Acid Rain
"Acid Rain" is a term that refers to processes more precisely characterized as acid deposition. Acid deposition may occur as a result of precipitation such as rain, snow, sleet, hail, or fog; acid deposition may also occur as dry particles or dust settling out of the atmosphere.
The term 'acid rain' was first coined in 1856 by a British chemist named Robert A. Smith when he observed that smoke and fumes from human activity could change the acidity of precipitation.
'Natural', or pure rain water is slightly acidic as a result of its reaction with carbon dioxide, forming a weak solution of carbonic acid. Typically, clean rainwater will have a pH in the range of 5.6 - 5.7, though actual pH values vary from place to place and depend on the presence of other gasses and particles in the air.
The term pH refers to the presence of free hydrogen ions in the water (or other liquid) and is measured on a scale of 1 to 14. A pH of 7.0 is considered neutral; a pH below 7.0 is considered acidic, while a pH above 7.0 is considered to be alkaline, or basic. The pH scale is a logarithmic function; each point on the scale represents a tenfold increase (or decrease) from its nearest neighbor.
The gasses in the atmosphere that contribute to the development of these acids come from both natural sources, such as volcanoes or the decomposition of organic matter, and from anthroprogenic, or man-made sources such as automobiles and boilers.
The man-made component of acid deposition is principally derived from fossil fuel combustion; that is the combustion of coal, oil, or gas in utility and factory boilers, exhausted from smokestacks, and gasoline and diesel fuel from cars, buses and trucks exhausting through tailpipes. Wood and biomass combustion also contribute to acid deposition. The pollutants creating the acids are primarily sulfur dioxide (SO2) and nitrogen oxides (NOx) which are found in the exhaust from both smokestacks and tailpipes.
These pollutants are emitted into the atmosphere, where they gradually settle back to earth as an acidic dust, or combine with water vapor and return to the earth as acidic rain.
Acid Rain Effects
Acid deposition changes the chemistry of the environment. It affects water bodies such as ponds and lakes, river and streams, and bays and estuaries by increasing their acidity, in some cases to the point where aquatic animals and plants begin to die off. The lowered pH may liberate metals bound in the minerals of the bedrock and soils surrounding a waterbody, sometimes to a toxic effect.
Acid deposition damages vegetation as well. Scientists have observed leaf damage attributable to acid rain that limits the plant's ability to grow and sustain itself. Damage to forests has also been well documented; acid deposition reacts chemically with forest soils, leaching away nutrients vital to tree growth while at the same time mobilizing toxic metals in the soil.
While it is less well documented, some scientists have expressed a concern that acid deposition may adversely affect land dwelling animals as well, through the mobilization of metals in drinking water and through the uptake of metals by plants that are later consumed by animals. It is likely that humans would be similarly affected. It is clear that human health is compromised in those populations chronically exposed to airborne concentrations of sulfates and nitrates found downwind of heavily industrialized areas.
Acid deposition damages man-made structures as well; limestone, marble, and sandstone are susceptible to damage from acid deposition, as are metals, paints, textiles and ceramics. Repairing the damage caused by acid rain to buildings and monuments costs millions of dollars per year.
While it is true that acid deposition is a type and consequence of air pollution, its effects are not evenly distributed. Geography, topography, meteorology, and the chemistry of soils and bedrock all play a role in what the effect of acid deposition will be. Alkaline or basic soils, for example, have some ability to resist a change in their pH due to the buffering effect of certain minerals in their makeup; less alkaline soils have less ability to resist a change. Similarly waterbodies situated on an alkaline bedrock is more resistant to lowering its pH that is less alkaline bedrock.
Distance from the source of the air pollution plays a role in the rate at which acid deposition occurs, as do prevailing wind direction and elevation. It is known for example that the eastern half of North America has been more heavily damaged by acid deposition than has the western half; it is also true that (in general) the most severe damage in the east has occurred to forests and waterbodies at higher elevations.
Clean Air Act
Public concern about the environment, and about air pollution as a public health issue led to the passage of the Clean Air Act in 1970. By the end of the 1980s the adverse effects of acid deposition had been so well documented that in 1990 specific amendments were added to the Clean Air Act to reduce acid deposition.
Title IV of the 1990 Clean Air Act Amendments, known as the Acid Deposition Control Program, was designed to reduce annual emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx). Recognizing that the largest sources of these emissions are electrical utilities, Congress set national emission caps for the utility industry, phased in over a period of years, and instituted a sulfur dioxide (SO2) banking and allowance system to help the industry achieve the emission reduction targets.
