Environmental Engineering Reference
In-Depth Information
be about one-third less, and the average pH of precipitation will go up by 0.175 units, compared to
1970s levels.
9.2.6.1 Acid Deposition Modeling
Before enacting costly emission control strategies, government legislators and administrators would
like to know the expected environmental benefits of these strategies. For example, if SO 2 emissions
were reduced by one-half, will sulfate ion deposition rates decrease proportionally everywhere? This
relates to (a) the linearity of transformation of primary emission (SO 2 ) to secondary pollutant (SO 2 4 )
and (b) the geographic distribution of the secondary pollutant. To answer these questions, one has
to resort to atmospheric transport and transformation models, also called source-receptor models.
In the 1980s in the United States, Canada, and Europe, literally dozens of models were devel-
oped, ranging from simple box models (no atmospheric dynamics, just chemical reactions in an
enclosed box the size of a subcontinent) to complex Eulerian models stretching over a subconti-
nent, with individual emission rates in grids of a few km 2 along with simulated wind, diffusion,
precipitation, and other meteorologic and topographic factors. These latter models are called super-
models; they require enormous computational capacity to exercise them. However, the supermodels
can only simulate a single precipitation episode; they cannot estimate seasonal or annual average
deposition rates at widely dispersed receptors from seasonal or annual emission rates at widely
dispersed sources. These models have proven, however, that there is a near-linear proportion of
SO 2 4 deposition rates to SO 2 emission rates. Probably, these models encouraged the U.S. Congress
to enact Title IV of the 1990 CAAA, in expectation that reducing about one-half of SO 2 emissions
will halve SO 2 4 deposition rates and the commensurate hydrogen ion deposition rates.
The authors developed a simple Eulerian model with time-averaged emission rates, wind
direction and speed, diffusion rate, transformation, and deposition rates. 8 Because all parameters
are time-averaged, the model can only be applied to longer periods, such as a season or year. The
model quite successfully matched the predicted and observed annual and seasonal deposition rates
of sulfate in ENA in the years 1980-1982. Following is a brief description of that model.
The model is a solution to the general atmospheric dispersion equation under steady-state
conditions with chemical and physical transformation processes. The latter are assumed to be first
order processes. Two solutions are obtained: one for the primary emitted pollutant SO 2 , the other
for the secondary pollutant SO 2 4 :
Q
) 1 ] K 0
C p =
hD exp[ ur cos
| θ ϑ | (
2 D
r
)
(9.21)
2
π
Q
) 1 ] K 0
r
)
K 0
r
)
C s =
exp[ ur cos
| θ ϑ | (
2 D
(9.22)
2
π
hD 2
τ c
γ
2
α
2
1
τ w s +
1
D +
u 2
4 D 2
1
τ ds
2
α
(9.23)
1
τ w p +
1
D +
u 2
4 D 2
1
τ dp +
1
τ c
2
γ
(9.24)
8 See Fay, J. A., D. Golomb, and S. Kumar, 1985. Atmos. Environ., 19, 1773-1782.
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