Air Quality Modeling
To predict air quality impacts from a project that is still
on the drawing board, and for which and Environmental Impact Assessment
(EIA) is being conducted, Air Quality Modeling may be the
only possible method to arrive at realistic estimates of possible impacts.
An air quality or dispersion model is a mathematical description
of the meteorological transport and dispersion processes
as well as chemical reactions and transformations and settling and wash-out,
that translate the emissions from one or several sources into
ambient concentrations or immissions of one or more pollutants.
Air Quality Modeling:
Environmental standards important for Impact Assessment are formulated
either in terms of
emission related limits including regulations on furnaces,
engines and fuel quality;
air quality guidelines or thresholds for ambient concentrations (immissions).
Model come in many different styles, depending on the intended purpose
but also the available data, but in principle they all are of a general form:
C(x,y,z,t) = f(Q,M)
where C is the ambient concentration at the location (x,y,z)
and time (t), which is a (more or less complex and dynamic)
function of the emissions (Q) and the meteorology (M), Q and
M being complex constructs themselves and in most cases of high
dimensionality, spatially distributed and dynamic.
A typology of models
Models can be based on a a number of priciples and approaches, and can
have quite different levels of complexity in the underlying theory and
the set of assumptions they represent:
steady-state models (the most commonly regulatory
model based on the Gussian equation are of this type)
dynamic models treat the metoeorology and/or the emissisons
as a function of time. This is essential where key meteorological
parameters such as wind speed and direction can be changing faster than
the system can reach steady state (e.g., in coastal locations), or where
temporal emission patterns are important.
spatially lumped (box models), irrespective of the
temporal dynamics and resolution or process complexity:
box models are useful when the topography can be approximated by
a box (like in a narrow valey under stagnant conditions.
spatially distributed: this can be interms of
one dimension: along the main wind axis)
two dimensional: downwind and crosswind, in a horizontal plane
multi-layer: two-diemensional models that describe several
(coupled) vertical layers;
three dimensional (comparable resolution along all three axis,
full coupling in all directions)
point sources, the most common type representing industrial
stacks; this includes a description of plume rise due to
momentum and thermal buoancy;
are sources: usually understood as an agglomeration of numerous small
point sources not treated individually; typical examples are residential
heating, or industrial parks with numerous small stacks;
area sources are also important in the modeling of particulates,
where they contribute particles due to wind induced entrainment.
line sources: typical for the analysis of traffic generated pollutants;
volume sources: used, for example in the analysis of aircraft emissions
that can be treated as a three dimensional volume of pollutnts
generated primarily at start and landing (TOL cycles).