Atmospheric Pollutants: Types, Impacts, and Dispersion
Concept of Atmospheric Pollutant
An air contaminant is any physical, chemical, or biological agent that alters the natural composition of the air or introduces a foreign element. This can occur due to abnormally high levels of a substance and involve risk, danger, or nuisance to people, ecosystems, or property.
Primary and Secondary Pollutants
a) Primary pollutants are those emitted directly into the atmosphere from emission sources. Examples include:
- Suspended particles
- Carbon oxides
- Sulfur oxides
- Nitrogen oxides
b) Secondary pollutants are formed when primary pollutants undergo chemical reactions with atmospheric constituents, sometimes facilitated by solar energy. Examples include:
- Sulfuric acid
- Nitric acid
- Tropospheric ozone
- Peroxyacetyl nitrate (PAN)
Sulfur Oxides: Origin, Health, and Environmental Impacts
The sources of sulfur oxides can be natural (e.g., volcanic eruptions) or anthropogenic (e.g., combustion of fossil fuels like coal and oil, which contain sulfur as an impurity).
Impacts on Humans: Sulfur oxides, when concentrated in the air, have a pungent odor and can cause respiratory problems and lung irritation.
Environmental Impacts: Significant concentrations of sulfur dioxide can lead to acid smog (classic or London smog). Furthermore, once in the atmosphere, sulfur dioxide can oxidize to sulfur trioxide, which, in the presence of moisture, produces droplets of sulfuric acid, a major component of acid rain.
Weather Conditions and Pollutant Dispersion
Geographical location and relief influence the formation of breezes that can either disperse pollutants or cause their accumulation. The figures below illustrate the influence of valley and mountain breezes, which form due to differential heating of slopes and valleys during the day-night cycle.
During the day, the slopes heat up, creating an updraft of warm air. Meanwhile, cold air collects in the valley floor, producing a temperature inversion that prevents pollutant dispersion. At night, the process reverses, forming mountain breezes, which also cause inversion and pollutant accumulation.
Temperature Inversion
Generally, temperature decreases with altitude (vertical thermal gradient). However, during a temperature inversion, temperature *increases* with altitude. This affects convective heat flow because colder, denser air masses are located at ground level, trapping pollutants. The drawing provides an example of this phenomenon.
Ozone Layer Depletion: Origin and Consequences
“Ozone layer depletion” refers to a significant thinning of the ozone layer, currently most prominent over the South Pole. Certain pollutants, especially chlorofluorocarbons (CFCs) and nitrogen oxides, have been shown to destroy ozone.
- CFCs were commonly used as refrigerants and in spray cans.
- Nitrogen oxides are produced during the combustion of fossil fuels, particularly at high temperatures and pressures, such as in thermal power plants.
The decrease in the ozone layer leads to an increase in ultraviolet (UV) radiation reaching the Earth’s surface. This can cause various health hazards, including:
- Skin cancer
- Mutations
- Cataracts
- Weakened immune systems
Increased UV radiation also damages plants and animals. Prolonged destruction of the ozone layer, with the consequent exposure to intense UV radiation, could destroy the primary producers upon which all life on Earth ultimately depends.