Pollutants in the Air and Acids in the Rain: Influences on Our Natural Environment and a Challege for Every Industrial Society

Ellis B. Cowling

Given at Berkeley, California, March 28, 1985

"As a rule rain is not acid far from towns. If it is acid, artificial circumstances must be suspected."
-Robert Angus Smith, 1872.

In the opinion of many philosophers, an important part of our contemporary environmental problems derive from the injunction stated in the sixth chapter of Genesis:

"Go, multiply, and subdue the earth and take dominion over every bird of the air and every fish of the sea and every living thing that moves on the surface of the earth. "

Many would agree that both western and eastern peoples have taken this injunction a bit too literally. If given the chance to revise these poetic words, I wonder if we would not write a bit more modestly (but much less poetically!):

"Go and multiply with caution - bearing in mind the carrying capacity of the land and seas of the earth. Take a responsible dominion over the renewable and the nonrenewable resources of the earth - remembering that the health and prosperity of the people will be determined by the sustainable productive capacity of the lakes and streams and the fields and forests and the oceans of the earth. "

In the Beginning . . .

Since the origin of life on Earth, plants, animals, and microorganisms have obtained an important part of their sustenance from the atmosphere - carbon, hydrogen, and oxygen for photosynthesis and respiration; nitrogen and sulfur for synthesis of proteins and amino acids; phosphorus for energy transformations and production of nucleic acids; calcium and magnesium for structural components; and a host of trace elements - iron, zinc, copper, cadmium, manganese, and boron - to facilitate metabolic transformations. All 16 of these essential elements are dispersed through the atmosphere and are taken up by plants directly through their foliage as well as through their roots.

As the industrial revolution gathered momentum in the middle of the 19th century, human activities of many sorts added more and more substances to those that circulate naturally among the air, water, soil, and all living things.

Some of these substances are beneficial for agriculture and forestry because they provide nutrients for the growth of crops and forests or accelerate the natural weathering of soil minerals. Other substances are less important because they are inert biologically. Still other substances are detrimental because they cause stress in plants, animals, or microorganisms, alter surface and ground water quality, aggravate nutrient deficiencies in soils, or accelerate the soiling, weathering, or corrosion of engineering and cultural materials.

Everything human beings do on a large scale influences the chemistry of the atmosphere and in turn the health and productivity of the ecosystems on which the abundance and quality of our life depends. The largest of all human influences on the chemical climate result from combustion of fossil fuels, urban development, and clearing of land by burning of natural vegetation. These activities include:

  • generation of electricity;
  • refining and use of petroleum and petrochemicals;
  • industrial processes of many sorts;
  • use of transportation vehicles;
  • space and water heating;
  • incineration and decomposition of sanitary and solid wastes;
  • use of explosive devices in peace and war;
  • launching of space vehicles; and
  • agricultural and silvicultural operations involving plowing, cultivating, spraying, disposal of plant and animal wastes, and burning of farm and forest residues.

In recent years, the words "acid rain" have captured the imagination of the people of Europe and North America in much the same way and for substantially the same reasons that Rachel Carson's "Silent Spring" captured our imagination in the early 1960s. But "acid rain" must be recognized as only one special feature of a series of more general subjects that include:

1) The emission, transport, transformation, deposition, uptake, and exchange of natural and man-made chemicals between the atmosphere and terrestrial and aquatic ecosystems;

2) Wet and dry deposition of beneficial nutrients and injurious gases, aerosols, and dissolved or suspended substances in rain, snow, hail, dew, and fog;

3) The short-distance and long-distance transport of air pollutants from one state or nation to another;

4) The role of human activities as a potent force in the biogeochemical circulation of matter in the earth; and

5) The responsibilities of human beings as managers of industrial societies and custodians of the natural resources of the earth.

The Objectives of this 25th Albright Conservation Lecture

The Albright Lectureship in Conservation seeks to "stimulate wide general interest in the preservation for this and future generations of the natural beauty of America." It is a very humbling experience to be asked to contribute to this noble purpose. But it is a challenge which I accept eagerly in the hope that some small part of what is said here may lead others to continue their own self-education about:

  • pollutants in the air and acids in the rain;
  • the influences of air pollutants on our natural environment; and
  • the challenge they present to every industrial society.

Pollutants in the Air and Acids in the Rain

The airborne chemicals of major concern to our society are listed in Table 1. They include two major types of pollutants (primary and secondary) and eight specific chemical or physical groups of substances. Primary air pollutants are gases or other volatile waste products emitted directly from stationary or mobile sources. Stationary sources include power plants, metal smelters, and other industrial or commercial installations, as well as domestic and commercial space and water heating units. Mobile sources are mainly cars, trucks, trains, aircraft, and ships. Secondary Pollutants include a wide variety of substances formed in the atmosphere by chemical transformation of primary pollutants.

The six most important primary air pollutants are sulfur, dioxide, nitrogen oxides, toxic elements such as lead and fluorine, a wide variety of volatile organic compounds, carbon monoxide, and particulate matter. The two most important secondary pollutants are photochemical oxidants (especially ozone) and acid deposition. Airborne acids and particulate matter can occur as both primary and secondary pollutants.

Table 1. Air Pollutants of Major Concern to Society
Chemical and Physical Nature of Air Pollutants:

1) Sulfur dioxide (SO2) - A colorless gas produced during combustion of sulfur-containing materials such as coal, oil, and biomass, and during smelting of sulfide metal ores. SO, is emitted mainly by large stationary sources such as fossil fueled power plants, metal smelters, and certain other industrial and commercial installations. Biomass burning is an important source of sulfur oxides in tropical regions of the world.

