• • • • • • • Air Pollution (primary and secondary). Types of air pollutants Sulfur oxides. Suspended particles Carbon oxides Nitrogen Oxides Ozone
• Air pollutants may exist in gaseous or particulate form. • Concentrations are commonly expressed either in mass per unit volume (µg.m-3of air) or as a volume mixing ratio (1 ppm =10-6 v/v). • Particulate air pollutants are highly diverse in chemical • composition and size. • They include both solid particles and liquid droplets and range in size from a few nanometers to hundreds of micrometers in diameter. • Primary air pollutants: Air pollutants emitted directly into the atmosphere from a source. Examples carbonaceous particles from diesel engine exhaust and sulfur dioxide from power stations. • Secondary pollutants: Pollutants that are formed within the atmosphere itself. Examples: sulfuric acid and nitric acid, formed respectively from sulfur dioxide and nitrogen dioxide oxidation and Ozone from photolysis of molecular oxygen in the stratosphere.
SPECIFIC AIR POLLUTANTS
1. Sulfur Dioxide: • Sources: The major source is the combustion of fossil fuels containing sulfur (coal and fuel oil contain high sulfur while natural gas, petrol and diesel fuels have a relatively low sulfur content). • Combustion of coal in power stations is far the most major single source of SO2 emissions.
• Total emissions declined in several places due to a cut in emissions from power stations effected by installation of flue gas desulfurization plant and a switch to electricity generation from burning of natural gas. • A main driver for reduction was international concern over acid rain problems.
2. Suspended Particulate Matter
• • • • Airborne particles include organic and inorganic substances. Diameters ranging from 100 µm. The major part (by mass) generally exists in the 0.1-10 µm range. Three modes for the particles: I. Transient nuclei mode: very tiny particles formed by condensation of hot vapors, or gas to particle. Examples: primary particles from motor vehicle exhaust and sulfuric acid formed from SO2 oxidation. II. Accumulation range mode: When last particles go through coagulation both with other fine particles and also with coarser particles already in the atmosphere. Such particles are capable of growth through the condensation of low volatility materials with size 100 nm and 2µm. • Removal of the accumulation range particles by rainwater scavenging or dry deposition to surfaces is inefficient. They last in atmosphere for about two weeks.
Particles mode, continue
III. Coarse particles: >2 µm to about 100 µm. • They are mechanically-generated particles such as wind-blown dust, sea spray and primary volcanic particles. • These are formed by attrition of bulk materials.
• Suspended particles in the atmosphere is known as atmospheric aerosol. • The term total suspended particulate matter (TSP) has been used to describe the fraction of particles collected on a filter using the US high volume sampler.
Modes of particles
Components of Suspended Particulate Matter.
I. Combustion particles comprising mainly fine (less than 2.5 µm diameter), particles of elemental and organic carbon derived predominantly from road vehicle traffic.
II. Secondary particles of mainly ammonium sulfate (or sulfuric acid) and ammonium nitrate predominantly within the fine (less than 2.5 µm diameter) size range. III. Coarse particles arising largely from soil and road surface dust re-suspended by traffic activity and made up mainly of inorganic mineral components.
• It refers to impure carbon particles resulted from the incomplete combustion of a hydrocarbon. • It is restricted to the product of the gas-phase combustion process but is commonly extended to include the residual fuel...
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