Quantifying the Air Pollution Exposure Consequences of - Houston ...

a plume grows in horizontal and vertical extent depending on various characteristics of
the atmosphere. Three reviews of the formulations have recommended that modelers use
parameters appropriate to the elevation of the source and roughness of the site (Gifford,
1976; AMS, 1977; Seinfeld and Pandis, 1998). Taking their advice, we have chosen four
sets of dispersion parameters to describe the differences in release conditions along two
dimensions: whether the source is elevated or near ground-level at the point of release
and whether the source is located in an urban or rural area, which affects its characteristic
roughness. The next three paragraphs detail the dispersion parameters used under each of
these circumstances.
Rural, Ground-level Releases A seminal paper by Pasquill (1961) related the
spread of a plume to the standard deviation of the wind direction. His work was based on
empirical observations of concentrations resulting from the release of a non-buoyant
tracer from a ground-level source over flat, smooth ground. The observations were
connected to atmospheric stability in six discrete categories, now known as the Pasquill
stability classes. Gifford (1961) then converted the results into families of curves of the
standard deviation of pollutant concentration (σz and σy) for each of the stability classes,
A through F. The resulting curves are known as the Pasquill-Gifford curves, to which
many authors have fit mathematical formulations. While strictly relevant only to release
and topographical conditions similar to those of the experiment (Project Prairie Grass),
curves based on this dataset are the most widely recognized and utilized in air dispersion
modeling. In this study, we use a fit employing the modified power law form σ = ax b+clnx ,
where a, b, and c are empirical parameters that are based on the original Pasquill-Gifford
parameters as modified by Davidson (1990). These are the dispersion parameters used for
any source in a rural area whose height of release is near ground-level.
Rural, Elevated Releases Briggs (1974) proposed a series of interpolation formulas
for σy and σz dispersion parameters intended for elevated releases. These formulas adhere
to the Pasquill-Gifford curves at short distances (when σz approaches 50-100 m) (Gifford,
1961), the Brookhaven National Laboratory (BNL) curves at mid- to long-ranges (when
σz approached 300 m) (Singer and Smith, 1966), and the Tennessee Valley Authority
(TVA) curves at very long ranges (σz > 300 m) (Carpenter et al., 1971). The BNL data
reflects release of a passive tracer from a 108 m stack at distances of several kilometers.
The TVA collected concentration measurements of buoyant gas (SO2) emissions from
~10 different tall stacks (150 – 750+ m effective stack heights) at distances of up to 30
km from the source. We apply the Briggs interpolation curves to cases of elevated stacks
in rural areas.
Urban releases: Ground-level and Elevated Releases Two reviews (AMS, 1977;
Hanna et al., 1987) and one recent paper (Hanna et al., 2003) have selected the Briggs
urban dispersion parameters as the most appropriate for Gaussian plume modeling in
urban areas (Briggs, 1974, as reprinted in Turner, 1994, and Gifford, 1975). Based on
analyses of recent urban dispersion experiments in Los Angeles and Salt Lake City
(Allwine et al., 2002; Hanna et al., 2003), Hanna et al. (2003) modified these standard
parameters to account for the large initial mixing of plumes behind urban canopy
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