Page 274 - Fundamentals of Air Pollution 3E

III. Contaminant Fate Analysis 233

A. Atmospheric Fate
The following numbered paragraphs refer to particular numbered boxes
in Fig. 16-3,
1. The atmospheric fate of contaminants must be assessed whenever it
is determined that significant gaseous or airborne particulate contaminants
are released from the site. The atmospheric fate of contaminants released
originally to other media, but eventually partitioned to the atmosphere
beyond site boundaries, must also be assessed whenever this intermedia
transfer is likely to be significant.
2. The predominant directions of contaminant movement will be deter-
mined by relative directional frequencies of wind over the site (as reflected
in area-specific wind rose data). Atmospheric stability and wind speeds
determine off-site areas affected by ambient concentrations of gaseous con-
taminants. Usually, high stability and low wind speed conditions result in
higher atmospheric concentrations of gaseous contaminants close to the
site. High stability and moderate wind speeds result in moderate concentra-
tions over a larger downwind area. Low stability or high wind speed condi-
tions cause greater dispersion and dilution of contaminants, resulting in
lower concentrations over larger areas.
For particulate contaminants (including those adsorbed to dust or soil
particles), ambient concentrations in the atmosphere and areas affected by
airborne contaminants are determined by wind speed and stability and
also by particle size distribution. High winds result in greater dispersion
and cause particles to remain airborne longer (which may also increase
release rates). Low winds and high stability result in rapid settling of
particles and in a more concentrated contaminant plume closer to the site.
Larger particles settle rapidly, decreasing the atmospheric concentrations
with distance from the site. Finer particles remain airborne longer, and
their behavior more closely approximates that of gaseous contaminants, as
described.

3. Settling and rainout are important mechanisms of contaminant trans-
fer from the atmospheric media to both surface soils and surface waters.
Rates of contaminant transfer caused by these mechanisms are difficult to
assess qualitatively; however, they increase with increasing soil adsorption
coefficients, solubility (for particulate contaminants or those adsorbed to
particles), particle size, and precipitation frequency.
Areas affected by significant atmospheric concentrations of contaminants
exhibiting the foregoing physical and chemical properties should also be
considered as potentially affected by contaminant rainout and settling to
surface media. Contaminants dissolved in rainwater may percolate to
ground water, run off or fall directly into surface waters, and adsorb to