Geoscience Reference
In-Depth Information
Similarly, we might write pounds per million pounds, tons per million tons, or liters per million
liters. In each expression, identical units of weight or volume appear in both the numerator and
denominator and may be canceled out, leaving a dimensionless term. An analog of parts per million
is the more familiar term “percent.” Percent can be written as
Parts
Hundred parts
To convert from parts per million by volume (µL/L) to µg/m
3
at standard temperature (25°C) and
standard pressure (760 mmHg), known as STP, it is necessary to know the molar volume at the
given temperature and pressure and the molecular weight of the pollutant. At 25°C and 760 mmHg,
1 mole of any gas occupies 24.46 L.
Problem
: 2.5 ppm by volume of sulfur dioxide (SO
2
) was reported as the atmospheric concentration.
What is this concentration in micrograms (µg) per cubic meter (m
3
) at 25°C and 760 mmHg? What
is the concentration in µg/m
3
at 37°C and 752 mmHg?
Note:
This example problem points out the need for reporting temperature and pressure when the
results are present on a weight to volume basis.
Solution
: Let parts per million equal µL/L, then 2.5 ppm = 2.5 µL/L. The molar volume at 25°C and
760 mmHg is 24.46 L, and the molecular weight of SO
2
is 64.1 g/mole.
At 25°Cand 760 mmHg(STP):
3
2.5 µL
L
1µmole
2
64.1 µg
µmole
1000 L
m
6.66
×
10
µg
×
×
×
=
3
3
4.46 µL
m
At 37°Cand 752 mmHg:
310°K
298°K
7
60 mmHg
752 mmHg
=
24 46
.
µL
×
25.72 µL
3
2.5 µL
L
ole
25.72 µL
1µm
64.1 µg
µmole
1000 L
m
6.2
×
10
µg
×
×
×
=
3
m
3
1.5.2 C
onversion
t
Tables
For
C
ommon
a
ir
p
ollution
m
easurements
To assist the environmental engineer in converting from one set of units to another, the following
conversion factors for common air pollution measurements and other useful information are pro-
vided in Tables 1.8 through 1.12. These conversion tables provide factors for
• Atmospheric gases
• Atmospheric pressure
• Velocity
• Atmospheric particulate matter
• Concentration
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