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 NOx (nitrogen oxides), including NO and NO2.  NOx 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 NOx. NOx can also come from the soil. NOx 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 NOx.

Another secondary pollutant in a sense is acid deposition. Primarily SO2, but also NOx 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.”


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.


Hardwoods: mottling, flecking, stippling. Color can be purplish or white. Can coalesce, leading to bleaching of portions of the leaf. Conifers: chlorotic mottling (which may progress to needle banding and tip necrosis), stunted needles, needle retention reduced to 2 years, and growth loss ​[2, 4, 9, 17]​. Ozone also increases susceptibility to bark beetles and annosus root disease ​[12, 18]​.  In general, ozone can be expected to promote necrotrophic pathogens and reduce obligate biotrophic pathogens ​[15]​.

Symptoms of oxidant injury are not uncommon, especially on understory plants and especially late in the summer.  Although we cannot know for sure, it is thought that natural levels may be about 10-20 ppb. It is pretty common to see levels of 40 ppb in many parts of the country. It is thought that 55-85 ppb is about the threshold for forest growth impact. 80 ppb causes visible symptoms on eastern white pine (or less, depending on exposure time). Levels up to 600 ppb have been recorded in the San Bernardino Mountains of California!

White ash, aspen, ponderosa pine and eastern white pine are sensitive .


Hardwoods: interveinal chlorosis and necrosis, growth loss.  Even at sub-symptomatic levels, N and S deposition and associated acidification lead to elevated foliar Al, Fe, Mn, Zn, N:P ratio, and reduced Ca and photosynthetic capacity ​[3, 6]​. Conifers: chlorotic spots and bands, brown tips, growth loss. Generally, conifers are most sensitive to SO2.  “Clean” air probably has less than 1 ppb. On Whiteface Mtn. in New York, 1-3 ppb are usually recorded. EPA Air Quality Standard, average annual maximum is 31 ppb. Levels in acute problem areas, close to the source, are 500+ ppb.

SO2 and the associated acidic deposition predispose trees to various other diseases and injuries ​[5, 16, 21, 24]​.  Winter injury is worse when trees are exposed to acid deposition.


Hardwoods: marginal chlorosis –> necrosis. Conifers: tip burn/necrosis.  Various aluminum plants and other chemical plants that release flourides have severe mortality and damage around them. In North America, most are in the West. Areas of severe mortality are as large as 2000 acres, and damage can be up to 50 sq. miles. Flouride analyses of leaves support the conclusion that flouride is the responsible pollutant.

Some case studies

Smelters and SO2 – Sudbury

Sunset skyline of Sudbury, Ontario, Canada, with the Inco Superstack seen across Ramsey Lake. GNU Free Documentation License, Version 1.2

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

Location of the Erzgebirge / Ore Mountains forming a natural border between Saxony, Germany and Bohemia, Czech Republic.

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 NOx 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 NOx, 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.


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.

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