Applications of Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 142

Since fog forms close to the earth's surface, its dissipation leaves particles in the near-
surface atmosphere that are highly concentrated with pollutant. This observation has
significant implications for the long-range transport and deposition of environmental
pollutants (Munger et al., 1983). Knowledge of fogwater chemistry also has relevance
to the ambient acid formation and acidic precipitation problems. SO 2 oxidation reac-
tions increase the acidity of fog. Fog droplets have greater deposition rates than dry
gases and aerosols. Most of the liquid water content in fog is lost to the surface. Thus,
in regions of heavy fog events, calculation of dry deposition and rain fluxes may not
account for the actual deposition of pollutants. Historically, fog has been a cause for
severe human health effects such as those of the infamous London fog of 1952, which
caused close to 10,000 deaths in 4 months.

E XAMPLE 4.11 T OTAL D EPOSITION OF A P ESTICIDE BY F OG

The concentration of a pesticide (chlorpyrifos) in air from a foggy atmosphere in Parlier,
CA, was 17.2 ng/m 3 at 25 ◦ C. The aerosol particle concentration in air was negligible.
If the moisture deposition rate from fog is 0.2 mm/h and it lasts for 4 h, how much
chlorpyrifos will be deposited over 1 acre of land surface covered by fog?
The Henry's constant for chlorpyrifos is K aw = 1.7 × 10 − 4 at 25 ◦ C. C i =
17.2 ng/m 3 . Particle deposition is zero; hence only gaseous species need be consid-
ered. W i = 1 /K aw . Flux to surface is J i = R I C i a /K aw = 2 × 10 − 12 g/cm 2 h. Amount
deposited = ( 2 × 10 − 12 ) (4) (4.04 × 10 7 ) = 3.27 × 10 − 4 g.

m contribute the bulk of liquid water in fogs.
However, droplets smaller than this will be more concentrated in pollutants. Large
concentrations of colloidal organic matter (between 10 and 200 mg/L) are sometimes
observed in fogwater. These colloids arise from dissolved humics and fulvics and
are surface active (Capel, Leuenberger, and Giger, 1991). The fogwater collected in
California was observed to contain a variety of inorganic ions (NO 3 ,SO 2 4 ,H + ,
and NH 4 ) , which were also the major components of aerosols collected from the
same region (Munger et al., 1983). This showed that highly concentrated fogwater
appeared to result from highly concentrated aerosols. Likewise, dissipation of highly
concentrated fog resulted in highly reactive and concentrated aerosols. This points to
the fact that fog is formed via condensation on aerosol nuclei. The chemical compo-
sition of fog is not only determined by the composition of their condensation nuclei
(e.g., ammonium sulfate or trace metal-rich dust), but also by the gases absorbed into
fog droplets. High concentrations of formaldehyde (

Droplets larger than about 10 μ

0.5 mM) were also observed
in California fog. Many other inorganic species such as Na + ,K + ,Ca 2 + , and Mg 2 +
have also been identified in the fog collected from different regions of the world. Fog
is an efficient scavenger of both gaseous and particulate forms of S(IV) and N(III).
The high millimolar concentrations of metal ions in fogwater and the resultant metal
ion catalysis for the transformation of S(IV) to S(VI) in a foggy atmosphere are inter-
esting research areas. A number of investigators have shown that fogwater contains

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