2) Nitrogen oxides (NOx) - Two colorless gases (NO and NO2) produced in any high temperature process such as combustion of coal, oil, gasoline, and natural gas. NOx are emitted by both stationary sources and transportation vehicles. In tropical countries burning of biomass is also an important source of NOx.

3) Toxic elements - Heavy metals such as lead, cadmium, nickel, and mercury and fluorine and other toxic elements am released mainly by large metal smelters and by transportation vehicles using leaded gasoline.

4) Volatile organic compounds (VOC) - A wide variety of carbon compounds ranging from such simple molecules as ethylene, gasoline, and cleaning and painting solvents to very complex molecules such as pesticides. VOC are emitted by many different, usually small stationary and mobile sources.

5) Carbon monoxide (CO) - A colorless and odorless but highly toxic gas produced during incomplete combustion of coal, oil, and natural gas and incineration of garbage and other solid and liquid wastes. Carbon monoxide inhibits respiration in humans and other animals. It is of concern to society mostly in urban areas where it accumulates in stagnant air mainly from transportation vehicles.

6) Particulate matter (PM) - A catch-all category of pollutants ranging from very coarse "fugitive dust" particles that cause soiling of textiles, windows, paints, etc., to very fine aerosol particles that cause atmospheric haze or are drawn into lungs where they induce respiratory disease. These substances are extremely diverse both chemically and physically. The larger particles range from almost pure carbon in the case of soot from oil burners to mineral dusts in the case o manufacturing facilities that process cement, asbestos, clay, ceramic, glass, textile, and other materials. The fine particles range from smoke to all sorts o sulfate, ammonium, organic, metallic, and other particles formed by condensation of gases, vapors, and other volatile substances in the atmosphere. Some of these particles have very remarkable and complex fine structures that are characteristic of the original sources of the emissions.

7) Ozone (03) and other photochemical oxidants - These substances include formaldehyde, hydrogen peroxide, peroxyacetyl nitrate (PAN), and peroxypropionyl nitrate (PPN). They occur mainly as secondary pollutants which are produced when NO, and VOC interact with atmospheric oxygen in the presence of sunlight. Ozone is the most important of these three pollutants because it is much more abundant even though it is somewhat less toxic than PAN or PPN. These compounds are among the most toxic gases to which plants, animals, and humans am exposed in the environment.

8) Acid deposition - A variety of acidic and acidifying substances produced when gaseous SO2, NOx, HCl, and certain other airborne chemicals interact with oxygen, ammonia, and moisture in the air to give aqueous solutions or aerosols of sulfuric, nitric, and hydrochloric acids. Wet deposition of these substances occurs during all rain, snow, hail, dew, fog, cloud, or rime-ice events; dry deposition Occurs at all times - by absorption or adsorption of gaseous SO2, NOx, HN03, and HCl and by impaction of sulfate, nitrate, and chloride aerosols on the surfaces of plants, soils, animals, microorganisms, surface waters, and materials. The acidic and acidifying substances in wet and dry deposition may be partially or completely neutralized by alkaline earth elements Such as calcium, potassium, sodium, or ammonium ions. Acidification of ecosystems also occurs when ammonium sulfate aerosol and certain other ammonium compounds o ammonia itself are taken up by plants, animals, or microorganisms after deposition into ecosystems.

The operation of power plants, metal smelters, and motor vehicles all illustrate the dilemma in which we find ourselves. Electricity, metals of various sorts, and transport vehicles are used for many worthwhile purposes in our society. Fossil fuels (mainly coal, oil, and natural gas), and sulfide metal ores are available near the earth surface. The fuels can be burned to generate electricity or to propel motor vehicles. The metal ores can be roasted to separate "the metal" from the sulfur and other "impurities." During these combustion and heating processes, certain waste products are produced. These waste products include sulfur and carbon oxides formed from the fuels and sulfide ores, nitrogen oxides formed mainly from nitrogen in the air, and certain noncombustible materials including various toxic metals. These waste materials accumulate either as a solid (ash) or are released into the atmosphere in a rapidly moving stream of exhaust gases, fine aerosol particles, and coarse particles.

Various engineering systems are available for decreasing emissions of these air pollutants. Sulfur, for example, can be removed from coal and oil in three ways - prior to combustion by desulfurizing the fuel, during combustion by modifying the combustion process, or after combustion by desulfurizing the flue gases. Use of lead-free gasoline together with catalytic converters and modified combustion systems in automobile engines can greatly decrease vehicle emissions of lead, carbon monoxide, and nitrogen oxides. Unfortunately, these engineering systems have not been installed or are not properly maintained in all power plants, metal smelters, and motor vehicles.

Earlier it was believed that most of the airborne waste products emitted from these stationary and mobile sources fell out of the atmosphere near the point of emission. Now it is recognized, particularly with increasing use of tall smokestacks at power plants and metal smelters, that meteorological processes can lead to extensive mixing and to both chemical and physical transformations of millions of tons of gases, fine aerosol particles, and coarse particles that are released into the atmosphere each year.

Recently, fog in industrial regions and cloud water high in the atmosphere were discovered to be much more rich in acids and other air pollutants than rain or snow. This discovery greatly increased concern about possible effects of airborne chemicals on high mountain forests.

These airborne substances and their reaction products are carried by wind and clouds wherever the wind blows and then deposited on the surfaces of vegetation, soils, surface waters, and engineering or cultural materials at short or long distances from the original sources of emission. Thus, the chemical composition of the air, and the rain, snow, dew, hail, fog, and cloud water within any region is a function of all the airborne substances dispersed, mixed, transformed, and transported into the atmosphere of that region and then deposited and taken up by the plants, animals, and microorganisms in terrestrial and aquatic ecosystems.

