EPA/600/A-97/083
6.7 THE PLUME-IN-GRID TREATMENT OF MAJOR ELEVATED POINT-SOURCE EMISSIONS IN MODELS-3
Noor V. Gillani1*, Arastoo Biazar1, Yu-Ling Wu'
James Godowitch2, Jason Ching2, Robert Imhoff
1 University of Alabama in Huntsville, Huntsville, AL
2 Atmospheric Sciences Modeling Division, Air Resources Laboratory,
National Oceanic & Atmospheric Administration, Research Triangle Park, NC
3 Tennessee Valley Authority, Muscle Shoals, AL
1. INTRODUCTION
Modets-3/CMAQ (Byun et ai., 1998) is the new
comprehensive, multiseale, multi-pollutant Euierian
modeling system currently in development under the
sponsorship of the U.S. EPA. in regional applications,
as with its predecessor (RADM), its coarse grid
resolution is likely to be between 20 and 80 km. Unlike
RADM, however, CMAQ can achieve finer spatial
resolution by the use of nested grids in urban-industrial
areas with concentrated sources, and by plume-in-grid
treatment (PinG) of major elevated point-source
emissions (MEPSEs). Most MEPSEs are related to
large fossil-fuel power plants. In 1993, the utility sector
was estimated to contribute more than 72% and 33%,
respectively, of the US emissions of SOx and NOx
(EPA, 1994). Just the largest 100 and 200 of the
thousands of "major point sources" in the US contribute
about 35% and 50%, respectively, of the total NOx
emissions of this source category. Thus, these largest
MEPSEs constitute an important class, by themselves,
of the sources of ozone and aerosol precursors. Since
these MEPSEs do not also emit VOC (the other key
precursor of the ozone-aerosol chemistry), the mixing of
MEPSE NOx and background VOC is critical for plume
chemistry which is, therefore, diffusion-limited. As this
chemistry is also very non-linear, proper simulation of
such mixing is of critical importance for accurate
determination of the impact of MEPSE emissions. In
RADM and its contemporary and predecessor regional
grid models, such emissions were instantly mixed with
the VOC of the entire emission grid cell, thereby
completely bypassing the diffusion-limited subgrid-
scale (SGS) evolution of the plumes. Current practice is
shifting towards use of PinG to simulate the SGS
transport and chemistry in grid models. PinG in Models-
3 is significantly different from that in other grid models.
It is aimed particularly at realistic treatment of rural
MEPSE plumes in a coarse regional grid. This paper
provides an overview of PinG in Models-3.
2. OPERATIONAL PinG APPROACHES
PinG has been implemented in the PARIS (Seigneur et
al., 1983), UAM-V (Morris et al., 1992), SAQM (Myer et
* Corresponding author address: NoorV. Giani, Earth
System Science Lab., Univ. of Alabama, Huntsville, AL
35899. email: gillani@atmos.uah.edu
al., 1996) and URM (Kumar and Russell, 1996) grid
models. In all of these, multiple SGS Lagrangian plumes
(or puffs) within a domain can be simulated, but they are
assumed not to interact directly with each other. Each
plume expands as it advects, entraining the back-
ground, and the plume-background mixture then reacts.
Spatial resolution is achieved through discretization of
the plume cross-section: PARIS used a rectangular
plume cross-section divided into vertically well-mixed
pillars; the other models use an elliptical plume section
divided into concentric rings. The grid size was 4 km in
the single application of PARIS, and as high as 12 km in
UAM-V and SAQM. In all three, the outermost pillars or
elliptic rings are sequentially shed off to the grid model
as the remaining plume attains the width of the grid cell,
and the SGS simulation stops when the core of the
plume attains grid size. In URM, the elliptic plume
simulation is limited to a fixed one hour, when the whole
plume is handed over. Strictly speaking, the elliptic
plume model formulation is valid only as long as the
plume is fully elevated. In the convective boundary
layer (CBL), this condition typically lasts for less than
one hour. The common observation in all of the above
PinG applications has been that the PinG treatment
does not make much difference except very locally.
This is not surprising, since the above
formulations/applications have been focussed on the
detailed simulation of plume-core chemistry only in the
early phase of plume evolution.
