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Chemical environmental control FAQ

Air Quality Conversions

Converting Air Pollutant Concentrations and Other Formulas by mbeychok
Posted: 11 Jan 06

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For those of you involved with the air quality and/or air pollution fields, the following should be quite useful.

CONVERTING ATMOSPHERIC POLLUTANT CONCENTRATIONS

The conversion equation depends on the temperature at which you want the conversion (usually about 20 to 25 degrees Centigrade).  At an ambient air pressure of 1 atmosphere, the general equation is:

ppmv = (mg/m3)(¦K)(0.08205) / MW

and for the reverse conversion:

mg/m3 = (ppmv)(MW) / [(0.08205) (¦K)]

where:
    ppmv = air pollutant concentration, in parts per million by volume
  mg/m3 = milligrams of pollutant per cubic meter of air
        ¦K = atmospheric temperature in degrees Kelvin = 273.15 + ¦C
0.08205 = universal gas law constant in (atm+liter)/(gmol+¦K)
     MW = molecular weight of the air pollutant (dimensionless)
     atm = absolute atmosperic pressure in atmospheres
    gmol = gram mole

NOTES:
(1) The pollution laws and regulations in the United States typically reference their pollutant limits to an ambient temperature of 20 to 25 ¦C as noted above. However, in other nations, the reference ambient temperature for pollutant limits may be 0 ¦C or other values.
(2) 1 percent by volume = 10,000 ppmv (i.e., parts per million by volume).
(3) For all practical purposes, degrees Centigrade and degrees Celsius are synonymous.

EFFECT OF ALTITUDE ON ATMOSPHERIC POLLUTANT CONCENTRATIONS

Atmospheric pollutant concentrations expressed as mass per unit volume of atmospheric air (e.g., mg/m3, ug/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:

Pa = 0.9877a

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:

Ca = ( C)(0.9877a)

where:
a = altitude, in 100's of meters
Pa = atmospheric pressure at altitude a, in atmospheres
C = Concentration at sea level altitude, in mass per unit volume
Ca = Concentration at altitude a, in mass per unit volume

As an example, given a concentration of 260 mg/m3 at sea level, calculate the equivalent concentration at an altitude of 1,800 meters:

Ca = (260)(0.987718) = 208 mg/m3 at 1,800 meters altitude

STANDARD CONDITIONS FOR GAS VOLUMES:

A normal cubic meter (Nm3) is the metric expression of gas volume at standard conditions and it is usually (but not always) defined as being measured at 0 ¦C and 1 atmosphere of pressure.

A standard cubic foot (scf) is the USA expression of gas volume at standard conditions and it is very often defined as being measured at 60 ¦F and 1 atmosphere of pressure. There are other definitions of standard gas conditions used in the USA besides 60 ¦F and 1 atmosphere, but that is the most common one ... and it is very widely used in the oil, gas and hydrocarbon processing industries.

That being understood:

1 Nm3 of any gas (measured at 0 ¦C and 1 atm pressure) equals 37.326 scf of that gas (measured at 60 ¦F and 1 atm pressure).

1 kgmol of any ideal gas equals 22.414 Nm3 of that gas at 0 ¦C and 1 atm ... and 1 lbmol of any ideal gas equals 379.482 scf of that gas at ¦F and 1 atm.


where:
kgmol = kilogram mole
lbmol = pound mole
  atm = atmosphere

WINDSPEED CONVERSION FACTORS:

Meteorological 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:

1 m/sec = 2.237 statute mi/hr = 1.942 knots
1 knot = 1.152 statute mi/hr = 0.515 m/sec
1 statute mi/hr = 0.868 knots = 0.447 m/sec


where: 1 statute mile = 5,280 feet = 1,609 meters

CORRECTING CONCENTRATIONS TO REFERENCE CONDITIONS IN REGULATED EMISSION LIMITS:

Many 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 O2.  As another example, a regulation might limit the concentration of particulate matter to 0.1 grain per standard cubic foot (i.e., scf) of dry exhaust gas corrected to 12 volume percent CO2.

A standard cubic foot of dry gas is often denoted as "dscf" or as "scfd".  Likewise, a standard cubic meter of dry gas is often denoted as "dscm" or "scmd" by environmental agencies in the USA.

Correcting Concentrations to a Dry Basis:

If a gaseous emission sample is analyzed and found to contain water vapor and a pollutant concentration of X, then X 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:

(1)  dry basis concentration = ( wet basis concentration ) / ( 1 - w )

where:
w = fraction of the emitted exhaust gas, by volume, which is water vapor

Thus, a wet basis concentration of 40 ppmv in an emitted gas containing 10 volume percent water vapor would have a dry basis concentration = ( 40 ) / ( 1 - 0.10 ) = 44.44 ppmv.

Correcting Concentrations to a Reference O2 Content in the Emitted Gas:

The following equation can be used to correct a measured pollutant concentration in an emitted gas (containing a measured O2 content) to an equivalent pollutant concentration in an emitted gas containing a specified reference amount of O2:

(2)  Cr = Cm ( 20.9 - r ) / ( 20.9 - m )

where:
Cr = corrected concentration in dry emitted gas having the reference volume % O2 = r
Cm = measured concentration in dry emitted gas having the measured volume % O2 = m

Thus, a measured nitrogen oxides (i.e., NOx) concentration of 45 ppmv (dry basis) in an emitted gas having 5 volume % O2 = ( 45 ) ( 20.9 - 3 ) / ( 20.9 - 5 ) = 50.7 ppmv (dry basis) when corrected to an emitted gas having a specified reference O2 content of 3 volume %.

Correcting Concentrations to a Reference CO2 Content in the Emitted Gas:

The following equation can be used to correct a measured pollutant concentration in an emitted gas (containing a measured CO2 content) to an equivalent pollutant concentration in an emitted gas containing a specified reference amount of CO2:

(3)  Cr = Cm( r / m )

where:
Cr = corrected concentration in dry emitted gas with a reference volume % CO2 = r
Cm = measured concentration in dry emitted gas with a measured volume % CO2 = m
  
And thus, a measured particulate matter concentration of 0.1 grain per dscf in an emitted gas that has 8 volume % CO2 = ( 0.1 ) ( 12 / 8 ) = 0.15 grain per dscf when corrected to an emitted gas having a specified reference CO2 content of 12 volume %.

Notes:

--  Although ppmv and grains per dscf have been used in the above examples, you may use other concentrations such as ppbv (i.e., parts per billion by volume), volume percent, grams per dscm, etc.
--  1 percent by volume = 10,000 ppmv (i.e., parts per million by volume).
--  Equation (1) above is from "40 CFR, Chapter I, Part 60, Appendix A-3, Test Method 4".
--  Equation (2) above is from "40 CFR, Chapter I, Part 60, Appendix B, Performance Spec. 2".
--  Equation (3) above is from "40 CFR, Chapter I, Part 60".
--   CFR = United States Code of Federal Regulations
.

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