When fuels are burned, as gasoline is burned in automobiles and coal burned in electric power plants, heat is converted to a useful form of energy such as electricity. These energy convertions have created a spectrum of environmental challenges, like the production of gases that affect the human body, lead the formation of photochemical smog or aid the formation of acid precipitation.

Carbon dioxide is a common product of cars and power plants, it has no direct effect on humans, but accumulation in the atmosphere may lead to a warming of the Eartrh, which might lead to a shift in agricultural patterns.

Settling of particulates in atmosphere

A solid plastic sphere falling in a fluid is pulled downward by gravity and held back by a viscous force exerted by the fluid on the sphere, and a buoyant force tending to float the sphere.

The viscous force increases as the falling sphere gains speed, so the gravitational force is balanced by the opposing viscous and buoyant forces; the sphere descends with constant speed.

A sphere of any size eventually achieves constant speed, but the limiting speed of spheres made of the same material increases as the diameter increases.

The motion of particulates in the atmosphere is greatly influenced by winds, but the larger ones settle faster by exactly the same reason. Smaller particulates sometimes become permanently airborne and do not settle at all.

An easy way to estimate the settling time is the relation between settling speed and distance travel.

Settling time = distance travel / settling speed

Particulates with a smaller diameter than one micrometer settle so slowly that they tend to migrate thousands of miles before settling to the ground.

Particulate collection devices

There is no single particulate colector that can capture particulates of all sizes. Accordingly, a utility employs combinations of collectors, each working efficiently for a range of sizes. The most common collectors are gravitational, cyclone, electostatic, precipitation and fabric filtrations.

Gases containing a suspension of particulates from fuel combustion enter a large chamber. These particulates are influenced by the gravitational force of the Earth (pulling down) and retarding forces (viscous and buoyant) due to the gaseous medium. Some particles settle enough to be collected at the bottom of the container. As in atmosphere, the ones with larger diameter settle faster and are the most likely to be collected.

Gravitational collectors function efficiently only for particulate diameter larger than 50 micrometers.

In a cyclone separator, the gas containing the particulates is forced down through a tapered cylinder to produce cyclonic action. The particulate move toward the walls of the cylinder; some strike the walls and falling to a collection bin. A cyclone separator collects particulates with diameter as small as 5 micrometers.

Is the most efficient device for removing particulates with diameter smaller than 5 micrometers. It is based on the attractive electric force between unlike charges. The precipitator is a hollow cylindrical container with a wire along the axis of the cylinder. A large voltage is maintained between the central and outer housing, with the wire being negative. The electric force experienced by electrons in the wire is large enough to pull electrons from the wire. Electrons (-), are attracted to the positive cylinder.

Photochemical Smog

The photochemical smog is produced from chemical reactions involving automobile effluents and sunlight. An easy way to perceived it is the eye-irritant effect; but many of the eye-irritating chemicals are vastly more complex than the relatively simple molecules produced in the combustion of gasoline.

The most stable form of oxygen is the two atom combination (O₂). Atomic oxygen (O) and the three-atom molecule ozone (O₃) are extremely reactive in the presence of other atoms and molecules. Normal air contains very little atomic oxygen and ozone. Molecules of nitrogen dioxide NO₂ from the combination of gasoline, absorb solar energy and separate into nitric oxide and atomic oxygen according to the reaction.

NO₂ + Solar energy ---> NO + O

More than 99% of the atomic oxygen got from this reaction, combines with molecular oxygen to form ozone.

O + O₂ ---> O₃

Nitrogen dioxide and ozone are regenerated and some very irritating chemicals such as fomaldehydes, peroxyacylnitrates, and acroleins are produced. The initially high NO concentration from automobile emissions is lowed by the reactions:

NO + O ---> NO₂

NO + O₃ ---> NO₂ + O₂