Air Pollutants

Air Pollutants and their Sources

The most important pollutants in causing tree disease are ozone, sulfur dioxide, and hydrogen fluoride.

O₃ (Ozone)

The visible pollution over and downwind from urban areas is photochemical smog.   It typically contains the secondary pollutants O₃ and PAN and the associated primary pollutants.

Ozone is a secondary pollutant formed from several primary pollutants under the influence of ultraviolet light.  Natural sources include  lightning and the stratosphere. The stratosphere is a separate pool of ozone that is beneficial because, ironically, it actually protects us from ultraviolet light!  Ozone is an oxidant.

Several pollutants are involved in the formation of ozone.  Don’t worry about the full equations, but you should know what pollutants are involved in making O₃.

One of the precursors of ozone is NO_x (nitrogen oxides), including NO and NO₂.  NO_x are produced during any kind of combustion under heat and pressure. The N comes as N₂ from the air, which is needed of course to provide O₂ for combustion. So automobiles, factories, electrical generating plants, etc. all produce NO_x. NO_x can also come from the soil. NO_x themselves are not very toxic, but they react in the atmosphere to make toxins, especially ozone.

Volatile organic compounds (VOCs) are light hydrocarbons, molecules of hydrogen and carbon, like CH₃CH₂CH₂– etc. There may be some double bonds, some oxygen here and there, etc. These are released during oil and gas extraction and from incomplete burning of fuels during combustion. If someone’s car needs a tuneup badly, it smells and puts out visible smoke. Much of that is hydrocarbons.  Forests also produce volatile hydrocarbons.

The reactions in the atmosphere work something like this.  They all involve the power of ultraviolet light (UV).

1. A VOC is hit with UV and becomes radicalized.
2. It is then able to oxidize NO to NO₂.
3. NO₂ and O₂ react to form O₃ and NO.
4. That begins the recycling.  The same atom of N that came out of the tailpipe is ready to be converted again to NO₂, creating more O₃, and it can be recycled like this indefinitely.  The radical VOC can also go back again, on average 5 times, before it is too short to be photochemically reactive.  The point is, a limited amount of pollution can generate a lot of O₃.

But it gets worse.  While the NO₂ is pretty reactive and doesn’t get far away from the urban area where concentrations are high, it can get stabilized in peroxyacetyl nitrates, or PANs.  They are formed from action of UV light on the VOC radicals, NO₂, and O₂ in smog. An example is CH₃COOONO₂, which is peroxyacetyl nitrate itself.

PANs are important, not as pathogens themselves, but because they dissociate very slowly in the atmosphere to release their N as NO₂, which can quickly react to form O₃.  PANs are smog time-bombs with a long, slow fuse.  They transport the problem far from urban and industrial areas, creating ozone far downwind from the source of the primary pollutants.

SO₂ (Sulfur Dioxide)

SO₂ is produced during combustion of fuel that contains sulfur impurities. This is mostly coal, but also oil. That is why there is a distinction between high-sulfur and low-sulfur coal. Many heavy industries release large amounts of SO₂, such as electrical plants and smelters.

A big coal-burning power plant, with two generators, can burn 650 tons of coal per hour at each generator. If that is 2% sulfur, that can generate 65 tons of SO₂ per hour, also NO_x.

Another secondary pollutant in a sense is acid deposition. Primarily SO₂, but also NO_x combine with water and oxidize to form various acids, primarily sulfuric and nitric acids. Although pure water has pH 7, water equilibrated with unpolluted air has pH 5.6 (CO₂ dissolves, making carbonic acid). Rain more acidic than that is considered to be “acid rain.” Rain in many parts of the world is pH 4-4.5. Rain with pH below 2 has been recorded (more acidic than lemon juice). Actually it gets deposited in forests in forms other than rain, so the immission is often called “acid deposition.”

Fluorine

Several flourine compounds are produced during smelting (high temperature treatment of ore to purify minerals), the production of aluminum, phosphate fertilizer, glass, bricks, steel and chemicals, and burning of coal.  The most common form is HF.  HF is highly reactive, so damage is usually close to a point source.

Air Pollutant Effects on Plants

We could try to distinguish between injury and disease, injury being damage from a brief exposure at a particular point in time; disease being damage resulting from exposure over a longer period. That distinction is pointless in this case, so we won’t worry about it.

There can be a range of symptoms ​[20, 22]​, depending on concentrations, length of exposure, physiological condition during exposure, temperature, etc.  There may be more or less obvious symptoms, usually on the leaves, or only growth loss or other subtle symptoms that may be difficult to detect, much less diagnose.

A note on concentrations. In the U.S., it is the practice to express pollutant concentrations in ppm or ppb (parts per million or billion). This drives scientists in other parts of the world nuts, and they are right. The units refer to parts of volume, although it is not clear and could just as easily be weight, which is much different.

Some case studies

Smelters and SO₂ – Sudbury

Sudbury (Ontario, Canada) has a long history of smelting that made it one of the worlds largest sources of SO₂ and wreaked ecological havoc over a large area ​[7, 10, 23]​.  At its peak in 1960, three large nickel and copper smelters released 2.6 Tg (about 2.9 million US short tons) SO₂ into the atmosphere.

