Differentiate between the primary and secondary pollutants. Explain how these pollutants are harmful for humans and environment in about 250 words.

 Primary Pollutants: These pollutants are emitted directly into the air as a result of natural or human activity (Fig. 9.2). Examples include sulphurdioxide, nitrogenoxides, carbondioxide, carbon monoxide, hydrocarbons and particulates released from fuel burning.

Secondary Pollutants: Secondary pollutants are produced as a result of chemical reactions between primary pollutants and normal atmospheric compounds under the influence of electromagnetic radiations from the sun (Fig.9.2). For example, the primary pollutant sulphur dioxide (SO2) reacts with oxygen (O2) in the atmosphere to form sulphur trioxide (SO3), a secondary pollutant.

Major air pollutants, their sources and effects on humans and environment.

Pollutants	Sources	Effects
·         Oxides of Carbon (COx )  - Carbon dioxide (CO2)  - Carbon monoxide (CO)	Combustion of coal, oil and other fuels for energy production, manufacturing and transport; biomass burning	CO2 has a major role in green-house effect, produces weak carbonic acid adding to acid rains; CO affects human health by binding to haemoglobin, which may result in asphyxia.
·         Oxides of Sulphur (SOx)  - Sulphur dioxide (SO2)  - Sulphur trioxide (SO3) - Sulphate (SO4)	Combustion of sulphur containing fuel e.g. coal, petroleum extraction and refining; paper manufacturing; municipal incineration; ore smelting for metal extraction	SO2 can cause severe damage to human and other animal lungs and is important precursor to acid rain; adverse effects include corrosion of paints, metals and injury or death to animals and plants.
·         Oxides of Nitrogen (NOx) - Nitrogen oxide(NO) - Nitrogen dioxide (NO2) - Nitrous oxide (N2O) - Nitrate(NO3)	Burning of fuels; biomass burning; byproduct in the manufacture of fertilizers	Form the secondary pollutants: peroxy acetyl nitrate (PAN) and nitric acid (HNO3); suppression of plant growth and tissue damage; cause irritation to eyes.
·         Hydrocarbons (HCs) also called Volatile Organic Compounds (VOCs) - Methane(CH4) - Butane (C4H10) - Ethylene(C2H4) - Benzene (C6H6) - Benzo pyrine (C20H12)  - Propane (C3H8)	Evaporation from gasoline tanks, carburators; burning of fuels, biomass; municipal landfills; microbial activity of sewage; industrial process involving solvents	Can have carcinogenic effect on humans; higher concentrations are toxic to plants and animals; can convert into harmful compounds through complex chemical changes that occur in atmosphere; some are more reactive with sunlight and produce photochemical smog
·         Other organic compounds] - Chlorofluorocarbons (CFCs), - Formaldehyde (CH2O) - Methylene chloride (CH2Cl2) - Trichloroethylene (C2H Cl3) - Vinyl chloride (C2H3Cl) - Carbon tetrachloride (CCl4)  - Ethylene Oxide(C2H4O)	Aerosol sprays; foam and plastics for making disposable fast food containers; refrigeration	CFCs cause reduction in stratospheric ozone thus allowing greater penetration of ultraviolet light at earth’s surface; intensified UV radiations cause skin cancer and can have lethal effects on various life forms
·         Metals and other inorganic compounds - Lead (Pb),Mercury(Hg) - Hydrogen sulphide(H2S) - Hydrogen fluoride(HF	Oil wells and refineries; transport vehicles; municipal landfills; fertilizer, ceramic, paper, chemical and paint industries; pesticides; fungicides; aluminium production; coal gasification	Cause respiratory problems, toxicity and even death to humans and other animals; damage to crops; prove to be carcinogenic
·         Liquid droplets - Sulphuric acid (H2SO4) - Nitric acid (HNO3) - Oil - Pesticides e.g. DDT and malathion	Agricultural pesticides; fumigation; oil refineries; reactions of pollutants in the atmosphere	Contribute to acid rains; corrosion; damage to various life forms
·         Suspended particulate matter (SPM-solid particles) - Dust, soil, sulphate salts, heavy metal salts, fine particles of carbon (soot), silica, asbestos, liquid sprays, mist etc.	Fuel combustion; building constructions; mining; thermal power stations; stone crushing; industrial processes; forest fires; refuse incineration	Have chronic effects on respiratory system; deposition on the surface of green leaves thus interfering with absorption of CO2 and release of O2; blocking of sunlight; particles size that range from 0.1 to 10 mm, cause lung damage
·         Photochemical oxidants - Ozone (O3), peroxy acyl nitrates (PANs), - Formaldehyde (CH2O) - Acetaldehyde (C2H4O) - Hydrogen peroxide (H2O2) - Hydroxyl radical (HO)	Photochemical reactions in the atmosphere that involve sunlight, oxides of nitrogen and hydrocarbons	Produce haze; irritation to eyes, nose and throat ;respiratory problems; blocking of sunlight

 

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