DESCRIPTION OF CONTAMINANTS. Yves Alarie, Ph.D Professor Emeritus U niversity of Pittsburgh,USA. A. GAS A state of matter in which the molecules are practically unrestricted by cohesive forces.
University of Pittsburgh,USA
A state of matter in which the molecules are practically unrestricted by cohesive forces.
A gas has neither shape nor volume for our use. It is a substance which has a critical temperature below 200C and thus cannot be condensed into liquid form at any pressure at this temperature.
Examples with their critical temperatures in parenthesis: Methane (-820C),
Fluorine (-1290C), Helium (-2680C).
B. VAPORSubstances dispersed in air as individual molecules, below their critical temperature and thus could be condensed to a liquid at 200C by increasing the pressure. A vapor has neither shape nor volume for our use. Examples: Iodine (5120C), benzene (2890C), carbon disulfide (2790C).
Stable or quasi-stable suspension of solid or liquid particles in a gas. Various terms are used to define an aerosol on the basis of its origin or state.
These terms are:- Fumes: Solid particles formed by condensation. Generally used for metals such as Cd, Pb, Zn, etc., but has also been used for any solid, after heat treatment, such as Teflon or PVC fumes. Strictly speaking Teflon or PVC are not fumes since the particles are made of liquids, solids and dissolved gases. The appropriate term is smoke or thermal decomposition products as given below. Usually well below 1 µm in diameter and fairly homogenous, in the case of metals.
These terms are:- Dusts: Solid particles, formed during disintegration processes of a mechanical nature; mining, grinding, i.e. coal dust etc. Usually above 1 µm in diameter and heterogenous, less stable because of larger size and polydispersed.- Mists: Refer to liquid particles, formed by condensation of a vapor (small particles, homogenous, stable) or by atomization of a liquid (larger particles and less stable).
- Thermal Decomposition Products: Combination of vapors, liquids and solids formed from synthetic polymers (plastics) during operations such as molding, cutting with hot wires, heat treatment of any kind.
- Haze: Combination of vapors, dusts, fumes, mists or smokes which appreciably reduces visibility. Sometimes haze is used instead of fog (i.e., hazy vs. foggy) to describe reduced visibility. If water vapor condensation is the main culprit, foggy should be used instead of hazy.
-Smog: Older definition was for the combination of smoke and fog as in the London smog which resulted in 4,000 deaths in 1952. More recently, used in the U.S. to describe the resulting combination of gases and aerosols formed during U-V irradiation of hydrocarbons, oxides of nitrogen and oxygen and the resulting ozone, etc., i.e., L.A. smog. This will occur where a large amount of automobile exhaust and sunlight are present under atmospheric conditions unfavorable to dispersion of pollutants (Magill, 1949). Chemical reactions involved were given by Haagen-Smith in 1955. A better term is photochemical smog.
Aerosols are also described according to
their chemical composition or their size
distribution by the following terms:
- Homogenous: Refers to chemical constitution, i.e., sulfuric acid.
- Heterogenous: Refers to chemical constitution, i.e., coal dust.
- Polydispersed: As above, geometric standard deviation larger than 1.2. The term heterodispersed is also used.
Magill, P.L. The Los Angeles Smog Problem.
Indust. Engin. Chem. 41, 2476-2486, 1949.