ES: Particulate pollution

Particulate pollution

Invisible particles, especially fine particles with diameters less than 10 microns (PM-10) and ultra fine particles with diameter less than 2.5 microns (PM-2.5) pose a significant health hazard, such particles are emitted by incinerations, motor vehicles, radial tires, wind erosion, wood burning places, and industrial power plants. The particulate matter that remains suspended in air is called suspended particulate matter (SPM).

Such tiny particles (i) are not effectively captured by modern air pollution control equipment (ii) are small enough to penetrate the respiratory system’s natural defenses against air pollution (iii) can bring with them droplets or other particles of toxic or cancer –causing pollutants that become attached to their surfaces.Once they are lodged deep within the lungs, these fine particles can cause chronic irritation that can (i) trigger asthma attack (ii) aggravate other lung diseases (iii) cause lung cancer (iv) affects the O2 carrying capacity of blood.

Photochemical smog (Brown air smog)

Any chemical reaction activated by light is called photochemical reactions. Photochemical smog is a mixture of primary and secondary pollutants. Formed under the influence of sunlight.

Photochemical smog formation

It begins when nitrogen and oxygen in air react at high temperature found inside automobiles engines and boilers in coal burning industrial power plants to produce colourless nitric oxide.

N2 + O2 → 2 NO

Then the nitric oxide slowly reacts with the oxygen in the troposphere leads to the formation of nitrogen dioxide, a yellowish-brown gas with a choking odour.

2 NO + O2 →2 NO2

The NO2 is responsible for the formation of brownish haze that hangs over many cities during the afternoons of sunny days. This is the reason why photochemical smog sometime is called brown air smog.

Some of the NO2 reacts with water vapour in the atmosphere to form nitric acid vapour and nitric oxide

3 NO2 + H2O → 2HNO3 + NO.

When the remaining NO2 is exposed to ultra violet radiation from the sun, some of it is converted to nitric oxide and oxygen atoms.

NO2 + UV radiation →NO + O

O2 + O → O3

Both the oxygen atoms and ozone then react with volatile organic compounds (mostly hydrocarbons released by vegetation, vehicles etc) to produce aldehydes. In addition hydrocarbons; oxygen, and nitrogen dioxide react to produce peroxyacyl nitrates (PANs).

Hydrocarbons + O2 + NO2 →PANs

Collectively, NO2, O3 and PANs are called photochemical oxidants because they can react with an oxidize certain compounds in the atmosphere. Photochemical smog can irritate respiratory tract and damage crops stress.

Industrial smog: Gray – air smog

Fifty years ago in cities such as London, Chicago, during winter, people in such cities were exposed to industrial smog consisting mostly of (i) sulphur dioxide (ii) suspended droplets of sulphuric acid (iii) a variety of suspended solid particles and droplets.

Chemistry of Industrial smog formation

The carbon in coal and oil is converted to carbon –di- oxide and carbon monoxide when it is burned.

C + O2 →CO2
2C + O2 → 2 CO

Some of the unburned carbon also ends up in the atmosphere as suspended particulate matters (soot).

The sulphur compounds in coal and oil also react with oxygen to produce sulphur dioxide, a colourless, suffocating gas.

S + O2 → SO2

In the troposphere, some of the sulphur dioxide reacts with oxygen to form sulphur trioxide,

2SO2 + O2 →2 SO3

which then reacts with water vapour in the air to produce tiny suspended droplets of sulphuric acid.

SO3 + H2 O →H2SO4

Some of these droplets react with ammonia in the atmosphere to form solid particles of ammonium sulphate.

2 NH3 + H2SO4 →(NH4)2 SO4

The tiny suspended particles of such salts and carbon(soot) give the resulting industrial smog, a gray colour, explaining why it is sometimes called gray air-smog.

Factors influencing the formation of photochemical and industrial smog , the frequency and severity of smog in an area depend on

Last modified: Thursday, 29 March 2012, 9:54 PM