INTRODUCTION:

The global air pollution is mainly due to five primary air pollutants namely oxides of nitrogen (NO_x), oxides of sulfur (SO_x), oxides of carbon, hydrocarbon and particulate^1-3.Out of all these, particulates are most dangerous ones because of their high toxicity and longer lifetime in environment^4-6 The particulates are the small solid particles, liquid droplets as well as semi volatile components found in the atmosphere in fairly large number varying from several hundred per cm³ in pure dry air to 10⁵per cm³in highly polluted air. Their diameter may vary from 0.001 to 100µm. The residence time (average life time) of the particulates in air depends on their size, composition, density and turbulence in the air and its value ranges in between few seconds to few months ^{4 .}The mass of a particulate remains within 10µg/m² in dry air to 2000µg/m³in polluted air. When the sizes of the particulates remain within 1µ, these induce cloud and fog formation, cause the formation of water droplets and ice crystals and heat balance of the atmosphere^. However, when the particle size exceeds 1µ, these become excellent sites for sorption of various inorganic and organic pollutants and allowing them to undergo chemical reactions producing potent secondary pollutants.

In the present work, an attempt has been made to discuss different sources of particulate emission, their catalytic action resulting in secondary pollutants, their detrimental effects in biosphere and some remedial measure to minimize the level of particulates in environment.

SOURCES OF PARTICULATES

The particulates are released into atmosphere both by natural processes and anthropogenic activities, the former being the main contributor (about 80%).

A) Natural processes: The natural processes include volcanic eruption, forest fire, blowing of dust and soil by wind, evaporation of organic matter, bacterial decomposition, sea salt sprays etc.^4-6

B) Anthropogenic processes: The anthropogenic processes include mining activities (during crushing and grinding), incomplete combustion processes (e.g. combustion of coal, fossil fuel, wood, natural gas etc.) industrial processes, emission from jets and aircrafts. ^7,8

Nature and formation of particulates

The chemical composition of particulates varies according to sources and combustion factors. In terms of constituents, the particulates are of two types

a) Inorganic particulates : Some important inorganic particulates and their secondary reactions are outlined below:

i) Metal oxides: The metal oxides are a major class of inorganic particulates in the atmosphere. The formation of some important particulate oxides are given below:

3FeS₂ +8O₂ à Fe₃O₄ +6SO₂

CaCO₃ à CaO+ CO₂

4V + 5O₂à 2V₂O₅

ii) Acids: The acidic gases like SO₂, SO₃, NO₂, and CO₂etc. react with water vapors resulting in acid droplets. ^9-11

iii) 2SO₂ +O₂ +2H₂O à 2H₂SO₄

SO₃ + H₂O à H₂SO₄

4NO₂ +O₂ +2H₂O à 4HNO₃

N₂O₅ +H₂O à 2HNO₃

CO₂ + H₂O àH₂CO₃

HCl (g) +H₂ O à HCl(aq)

iv) Bases: The bases like ammonia, oxides and hydroxides of alkali and alkaline earth metals etc. react with water vapours resulting in basic droplets.

NH₃ + H₂O à NH₄OH

CaO + H₂O àCa(OH)₂

v) Salts : The acid droplets react with basic air pollutants like CaO,MgO,NH₃ etc. to produce salts

H₂ SO₄ + 2NH₃à (NH₄)₂SO₄

CaO + H₂SO₄ à CaSO₄ +H₂O

MgO +2HNO₃à Mg (NO₃)₂ + H₂O

CaO +H₂CO₃ à CaCO₃ +H₂O

In addition, NaCl in the atmosphere can also react with acids to give corresponding salts.

2NaCl + H₂SO₄àNa₂SO₄ + 2HCl

vi) Lead containing particulate: The combustion of tetra ethyl lead (an antiknock compound in Gasoline) resulting a potential source of particulate containing lead ⁴

Pb(C₂H₅ )₄ + C₂H₄X₂ +16O₂ à PbX₂ +10CO₂ +12H₂O

(Where X = Cl, Br, I)

PbX₂can also undergo hydrolysis producing PbO particle.

