Air

AIR 2.1 Physics of Air . I. Shape

5. Surface electrical charge

2: Shape factor

6. Dust

3. Specific Surface Area

7. Condensation nuclei

4.Particle surface property

8. Visibility

1. shape The shape of air may be in any,of the form such as spher irregular, cubical or flocks (a) Spherical The spherical shape may be due to. pollen grains, condensat solids fly ash, cotton, asbestos, synthetic materials etc. (b) Irregular Air may take irregular shape due to the presence of minera. (c) Cubical Air takes a cubical shape due to theipresence of flake and minerals, large mica, fibrous condensation nuclei. . (d) Flocs, A chain like appearance of air is formed during incompl combustion of fuels and contains, a large amount of carbon particle size 0.1 micron, 0.5 micron, 0.8 microns 1.5 microns so. on. The coal particles are largest, 40% by weight of irregular} cubical shape compared to spherical smog and fly ash. The density of air borne particles vary from 0.5 to 6.5 g/c with an average of 2.2 g/cm3. 2. Shape factor The simplest particle shape is sphere. Coalescence (comtogether) of liquid particles will result in larger spherical particle But coalescence of solid spherical particles result in flocculent thread like aggregation. Various shape factors are used to expre the degree of irregularity. The coefficient of sphericity is the ral of surface area of sphere with the same volume as the given parti to the surface area of the particle. 3. Specific Surface Area Air borne particles have a larger surface area due to the irregularity and internal pores.

smaller size, surface

Particle Surface Property Some of the static properties are in the form of their size, are volume, density and weight. Some other static properties of such air particles are in the form of surface electrical charge, adhesion rate of evaporation, condensation of moisture from the surface. 5; Surface electrical charge Several mechanism provide the electrical charges in a particle It may be by promoting the transfcr.of electrons to the surface providing a negatively or positively charged particles. The other mechanism is the diffusion of ions to the particle surface and the maximum surface charge. In dry air a particle can retain about 1.6 x JO elecrons/cm Even Freshly generated smoke particles contains a fraction of this maximum charge. 6 Aerosol It is a colloidal system such as mist or a fog in which dispersion medium of gas, presence of solid or liquid in gas Aerosol may be classified as (i) Dispersion aerosol (ii) Condensation aerosol (i) Dispersion aerosol It is formed during the grinding or atomosiation of solids and liquid by putting their powder into a state of suspension through the movement of air. (ii) Condensation aerosol It is formed when the saturated vapours are condensed or when gases react chemically to form a volatile or nonvolatile aerosol product. Mist: This include the condension and dispersion aerosol particles.characteristic bright pink colour of flesh, due to the presence carboxy haemoglobin in the blood. Carbon monoxide can reduce visual and mental activity., 3 Oxides of Sulphur The oxides of Sulphur includes both Sulphur trioxidc Sulphur dioxide. Sulphur trioxide (SO,) is emitted along Sulphur dioxide (SO,) in the proportion of 1 to 5% of the quanti of elemental sulphur. Sulphuric acid manufacturing industri electroplating, phosphate fertilizer industry etc., emit SO, on higher side. S03 in the atmosphere combines with moisture or wat

with

liquid

(H20) to form H2S04 sulphuric acid causing acid rain. It has low dew point and aerosol could be easily formed. SO} and sulphti acid generally induce poor visibility. (i) Sulphur dioxide Sulphur Dioxide is one of the chief air pollutant. It's cont in air is estimated to be 150 x 10* tonnes. S02 emission can controlled either by treating coal, oil or collecting it at the soure It is a colourless, pungent irritating gas and can be delected in air at a concentration of 0.5 ppm. S02 is highly soluble in wati About. 11 grams of sulphur dioxide Combines with 100 gms of wall forming weak sulphurous acid. H20 + S02 -> H2 S03 (Sulphurous Acid) S02 combines with oxygen in the atmosphere forming So which on reaction with water forms sulphuric acid which leads' the Acid Rain. The sulphur compounds in general causes sensory an respiratory irritation on humans and other adverse effects on rai and health. It also can cause plant damage, and also enhances the corrosion of metals. (ii) (H2S) Hydrogen Sulphide Hydrogen Sulphide is a gas with a rotten egg smell. It is toxi and can be detected at very low concentration of 0.5 pp. The natural soures of H2S arc anaerobic decay process on land, decomposition of marshy land, decomposiiion of sea weed, algae etc. 100 x 106 connes are emitted by pollution sources. The problem with H2S comes to the picture if it gets concentrated in small areas. The main Sources of H2S includes rayon industry, coke oven, oil refineries, paper"industries etc., Its concentration over the cities arc more than 0.1 ppm. H2S oxidises with 0.1 ppm. H2S oxidises with air to form SO2 within an hour H2 oxidation SO2 o2 SO3H2O H2SO4 4. Nitrogen Compounds Nitrogen compounds are very stable and constitute about 78% f the atmosphere. It tempers with the oxidative power of the atmosphere. The various nitrogen compounds include Nitrous oxide

(N20)

Nitric oxide

(NO)

Nitrogen oxide

(NO2)

Nitrogen Trioxide

(NO3)

Nitrogen Tetroxide

(N2O4)

Nitrous acid

(HNO2)

Nitric acid

(HNO3)

