Environmental Engineering Reference
In-Depth Information
In addition to baseline modeling, brute force “zero-out” simulations and
high-order decoupled direct method (HDDM) sensitivity analysis (Dunker, 1984;
Hakami et al., 2003; Koo et al., 2007) were conducted to simulate the impacts of
each of six power plants (Table 1). Four of the facilities (Martin Lake, Limestone,
Monticello, and Welsh) are among the largest NO
x
emitters in Texas and located
amidst dense biogenic emissions of isoprene (
Fig. 1
). The other two, Deepwater
and Parish, are in the Houston metropolitan region.
The zero-out source contributions (ZOC) of the facilities were computed by
differencing concentrations from the baseline and zero-out simulations:
(1)
ZOC
=
C
(
base
)
−
C
(
zero
out
j
)
i
,
j
i
i
where
C
i
is concentration of species
i
, and
j
is the facility. The HDDM first- and
second-order sensitivity coefficients, defined as the first- and second-order
derivatives of concentration to emissions respectively, characterize the responsive-
ness of concentrations to small changes in emissions.
The zero-out and HDDM results were used to develop metrics characterizing
the efficiency, spatial extent, and nonlinearity of ozone formation in each plume.
Ozone production efficiency (OPE) is the number of ozone molecules produced
per NO
x
molecule lost and is a key metric of pollutant formation in a plume (Liu
et al., 1987). Here, we approximate OPE by the ratio of ozone to NO
z
(NO
z
=
reactive nitrogen (NO
y
) - NO
x
) contributed by the plume:
ZOC
(2)
O
,
j
OPE
=
3
ZOC
NO
,
j
z
This equation assumes that all ozone formed and NO
x
lost in a plume remain as
ozone and NO
z
, respectively. Over time, deposition of nitric acid and ozone can
influence the ratio (Neuman et al., 2004; Nunnermacker et al., 2000).
To characterize the spatial extent of ozone formation, we define a Distance of
Ozone Impact metric by the following equation.
+
∑
d
(
x
)
⋅
ZOC
(
x
)
(3)
j
O
,
j
DI
=
3
x
+
∑
ZOC
(
x
)
O
,
j
3
x
Here, d
j
(x) is the distance from power plant j to cell x, and ZOC
+
O3,j(x)
is the
contribution of NO
x
emissions from j to ozone at x (excluding cells with negative
source contributions).
A final metric, the nonlinearity index ( ), characterizes the nonlinearity of
pollutant responsiveness to emissions changes (Cohan et al., 2005).
α
(
,
2
)
(4)
S
i
j
α
=
i
,
j
(
,
2
S
i
j
High levels of typically indicate that a large reduction in a NO
x
source
would yield more ozone reduction per ton than a small reduction, whereas a small
indicates that the ozone impact scales linearly with the amount of emissions
reduction.
α
α
