Environmental Engineering Reference
In-Depth Information
evaporation and volatilization of organic molecules from vegetation, wetlands, and surface waters.
For example, evergreen trees exude copious quantities of isoprene, terpenes, pinenes, and other
compounds, which, by virtue of double bonds in their molecules, are very reactive with a hydroxyl
radical.
After the discovery of the presence in the atmosphere of the various oxidizing agents and other
compounds that participate in the ozone formation process, researchers developed models that
attempted to show how observed concentrations of ozone and other smog ingredients are reached
as a function of precursor concentrations, meteorological conditions, and insolation.
9.2.5.1 Photo-oxidant Modeling
Because ozone and the other photo-oxidants are secondary pollutants, the regular dispersion models
described in Section 9.2.4, pertaining to primary pollutants that do not transform while dispersing,
are not applicable. For photo-oxidant modeling, in addition to meteorological parameters, the
reactions of the primary pollutants among themselves, and those with atmospheric species, plus the
interactions with sunlight need to be included. A further complication is that some of the chemical
kinetic processes are not linear; that is, they are not first-order rate reactions.
The rate of the reaction (9.13) depends on the kind of VOC. Molecules with a double or triple
bond are very reactive, followed by aromatics, then by branched- and extended-chain aliphatics.
The simple molecule methane, CH
4
, reacts very slowly with OH; therefore CH
4
is not included in
the rate equations.
The model that has been used most frequently in the past is called the Empirical Kinetic
Modeling Approach (EKMA). It is a Lagrangian model; that is, the coordinate system moves with
the parcel of air in which chemical changes occur. The architecture of EKMA is as follows. A
column of air is transported along a wind trajectory, starting at 0800 local time (LT). The column
height reaches to the bottom of the nocturnal inversion layer. The concentration of chemical species
in the column is estimated from the 0800 LT emission rates of the following pollutants: NO, NO
2
,
CO, and eight classes of VOC: olefins, parafins, toluene, xylene, formaldehyde, acetaldehyde,
ethene, and nonreactives. Other input parameters are date, longitude, and latitude, which define
the insolation rate. As the column moves with the wind, fresh pollutants are emitted into the
column. As time progresses, the solar angle increases and the height of the column (mixing height)
increases, with a consequent dilution effect. Inside the column the photochemical reactions occur in
which ozone is generated. The rate constants for the chemical reactions are empirically determined
based on smog-chamber experiments. The model calculations stop when the ozone level reaches a
maximum value, which usually occurs between 1500 and 1700 LT.
Figure 9.6 presents an isopleth plot of maximum ozone concentrations versus 0800 LT con-
centrations of the sum of eight VOC compounds on the horizontal axis, measured in units of parts
per million carbon (ppmC), and the NO
x
concentration on the vertical axis. The isopleths have the
shape of hyperbolas. The diagonals drawn through the isopleths represent the morning ratios of
VOC/NO
x
. A ratio of 4:1 corresponds to a typical urban ratio; 8:1, suburban; 16:1, rural. Suppose
the maximum ozone level reached is 200 ppbV. The “design” (i.e., NAAQS) value is 120 ppbV. To
reach the design value, one can go in two directions: reducing either the 0800 LT VOC or the NO
x
concentrations, represented by the horizontal and vertical dashed lines in Figure 9.6, respectively.
It can be seen that in an urban environment (4:1 diagonal), the 120 ppbV level can be reached with
a much smaller reduction of VOC concentrations than NO
x
. In a rural environment (16:1 diagonal),