(a) Nitrous oxide: (N20) This is a colourless, non toxic sweet gas present in largest amount in the air as compared to other gases. About 0.25 ppm is used in anaesthesia, surgery and dentistry. It is also called as laughing gas. When nitrous oxide is present in largest amount, on inhalation produces loss of feeling. The major natural sources of nitrous oxide are the biological action of soil and oceans. The world wide production of Nitrous oxide is estimated to be 109 tonnes year. Nitrous oxide is found to be, associated with photochemical reaction in the atmosphere. (b) Nitric oxide: (NO) It is a colourless, odourless and tasteless gas produced main by the biological and the combustion processes. The Nitric oxide in the air is oxidised rapidly by the ozone by the process photochemical reaction and slowly by oxygen to form N02. world wide natural emission of NO is about 4.3 x 108 tonnes/yet The nitric oxide is mainly contributed by the forest combustion the high temperate zone. Nitrogen reacts with oxygen to form (c) Nitrogen Dioxide It is a reddish brown pungent smelling gas. It's natural emissior are due to the biological decay where nitrates arc reduced to Nitrital and ultimately converted to nitrous acid Oxidation NO2 ------> N03 ------> HN03 (Nitrous Acid) The natural emission of Nitrogen dioxide amounts to abot 650 x 106 Tonnes. Presence of Nitrogen Dioxide in the air irritate the mucous membrane of the body. It also leads to the corrosion of, » metals. It also serves as the energy trap by absorbing sunlight form nitric acid and atmospheric oxygen. Atmospheric oxygen it very active in forming Ozone and initiates number of photochemica chain reactions. Nitrogen oxide absorbs the light strongly especially the ultraviolet radiations and as such the ozone job is helpe by NO2 Nitrogen oxide reduces the visibility, causes sensory irritatio and causes damage to the plants. (d) Ammonia Nature produces ammonia in .huge, quantities. It has a share odour and can Be detected by taste at high concentration of 20 ppm At low concentration it is harmless'. But at higher concentration it forms ammonium salts with' Sulphuric acid. Nitric acid and Hydrochloric acid. Ammonia causes adverse health effects and toxicity to fish and other aquatic life. 5 Clilorine compounds souresThe chlorine compounds are most widely,used in the disinfection of water to kill the bacteria. The Chlorine Dioxide is being used in place of chlorine has a residual effect, frequently Hydrochloric acid and-.vapours of chlorinated hydrocarbonschlaoroform(CHC13), Carbon Tetra, Chloride (CC14) etc are formed and joins the.atmospheric air..The freon vapours are. widely used is refrigerants, as aerosols, spray propcllants, have been found to be building up in the air. Generally the concentration of

chlorine in he 'atmosphere is very low.'It is feared that the ferrous ultimately ise into the stratosphere 10 to 20 miles and destroy the ozone layer. Elemental chlorine is widely used in water disinfection, sewage works, chemical and plastic industries,; in bleaching power, pesticides and herbicides which release chlorine to kill the pests, chlorine is a heavy yellow green pungent gas. It is highly reactive and irritates the mucus membrane like lungs and throat. Hydrochloric Acid This is commonly emitted from industries and domestic actions. These are found in lesser concentrations. This is characterized as lead scavenger. 6. Ozone Ozone in a bluish gas which is about 1.6 times as heavy as air but highly reactive at high altitudes. Ozone is formed by the petrocliemical reaction. The concentration of ozone is greatest in the stratosphere depending upon the altitude. The ozone concentration in air varies between 0.02 ppm to about 0.2 ppm and at-height of about 20 km, lightening and thunders, are also responsible for producing some ozone. Ozone strongly absorb the ultraviolet radiation from the suns rays, thereby causing heavy, damages to the living tissues in the earth. Ozone and other oxidants such as perroxy acetyl Nitrate are formed in the atmosphere due to the various photochemical reactions. 2.2.3 Biology of Air Biological Aerosols Aerosols of biological origin are ofcourse not important in . direct chemical reaction occurring in the atmosphere, It. indu the transport of other materials in the atmosphere. Biologist aerosols"have been found even at great distances from their point of source. Living spores of various fungi arc carried by aeroplane . above.the Caribbean sea, 1000 kms from their nearest source Marin Bacteria are removed from the sea only when the surface is stirre sufficiently to produce spray. On an average the number of marin bacterias exceed 300/m/ of sea water. The population of marin bacteria in the air must be sparce everywhere except perhape sometimes when the sea is rough or in the vicinity of coastal breaker. Occasionally sea water is highly contaminated with micre organisms that are carried into the atmosphere for example Dine flagellate Gimonodium was present in quantities sufficient to cause the sea to appear red. Ocean spray under these conditions produce severe respiratory irritation among person coming in contact with the spray. Micro organisms are reported to exist at various levels in air upto 20,000 ft high. Spore forming bacterias are predominantly found and about 29 varieties of such bacterias have been identified in the atmosphere the ratio of dust to the,micro organisms such as bacteria was found to be about 100:1. There is great variation in it concentration atsdifferent levels of the atmosphere heavy at higher altitude than at lower altitudes. Bacterial stratification takes place for example spora ceilings are often marked by a cloud layer or visible haze.

Table 2 3 Permissible Concentration of elements in air Elements

Tolerance limit

Physical 1. Dust

Dust Free

2. Colour

Colourless

3 Odour and laste

Odourless and tastily

4Temperature

85° F to 29" F

5. Turbidity

Turbid free

6. Visibility

Clear

(b) Chemical 1:'Ammonia

1.2 mg// as Nitrogen

2. Arsenic

I mg//

3. Fluoride

2 mg//

4. Lead

0.05 mg//

5. pH

5.5 to 9 mg//

6. Carbon monoxide 10 mg// for 8 hours 7. Particulate matter 55 to 110 ug/m5 (annual) 8. Nitrogen Dioxide

100µg/m3

9. Ozone

235 µg/m' (once in a year)

10. Sulphur Dioxide

80 Hg/m3 (annual)