In February, 1903, 10 million tonnes of red dust from northwest Africa was deposited over England. Dust from the American dust bowl was clearly visible in the city of Washington DC.

The dust in the atmosphere has a wide range of chemical composition. The composition of dust from natural sources varies as the various portions of the earth’s crust.

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Human activities, such as the manufacture of various products, introduce organic and inorganic particles into the atmosphere. These products include steel, rubber, lime and a variety of other items. In fact, almost all operations involving the burning of coal introduce some dust, i.e., soot and fly ash, into the atmosphere. A large quantity of rubber (50 tonnes/ day in Los Angeles) is worn off tyres on city streets. Much of this is possibly injected into the atmosphere in the form of dust.

2. Condensation Nuclei:

Vapours in general must be greatly supersaturated before condensation can occur in the absence of foreign particles. A super saturation of about 4.2 times is required for water vapour. However, when the vapour contains particles which can act as nuclei for condensation, little super saturation is required.

Condensation nuclei are important because of the way in which they affect our everyday life. They are largely responsible for fogs and rain. Without them, condensation would occur mainly on the walls of buildings and other exposed surfaces of the earth.

Natural sources appear to be the main producers of nuclei. Such sources include volca­nic eruption, ocean spray and natural combustion, e.g., forest fires. Man-made sources include many industrial operations, modern transportation facilities (locomotives, boats, automobiles, and aero planes) and domestic combustion processes.

Enormous quantities of nuclei are often produced by a single source. An average grass fire extending over 1 acre produces some 20,000 billion-billion (2 x 1022) nuclei. If these nuclei were distributed through a column of air, having a cross section of one acre and a height of 10,000 ft, there would still be a concentration of about two billion particles per cubic centimeter. Costs and Wright showed that a flame of commercial coal gas, lit for only 15 minutes, increased the number of nuclei in a chamber from 109,000 to 860,000 per cubic centimeter.

Lands berg found that the concentration of nuclei in a ventilated kitchen containing a large operating gas range ex­ceeded 500,000 nuclei per cubic centimeter while the outside air contained only about 25,000 nuclei per cubic centimeter.

Most nuclei range in size of about 0.0010µ. Their weight has been estimated at 10 to 1015g or an equivalent aggregate of about 106 molecules. It is interesting to note that water vapour is supersaturated enough to be self-nucleating, i.e., when water condensation occurs in the absence of foreign nuclei, the nuclei consist of aggregates containing only about 80 water molecules.

The number of nuclei in the atmosphere usually exceeds the number of dust particles by a factor of several thousands. This has been attributed to the fact that the dust particles being larger are subjected to considerable sedimentation, while the nuclei remain suspended because of convection currents and Brownian movement. However, it is probably also true that compared to dust particles; a larger number of nuclei are produced by many dispersion processes.

3. Biological Aerosols:

Aerosols of biological origin are probably not important in direct chemical reaction that may occur in the atmosphere. However, the study of aerobiology has provided information that may be important as it applies to the transport of other materials in the atmosphere.

Biological aerosols have been found even at great distances from their sources. Living spores of various fungi have been collected by an aero plane above the Caribbean Sea, 600 miles from their nearest source, while all organs have been identified at least 1,500 miles from their probable origin. In spite of the usually low concentrations at the source, marine bacteria have been collected 80 miles inland from the nearest sea coast.

Marine bacteria are removed from the sea only when the surface is stirred sufficiently to produce spray. Since, on an average, the number of marine bacteria seldom exceeds 500 per milliliter of sea water, the population of marine bacteria in the air must be sparse everywhere, except, perhaps, at times when the sea is rough or in the vicinity of coastal breakers.

Occasionally, sea water becomes highly contaminated with micro-organisms that are carried into the atmosphere, e.g., dinoflagellate gymonodium was present in quantities suf­ficient to cause the sea to appear red. Ocean spray, under these conditions, produces severe respiratory irritation among persons who come in contact with the spray.