PbX₂ +H₂O à PbO +2HX

vii) Mining and metallurgical operation: The metallurgical operations like crushing, grinding, roasting etc. may release a lot of particles in to environment (e.g. SiO₂, Fe₂O₃, Fe₃O₄, CaO, MgO, Al₂O₃, etc.)

viii) Fly ash: The combustion of high ash containing fuels like fossil fuel, coal etc. release a lot of particles into environment known as fly ash. For example, combustion of pyrite containing coal releases fine particles of Fe₃O₄ and SiO₂.The main components of fly ash are: SiO₂ 17-65%, Al₂O₃10-60% ,K₂O 2.5-3.0% , Fe₂O₃ 2-27%.

viii) Asbestos particles: These are fibrous silicate minerals released from asbestos industry and hence have comparatively longer life time

ix) Vehicular exhaust: Fine and ultrafine particles generated from vehicular exhaust are highly unstable and persist briefly, aggregating to form large particulates.⁹

b) Organic Particulates: The Organic particulates are produced either through some atmospheric reactions or through some combustion processes. The generation of some organic particulates and their characteristics are outlined below:

i) Polycyclic aromatic hydrocarbons (PAH): The polycyclic aromatic hydrocarbons are produced through pyrosynthesis of saturated hydrocarbons of low molecular mass under oxygen deficient conditions. At higher temperature, C-H and C-C bonds undergo homolytic cleavage resulting in free radicals which combine chemically to produce PAHs.

XC_nH_{2n +
2} à PAH

PAHs can also be generated by the pyrolysis of higher alkanes

Both the processes of pyrosynthesis and pyrolysis occur during the combustion of fossil fuel. Some important examples of PAH are: Benzo-(α)-pyrene, Benz-(α)-anthracene, chrysene, Benzofluoranthene etc.¹²

i) Bifunctional substituted alkanes : Such type of particulate organics are characterized as bifunctional substituted alkane derivatives of type X-(CH₂) _n-X where the values of n ranges between 1-5 and X may be: -COOH, -CH₂OH, -CHO, -CH₂ONO, -CH₂ONO₂, -COONO,-COONO₂ etc. These are obtained from auto mobile exhaust or from decomposition of vegetation.

iii) Soot particles: A soot particle is composed of thousands of interconnected graphitic platelets and each graphitic platelet is formed by the condensation of a large number of aromatic rings .The large surface area of soot particles favours adsorption of several organics like PAHs, occurrence of a number of environmental reactions providing the necessary catalytic surface for the adsorption of toxic elements like Be, Cd, Pb etc.

SO₂ + ½ O₂ à SO₃

SO₂ + SO₃ à SO₃+ O₂

SO₃ + H₂O à H₂SO₄

R-CHO + HO^•à R-CO^•+H₂O

R-CO^•+ O₂à R-CO-O-O^•

R-CO-O-O^• + NO₂à R-CO-O-O-NO₂ (PAN)

iv) Quinones: (1, 2- and 1, 4-Naphthaquinones, 9, 10- anthraquinones etc.) are considered to be of toxicological importance because these can generate reactive oxygen species through redox cycling. Quinones can also react with –SH group of compounds and could deplete protective nucleophiles. ¹³

DETRIMENTAL EFFECT OF PARTICULATES

A)Effect on Human beings :

i) Asbestos particles cause lungs disorder

ii) Lead containing particles affect children’s brain and interfere with the development and maturation of red blood cell.

iii) Particulates of less than 1m reach alveoli of lungs and damage lungs tissues

iv) Soluble aerosols get absorbed into blood from alveoli while the insoluble aerosols are carried to the lymphatic stream and get deposited in the pulmonary, lymphatic depot point or lymph glands.

v) Black lung disease of coal miners, pulmonary fibrosis of asbestos workers and emphysema of urban peoples are due to the particulate accumulation.

vi) The lodged particles in the lungs (less than 3 m) can cause severe breathing trouble by physical blockage and irritation of lung capillaries.

vii) Particulates having heavy metals and PAHs result in higher oxidative cellular damage and toxicological effect.¹³

viii) Deposition of particulates in the lungs alveoli stimulate the formation of ROS specially ^•OH and O₂^• ^-which can cause oxidative damage to cellular lipids, proteins and DNA. The ROS can also cause pulmonary inflammation, impairment of excision repair mechanism of DNA and activation of oncogenes. ^12,14

The basic health effects are cardiopulmonary diseases, irritation of eyes and respiratory system, excess mortality of elderly and chronically ill persons etc.^15-17

B) Effect on plants

i) The deposition of particulates on soil makes the soil unsuitable for plant growth.

ii) Deposition of particulate on leaves prevents CO₂ absorption and hence decreases the rate of photosynthesis.

iii) Particulates deposited on plant leaves block the stomata of plants and thus inhibit the rate of transpiration

iv) In case of some plants, which are sensitive to traces of toxic metals, their enzyme activity is disturbed in presence of particulates containing trace elements.

C) Effects on materials

i) Particulate fumes and mists react directly with painted surfaces and cause cracks

ii) The particulates containing acidic or basic substances induce corrosion of metals.

iii) These accumulate on the soil surface causing soil erosion .Particles including fues, dusts , soot, mists and aerosols can bring about severe damage to buildings, sculpture and monuments

D) Depletion of ozone layer

The surface of the particulate matter (particularly the surface aerosol) can catalyze the photodecomposition of CFCs generating Cl^•radical capable of depleting ozone layer.

