Tuesday, March 8, 2011

Polluted air

Man’s activities take place, for the most part, on the earth’s surface within the first 2 kilometers of the atmosphere. The pollutants produced by these activities get injected directly into the troposphere where they are mixed and transported. The background concentrations of the relative gases have remained, to the best of our knowledge, constant with time. It implies that the sources and sinks (as the information and removal processes are generally called) are in balance, and also, for gases with a high pollutant contribution, that the sinks are able to cope with the additional burden from man. It could be partly explained by reactivity and partly by the fact that these gases’ natural source greatly exceeds the pollutant one in the majority of cases. Nature produces over 10 times as much hydrogen sulfide, at least an equivalent amount of nitrogen oxides, and over 100 times as much ammonia as is produced by man. Sulphur dioxide looks like an exception to the rule, however, Hydrogen sulphide has been ultimately converted to sulphur dioxide in the atmosphere and is, therefore, a source of sulphurtri oxide.
Pollution problems associated with the gases like Nitrogen, Oxygen, water, Argon, Carbon dioxide, Neon, Helium, Methane etc. arise not because of the magnitude of the man made emission gets concentrated in the areas where people live  and work, and more specially in the cities of the industrial world. Further most of the world’s industries have been concentrated in the northern hemisphere and the great majority of this between latitudes of 30N and 60N. The estimated world-wide production of the past century of one such pollutant-sulphur dioxide was about 5 million tones, mainly from coal combustion. Today, about 190 million tones get produced, f which about 100 million is from coal (which generally contains about 0.5 to 4% sulphur). 50 million tones from the refining and burning of oil, and the remainder chiefly from the smelting of copper, lead, and zinc ores. Any pollutants will tend to increase in concentration with time if there have been no sinks available. For carbon dioxide the problem has been lack of balance between the sources and sinks over a scale that is relevant to the atmospheric concentration. The major vapor phase atmospheric species has been water.  The persists as a vapor until supersaturation occurs, when it condenses, the vapor concentration for condensation being a function of the temperature. In practice, the water content of air has been very variable and ranges from 0 to about 4% by weight. Where the pollutant emission has been typical of the process, as in the case of alkali industry, and there has been only one plant carrying out the process ion the area, then an association between effects and pollutant readily made and control measures can be taken where necessary. In cities pollution comes from a multiply of sources, not least of which is the motor car, which contributes more than 50% of the hydrocarbons and nitrogen oxides to most urban air sheds and 90% of the carbon monoxide.



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