As indicated earlier, some of these substances are beneficial nutrients that help living things grow. Unfortunately, however, some of these substances are also toxic or otherwise injurious to plants, animals, microorganisms, human beings, engineering materials, and such cultural resources as the Parthenon in Greece, the Taj Mahal in India, and the Statue of Liberty in the United States.

Natural Sources vs Human Sources of Airborne Chemicals

There are some in our society who argue that natural emissions of airborne chemicals are just as important as emissions from human activities. There is a measure of truth in this contention so long as the whole earth is used as the basis for reference rather than just the industrial regions. A Russian soil scientist named Kovda (1975) was among the first to compare the amounts of substances involved in natural processes and human activities. His data are shown in Table 2. They permit two major generalizations about the relative magnitudes of natural (biogeochemical) and human (anthropogenic) processes:

1) Garbage, urban wastes, and byproducts are now produced at about twice the rate at which photosynthesis occurs in the whole earth; also

Table 2. Biogeochemical and Technological Forces in the Biosphere of the Earth (Data of Kovda), 1975
Biosphere Components
Tons per Year
Biogeochemical Processes:
Yield of photomass
1 x 1010
Cycle of inorganic elements
1 x 1010
River discharges:
Dissolved substances
Suspended substances
3 x 109
2 x 1010
Anthropogenic Sources:
Output of fertilizers
3 x 108
Industrial dusts
3 x 108
Garbage, urban wastes, and byproducts
2 x 1010
Mine refuse
5 x 109
Aerosols and, gas discharges
1 x 109

2) Industrial dusts, aerosols, and gases are now discharged into the atmosphere at about the same rate as dissolved chemicals drain from all the rivers of the world.

When the question of relative magnitude of human and natural sources of airborne chemicals is restricted to the industrial regions of the world, an even more impressive picture emerges. Galloway and Whelpdale and later Husar have estimated that human activities in North America release about 20 times more sulfur oxides and about 10 times more nitrogen oxides than are produced by all natural sources in this same continental area (NAS, 1986).

The remarkable rapidity of change in emissions of sulfur and nitrogen oxides in the northern and southern United States during the past century is shown in Figures I and 2. Note that the southern states are rapidly catching up with the heavily industrialized northeastern states in terms of total emissions (NAS, 1986). Human activities have indeed become a major force in the biogeochemical circulation of matter in the earth!

Figure 1. Changes in emissions of sulfur dioxide in the areas north and south of the Ohio River in the eastern United States between 1880 and 1980.

Figure 2. Changes in missions of nitrogen oxides in the areas north and south of the Ohio River in the eastern United States between 1880 and 1980.

The Influences of Air Pollutants on Our Natural Environment

As indicated more fully in Table 3, airborne chemicals cause eight different kinds of detrimental or beneficial effects on our society:

  • direct effects on human health;
  • indirect effects on human health;
  • damage to materials;
  • increased haze in the atmosphere;
  • acidification of lakes, streams, ground waters, and soils;
  • fumigation of crops and forests near point sources;
  • regional changes in health and productivity of forests; and
  • fertilization of crops, forests, and surface waters.

All except the last of these eight effects are detrimental to the interests of society. In fact, even the last has proven to be detrimental in the case of some surface water systems such as Lake Erie and Lake Ontario (NAS-RSC, 1985).

Table 3. Major Effects of Air Pollutants on Society

Type of Effect
Nature of Effect
Pollutants Involved
1) Effects on human health due to inhalation of airborne chemicals Pulmonary dysfunction, respiratory disease, and mental retardation (especially in children) Ozone
Sulfur dioxide
Nitrogen oxides
Particulate matter
Carbon monoxide
Toxic elements
2) Effects on human health due to atmospheric deposition or leaching and later ingestion of airborne or soilborne chemicals via drinking water, fish or other food products Diarrhea and mental retardation in children and poisoning of adults by lead, mercury, copper, cadmium, or other toxic elements Toxic elements
Acid deposition resulting from sulfur and nitrogen oxide emissions
3) Damage to engineering materials, statuary monuments, and other cultural resources Increased corrosion of metals; accelerated weathering of stone and masonry; soiling of textiles, glass, paints, and other materials; deterioration of paints, plastics, and rubber Sulfur dioxide
Nitrogen oxides
Particulate matter
4) Increased haze in the atmosphere Decreased visibility in urban and rural arm with attendant decreases in safety of air transport and enjoyment of scenic vistas from aircraft and in parks Particulate matter
Sulfur dioxide
Nitrogen oxides
Volatile organic compounds
Photochemical oxidants
5) Acidification of lakes, streams, ground waters, and soils Death and reproductive failure in fresh-water fish, decreased fertility of soils Sulfur dioxide
Nitrogen oxides
Acid deposition
6) Fumigation of crops and forests near point sources of pollutants Decreased growth and yield of crops and forests Ozone
Sulfur dioxide
Nitrogen oxides
Toxic elements
7) Regional changes in the health and productivity of forests Decreased growth, increased mortality, and predisposition of forest trees to biotic and abiotic stress factors Ozone
Nitrogen oxides (?)1
Ammonia and ammonium nitrogen (?)1
Sulfur dioxide(?)1
Acid deposition(?)1
Toxic elements (?)1
8) Fertilization of crops, forests, and surface waters Increased productivity of crops, forests, and surface waters Increased productivity of crops, forests, and surface waters

1 Question marks indicate major continuing uncertainty about the role of specific pollutants other than ozone in regional changes in the health and productivity of forests even though the involvement of air pollutants generally is widely assumed.