F₃CCl + hν à F₃C^• + Cl^•

F₂ CCl₂ + hν à F₂ClC^• + ClO^•

Cl^•+ O3 à ClO^•+ O₂

E) Influence on global climate

i) Anti greenhouse effect: The particulates may act as nuclei for the condensation of water vapors to form clouds, rains and snow which make the atmosphere cooler. Such an effect is called as anti green house effect.

ii) Global warming effect: The particulates like soot, desert dust etc. can absorb solar radiation and infrared radiation emitted by earth. Such a process can cause global warming (Green house effect).

MINIMIZATION PARTICULATE

The particulate emission from natural sources cannot be checked. However, the steps may be taken for the minimization of particulates emitted from anthropogenic sources.^18,19

The effluent gases containing particulates should be discharged into environment after suitable treatment. Some important techniques employed for this purpose are passing the effluent gases through

i) Gravity settling chamber to remove the particles of size more than 50m

ii) Cyclone collector

iii) Suitable wet scrubbers which absorb solid, liquid and gaseous particulates.

iv) Electrostatic precipitators which cause precipitation of charge particles.

v) Cyclonic separators and trajectory separators.

In addition, the level of particulates in the environment can be minimized by massive afforestation.

REFERENCES:

1. Panda, S. Environmental degradation due to the accumulation of particulates. J. Env. Res. Dev., 1, 2006 , 22-25

2. Panda, S. Environment and Ecology,2^nd Ed. Vrinda Publication(P) Ltd.,Delhi,2007

3. Tripathy, S.N. and Panda, S Fundamentals of Env. Studies, 3rd.Ed. Vrinda Publication (P) Ltd., Delhi. 2009.

4. Das, A. Environmental chemistry with Green Chemistry Books and Allied (P) Ltd, Kolkata. 2010.

5. Dey, A. K. Environmental chemistry 6^th Edition , New Age International (P) Ltd New Delhi,2006 .

6. Sharma, B.K Environmental Chemistry, 6^th. Ed., Goel Publishing House, Meerut, India, 2004

7. Valavanidis, A., Fiotakis, K., Vlahogianni,T. and Pantikaki,V Determination of selective quinones and quinoid radicals in air borne particulate matter and vehicular exhaust particles, Environmental Chemistry 3; 2006 :118-123.

8. Valvanidis A., Fiotakis K. and Vlachogianni T. Airborne particulate matter and human health: Toxicological assessment and importance of size and composition of particles for oxidative damage and carcinogenic mechanism, J. Environ. Sc. Health Part-C 26; 2011 : 339- 362.

9. Aust, A. E., Ball, J. C., Hu, A. A., Lighty, J. S., Smith, K. R., Straccia, A. M., Veranth J. M. and Young, W. C. Particle characteristics responsible for effects on human lungs epithelial cells, Research Report Health Effect Institutes, 100; 2002 :1-65

10. Ellision, J.M. and Walker, R.E A Review of sulfur oxides and particulate matter as air pollutants with particular reference to effect on health in United Kingdom, Environ. Res. 16; 1978 : 302-308.

11. Nel, A., Xia, T., Madler, L. and Li, N. Toxic potential of materials at nano level, Science 311; 2006: 622-627.

12. Sagai, M., Lim, M.S.P. and Ichinose, T Lungs carcinogenesis by diesel exhaust particles and the carcinogenic mechanism via reactive oxygen, Inhalation Toxicology. 12; 2000: 215-223.

13. Seaton, A., MacNee ,W., Donaldson, K. and Godden, D. Particulate air pollution acute health effect, Lancet, 345; 1995: 176-178..

14. Prahalat, A.K., Inmon, J., Dailey, L.A., Madden, M.C., Ghio, A.J. and Gallaghar, A.E., Air pollution particles mediated oxidative DNA damage in a cell free system and in human airway epithelial cells in relation to particulate metal content and bioreactivity. Chemical Research in Toxicology. 14; 2001: 879-887.

15. Bates, D. Health indices of the adverse effects of air pollution: the question of coherence, Environmental Research 59; 1992: 336-349.

16. Pope, C. A., Bates D. V. and Raizenne M. E. Health effects of particulate in air pollution : time for reassessment, Environmental Health Perspectives ,103; 1992:472-480..

17. Pope, C. A. and Dockery D. W. Health effects of fine particulate air pollution: Lines that connect. J. Air and Waste Mgt.Asso. 56; 2006:709-742 .

18. Holgate, S., Samet J., Koren H. and Maynard R(eds) Air pollution and health, Academic press, Sandi ego,1994.

19. Manahan, S.E Environmental Chemistry, Lewis Publisher, New York, 1994.