Point sources are characterized exactly as in the ISC3 model (U.S.Environmental Protection

Agency 1995). The input to the model includes the location, elevation, emission rate, stack

height, stack gas temperature, stack gas exit velocity, and stack inside diameter. The temperature,

exit velocity, and diameter are required for plume rise calculations.

Similarly, volume sources require the same input as the ISC3 model. This includes the

location, elevation height (optional), height of release, emission rate, the initial lateral plume size

(Fy) and initial vertical plume size (Fz). AERMOD differs from ISC3 in the treatment of volume

sources only in how the initial plume size is implemented. Where ISC3 uses the virtual source

technique to account for initial plume size, AERMOD adds the square of the initial plume size to

the square of the ambient plume size:

where Fyo is the initial horizontal plume size, Fyl is the plume size before accounting for the initial

size, and F

y is the resultant plume size after accounting for the initial size.

The area source treatment is enhanced from that available in ISC3. In addition to being input

as squares or rectangles, area sources may be input as circles or polygons. A polygon may be

defined by up to 20 vertices. A circle is defined by inputting its center location and radius. The

AERMOD code uses this information to create an equivalent nearly-circular polygon of 20 sides,

with the same area as the circle.

As with ISC3, AERMOD allows for the calculation of a simple half-life decay.

5.8 Adjustments for the Urban Boundary Layer

Although urban surface characteristics (roughness, albedo, etc.) influence the boundary layer

parameters at all times, the effects of the urban sublayer on the structure of the boundary layer is

largest at night and relatively absent during the day (Oke 1998). An urban “convective-like”

boundary layer forms during nighttime hours when stable rural air flows onto a warmer urban

surface. Following sunset, the urban surface cools at a slower rate than the rural surface because

buildings in the urban area trap the outgoing thermal radiation and the urban subsurface has a

larger thermal capacity. AERMOD accounts for this by enhancing the turbulence above that

found in the rural stable boundary layer (i.e., a convective-like urban contribution to the total

turbulence in the urban SBL). The convective contribution is a function of the convective

velocity scale, which in turn, depends on the surface heat flux and the urban mixed layer height.

The upward heat flux is a function of the urban-rural temperature difference.

The urban-rural temperature difference depends on a large number of factors that cannot

easily be included in applied models such as AERMOD. For simplicity, the data presented in

Oke (1973; 1982) is used to construct an empirical model. Oke presents observed urban-rural

temperature differences for a number of Canadian cities with populations varying from about

 

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