In examining the summary information in Table 3, please note especially that:

1) Sulfur dioxide (S02) emissions are involved in all eight effects;

2) Nitrogen oxides (NO and N02) or their photochemical derivatives are involved in seven of the eight effects;

3) Volatile organic compounds (VOC) or their photochemical derivatives such as ozone (03) are involved in five of the eight effects;

4) All three of these primary pollutants (S02, NOx, and VOC) are produced during combustion of fossil fuels in power plants, metal smelters, transportation vehicles, and ocher industrial, commercial and domestic uses of energy; and

5) SO2, NOx, and VOC are also the most important chemical precursors of photochemical oxidants, acid deposition, atmospheric haze, and certain types of particulate matter which have a wide range of detrimental effects on our society. For this reason, these three primary pollutants are major keys to the proper management of air quality in most industrial regions of the world (NAS, 1981, 1983, 1986).

Ecosystem Responses to Change in the Chemical Climate

During all the millennia that followed the first development of life on our planet, microorganisms, plants, and animals adapted their habits of growth, nutrition, and metabolism to fit within the dominant physical and chemical features of their environment. The processes of natural selection determine which organisms survive and which organisms perish. If a given organism is well adapted to its environment, it will thrive and reproduce. If it is not fit, it will not survive. When the climate changes, new pressures of natural selection are applied to the population of surviving organisms. These continuing processes of evolutionary change and adaptation have led to the development of the marvelously diverse communities of living organisms that we find everywhere over the land and in the surface waters of the earth.

These natural ecosystems, both terrestrial and aquatic, come as a gift from the evolutionary history of our planet. They are ours to use as we see fit - to manage within the sustainable productive capacity of the resource in question, or to exploit carelessly with little regard to the long term stability of the ecosystems themselves.

When human beings first appeared on the earth and the process of civilization began, our collective impact on the processes of natural selection and evolution were hardly perceptible. But as we increased in numbers and particularly after we learned to

  • harness the energy stored in fossil fuels and to
  • apply this energy to the processes of urbanization, industrialization, and intensive agriculture and forestry,
  • our collective impacts on the processes of natural selection and evolution became progressively more impressive.

Today, the aquatic and terrestrial ecosystems in certain high elevation, industrial, and urban locations in North America, Europe, Japan, China, South Africa, and some developing countries are receiving much heavier loadings of airborne nutrients, acidic, toxic, and growth-altering chemicals than were deposited in the preindustrial period in which these ecosystems evolved. In some locations, essentially all the nutrients needed to sustain some of these ecosystems are now provided from atmospheric sources (see Tables 1 and 2 and Figures I and 2). Never before in their evolutionary history have the terrestrial and aquatic ecosystems of North America and Europe been "fed from above" to the extent that they are today!

Under these new conditions of continuing change in our chemical climate, additional pressures for further adaptation of plants, animals, and microorganisms are applied and have their influences within these ecosystems. At present we have only limited experience and even less scientific evidence with which to identify regions where the rates of continuing change in the chemical climate are within the elastic limits of ecosystem resiliency and adaptability and where these rates of change will exceed those limits. Nowhere are these uncertainties more evident than in the forests of central Europe and certain high elevation forests in eastern North America.

Acute vs Chronic Exposures of Forests

As suggested by the question marks in Item 7 of Table 3, a great disparity exists in our present understanding of the detrimental effects of airborne chemicals on forests. This disparity is between:

  • Acute exposure to locally dispersed primary pollutants such as sulfur dioxide and hydrogen fluoride (Item 6 in Table 3); and
  • Chronic exposure to lower concentrations of regionally dispersed secondary pollutants such as ozone and acid deposition (Item 7 in Table 3).

Steep gradients around point sources of primary pollutants have made it fairly easy to establish a strong correlation between pollutant concentration and visible symptoms of damage and/or death of vegetation. When this strong correlative evidence has been coupled with the results of a few controlled exposure tests, little scientific uncertainty usually remained about:

  • what species of plants are susceptible,
  • the source of the injurious substance(s),
  • their chemical nature, or
  • the concentrations of chemicals that are injurious.

Thus, sensible air-quality and forest management recommendations could be formulated on the basis of straight-forward relationships between the concentration and time of pollutant exposure (dose) and the change in health or productivity of the forest (response). Only at the fringes of the area affected was there usually much uncertainty about cause-and-effect and dose-response relationships or about the complicating role of competition, drought, frost, biotic pathogens, or other natural stress factors.

Cause-and-effect relationships have been much more difficult, to establish when chronic exposure to regionally dispersed secondary pollutants rather than locally dispersed primary pollutants are involved. There are several reasons for this:

  1. visible symptoms may be subtle or lacking;
  2. concentrations of regionally dispersed airborne chemicals are often highly variable;
  3. exposure to two or more chemicals may occur simultaneously or sequentially and they may act additively, synergistically, or antagonistically; and,
  4. rigorous scientific methods must be employed to distinguish the effects of regionally dispersed airborne chemicals from those of natural stress factors which may act as predisposing, inducing, or contributing causal factors (McLaughlin et al, 1985).

Regional Changes in Forest Health and Productivity

Although a total of 18 regional changes in forest health and' productivity have been observed in Europe and North America during the past several decades (Cowling, 1985), regionally dispersed airborne chemicals are considered to be a probable or possible causal factor in only 5 of them. These five are described below in order of decreasing quality of evidence that airborne chemicals have played a crucial role.

1) Ozone Damage to White Pine in the Eastern United States. - The most rigorous scientific evidence for a regional forest health problem resulting from regionally dispersed pollutants is the case of eastern white pine damaged by ozone within much of its natural range in the eastern United States and Canada. In this case:

a) Visible symptoms of damage are correlated with measured geographical gradients in mean ozone concentration or with periodic episodes of high peak concentrations;

b) The major visible symptoms of damage have been duplicated in controlled exposures which are similar to the exposures occurring in forests; and

c) Observed variation in susceptibility to damage between individual trees in the forest has been duplicated in controlled exposures with clonal lines of eastern white pine.

