Page 48 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

1.7 Units and Dimensions 21

concentration, ppm is used more often than percentage to quantify the concentration
of an air pollutant or GHG in the air. One frequently encountered unit conversion is
between volume (or mass) percentage and ppmv (or ppmm):

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x % ¼ 0:01 x 10 ppm ¼ 10000x ðppm) ð1:3Þ
y y
y ppm ¼ ¼ % ð1:4Þ
10 6 10 4
Sometimes, it is necessary to specify the concentration by volume or mass.
Accordingly,ppmvand ppmm are usedtoexpress parts per millionbyvolumeand mass,
respectively. The density of a species is needed in order to convert ppmv to ppmm:

q spieces
x ðppmmÞ¼ y ðppmvÞ ð1:5Þ
q
mix
The conversion between the volume concentration and the mass concentration as
determined by the density of the subject of concern can be explained using the
following example (Example 1.1).
Example 1.1: Unit conversion The National Ambient Air Quality Standard for
carbon monoxide (CO) is 35 ppmv measured over a one-hour averaging time. What
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is the equivalent concentration in (a) percentage, (b) mg/m under standard con-
3
dition when its density is 1.145 kg/m ?
3
3
Solution 35 ppmv means that there is 35 m of CO in one million m of air.
Therefore,
(a) by volume percentage, 35 ppmv is 35 % = 0.0035 %
10000
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(b) by mass concentration, 35 ppmv of CO in air becomes 1.145 kg/m
6
3
35=10 = 40.075 10 6 kg/m = 40.075 mg/m 3
Unless otherwise stated, the standard ambient condition is at a temperature of
5
25 °C and standard atmospheric pressure (1 atm or 1.013 10 Pa).

1.8 Practice Problems

1.8.1 Multiple Choice Problems

1. Which one of the following gases is NOT considered as a greenhouse gas

a. Carbon dioxide
b. Methane
c. Water vapor
d. Ammonia