When it became obvious over fifty years ago that inversions and high spikes downwind of polluting sources were killing vegetation and sickening people, industries very cleverly learned to disburse their emissions.  They built tall stacks and regulations restricted the most visible auto emissions, thus reducing the smoky elements of smog.  Localized, isolated extreme peaks of ozone concentration were reduced.  But because the precursors travel across continents and oceans, over decades the global background concentration has been inexorably rising - damaging trees everywhere on earth at a rapidly accelerating rate. Complicating matters is the unregulated rise in emissions of reactive nitrogen, from burning fuel and agriculture, and methane from melting permafrost and leaks from natural gas extraction processes and storage.

The following is excerpted from a book called "Air Pollution and Plant Life" edited by J.N.B. Bell of the Imperial College of Science, Technology and Medicine, University of London.  It's a wonderful text, which begins with the history of our understanding of the effects of air pollution.  I can only reproduce a small fraction of the wealth of facts presented in the book, so I won't even include the issues of sulfur, acid rain, and myriad lesser pollutants, but will stick to ozone, which is a far greater and more lasting problem for the world's trees and plants:

2.  Historical perspectives - p. 5

The quiet, muggy fall air clung over the Los Angeles Basin in California that historic day in 1945.  The County Agent was puzzled.  Plant pathologists from the University of California Citrus Experiment Sate in Riverside were called out.  Never before had they seen the silvery bronze glaze that appeared on the leaves of vegetable crops throughout the Basin.  The leafy crops, especially near the oil refineries in the Dominguez area were ruined.  The glazing, silvering and bronzing, sometimes coupled with necrosis of the lower leaf surfaces, appeared on such species as endive, lettuce, spinach and romaine.

A decade later, on the eastern seaboard of the United States, another mysterious syndrome appeared.  First observed on the tobacco leaves grown for cigar-wrappers, the characteristic, straw-coloured flecks and lesions rendered the leaves totally unsuited for cigars.  The disease characteristically appeared following periods of high humidity and termperature inversions common in the summer and fall months.  Similar symptoms subsequently appeared on numerous other crops both in the east and in California.  Pathologists suspected that weather was somehow involved, but the specific cause remained a mystery.  The disease was called weather fleck.

Because of the diversity of crops that were affected, and the absence of known biotic or viral pathogens, thoughts turned to air pollutants as a possible cause.  But if so, which ones?

The earliest concerns

The first reports of air pollution problems appear to have been made by writers in ancient Rome, who were aware of adverse effects on human health.  Perhaps the best documented examples of concern about air pollution occurred in Medieval London, where importation of coal by sea from collieries of north east England and its combustion in various industrial processes led to legislation aimed at prohibiting the latter as early as the thirteenth century.  Needless to say the legislation proved completely ineffective and by the seventeenth century ever increasing coal combustion in London led to a serious and growing problem.

It is at this time that the first records appear to have been made of damage to vegetation.  In 1661, the English diarist, John Evelyn, published his famous treatise, Fumifugium:  Or the Inconvenience of the Aer and Smoake of London Dissipated, in which he described the contemporary air pollution problems in the English capital, making recommendations for their amelioration.

Fumifugium contains graphic descriptions of effects on vegetation, such as '...Our Anemonies and many other choycest Flowers, will by no Industry be made to blow in London or the Precincts of it, unless they be raised on a Hot-bed and governed with extraordinary Artifice to accelerate their springing; imparting a bitter and ungrateful Tast to those few wretched Fruits, which never arriving to their desired maturity, seem, like the Apples of Sodome, to fall even to dust, when they are but touched.'

Fascinatingly, Evelyn also describes what can be described as the first experiment, albeit inadvertant, on air pollution impacts on plants, when coal smoke was eliminated in London one summer as a result of the English Civil War stopping the coastal trade concerned, with him noting how the trees produced uprecedented quantities of high quality fruit.  Clearly air quality deteriorated even further over the next 100 years, as a preface to a second ediition of Fumifugium, written in 1772 noted 'It would now puzzle the most skilful gardener to keep fruit trees alive in these places: the complaint at this time would be, not that the trees were without fruit, but that they would not bear even leaves.'

Photochemical pollution p. 9

When thoughts in Los Angeles and the eastern USA turned to atmospheric issues, it was quite natural to consider SO2 first, clearly the most studied and best understood air pollutant.  In California, especially, there was a number of sources of this well-studied pollutant, largely refineries, smelters, and the combustion products of coal and petroleum.

With the appearance of symptoms of unknown origins the suspicion of SO2 injury provided impetus for study and legislation directed toward its control from industries small and large.  While this programme was sucessful in reducing the amount of SO2, it did nothing to reduce the glazing or bronzing on such crops as endive, lettuce or spinach.  Nor did it improve the visibility or lessen the sore throats, running noses, smarting eyes or headaches that often accompanied the hazy, irritating atmosphere that was becoming so pervasive in the Los Angeles area.  'Smog' was as bad as ever.

p. 10

When Ed Stephens and his colleagues at the Citrus Experiment Station analysed the 'smog' products in a new 250-500 m infrared cell, they found that the ozone was formed as nitrogen dioxide disappeared.  Another, unidentifiable compound arose that was considered to be particularly important.  They called it compound X.  A few years later they found it consisted of a group of peroxyacyl radicals that in turn could add nitric oxides forming peroxyacyl nitrates, or PAN.  This series of compounds was subsequently found to be the principal constituent causing the glazing type of plant damage.

p. 11

Also, years earlier, the door had been opened to the traditional question:  what, if any, were the adverse effects before the appearance of any symptoms visible to the naked eye.  Such effects have traditionally been referred to as chronic, hidden or subliminal injury...

During the 1960's, ozone injury became widely recognised throughout the USA on numerous crops and forests.  Agricultural areas of Canada were also impacted.  By 1970, ozone concentrations were found to be elevated in Europe and injury reported on indictor plants and forest species.

Subsequently O3 was demonstrated to be the most important phytotoxic pollutant in Europe as well as in North America, with studies using filtered chambers or chemical protectants demonstrating growth reductions and/or visible injury in many locations.  Over the same period it also became apparent that there were major oxidant problems in Japan, but it took much longer to recognise that this presented a significant environmental issue in the developing world.  This is unfortunate, because growing emissions of precursors and meteorological conditions favourable to its formation are both prevalent in many developing countries.  Yet, there is little O3 measurement carried out in such places, and even this is normally confined to the cities.  However, evidence is now accruing, either on a predicted basis or by experimental demonstration that there may be major O3 effects on crop growth in China, Egypt, India and Pakistan.

Air quality standards

An air quality standards, that could be regulated by law and hopefully achieved, could only be set once some basic parameters were established.

Concentrations of ozone or other pollutants at which injury first appeared, would have to be determined, and reliable analytical methods would be required to measure the pollutants.

Of course, even if such concentrations and thresholds of effects could be determined, it would still be quite another matter to regulate emissions to meet them.