2) Ozone Damage to Mixed Conifer Forests in the San Bernardino Mountains of California. - The evidence in this case is slightly less rigorous than that for eastern white pine:

a) the frequency of damage to trees is well correlated with geographical gradients in ozone concentrations and with exposure to air masses arriving from Los Angeles - the principal source of precursor nitrogen oxides and volatile organic carbon compounds from which ozone is formed; and

b) the major damage symptoms have been reproduced in controlled exposures to ozone. In contrast to the eastern white pine case, death of severely affected trees usually is induced by bark beetles and root-rotting fungi which preferentially attack the ozone-damaged trees.

3) Waldsterben in Central Europe. - In this case, essentially all native and introduced commercial softwood and hardwood tree species are affected - though not to the same degree nor in exactly the same way. At least 10 tree species grown under a wide range of climatic, soil, and site conditions have shown marked decreases in rate of growth over the past 5 to 30 years. During the past 5-8 years, they also have shown various visible symptoms of damage including:

a) premature loss of older needles on conifers beginning with the innermost parts of the crown;
b) chlorosis in needles;
c) atypical branching habit;
d) smaller and more irregularly shaped leaves;
f) decreased radial growth; and
g) decreased abundance of feeder roots and mycorrhizae
(Schutt and Cowling, 1985).

Damaged trees are generally distributed at random mainly in forests over 60 years of age.

Several of these symptoms have never been seen before. The cause(s) are unknown. Less then 5 percent of the damaged trees have symptoms which can be attributed to insects or disease. Some trees in regions of high sulfur dioxide and ozone concentrations 0 show symptoms that are typical of damage by these pollutants; some trees on soils with known nutrient deficiencies have shown foliar symptoms typical of these deficiencies; but most damaged trees have no identifiable causes.

Airborne chemicals are suspected mainly because no other more plausible explanation has been advanced that can account for the wide variety of symptoms on so many different species of trees and types of forests, growing over such large geographical areas, under so many different soil, site, and elevational conditions.

In the West German province of Baden-Württemberg, a survey of forest damage indicated that the frequency and severity of symptoms was greatest on trees with the greatest exposure to moving air masses. This was shown with regard to predominant wind direction, altitude, position within a stand (edge vs. center, position within the canopy (dominant vs. nondominant trees), type of forest (pure softwood vs. mixed softwood and hardwood), etc. Also, no known fungi or insects, physical climate, or soil-chemical factors were well correlated with damage.

The only direct experimental evidence suggesting which particular airborne chemicals might be inducing some of these symptoms was developed with Norway spruce seedlings. A combination of ozone and acid mist treatments in controlled exposure chambers reproduced the magnesium deficiency symptoms observed in high elevation spruce forests (See Prinz in McLaughlin et al, 1985).

4) High Elevation Spruce-Fir Forests in the Appalachian Mountains. - Also entirely circumstantial is the evidence suggesting that airborne chemicals might be involved with the health problems of red spruce and balsam and Fraser firs in the Appalachian Mountains of the eastern United States. The major symptoms of damage on red spruce include dieback of terminal leaders and branches, premature loss of needles, unexplained decreases in diameter growth beginning around 1960, and loss of feeder-root biomass. Unexplained mortality in high elevation red spruce stands has also been reported (Johnson and Siccama, 1983; Bruck, 1986).

The frequency and severity of these symptoms increases with altitude and thus could be associated with known altitudinal gradients in: a) temperature; b) high winds; c) acidity of precipitation, d) concentrations of ozone and other known pollutants; e) timing of exposure to nutrient-rich and acidic cloud water; and f) accumulation of lead and other toxic metals in forest floor materials. Few, if any, controlled exposure tests have been completed with either red spruce or Fraser fir.

5) Three Cases of Decreased Growth Without Visible Symptoms. - The recent forest health problems with the least scientific evidence suggesting a link with air pollution include three cases of decreased diameter growth in the absence of other visible symptoms. These three cases include:

a) shortleaf and pitch pines in the Pine Barrens region of New Jersey;
b) low elevation red spruce forests in New Hampshire, Vermont, New York, and Maine; and
c) some naturally regenerated forests of southern pine in Virginia, North and South Carolina, Georgia, and Alabama.

A number of plausible hypotheses involving natural stress factors have been suggested to explain these problems, e.g., increased frequency of droughts, aging of tree populations, increased hardwood competition, and loss of "old field" conditions (decreased fertility of soils after reversion from agriculture). Although it is possible that airborne chemicals may have adversely altered the ability of these trees to withstand natural stress factors, experimental studies to test this idea in a scientifically rigorous way have only recently been initiated.

Conclusions Regarding Effects on Forests

A great deal is known about the effects of sulfur dioxide and hydrogen fluoride on forests in the vicinity of strong point sources of these pollutants. By comparison, however, very little is known at present about the possibility that regionally dispersed airborne chemicals might be involved in the last three of the five regional changes in forest health and productivity discussed above. Ozone is the only airborne chemical which so far has been rigorously proven to cause regional effects on forests (cases I and 2, above). On the basis of general knowledge of the responses of forests to stress, some circumstantial evidence, and a very few controlled exposure tests, however, a consensus of informed judgment is developing which suggests that the following airborne chemicals may be involved. These five airborne chemicals are listed below in order of decreasing probable importance; the detailed rationale for this ranking is summarized elsewhere (Cowling, 1985):

  • ozone;
  • excess nutrient substances - especially greater-than-normal atmospheric deposition of biologically available nitrogen compounds including nitrate and ammonium nitrogen and ammonia and nitric acid vapors;
  • other phytotoxic gases including sulfur dioxide, nitrogen oxides, hydrogen fluoride, and peroxyacetyl nitrate;
  • acidic or acidifying substances including sulfate, nitrate, chloride, ammonia vapor, and ammonium ion;
  • toxic metals such as lead, cadmium, mercury, and zinc;
  • growth-altering organic substances such as ethlyene, aniline, and dinitrophenols.

