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Photochemical oxidants are secondary air pollutants formed under
the influence of sunlight by complex photochemical reactions in air
which contains nitrogen oxides and reactive hydrocarbons as
precursors. The most adverse components formed by photochemical
reactions in polluted air are ozone (0 ) 3 and peroxyacetyl nitrate
(PAN), among many other products such as aldehydes, ketones,
organic and inorganic acids, nitrates, sulfates etc. An analysis
and evaluation of the available knowledge has been used to
characterize the relationships among emissions, ambient air
concentrations, and effects, and to identify the important
controlling influences on the formation and effects of
photochemical oxidants. The biological activity of photochemical
oxidants was first clearly manifested during the early 1940's, when
vegetation injury was observed in the Los Angeles Basin in the
United States. Since that time, as a consequence of the increasing
emissions of photochemical oxidant precursors, the photochemical
oxidants have become the most important air pollutants in North
America. In other parts of the world, for example South and Central
America, Asia, and Australia, photo chemical oxidants threaten
vegetation, particularly the economic and ecological performance of
plant life. According to my knowledge, the first observations of
ozone and PAN injury to vegetation in Europe were made by Dr. Ellis
F. Darley (Statewide Air Pollution Research Center, University of
California, Riverside, California) during a study visit (1963/64)
to the Federal Republic of Germany."
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