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by Lidia LISITSYNA, Cand. Sc. (Geogr.), Nikolai YELANSKY, Dr. Sc. (Phys. & Math.), A. M. Obukhov Institute of Physics of the Atmosphere, Russian Academy of Sciences; Dr. Ludwig WEISSFLOG, the Center for Environmental Studies, Leipzig, Germany; Prof. Erich PUTZ, the Institute for Astrophysics, Geophysics and Meteorology, Graz, Austria
It is an open secret that climatic changes and human economic activities alike are responsible for harmful gas products polluting the earth's atmosphere. Killing vegetation and causing desert encroachment, they pose the gravest menace to the fragile, vulnerable plainland regions. Lately a new class of such deleterious substances, the halogenated hydrocarbons caught scientists' attention.
Perhaps the worst pollution of the atmosphere is caused by oxides of sulfur, carbon, and nitrogen as well as by certain organic compounds. Ozone is the most aggressive oxidizer. Diffuse, it penetrates a plant through its stoma* and suppresses as good as all physiological processes in its cells and, worst of all, inhibits photosynthesis. The water balance is upset thereby, and the increment of overall vegetative biomass goes down. Some of the contaminants, though, exert a negative effect on plant productivity indirectly. For instance, enhanced concentrations of sulfur dioxide modify the actual acidity of precipitation and thus impair the chemical composition of topsoil.
Halohydrocarbons (halocarbons), as we know now, are largely responsible for negative effects like that. Many of them enter readily into chemical reactions whereby toxic compounds and ozone are produced. Others (like, e.g. freon-11 and -12), inert in the lower atmosphere, disintegrate in the upper atmosphere under the action of ultraviolet radiation and give off chlorine, bromine and other halogens that immediately react with ozone, a process resulting in the rarefaction of its stratospheric layer. Still other contaminants absorb infrared radiation and, to make injury double sure, exhibit greenhouse effect properties to boot. Incidentally, commercial production of such substances is restricted by the Kyoto Protocol in force as of this year (2005), which imposes rigid controls on the discharges, production and consumption of greenhouse gases.
Chlorohydrocarbons and products of their oxidation coming, as a rule, from anthropogenic (man-made) sources are the worst hazard to the plant kingdom. These toxic halogenated compounds pollute the air and other media. As it is, chemical enterprises the world over are still producing large amounts of chloroform
* Stoma (pi. stomata), here a microscopic pore in the skin of plant leaves and grassy stems bounded by two guard cells. Now opening and now closing, they regulate gas exchange and evaporation. - Ed.
(trichloromethane), tetrachlorethene, trichlorethane and other deleterious substances used in the metal-working, textile and light industries as cleaners, solvents and degreasing agents. Such pollutants are also formed in the course of chemical reactions during coal burning at metallurgical mills, waste disposal, cellulose bleaching, and potable water chlorination. These killer pesticides are consumed from the air by plant leaves during respiration and by plant roots from soil water.
It's a fact: their release into the atmosphere is steadily on the increase, especially in the midlatitudes of the Northern Hemisphere where most of their producers and consumers are concentrated. However, as found out recently, chlorohydrocarbons may also be generated by savannah fires in East and South Africa (by incinerating the biomass) and also during fires in the woodland zones of northern Russia.
One of the oxidation products of tetrachlorethene and trichlorethane, the trichloroacetic acid (TCA), is now in the focus of attention owing to its highly pronounced toxic qualities-it can be detected even if present in insignificant amounts. A few decades ago it was much in use as herbicide and introduced directly into soil. As it seems, this chemical should not be present in the environment now, what with the ban on its use in agriculture (besides, its half-life period is in the range of 14 to 90 days, and it dissolves in water readily). Why then is it still detected in the air, soil and vegetation near industrial centers and elsewhere?
The point is that TCA is also formed under natural conditions-in chemical reactions involving certain chlororganic compounds, mostly tetrachlorethene and trichlorethane, which are oxidized in the presence of ozone, oxides of hydrogen, nitrogen, carbon, and other elements. There are signs that it may also be synthesized within plant tissues. Evaporating much of the moisture, the affected plants consume more of the contaminated water from the topsoil. Their photosynthetic processes slow down as a result, and so does the consumption of carbon dioxide from air. The water and gas imbalance in plants has a dual effect: it provokes soil aridity and accumulation of hothouse gases in the air. If this process affects a large territory, its ecosystem may come to be upset, and the sad aftermath thereof is known to all-desertification (desert encroachment) and regional climate changes.
The most convenient and thus routine procedure of detecting the presence of TCA consists in assaying for it needles of the Scotch (common) pine Pinus silvestris, which has the largest propagation area on earth. A body of data obtained in different regions allows to trace the transfer and global spread of the "precursors" of this harmful compound, i.e. tetrachlorethene and trichlorethane, and predict the aftermath of air pollution-which may be bad indeed under certain conditions.
This work was carried out in European Russia between 1997 and 2004 within the framework of the international project INCO-COPERNICUS. Experts of four research centers joined hands: from the A.M. Obukhov Institute of Physics of the Atmosphere (Russian Academy of Sciences), the Center for Environmental Studies (Germany), the Institute for Astrophysics, Geophysics and Meteorology (Austria), and Potchefstroom University (Republic of South Africa). Closest studies were made into the situation in Kalmykia*, Daghestan, Astrakhan region, Stavropol territory, in the mountain regions of the North Caucasus, near Moscow and on the Kola Peninsula. Besides taking samples for the presence of TCA, field teams measured the concentration of chemically active components in pine needles and determined radiation
* See: N. Yelansky, "Can Climate Warming Save Kalmykia?", Science in Russia, No. 2, 2004. - Ed.
characteristics of the atmosphere. The data thus amassed made it possible to monitor chemical processes in the atmosphere over Russia, learn more about the condition of respective ecosystems, and about the mode and degree of deleterious effects.