The Title IV program and its market-based mechanisms proved to be successful beyond expectations, both in terms of actual emission reductions and in terms of the cost of making those reductions. Many people believed that with Title IV, the 'Acid Rain' problem was solved. Scientists did, in fact, find improvement and the beginning of recovery in some locales. For many other areas, however, the emission reductions to date have merely slowed down the rate at which the damage is occurring, and for these areas, acid deposition remains a serious problem. It is worth noting here also that the emission reductions achieved were primarily those of sulfur dioxide (SO2), while emissions of nitrogen oxide (NOx) have actually increased in some areas of the eastern United States.
Still, there is reason to be optimistic. Studies suggest that it is possible for eco-systems damaged by acid deposition to recover. The rate at which recovery occurs and the extent to which the recovery happens is dependent upon the magnitude of the reductions in sulfur dioxide (SO2) and nitrogen oxide (NOx) emissions, and the time it takes to achieve these emission reductions. For much of the northeastern U.S., it has been estimated that upwards to an 80 percent reduction in utility emissions of sulfur dioxide (SO2) (beyond those called for under Title IV) and the implementation of controls for nitrogen oxides (NOx) will be required for eco-system recovery. Even with these emission reductions, substantial eco-system recovery may not occur for another 25 years or more.
Critical Loads
The term 'critical load' implies a tipping point, or threshold. Most generally, the critical load may be defined as the maximum load that a system can tolerate before failing. As applied to environmental issues, however, critical load usually refers to exposure to pollutants. An environmental critical load is an estimate of the level of exposure to one or more pollutants below which no harmful effects are known to occur to specified elements within an ecosystem.
The use of critical loads within the context of air quality management is premised on the notion that the effectiveness air quality policy is reflected in ecosystem impacts. The critical load concept is uniquely well suited toward informing air quality policy because its receptor-based approach takes into account both the spatial and topographical variables of atmospheric deposition.
As it applies to the atmospheric deposition of acid forming compounds then, a critical load is that level of exposure to sulfur and nitrogen compounds below which no harmful effects are known to occur within a specified environment (or ecosystem).
A critical load map for Maine has recently been completed; for this map, critical loads have been calculated for Maine 's forest ecosystem.
The approach used to identify critical loads for sulfur and nitrogen in Maine's forest ecosystem is an ecological assessment based on an overall (steady-state) ecosystem budget for nutrient cations of calcium (Ca 2+ ), magnesium (Mg 2+ ), and potassium (K + ). This budget exists within a dynamic system of nutrient inputs, exports, and recycling.
In its simplest terms, the inputs to the nutrient budget for the Maine forest ecosystem include the addition of the nutrients Ca, Mg, and K through atmospheric deposition; acid forming compounds of sulfur (S) and nitrogen (N) are also introduced through deposition. Additional inputs of Ca, Mg, and K result from the chemical weathering of the bedrock and soils.
Nutrient losses or exports from the system occur as a result of chemical reactions within the root zone which may render a portion of nutrients unavailable for plant nutrition, and through soil leaching in response to the presence of acids. Additional losses or exports occur as a result of forest fires and through the harvesting of trees from the forest.
Nutrient recycling occurs throughout the lifecycle of the trees in the forest through the shedding of leaves and/or needles and through the decay of vegetative and woody debris on the forest floor.
The overall ecosystem budget is based upon the relative values of the inputs to and exports from the system. A condition where nutrient inputs exceed exports suggests that a sufficient state of biologic capacity exists for an ecosystem. Conversely, a condition where nutrient exports exceed inputs suggests a net nutrient deficit and increasing soil acidification; conditions ultimately unsustainable for a ecosystem over the long term. Many studies have demonstrated that inadequate nutrient levels lead to poor forest health and reduced growth rates.
The critical load map developed for Maine is derived on the basis of steady-state or static models. Consequently the map reflects conditions of long-term nutrient sustainability rather than absolute measures of current soil acidity/fertility. Nevertheless one might observe that where a negative nutrient imbalance is small, forest health problems and growth decline may not yet be evident; in those locations where the imbalance is significant, the impacts on forest health are likely to be observable today.
Critical load approaches offer air quality and natural resource managers a powerful tool with which to identify ecosystems at risk and to tailor monitoring and management strategies to address specific resource issues.
