Page 333 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

LEED Indoor Envir onmental Quality 295

If the concentration of a gas or a mixture of gases is given in moles per volume (n/V),
then the changed concentration due to temperature variations can readily be estimated
by multiplying the initial concentration by the ratio of the two temperatures. Note that
the volume increases with an increase in temperature, so the concentration decreases
with an increase in temperature, and the temperatures used must be based on one of the
absolute scales (Kelvin or Rankine).
If the concentration of a gas or a mixture of gases is given in moles per volume (n/V),
then the changed concentration due to pressure variations can readily be estimated by
multiplying the initial concentration by the ratio of the two pressures. Note that the volume
increases with a decrease in pressure, so the concentration decreases with a decrease in
pressure, and the pressures used must be absolute pressures, not gauge pressures.

Some Typical Properties of Ambient Air
The following properties of ambient air are useful in performing indoor air pollutant
concentration calculations:
• At room temperature and 1 atm, there is approximately 24 liters per mole (L/mol)
of air.
• At the earth’s surface, dry air is predominantly composed of 78 percent nitrogen,
21 percent oxygen, and 1 percent argon. This is a mole or volume percent.
• Dry air composed of approximately 78 percent nitrogen, 21 percent oxygen, and
1 percent argon has an average molecular weight of approximately 29 g/mol.

Concentrations
Air pollutant concentrations are typically given either by mass of the pollutant per volume
of air or by a molar ratio. The first unit type (mass/volume) can be used for gaseous,
gaseous mixtures, liquid, or solid air pollutants. One common unit is micrograms per
meter cubed (μg/m ).
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The molar ratios can only be used for the gaseous pollutants. They are usually given
in parts per million (ppm) or parts per billion (ppb). When these units are used for
gaseous phase concentrations, they are in parts per million or billion mole. They are not
based on mass. Since most of the air pollution concentrations that are of interest inside
buildings and in ambient air can be modeled by the ideal gas law, the ppm or ppb on a
molar basis is equal to the ppm or ppb on a volumetric basis.
If masses of air pollutants are known, the molecular weights must be used to convert
the units to ppm or ppb. This cannot be done for the air pollutant categories that
combine several types of pollutants, such as total VOCs, even though the VOCs are
gases. Therefore, these concentrations are always given in mass per volume.

Box Models
Simple air pollutant box models based on the conservation of mass can be used to
estimate indoor air pollutant concentrations (see Chap. 10 for a description of simple
box models). Although the examples given in Chap. 10 are based on water and water
pollutant concentrations, similar box models can also be used for both air mass balances
and air pollution mass balances. However, care must be taken when balancing the air
masses and air pollutant masses so that variations in flows, volumes, and concentrations
due to temperature and pressure changes are addressed.