By and large, in our country the concentration of TCA in plants is very low, on a par with background values. Enhanced concentrations were detected only on the Kola Peninsula and in the Astrakhan administrative region; in northern Kalmykia, though, the indicators were found to be anomalously high. The pollution of the Kola Peninsula is the aftereffect of the booming wood-pulp and paper enterprises of neighboring Finland where, up until recently, chlorine was used for cellulose bleaching. But farther east the concentration of this herbicide in pine needles declines.
Numerical values for pollution effects on the atmosphere's oxidation characteristics were obtained on the basis of Stockwell's mathematical model which we complemented with as many as 119 reactions (involving 42 compounds, such as methyl chloroform, trichlorethane, trichlorethene and tetrachlorethene) responsible for the conversions of chlorohydrocarbons in the air. Research teams also monitored changes in the concentration of impurities, in the TCA formation rate in manufacturing and in rural districts. The formation process is more vigorous at higher temperatures, humidity and solar illuminance-especially in the air contaminated by "precursor" agents.
Yet the higher presence of TCA in Kalmykia's rural areas (in the community of Godzhur, for instance) actually having no industrial or manufacturing enterprises could not be rationally explained by man-caused (anthropogenic) factors alone. The TCA formative process is exceedingly vigorous there, with the pollutant being present at anomalously high concentrations.
Postulating the possibility of natural hot spots of halo-hydrocarbons (halocarbons), the international expedition made a discovery: such agents are formed due to halobacteria, the halophilic ("salt-loving") microorganisms found only in salt lakes, e.g. north of the Caspian and in South Africa. Let us stress: abundant precipitation, and inflow of fresh water result in their partial destruction. However, the weather conditions in southern Russia-with temperatures ranging from minus 20°C in winter to plus 45°C in summer-are not dangerous for halobacteria. They survive the long arid period in the summertime, when salt lakes dry up, by ensconcing themselves in between crystals of salt.
Field workers were amazed: these smallish organisms were intensive producers of trichlorethene, tetrachlorethene and some other similar hydrocarbons; the higher the water salinity, the higher their activity. This process is likewise dependent on the presence in water of archaebacteria of halophytes (plants growing on high-salinity soils), coeval with nascent life on earth, which spread all over the planet together with salt aerosols. Subsequent investigations by other research teams, too, confirmed the existence of a source of pollution like that. For instance, the presence of decomposition products of volatile halocarbons is registered in the upper Antarctic ice layer formed about 250 years ago, i.e. before the industrial age.
The ongoing climatic changes can stimulate the growth of salt lakes, both in number and in area, and step up microbial activity there, with a sequential
increase in the concentration of ozone-destructive volatile halocarbons in the air, alongside toxic compounds of the TCA type. Their transfer over long distances can damage vegetation in large tracts of land in the earth's arid areas; this, in turn, will entail further soil erosion and degradation, water shortages and climatic warming. A vicious circle!
There are indications that such problems are going to become most acute in the next few years for the republics of Central Asia. One of the harbingers of that is a decrease in the transport of corresponding aerosols into Russia's territory, which means activation of heterogenic processes in the numerous salt lakes and saline soils, the major natural sources of chlorohydrocarbons.
This will have an adverse effect on the plant life of Central Asia and on crop productivity there because of the polluted water used by farmers; this water will contain a higher concentration of such harmful agents. Given the pollution with toxic substances of anthropogenic origin (volatile organic compounds, phends inorganic salts, etc.), the situation is bound to become worse in the future.
By dint of its geographical position, and its hydrological and geological conditions, Central Asia suffers from water shortages. Such things as the current global warming, and the growth of industries and population will be instrumental in further depletion of water resources and in soil aridity. According to informed expert opinion, climatic changes are more intensive there than elsewhere on the globe. Changes are taking place in the hydrological balance, solar illumination intensity and evaporation. Consequently, the area of natural and man-made deserts-east, north and west of the Caspian in particular-may expand (which will inevitably affect the farming industry and economy of Kazakhstan, Azerbaijan and other states in the area).
The prognosis is lackluster indeed. Further programs have been mapped out so as to assess the actual impact of natural and technogenic factors on this dire process. To begin with, we are all set to study the sources of halo-carbons emission in the gulf of Kara Bogaz Gol in the eastern Caspian and their dependence on climatic characteristics, the composition and total concentration of salts in water. Furthermore, we aim to study the transport of the above toxic agents to remote districts of Daghestan, Kirghizia and Turkmenia; and we shall assess the condition of the local flora and its resistance to deleterious effects. Using numerical modeling methods, we shall compute the possible extreme concentrations of harmful compounds. We should also find out how their discharges are going to affect the vegetation of the worst polluted regions within Central Asia in the first place, test the quality of drinkable water and that used in irrigation and incorporate such data for recommendations for effective ecomanagement under conditions of limited water consumption and for sustained crop productivity.
These plans are bound to be realized. The guarantee thereof is in our expanding cooperation with colleagues in the endangered areas toward achieving practical results in counteracting desert encroachment across the world.
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Lidia LISITSYNA, Nikolai YELANSKY, Ludwig WEISSFLOG, Erich PUTZ, NATURAL PESTICIDES // London: Libmonster (LIBMONSTER.COM). Updated: 01.10.2018. URL: https://libmonster.com/m/articles/view/NATURAL-PESTICIDES (date of access: 20.04.2021).
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