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## Useful conversions and formulas for air dispersion modelingVarious governmental agencies involved with environmental protection and with occupational safety and health have promulgated regulations limiting the allowable concentrations of gaseous pollutants in the ambient air or in emissions to the ambient air. Such regulations involve a number of different expressions of concentration. Some express the concentrations as ppmv and some express the concentrations as mg/m³, while others require adjusting or correcting the concentrations to reference conditions of moisture content, oxygen content or carbon dioxide content. This article presents a set of ## Additional recommended knowledge
## Converting air pollutant concentrationsThe conversion equations depend on the temperature at which the conversion is wanted (usually about 20 to 25 degrees Celsius). At an ambient air pressure of 1 atmosphere (101.325 kPa), the general equation is: and for the reverse conversion:
Notes: - Pollution regulations in the United States typically reference their pollutant limits to an ambient temperature of 20 to 25 °C as noted above. In most other nations, the reference ambient temperature for pollutant limits may be 0 °C or other values.
- 1 percent by volume = 10,000 ppmv (i.e., parts per million by volume).
- atm = absolute atmospheric pressure in atmospheres
- mol = gram mole
## Correcting concentrations for altitudeAtmospheric pollutant concentrations expressed as mass per unit volume of atmospheric air (e.g., mg/m³, µg/m³, etc.) at sea level will decrease with increasing altitude because the atmospheric pressure decreases with increasing altitude. The change of atmospheric pressure with altitude can be obtained from this equation: *P*_{a}= 0.9877^{a}
Given an atmospheric pollutant concentration at an atmospheric pressure of 1 atmosphere (i.e., at sea level altitude), the concentration at other altitudes can be obtained from this equation:
As an example, given a concentration of 260 mg/m³ at sea level, calculate the equivalent concentration at an altitude of 1,800 meters:
## Standard conditions for gas volumesA normal cubic meter (Nm³ ) is the metric expression of gas volume at standard conditions and it is usually ( A standard cubic foot (scf) is the USA expression of gas volume at standard conditions and it is often ( That being understood:
Notes: - kmol = kilomole or kilogram mole
- lbmol = pound mole
## Windspeed conversion factorsMeteorological data includes windspeeds which may be expressed as statute miles per hour, knots, or meters per second. Here are the conversion factors for those various expressions of windspeed:
Note: - 1 statute mile = 5,280 feet = 1,609 meters
## Correcting for reference conditionsMany environmental protection agencies have issued regulations that limit the concentration of pollutants in gaseous emissions and define the reference conditions applicable to those concentration limits. For example, such a regulation might limit the concentration of NOx to 55 ppmv in a dry combustion exhaust gas corrected to 3 volume percent O Environmental agencies in the USA often denote a standard cubic foot of dry gas as "dscf" or as "scfd". Likewise, a standard cubic meter of dry gas is often denoted as "dscm" or "scmd" (again, by environmental agencies in the USA). ## Correcting to a dry basisIf a gaseous emission sample is analyzed and found to contain water vapor and a pollutant concentration of say 40 ppmv, then 40 ppmv should be designated as the "wet basis" pollutant concentration. The following equation can be used to correct the measured "wet basis" concentration to a "dry basis" concentration:
Thus, a wet basis concentration of 40 ppmv in a gas having 10 volume percent water vapor would have a dry basis concentration = 40 ÷ ( 1 - 0.10 ) = 44.44 ppmv. ## Correcting to a reference oxygen contentThe following equation can be used to correct a measured pollutant concentration in an emitted gas (containing a measured O
Thus, a measured NOx concentration of 45 ppmv (dry basis) in a gas having 5 volume % O ## Correcting to a reference carbon dioxide contentThe following equation can be used to correct a measured pollutant concentration in an emitted gas (containing a measured CO
Thus, a measured particulates concentration of 0.1 grain per dscf in a gas that has 8 volume % CO Notes: - Although ppmv and grains per dscf have been used in the above examples, concentrations such as ppbv (i.e., parts per billion by volume), volume percent, grams per dscm and many others may also be used.
- 1 percent by volume = 10,000 ppmv (i.e., parts per million by volume).
- Care must be taken with the concentrations expressed as ppbv to differentiate between the British billion which is 10
^{12}and the USA billion which is 10^{9}.
## See also- Standard conditions of temperature and pressure
- Units conversion by factor-label
- Atmospheric dispersion modeling
- Roadway air dispersion modeling
- Air pollution dispersion modeling books
- Accidental release source terms
- Choked flow
## References**^**Air Dispersion Modeling Conversions and Formulas**^**Beychok, Milton R. (2005).*Fundamentals Of Stack Gas Dispersion*, 4th Edition, author-published.__ISBN 0-9644588-0-2__.**^**40 U.S. Code of Federal Regulations, Chapter I, Part 60, Appendix A-3, Test Method 4.**^**40 U.S. Code of Federal Regulations, Chapter I, Part 60, Appendix B, Performance Specification 2.**^**40 U.S. Code of Federal Regulations, Chapter I, Part 60.
Categories: Chemical engineering | Environmental engineering |
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Useful_conversions_and_formulas_for_air_dispersion_modeling". A list of authors is available in Wikipedia. |