Micro-organisms are reported to exist at various levels in the air, up to a height of 20,000 ft. Spore-forming bacteria were predominant and 29 bacteria were identified. The ratio of dust particles to micro-organisms (bacteria and molds) was found to be about 100:1.

Many observers have reported great irregularities in concentrations at different levels of the atmosphere, often finding heavier concentrations at higher altitudes than at lower levels. There are several recorded instances of such biologic stratification, e.g., spera ceil­ings are often marked by a cloud layer or a visible haze.

4. Gaseous Impurities:

A large number of gaseous impurities is emitted into the air, e.g., approximately 700 tonnes of volatiles are emitted daily into the atmosphere of Los Angeles from automobiles. The concentrations of impurities can range from practically zero to about 100 percent at the source.

Furthermore, the variety of gaseous impurities is enormous. However, there are a few gases and classes of gases which seem to deserve special attention because of their prevalence in the atmospheres of many industrial cities.

5. Sulphur Dioxide:

Sulphur dioxide occurs in the contaminated atmospheres of cities at concentrations of up to several parts per million. It produces acidity in rain water and fogs and is a major source of corrosion of buildings and metal objects.

The combustion of fuels, smelting operations, refineries, and chemical plants are the chief sources of sulphur dioxide in industrial cities.

A power station burning 5,000 tonnes of coal per day may discharge 500 tonnes of sulphur dioxide into the air.

6. Hydrogen Sulphide:

Hydrogen sulphide is usually an important contributor to air pollution problems only under rather localized conditions. This probably results in part from the fact that hydrogen sulphide is easily burned to form sulphur dioxide.

Thus, it is not generally released into the atmosphere following combustion. However, because of its great toxicity, it is a hazard in the oil- refining and rubber industry, in tanneries and other industries where animal matter is handled, as well as at plants manufacturing sulphur dyes, and artificial silk by the viscose process. Even when present in the air in concentrations below the level of physiological significance, it discolors lead particles.

7. Oxides of Nitrogen:

Oxides of nitrogen are produced by the combustion of organic matter and are, thus, introduced into the atmosphere from automobile exhausts, furnace stacks, incinerators, and many other similar sources. The oxide of nitrogen may be forced from nitrogen compounds in the organic material, but they are also formed by nitrogen fixation.

Concentrations as high as 0.5 ppm have been found in the Los Angeles atmosphere and as high as 0.8 ppm in Baltimore. The oxides include nitric oxide (NO), nitrogen dioxide (NO2), nitrogen pentoxide (N2O5) and the hydrated nitrogen pentoxide, nitric acid (HNO3). Nitrogen dioxide is in equilibrium with its dimer, nitrogen tetroxide.

Nitrogen dioxide has received considerable attention as an air pollutant because it is considered a hazard in numerous industries. Its insidious nature as a poison was emphasized by the Cleveland Clinic Fire (USA) which occurred in May, 1929. Many deaths occurred following that lire as a result of the inhalation of the nitrogen dioxide produced from burning X-ray film.

8. Ammonia:

Ammonia is another nitrogen compound which is frequently present in the air. It has been found in the atmospheres of Los Angeles, and of Charleston, USA, at concentrations as great as 0.2 ppm, and in the atmosphere of other cities at lower concentrations. The relatively high concentrations at Charleston have been attributed to the fact that ammonia is a by­product and raw material in the extensive organic-chemical industry of the area.

Ammonia is a product of many combustion processes, including domestic incineration and the operation of automobiles. It is also discharged from certain refinery operations and is always detectable in the air near stockyards.

9. Hydrogen Fluoride:

Fluorides, in general, including both hydrogen fluoride and various solid fluorides dispersed as aerosols, have received considerable attention because relatively small amounts produce flurosis in cattle. The contaminated atmospheres of cities generally contain lower concen­trations of fluorides than most of other commonly determined contaminants.

The maximum concentration found in most cities is about 0.025 ppm. The highest maximum concentration reported has been 0.08 ppm in an industrial area of Baltimore, U.S.A.

