Page 32 - Materials Chemistry, Second Edition

Site Assessment and Remedial Investigation 15

Discussion:
1. The default air intake rate for children of 6–8 years old is 10 m /
3
day (ATSDR, 2005). With a total air intake of 2 m , this child
3
would have been playing out there for a few hours.
2. The average soil ingestion rate for children is 200 mg/day, while
that for pica children is 5,000 mg/day [13]. (This rate should only
be used when assessing acute exposure situations.)
3. Benzene/toluene/ethyl benzene/xylenes (B/T/E/X) are the main
COCs in gasoline because of their toxicity. Please note that the
chemical formulas of ethyl benzene and xylenes are the same
(and so are their molecular weights).

Example 2.6: Gas Concentration in ppmV
The vapor pressure of mercury at T = 25°C and P = 1 atm is 0.0017 mm-Hg. If
mercury is allowed to evaporate to equilibrium in an enclosed space, deter-
mine the theoretical mercury concentration (in ppm) in air.

Solution:
(a) Mole fraction of mercury in air = P mercury /P total
= (0.0017 mm-Hg)/(760 mm-Hg) = 2.24 × 10 −6
(b) Vapor concentration of mercury = mole fraction of mercury in air
= 2.24 × 10 = 2.24 parts per million = 2.24 ppm (or ppmV)

−6

Discussion:
1. The calculation for this question is relatively simple. To correctly
answer the question, we need to have the right concept about
vapor concentrations in the units of ppm (or ppmV).
2. An engineer should be familiar with units commonly used for
pressure. One atmosphere = 1.013 × 10 Pa = 101.3 kPa = 1.013 bar
5
= 1,013 mbar = 760 mm-Hg = 760 torr = 29.92 in.-Hg = 14.696 lb/in.
2
(psi) = 33.9 ft-H O = 10.33 m-H O.
2
2
Example 2.7: Conversion of Gas Concentrations between
ppmV and Mass Concentration

The National Ambient Air Quality Standard (NAAQS) for nitrogen diox-
ide (NO ) is 100 ppb (1-h average). A dispersion modeling analysis of NO
2
2
emissions from a source shows a maximum ambient receptor concentration
of 180 μg/m . The receptor elevation is 6,000 ft; the barometric pressure is
3