in pH from 4.52 to 4.3 over a period of twenty years. Figure 5, a more recent map shows that the eastern US continues to have low pH levels despite significant reductions in emissions. In a later lecture, we will discuss another aspect of acidic materials in the atmosphere, namely their contribution to global cooling.

The National Acid Deposition Program provides annual nationwide plots of deposition of numerous chemical compounds for the last several years.

Regional and Global Effects

Most air pollutants contributing to acid deposition are generated at the surface of the Earth and are deposited back to Earth by atmospheric processes in the troposphere. However, as source regions expand regional and global problems are beginning to emerge. A cloud of pollutant material has developed across South Asia that is so thick and expansive that it is causing damage to agriculture, weather patterns, and human health for the countries of Afghanistan, Bangladesh, Bhutan, India, Maldives, Nepal, Pakistan, and Sri Lanka. A United Nations Environment Programme indicates that this mixture of haze, ash, and acid particles from burning fossil fuels and agricultural materials is estimated to have caused 37,000 premature deaths per year in the mid 1990s. By blocking solar radiation to the ocean surface, the cloud may be reducing evaporation and contributing to disruption of the monsoon system causing large-scale changes in patterns of drought and flooding. The report was compiled by a team of some 200 scientists. In addition to acidic materials, soot emitted during combustion on large regional scales is contributing to global and regional effects.