Applications of Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 141

shown that for most nonreactive organic compounds such as PAHs in Table 4.7, the
vapor scavenging washout ratios indicated that equilibrium was attained between the
falling raindrops and the gas phase within about a 10-m fall through the atmosphere
(Ligocki, Leuenberger, and Pankow, 1985). Hence, these values can be satisfactorily
predicted from the temperature-corrected Henry's constants for the compounds. The
aerosol scavenging washout ratios of organic compounds were observed to be similar
to the washout ratios of fine-particle elements such as Pb, Zn, As, and V.

E XAMPLE 4.10 A T YPICAL C ALCULATION OF W ASHOUT R ATIOS
Given typical values of particle washout ratios, W p = 2 × 10 5 , aerosol surface area,
A v = 3.5 × 10 − 6 cm − 1 ofairandtheconstant ρ i = 1.7 × 10 − 4 atm cm.Letusestimate
the total washout ratios and the relative contributions of gas and aerosol scavenging for
benzene, benzo-[ a ]-pyrene and γ -hexachlorocyclohexane (HCH). Let us also assume
that ρ i is approximately a constant between these compounds.
The necessary properties are given below.

P i

Compound (atm) K aw ( − )

Benzene 0.125 0.228

3.1 × 10 − 7 2.0 × 10 − 5 γ -HCH

1.2 × 10 − 10 2.3 × 10 − 6 Benzo-[ a ]-pyrene

P

The calculated values of φ

i and washout ratios are as follows.

p
i

p
i )

p
i

Compound φ W g ( 1 − φ W p φ W T

4.7 × 10 − 9 9 × 10 − 4

Benzene 4 4

1.6 × 10 − 3 5 × 10 4 3 × 10 3 5.3 × 10 4

γ -HCH

7 × 10 4 2 × 10 5 2.7 × 10 5

Benzo-[ a ]-pyrene 0.83

For benzene the deposition is wholly via gas scavenging. For γ -HCH, although the
vapor scavenging dominates, there is a small contribution from the aerosol fraction.
However, for benzo-[ a ]-pyrene, the contribution is almost completely from the particle
(aerosol)-scavenging process.

Fog forms close to the ground, where most of the gases and aerosols are con-
centrated. It is formed as a result of a decrease in air temperature and an increase
in relative humidity, whereby water vapor from the atmosphere condenses on tiny
aerosol particles. Fog droplets have diameters generally between 1 and 10

m. Since
a fog droplet is approximately 1/100th of a raindrop, it is more concentrated than rain.
In addition to its importance as a site for chemical reactions, fog exerts a significant
influence as a scavenger of both organic and inorganic materials from the near-surface
atmosphere. Hence, it can have a significant effect on human health and vegetation.

μ