Fluorides are emitted into the atmosphere by aluminium plants, steel plants, and phosphate-fertilizer plants. Fluorides in the atmosphere have also been attributed to the burning of coal. Hydrogen fluoride itself is used in the chemical industry and in refining processes.

10. Hydrocarbons:

Recently, hydrocarbons have received considerable attention as air pollutants because they may participate in reactions, in the atmosphere, which produce-objectionable intermediate compounds and products.

Such intermediates and products may cause irritation and plant damage resulting from contaminated air. The concentrations of hydrocarbons in the atmospheres of most cities have not been measured. This is probably largely due to the difficulty of making such determinations.

The concentration of hydrocarbons in Los Angeles smog has been estimated to be about 2 ppm, of which about 1.6 ppm is paraffin and 0.4 ppm is olefins. Nearly 700 to 1,000 tonnes of volatile hydrocarbons are emitted daily into the Los Angeles atmosphere from automobile exhaust. Hydrocarbons are also emitted into the atmosphere from automobiles by oil refineries and by the evaporation of gasoline at service stations.

Local high concentrations of hydrocarbons in the air are common in garages, service stations, dry-cleaning establishments, oil refineries, and many other industries. In general, aliphatic hydrocarbons are not considered to be hazardous if the concentration is less than 20 percent of the lower inflammable limit, although such a high concentration (2,000 ppm) in the case of gasoline may produce unpleasant effects on the human system.

Benzene and other aromatic hydrocarbons are used extensively for solvent extraction purposes. They often occur locally in the air at relatively high concentrations where they can be quite a health hazard. The concentration of aromatic hydrocarbons in city air is probably generally less than 0.1 ppm although some aromatic hydrocarbons are present in automobile-exhaust gases.

11. Aldehydes and Ketones:

Aldehydes and ketones are introduced into city air from automobile exhaust gases, incinerator smoke and stack gases from the combustion of various organic substances.

Aldehydes and ketones are evidently also produced by the oxidation of hydrocarbons after they are admitted into the atmosphere. Such oxidations are discussed later in this section.

Concentrations of aldehydes and ketones have been determined in the air of several cities in the USA. The maximum concentrations ranged from 0.12 ppm (Baltimore) to 1.0 ppm (Los Angeles). The variations may represent differences in analytic?’ techniques rather than significant difference in concentrations.

Studies at the Stanford Research Institute revealed that replacement of the usual sodium bisulfite absorption train by an efficient cold-trap system greatly increased the amount of aldehydes and ketones collected from a given vol­ume of Los Angeles air. In the Los Angeles atmosphere, at least, less than half of these substances are formaldehyde, the remainder being ketones and higher aldehydes.

12. Organic Acids:

Most of the processes which emit aldehydes and ketones into the atmosphere also emit organic acids. The present of formic acid in Los Angeles smog has also been reported. The Los Angeles County Air Pollution Control District has reported acids varying from 2 to 12 carbon atoms in chain length, some of which contained hydroxyl and carboxyl groups.

13. Organic Halides:

Organic halides have been identified in Los Angeles smog by several workers, and they are probably present in the air of other cities also. The concentrations are probably generally below 0.1 ppm.

Organic halides have been the subject of considerable speculation as possible eye irritants to contaminated city air. Some of these compounds produce eye irritation at very low concentrations.

The threshold lachry-matery concentrations for cyanogen chloride, chloroacetophenone, and bromobenzyl cyanide are 0.05 and 0.02 ppm, respectively. Unfortunately, the individual compounds are very difficult to identify at such low concentrations.

14. Carbon Monoxide:

Carbon monoxide is commonly found in city air at concentration of up to about 55 ppm Concentrations much higher than this occasionally occur in the open atmosphere or locally in garages, tunnels, behind automobiles, etc. Carbon monoxide is produced by the incomplete combustion of organic material, and automobiles are notorious for their production of this gas.