It is known that all biogeochemical work of the biosphere is performed by the global biogeochemical cycles. The principal ones, which are, in particular, responsible for the contemporary global climate change, are the global carbon, nitrogen, and sulfur cycles.
Carbon. Gaseous carbon compounds of the global cycle include carbon oxides (CO2, CO), methane (CH4), and a great amount of different volatile hydrocarbons that are released as a result of vegetation metabolism and fuel combustion. The main problem here is to estimate flows of the main 'greenhouse gases', such as carbon dioxide and methane, into the atmosphere, and their anthropogenic components.
The CO2 flow into the atmosphere from anthropogenic sources results mainly (75%) from organic fuel combustion (coal, oil, gas) and also from other kinds of economic activities (cement production, flue gas burning), making 20 billion tons yr-1. One should add about 7 billion tons of CO2 due to annual destruction of forests and loss of vegetative cover. The overall CO2 anthropogenic flow into the atmosphere reaches about 27 billion tons yr-1, that is, less than 0.01% from the CO2 total amount in the atmosphere. According to earlier data, the CO2 anthropogenic emission into the air amounted to 21.3 billion tons yr-1 in 1990. Thus, estimating the proportion of anthropogenic and natural components in the CO2 flux into the atmosphere, one should note that the natural component is approximately 25-30 times more than the human-made one.
Methane inflows to the atmosphere are subdivided into two groups:
• natural biogenic and abiogenic;
• anthropogenic that consists of two subgroups: sources relating to human activity as a biological species and technogenic sources.
An analysis of different data by Adushkin et al. in 1998 allows us to conclude that:
1. natural biogenic sources are responsible for an annual average flow of methane equal to about 540 million tonsyr-1;
2. abiogenic natural sources from lithosphere and hydrosphere make up c. 1360 million tons of methane annually (therefore, a ratio between biogenic and abiogenic methane is 1:2.5 in natural sources);
3. anthropogenic sources, including methane resulting from human agricultural activity, losses of methane during extraction of fossil fuels, and its industrial emissions produce an average annual flow ofmethane equal to about 1100 million tons yr-1.
Therefore, the natural component ofmethane in the atmosphere estimated at 1900 million tons yr-1 is 1.7 times larger than its anthropogenic component.
Nitrogen. There are three kinds of nitrogen oxides -nitrous oxide (N2O), nitrogen oxide (NO), nitrogen dioxide (NO2) - and some ammonia. Nitrous oxide has the greatest concentration in the atmosphere (=270-280 ppbv).
Nitrogen oxides reach the atmosphere from different natural sources, such as decomposition of nitrogen-based compounds in the ground by anaerobic bacteria, forest and peat fires, hydrolysis, and sedimentation of nitrates. Nitrogen oxides give rise to aerosols of nitric acid, which is one of the basic components of acid deposits. Total emissions of nitrogen oxides from natural sources are estimated to be 310 million tons yr-1, 540 million tons yr-1, or 1090 million tons yr-1 depending on the source.
Sources of the anthropogenic flux of nitrogen oxides are industrial emissions of thermal power stations, chemical and iron and steel industry enterprises, waste dumps ofcoal and sulfur mines, motor transport, burning of biomass, etc. Total emissions of nitrogen oxides from anthropogenic sources are estimated to be from 30-55 million to 100-110 million tons yr-1.
Therefore, a ratio of anthropogenic and natural components in a flux of nitrogen oxides is 1:10, that is, the anthropogenic flux is 10 times less than the natural one.
Sulfur. In nature, sulfurous gas, hydrogen sulfide, and other gaseous compounds containing sulfur are formed in large quantities as a result of processes of biological decomposition, decomposition of sulfur-containing ores, volcanic activity, and geothermal sources. Hydrogen sulfide getting in the atmosphere is quickly oxidized to make sulfurous gas; therefore, it can be considered one of the significant sources of SO2.
A wide spectrum ofgaseous sulfur compounds is released in the atmosphere after eruptions of volcanoes. Over a 25-year period, annual SO2 emissions by subareal volcanoes changed from 10 to 30 million tons yr-1. Volcanoes are responsible for approximately 7% of sulfur compounds getting to the atmosphere.
Thus, a total flux of gaseous sulfur compounds from natural sources (mainly gaseous sulfur dioxide) is estimated at 200-300 million tons yr~ .
Anthropogenic sources of gaseous sulfur compounds are metallurgical enterprises, thermal power stations, chemi-nasescal and coke plants, oxidated landfills of collieries and sulfidic ores, transport, and explosive works. In addition, anthropogenic hydrogen sulfide is formed at factories manufacturing kraft pulp, mineral oil and natural gas treatment facilities, and enterprises making artificial silk and nylon. Global emissions of anthropogenic sulfur dioxide increased during 1950-90 from 20 to 160 million tons yr-1.
The total emissions of anthropogenic sulfur oxides in the world are estimated at 130-200 million tons yr~ . As a result, we observe that the anthropogenic flux of sulfur oxides is practically same, as its natural counterpart. Hence, an impact ofanthropogenic sulfur oxide emissions on the environment, in particular, as regards atmospheric pollution, is comparable to the one from natural sources (Table 1).
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