3. Models-3/PinG : CONCEPTUAL APPROACH
Models-3 is scheduled to be released in 1998. It will
include a preliminary developmental version of PinG
which will be best suited for rural MEPSEs within a
coarse regional grid (A > 12 km or so) where the SGS
error in the representation of MEPSEs is largest without
PinG treatment. Figure 1 provides a graphical perspec-
tive of the number and distribution of such MEPSEs in
eastern USA. The "rural" cells represent likely locations
of rural MEPSEs with total NOx emission rate (QNO*)
exceeding approximately 0.3 kg/s or 10,000 tons/y
(approximately, the cut-off source strength for the top
200 MEPSEs in the US); "urban" ceils are likely
locations of urban areas (with or without MEPSEs). The
urban and rural cells together occupy only 3% of the
land mass shown, but contribute more than 50% of the
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total NOx emissions of the region (Gillani and Pleim,
1996). Some of the "rural" cells represent suburban
MEPSEs, but many are truly rural. There is a concen-
tration of MEPSEs in the Ohio River valley, but many of
the other rural MEPSEs are quite isolated. PinG
currently assumes linear superposition of SGS MEPSE
plumes. Rural MEPSEs have the lowest likelihood of
interactions, not only with other MEPSEs, but also with
the thousands of non-MEPSE "major point sources".
Figure 2 illustrates the typical chemical evolution of
a large isolated rural power plant plume, consisting of
three stages (Gillani and Wilson, 1980). In stage 1, the
relatively fresh plume is dominated by primary
emissions and a deficit of ozone due to its titration by
NO. The chemistry is mostly inorganic and VOC-limited,
In stage 3, at the other end, the plume is substantially
diluted and has a high VOCrNQx ratio, and is chemically
mature, with little NOx remaining and an excess of
ozone; the chemistry here is mostly NOx-Iimited, much
as in the background. Stage 2 marks the transition of
the chemistry from VOC-limited (Stage 1) to NOx-Iimited
(Stage 3). It is marked by growing ozone wings at plume
edges, resulting from vigorous chemistry there fed by a
good supply of both VOC (background entrainment) and
NOx (diffusion from plume core). This is the diffusion-
limited chemical stage with a complex SGS structure.
Its proper simulation is the key to accurate characteri-
zation of SGS plume chemistry. Indeed, such charac-
terization requires that a chemical criterion of success-
ful completion of stage 2 chemistry be satisfied in the
PinG solution before plume handover to the grid solution
(Gillani and Pleim, 1996), This chemical criterion is not
observed in the PinG treatments of UAM-V, SAQM and
URM, because they are necessarily limited to near-field
and plume core chemistry (as in stage 1) as a result of
Figure 1. Distribution of 20 km x 20 km "rural" (D) and
"urban" ( •) cells with NO, emission flux, qNO, , greater
than 101J molecules cm"J s"1, "Urban" cells are defined
by qvoG/QNo.3" 1 (C:N, by volume), and "rural" cells by
quoc/qNOl[< 1. (Figure from Gillani and Pleim, 1996).
their elevated elliptic ring formulation. As shown in
Figure 2 (stage 2), there is much fine structure in the
plume even when it is about 20 km wide. The plumes of
very large MEPSEs in the eastern USA (QNOx - 3 kg
NOj/s or more) generally do not attain full chemical
maturation until their width is about 30 km; smaller ones
mature at smaller widths. In rural Tennessee, such
maturation typically requires about 100 km and 6 h of
plume transport in the summer daytime CBL for large
SO, Stage I
100
ppb
Stage2
••.A /'-.Ozone
Sulfate
20
ppb
Bscat
Stage 3
_ Bscat
.*' Sulfate
10x10
1131
1138
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1231
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TIME
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Figure 2. Aircraft data of SO2 (surrogate for NOx), ozone, sulfate and Bscat (surrogate for fine aerosol mass)
during crosswind traverses on 23 Aug 1978 through the plume of a large MEPSE (TVA's Cumberland power plant
in northwest Tennessee, QNOx - 3 kg/s), illustrating the three stages of plume chemical development. The three
sets of profiles shown represent quasi-Lagrangian information at three downwind sections of the - 0700 plume
release. Plume (age.width) at the distances shown are approx. (4 h, 10 km), (5.5 h, 20 km) and (8 h, 32 km). The
plume was in elevated stable layers until almost 1030, and had just completed fumigation at 80 km, where it was
still more or less in stage 1. (Figure taken from Gillani et al., 1981).
