Modelling of Pollutant Transport in the Atmosphere - MANHAZ

Figure 11 shows relative excess density ∆ρ ρ air as a function of mixing
concentration c calculated by four models of different complexities. Each
model assumes adiabatic mixing, i.e. it neglects possible heat transfer
from the ground. The chosen atmospheric conditions match the most
humid case in a set of field experiments with liquefied ammonia (Nielsen
et al, 1987). Ammonia is a hygroscopic substance (i.e. readily reacting with
air humidity) and the thick solid curve, calculated by Wheatley´s (1987)
model, is the most accurate one. This solution may be divided into three
domains: dry mixing, nearly pure water aerosols and nearly pure ammonia.
The moisture affects the aerosol formation in two ways: the relative
humidity determines the limit of transition between dry and wet mixing,
while the absolute humidity (depending on air temperature) determines
the magnitude of the deviation from dry mixing. The immiscible aerosol
model (dashed line) does surprisingly well with just a slight over
prediction of the density in the domain of almost pure water aerosols.
Most heavy gas sources produce an initial dilution, and the domain of
almost pure ammonia is unimportant for heavy-gas dispersion. The
relatively simple pure water condensation model (thin line) is therefore
adequate for most heavy-gas calculations. The straight dotted line,
∆ρρair ≈ c ∆M∗M
air , is based on the effective molecular weight,
air
M∗ = M−∆H0c c p Tairh, which approximates the dilute limit of pure gasphase
mixtures (Nielsen & Ott, 1999). M is the contaminant molar weight
[kg mole -1 air
], cp and
T air are the heat capacity [J (kg K) -1 ] and absolute
temperature [K] of the entrained air, and the enthalpy difference [J kg -1 ]
between source and ambient conditions, ∆H 0 , is assumed constant. The
simple M∗ approximately describes the domain of dry mixing quite well,
but has a deviation of up to 78% in the domain of almost pure water
aerosols. The test scenario is however a demanding one, partly because of
the high relative humidity and partly because of the large heat of
evaporation and low molecular weight of ammonia. The M∗ approximation
will be more successful for other release conditions and it is a useful
concept when comparing the results of wind-tunnel test with those from
large-scale releases of liquefied gasses.
Cloud Dynamics. The combined forces of gravity and the ambient wind
drive heavy gas dynamics. The negative buoyancy makes the cloud spread
horizontally and gives it a tendency to move downhill in sloping terrain.
The density stratification between the cloud and the air above reduces
the turbulent kinetic energy level and vertical mixing. Thus, a heavy-gas
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