The Challenge for Every Industrial Society

Acid deposition in particular and air pollution in general have become major environmental issues in both Europe and North America during the past two decades. Much has been learned already but much more remains to be learned about various aspects of these twin problems.

The challenge for many of us in science on both continents is to satisfy the public need for additional information by developing more comprehensive understanding of the atmospheric processes, soils transformations, changes in water quality, effects on materials, and the physiological and ecological influences of acid deposition and air pollution on forests. Among all these areas, regional changes in forests is the area of greatest current concern. That is why an important part of this lecture has been devoted to analysis of the present state of scientific knowledge about regional air quality and its possible influences on the health and productivity of forests.

The challenge for industrial and political leaders in North America and in Europe continues as always - to make decisions about complex issues under conditions of substantial uncertainty. The uncertainties involved in management of air pollution and acid deposition are analogous with those that surrounded the debates about the role of phosphorous in the eutrophication of Lake Ontario and Lake Erie in the early 1970's:

The experts then could not agree. Some said a 40% decrease in phosphorus loading would be needed. Others said 75% Still others said nitrogen was to blame, not phosphorous

Environmentalists warned of "possible irreversible harm." Industry said more research was needed. Finally, a political decision was reached - a plan should be developed. More debates were held. A theoretical model was wed to predict that a 50% decrease in phosphorus loading might be sufficient.

A management plan was finally implemented Linder conditions of continuing uncertainty. After some time the lakes began to improve. The theoretical estimate proved to be too low, but with some further adjustments the plan worked and the lakes am now on the road to recovery (NAS-RSC, 1985).

My friend, Chris Bernabo, former leader of the National Acid Precipitation Assessment Program in the United States, and others (Freeman, 1983) have discussed the matters of scientific uncertainty in public decision making at recent conferences on the effects; of air pollutants. In essence, they believe that the degree of scientific uncertainty which can be accepted in making a political, decision is an inverse function of the degree of public consensus, about the issue itself If a strong public consensus already exists about a given course of action, substantial uncertainty can be tol- erated in choosing a course for political action. On the other hand, if there is only a weak public consensus about what ought to be', done, a very high degree of scientific certainty will be required, before any particular course of action will be acceptable politically. .

The information summarized in Tables 1 and 3 shows that, there is a substantial base of scientific knowledge about the specific airborne chemicals involved in each of the eight major effects of air pollutants in our society. What irony there is in the fact that the,, area of greatest current public concern about air pollution (effects on forests) is the very same area in which we have the largest degree of continuing scientific uncertainty!

The Scientific Foundation For Public and Private Decision Making

During the past 25 years, scientific understanding about air-` borne chemicals and their effects on our natural environment has increased enormously. At present, we know a great deal about:

  • the chemical nature of primary air pollutants and their transformation products;
  • the meteorological and climatological processes by which they are dispersed;
  • the mechanisms by which pollutants are transferred from the atmosphere and taken up by plants, animals, and microorganisms;
  • the chemical, physical, and biological mechanisms of action on human health, materials, atmospheric haze, agricultural crops, aquatic ecosystems, and certain types of forest trees; and
  • the industrial process modifications and control technologies by which emissions of air pollutants can be managed within limits acceptable for society after thorough review of industrial standards of performance and government regulations.

In the numbered paragraphs that follow, I have listed 12 principles regarding air pollution and its control. These principles were developed in an attempt to summarize some important features of our present knowledge about air pollution and its control. They have been carefully reviewed by both atmospheric scientists and biologists, by colleagues in industry, and by officials in federal and state regulatory organizations. These principles provide a part of the foundation for further public and scientific discussions about the phenomena, effects and management of air quality in industrial societies:

1) Combustion of fossil fuels is the single most important source of air pollutants (NAS, 1981, 1986).

2) The concentration and deposition of primary air pollutants decrease progressively with increasing distance and time after emission from any particular emission source (Altshuller and Linthurst, 1983).

3) The concentration and deposition of secondary pollutants is a complex function of meteorological, seasonal, altitudinal, temporal, geographical, and other factors (NAS, 1983).

4) The elapsed time between emission and deposition varies with the pollutant in question but ordinarily ranges from a few minutes or hours to a maximum of 4 to 5 days (NAS, 1983).

5) This time is sufficient for dispersal over both short distances (0-500 Km) and long distances (>500 Kin) (NAS, 1983, 1986).

6) No state or nation can control the quality of air within its own borders without cooperation by other nearby states or nations (NAS 1983, 1986).

7) The average zone of influence of any particular emission source extends in all directions from the source, and is roughly symmetrical - with the center of the deposition field generally displaced from the source by a distance of only a few tens or perhaps hundreds of Km. Thus, air pollutants generally have their greatest effects within a few hundred Kin from their source of emission (Bolin et al, 1972; NAS, 1983, 1986).

8) The atmosphere over most industrial regions (especially in Europe and North America) is very well mixed; also the distance between most sources of air pollution is much smaller than the average distance of dispersal from any given source. Thus, pollutants rarely, if ever, occur alone; in addition, they frequently react with each other leading to new chemical transformation products. They also can interact so as to produce combined effects which are additive, synergistic, or antagonistic (NAS, 1981, 1986; Ministry of Agriculture, 1982).