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MEPSEs, and less for smaller ones (Giilani et al, 1997),
During much of this transport, the plume is fully mixed
over the CBL and in contact with the ground. The
elevated elliptical plume formulation of UAM-V, SAQM
and URM is not the appropriate framework for such
plume transport-chemistry simulation. The rectangular
plume geometry of PARIS is much more appropriate.
Unfortunately, the single application of PARIS (Seigneur
et al., 1983) was within a 4 km grid and the detailed
plume simulation was confined within the inner 4-km
core, dumping the critical plume edges with much SGS
structure prematurely to the coarse grid solution.
Based on such observational evidence, our goal in
PinG of Models-3 is to perform detailed plume physical
and chemical simulations until plume chemical
maturation. Figure 3 illustrates our approach in concept,
in keeping with the available hourly meteorological
information, our detailed SGS plume simulation in PinG
is designed, in essence, to transform each steady
hourly emission rate (™v) of primary species "p" from
the target MEPSE source to a steady hourly delivered
rate (™j , excess over background) of all modeled
species *f, at the appropriate plume handover time (two)
and location, to the grid model. Such transformation is
achieved by means of a detailed Lagrangian simulation
of plume dynamics and kinetics in which plume transport
after touchdown (within the CBL) is expected to last
typically about 3 to 7 hours in the summer in eastern
USA (depending mainly on source strength, plume
spread rate, background chemical composition, and
jre_3. Conceptual model of a subgrid scale
plume simulation in PinG of Models-3,
sunlight). Such simulation is facilitated by a formulation
based on a rectangular plume cross-section. A major
limitation of this Lagrangian plume approach is that
conditions of high plume shear (particularly speed
shear) cannot be modeled accurately. Such conditions
are most likely during the nocturnal transport of a
vertically well-mixed daytime plume of 1-2 km depth.
Most daytime releases, except those of the late
afternoon, are likely to be chemically mature before the
collapse of the CBL in the evening. Night-time releases
of MEPSEs in the elevated stable layers typically result
in vertically confined plumes which are spared much
shear because of their small vertical extent. Our
practice is to hand the plume over to the grid model
whenever conditions prevail, e.g., high shear, which the
plume model cannot simulate with acceptable accuracy.
4. Mode!s-3/PinG : THE BUILDING BLOCKS
For each MEPSE plume treated in PinG, its SGS
plume dynamics and chemistry must be simulated for
each release. We have developed a new plume
dynamics model (PDM). For chemistry, we have
adapted the U. of AL Huntsville Lagrangian Reactive
Plume Model; UAH-LRPM (Giilani, 1986), as the building
block. The Lagrangian entity tracked is the expanding
plume section representing emissions into a thin semi-
infinite vertical slab as it passes the target MEPSE
source. The new PDM simulates plume rise, plume
transport and plume crosswind spread (horizontal and
vertical). For releases into the nocturnal stable layer
above the shallow mixing layer, it also simulates fumiga-
tion into the rising mixing layer in the morning. PDM
uses state-of-the-art parameterizations of plume rise,
and of oy and o, representing horizontal and vertical
plume spread, or is assumed to consist of two
components representing turbulent spread (o,,) and
shear spread (a^), as follows: af = oy/ + oyihz. oyjh is
due to directional shear in the mean wind field, it is
small in coherent nocturnal plumes, but within the CBL,
its contribution can be significant after the plume has
attained substantial vertical depth. The implementation
of PDM, of course, requires appropriate meteorological
inputs, including dynamic 3-D temperature and wind
fields, as well as 2-D information about mixing height, u*
and w* (friction and convective velocities). PDM outputs
(typically every 10 minutes) include, most importantly,
the plume cross-sectional geometry, i.e., the co-ordi-
nates of the centroid and of the four plume edges,
left/right and top/bottom,
The chemistry model (UAH-LRPM; Giilani, 1986)
simulates the complete mass balance of reactive
species within such an expanding plume section. Figure
4 illustrates the processes simulated in the LRPM. The
upper plot shows a time-height view of night-time and
daytime plume releases. The lower plot shows the
plume cross-section, discretized into vertically well-
mixed pillars, and its expansion from t to t+At, where At
-------
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jre4. An illustration of the formulation of the UAH-Lagrangian Reactive Plume Model showing
the processes simulated : (a) the time-height view ; (b) the cross-sectional view.