The result was 20,000 ha nearly completely barren, 80,000 semi-barren, and about 7,000 acid-damaged lakes.  Barren soils were often eroded to bedrock on hills.  White pine is particularly sensitive and was lost over about 180,000 ha.  Soils within about 12 km are toxic due to acidity combined with heavy concentrations of aluminum, copper and nickel.

One of the smelters closed in 1972 and the other received what was then the world’s largest smokestack – 380 m high. This has been done in many areas, so we don’t get the acute damage as often. This is a classic case that led to the phrase, “Dilution is the solution to pollution.”  The situation was further improved in the 1990s when technological changes were made to reduce emissions. Elevated nickel and copper remains in the soil. Another example is Copper Basin in Tennessee, and various smelters, generating plants, etc. in the West.

Erzgebirge on the border of Czechoslovakia and eastern Germany – SO₂

The former “Black Triangle” in the western Czech Republic is the Sudbury of Europe ​[8, 14, 19]​.  Beginning in the 1950s, when eastern Europe was heavily influenced by the Soviet Union, the area (then Czechoslovakia) was heavily industrialized.  Coal-burning industries were concentrated east of the Ore Mountains (German: Erzgebirge) of the western Bohemia region. The heavy pall of coal dust led to the name “Black Triangle.”

By the late 1980s, when the Berlin wall came down and the Soviet bloc began to disintegrate, there was scant vegetation in the vicinity of the industrial complexes, and the spruce-fir forests on the eastern slopes of the mountains had very heavy mortality.  Surviving trees had heavy branch dieback.  The United Nations Environment Programme (UNEP) officially designated the area as an “ecological disaster zone.”

Many of the most heavily affected stands were salvaged.  Beginning in the 1990s, SO₂ and NO_x emissions in the Czech Republic were dramatically reduced.  Survivors are now showing a growth increase.  This increase has been associated with decreased pollution, but also a warmer climate.

Waldsterben and Northeastern U.S. Forest Decline

As areas of severe damage began to appear in eastern Europe in the 1970s into the 1980s, clearly due to SO₂ and NO_x, forest scientists in western Europe began to question whether there was more subtle or unattributed damage farther from the point sources.  Frankly, there was some less-than-stellar science as people competed for the most dramatic findings.  Much like people see an image of Jesus Christ in a piece of toast, people began seeing the fingerprints of acid rain in all sorts of natural phenomena: the variation in branching habit of Norway spruce, variation in structure of the root system, variation in crown density, and of course branch dieback and mortality.  It didn’t help that there was not widespread appreciation of the role of diseases in natural forests, because some of them were attributed to acid rain as well.

The problem became known as Waldsterben, German for “forest dying.”  As the social-science phenomenon reached a fever pitch, it spread to eastern North America.  Scientists began to blame dead or sick-looking trees from the southern Appalachians into New England on acid rain.  Large old red spruce died in large numbers in the mountains of New Hampshire, and it was attributed to acid rain in numerous scientific papers.  Hardly anyone noticed (or had the knowledge to recognize) that there was a spruce beetle outbreak killing the trees.  A more severe spruce beetle outbreak around and before 1900 killed many more spruce over a wider area, well before substantial pollution ​[1, 11]​.

Don’t get me wrong, there was acid deposition.  There was evidence that it affected certain types of sensitive subalpine lakes, and that it predisposed trees to winter injury at high elevations ​[5, 21]​, but no real evidence that it otherwise affected forests.

Why do I tell this story then?  Because people who are learning science should know that scientists are fallible.  They are susceptible to the same kind of hysteria and group-think that other people are.  Forest ecologists should take courses in forest pathology and entomology before they publish papers saying what’s killing trees.  Always look beyond the conclusions and abstract to find the real evidence.

San Bernardino Mountains – O₃

The most infamous area of ozone damage in North America is the San Gabriel and San Bernardino Mountains about 50 miles east of Los Angeles ​[2, 4, 9, 12, 13, 17, 18]​. The mountains present a barrier to the eastward movement of air, and inversions are frequent. In ponderosa pine, ozone causes chlorotic mottling, stunted needles, needle retention reduced to 2 years, and growth loss. Ozone also increases susceptibility to bark beetles and annosus root disease.  Growth impact on ponderosa pine is dramatic:

• low pollution, 1910-1940 – 0.5 cm/yr radial growth
• high pollution, 1941-1971 – 0.3 cm/yr

Forests there are changing as a result.  There is a reduction in pine.  So far there does not seem to be a decrease in ozone levels.

Conclusion

This is a complex subject, and one should not rush to judgement. Some people have their minds made up one way or the other about whether pollutants are causing forest damage in region-wide situations where there are no symptoms. If you develop an opinion on the question, I advise you to do it based on an independent evaluation of the facts. Even in the hallowed halls of academia, there are those that will try to push their opinions on you. Study the issue, find your own facts, and think for yourself.

More general information is available on abiotic diseases and injury.