9) For many different combinations of air pollutants and specific biological effects, there is no distinct "threshold dose" or "safe concentration" below which we are certain there will be no adverse effects; thus, any important decrease in emissions is very likely to result in decreased adverse effects (NAS, 1981); Ministry of Agriculture, 1982).

10) If emissions of primary pollutants are decreased (or increased) by a significant amount in an area of several hundred square Km, the resulting average decrease (or increase) in air concentrations and/or deposition of pollutants will be roughly proportional to the magnitude of change in emissions (NAS, 1983).

11) Significant decreases in air emissions of S02, NOx, CO, and probably VOC as well, are very likely to have significant and simultaneous beneficial effects on human health, visibility, materials damage, surface water quality, and both crop and forest productivity (OTA, 1981; Ministry of Agriculture, 1982; NAS, 1986).

12) If a decision were reached to further decrease air emissions of S02, NO., and VOC, it is very likely to take at least five years to develop specific industrial, state, provincial, regional, national, or international implementation plans. Further, at least another five years will be required to design, build, and install the pollution-control machinery and management systems that will be necessary to achieve the decrease in emissions that is planned (OTA, 1981).

For all the above reasons, it seems prudent that the United States and Canada, and various countries in Europe, earnestly continue their historical air-quality planning and evaluation procedures. In doing so, leaders of industry and government should recognize that:

1) Research is continuing and will continue to increase scientific and public understanding of air pollution and its direct and indirect effects on our society;

2) An integrated (multiple pollutant and regional) program for management of air quality would have many advantages over the present single-pollutant and state-by-state and province-by-province methods of implementation;

3) Cooperation, consultation, and statesmanship are likely to produce more economically and scientifically sound management than legislatively mandated solutions; and

4) It is likely to take at least 10 years to implement a decision to make a significant change in the air quality in any large part of the United States, Canada, or Europe.


The health and welfare of our society is inextricably bound up with the health and welfare of the ecosystems on which our life depends. Human activities are changing the chemical climate of the industrial regions of the world. These changes in our chemical climate are having important influences on human health, the stability* of the atmosphere, the acidity and biological functions of aquatic ecosystems, and the health and productivity of crops and forests. For all of these reasons it is important that the informed citizens of every industrial democracy have some awareness about:

  • pollutants in the air and acids in the rain,
  • the effects of these substances on our natural environment, and
  • the challenge they present to our society.

A selected list of publications follows to facilitate this continuing self-education.
*of engineering materials and cultural resources, the haziness



Selected Publications on Airborne Chemicals and Their Effects on Our Natural Environment

Altshuller, A. P., and R. A. Linthurst, eds. 1983. The acidic deposition phenomenon and its effects: Critical assessment review papers. Volume I - Atmospheric Sciences. Volume II - Effects Sciences. Office of Research and Development, Environmental Protection Agency, Washington DC

Bennett, D. A., R. L. Goble, and R. A. Linthurst. 1985. The acidic deposition phenomenon and its effects: Critical assessment document. EPA/600/8-85/001. Environmental Protection Agency, Washington DC. 159 pp.

Bolin, B. et at. 1972. Sweden's Case Study for the United Nations Conference on the Human Environment: Air Pollution Across National Boundaries. The impact on the environment of sulfur in air and precipitation. Norste it and Sons, Stockholm, Sweden. 97 pp.

Bruck, R. 1. 1986. The forest decline enigma - it is air pollution related? Plant Disease 70:ln Press.

Carson, R. 1962. Silent Spring. Houghton Mifflin, Boston, Massachusetts. 368 pp

Comptroller General. 1984. An analysis of issues concerning "acid rain". Report to the Congress of the United States. General Accounting Office. Washington DC. 185 pp.

Cowling, E. B. 1982. Acid precipitation in historical perspective. Env. Sci. & Tech. 16:11OA-123A.

Cowling, E. B. 1985. Comparison of regional declines of forests in Europe and North America: A possible role of airborne chemicals. pp. 217-234. In Air Pollutants: Effects on Forest Ecosystems. Acid Rain Foundation, St. Paul, Minn.

Drablos, D., and A. Tollan, eds. 1980. Ecological impact of acid precipitation. SNSF Project, Norwegian Forest Research Institute, As, Norway 383 pp.

Freeman, G. C. 1983. The politics of acid rain. Technical Advisory Committee, Virginia State Air Pollution Control Board. Virginia Beach, Virginia.

Johnson, A. H., and T. G. Siccama. 1983, Acid deposition and forest decline. Env. Sci. & Tech. 17:294A-305A.

Kovda, V. A. 1975. Biogeochemical cycles in nature, their disturbance and study (In Russian). Nauka Publishing House, Moscow, USSR.

Kozlowski, T. T., and H. A. Constantinidou. 1986. Responses of woody plants to environmental pollution. Part 1. Sources and types of pollutants and plant responses. For. Abstr. 47(l):1-51; Environmental pollution and tree growth. Part 11. Factors affecting responses to pollution and alleviation of pollutant effects. For. Abstr. 47(2):105-132.

McLaughlin, S. B., B. Prinz, W. H. Smith, E. B. Cowling, and F. D. Manion. 1985. Effects of air pollutants on forests: A critical review. J. Air Poll. Cont. Assn. 35:511-533; 913-924,

Ministry of Agriculture. 1982. Reports and Background Papers for the 1982 Stockholm Conference on Acidification of the Environment: Acidification today and tomorrow, 231 pp; Proceedings, 127 pp; Ecological effects of acid precipitation, 340 pp; Strategies and methods to control emissions of sulphur and nitrogen oxides, 192 pp; Acidification: A boundless threat to our environment, 40 pp. Ministry of Agriculture, Stockholm, Sweden.

Office of Technology Assessment. 1981. Acid rain and transported air pollutants: Implications for public policy. Office of Technology Assessment, United States Congress, Washington DC. 323 pp.

National Academy of Sciences. 1981. Atmosphere-biosphere interactions: Toward a better understanding of the consequences of fossil fuel combustion. Narioml Academy Press, Washington DC. 263 pp.

National Academy of Sciences. 1983. Acid deposition: atmospheric processes in eastern North America. National Academy Press, Washington DC. 373 pp.

National Academy of Sciences. 1985. Acid deposition: Effects on geochemical cycling and biological availability of trace elements. National Academy Press, Washington DC. 83 pp.

National Academy of Sciences. 1986. Acid deposition: Long-term trends. National Academy Press, Washington DC. 506 pp.

National Academy of Sciences - Royal Society of Canada. 1985. The great lakes water quality agreement: An evolving instrument for ecosystem management. National Academy Press, Washington DC. 224 pp.

Schutt, P., and E. B. Cowling. 1985 Waldsterben, a general decline of forests in central Europe: Symptoms, development, and possible causes. Plant Disease 69:548-585.

Smith, R. A. 1872. Air and Rain: The Beginnings of a Chemical Climatology. Longmans-Green, London. 600 pp.

Turk, J. T. 1983. An evaluation of trends in the acidity of precipitation and the related acidification of surface water in North America. U. S. Geological Survey Water Supply Paper 2249. Superintendent of Documents, Washington DC. IS pp.



Introducing: Ellis B. Cowling

Ellis B. Cowling, the 25th Albright Lecturer, is an advocate for greater use of scientific understanding in the conservation, management, enjoyment, and wise use of natural resources,

His adventures in science have included studies on: the enzymology and biotechnology of cellulose and lignin in wood; the development of disease-management systems in forests; the conservation of essential elements by forest trees; and the impact of change in the chemical climate on the health and productivity of ecosystems.

Dr. Cowling was born in Waukeegan, Illinois in 1932 - the son of a minister and a church musician. After completing B.S. and M.S. degrees at the State University College of Forestry at Syracuse University, he earned a Ph.D. degree in plant pathology at the University of Wisconsin. In 1970, he completed a second Ph.D. degree in physiological botany at the University of Uppsala in Sweden.

Dr. Cowling spent a postdoctoral year, 1959-60, in Sweden as a US Public Health Service Fellow conducting research on the enzymatic degradation of cellulose. During that year he and Goran Pettersson discovered and characterized the smallest known enzyme protein - a cellulose in the wood-destroying fungus, Polyporus versicolor.

In 1970-71 he spent a sabbatical year as a visiting professor at the University of Uppsala. During that year he was inspired by Svante Wen, Carl Olaf Tamm, Erik Eriksson, and Bert Bolin to invest the next 15 years of his career in research on the impact of airborne chemicals on ecosystems in Europe and North America.

In 1983-85 he collaborated with Peter Schutt of the University of Munchen in publishing a comprehensive description of the unprecedented multiple-species decline of forests that developed in central Europe beginning in the late 1970s. This publication led to an improved understanding of one of the most important probable effects of airborne chemicals in forests.

Most of his 25 years at Yale and North Carolina State Universities were devoted to research together with 63 graduate and postdoctoral students. They are now engaged as scientists, administrators, and advisory-service agents in 14 countries.

Since 1978, Dr. Cowling has served the School of Forest Resources at North Carolina State University as Associate Dean for Research. Both as Professor and as Associate Dean he participated in a series of institutional development and advisory service efforts together with many faculty and student colleagues:

  • Design of the Fusiform Rust Resistance Testing Center now maintained by the U.S. Forest Service at Asheville, North Carolina.
  • Development of the National Atmospheric Deposition Program (NADP). This Interregional Research program (IR 7) now involves more than 200 scientists in the United States and Canada.
  • Design and organization of the National Acid Precipitation Assessment Program (NAPAP). This ten-year multi-agency program of research was initiated by President Carter in 1979 and funded by the Congress under Public Law 96-294. It currently involves more than 500 scientists throughout the United States.
  • Leadership of the Triacademy Committee on Acid Precipitation for the National Academy of Sciences in the United States, the Royal Society in Canada, and the Academy of Sciences in Mexico.
  • Development of the Acid Deposition Program at North Carolina State University under a series of cooperative agreements with the U.S. Environmental Protection Agency, the U.S. Forest Service, and other organizations.
  • Leadership of a multidisciplinary Task Force on Basic Research needs in Forestry and Renewable Natural Resources. This effort led to the creation of an $8,000,000-per-year competitive grants program in forestry and renewable natural resources.
  • Development of the Natural Resources Research Center at North Carolina State University.
  • Development of the Acid Rain Foundation-a public foundation devoted to education, public awareness, and research on acid deposition and air pollution in the United States.
  • Testimony on the "acid rain" issue before the National Commission on Air Quality and various Committees of the United States Senate and the House of Representatives.

These activities led to Dr. Cowling's election as a member of the National Academy of Sciences in the United States, and the International Academy of Wood Science in Vienna. He was also elected as a Fellow of the American Phytopathological Society. He was named "Adventurer in Agricultural Research" by the International Congress of Plant Protection, and as "Air Conservationist of the Year" by the Governor of North Carolina. He also received the North Carolina Award for Achievement in Science and the 0. Max Gardner Award for contributions to the "welfare of the human race" by the Consolidated University of North Carolina.