denotes the transport (advection) time step. In the
figure, the pillar widths are shown as equal, but the
model can simulate any arbitrary non-uniform
discretization also (e.g., "Gaussian" framework, as in
PARIS). At each plume expansion, horizontally and
vertically, the plume mass not only dilutes, but there is
also entrainment from the sides (the background) and
from aloft, as well as entrainment and detrainment
between adjacent pillars. Also, there is (turbulent and
shear) diffusion of mass across pillar interfaces. In
addition, the model permits uniform surface emissions
and dry deposition to/from each pillar. Finally, LRPM
performs detailed gas-phase chemistry in each pillar,
with the same options of chemical mechanisms as
available in the Chemistry-Transport Model (CTM) of
Models-3. LRPM also uses the same chemistry solvers
as does CTM. It derives background and aloft
information from the appropriate grid cell solution. It
receives dynamic plume geometry from PDM
simulations.
5. Models-3/PinG : IMPLEMENTATION
Figure 5 shows the relationship of PDM and PinG to
Models-3/CMAQ. The main Chemistry-Transport Model
(CTM) of CMAQ has options to use three levels of
nested gridding, any one of several state-of-the-art gas-
phase chemical mechanisms, aqueous chemistry and
an aerosol module. It has two main pre-processors:
MEPPS for emissions, and MM5 for meteorology. MEPPS
also performs the selection of MEPSEs from the "major
point sources", based on the following two criteria:
1. QNO, > Qc, where Qc is a critical cut-off value
(default Qc = 0.3 kg/s = 10 kT/y as NO2);
2. h(stack) > hc, where hc is a critical cut-off value of
stack height (default hc = 100 m).
Interface processors ECIP and MCIP process the
outputs of MEPPS and MM5, respectively, to generate
appropriate inputs for CTM, PDM and PinG. PDM runs as
a pre-processor of PinG, as shown. It performs the
complete plume dynamics for each hourly release of
each MEPSE selected for PinG treatment, generating
Emissions
Chemistry-
Transport
Plume
Dynamics
Meteorology
Figure 5. Placement of PDM and PinG in Models-
3/CMAQ
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output files containing dynamic plume geometry as well
as other related information. The actual plume transport-
chemistry simulation (as in LRPM) occurs in PinG, which
is embedded in CTM and operates interactively with it at
the level of each CTM advection time step (= 10 min).
For a typical 5-day run of CTM including 100 MEPSEs,
PDM and PinG must process a total of 12000 hourly
plume releases. At each interaction with CTM, PinG
initiates new plumes (if any) and expands every "live"
plume (typically 500 - 1500 for a 100 MEPSE run,
depending on time of day) according to the PDM-
supplied plume dynamics. Consequently, each plume
entrains mass from the grid solution, and also receives
surface emissions (after plume touch-down). Also, gas-
phase chemistry (same as in CTM except for night-time
plume releases, for which a much simpler chemistry is
implemented), dry deposition and other process
operations are implemented. PinG then checks each
live plume for the handover decision, and if necessary,
performs plume handover to the appropriate grid cells,
updates the grid solution and returns control to CTM.
The following handover criteria are used.
1. Plume reaches the boundary of the grid domain;
2. Plume is exposed to precipitation > critical level;
3. Plume enters an "urban" cell, as defined previously,
after it has touched down. Night-time releases of
MEPSEs in such cells may continue to remain alive
as elevated plumes transporting above the city.
4. Major point source emissions of NOx in the cells on
either side of the plume exceed a cutoff level. This
criterion is forced because of the uni-layer plume
limitation of the current LRPM.
5. Plume chemistry attains maturity based on the
criterion that plume average value of the chemical
age parameter, NOz/NOy, exceeds a prescribed
fraction (= 0.9) of the corresponding average value in
plume environment.
6. Plume width exceeds grid size.
In addition, PinG also performs partial plume handover.
If along a plume trajectory in the CBL, the mixing height
decreases by more than a prescribed fraction, then the
portion of the plume above the diminished mixing layer is
handed over, since LRPM currently has only one
vertical plume layer. According to this criterion, a well-
mixed daytime plume will be handed over piecemeal as
the mixing height collapses at the end of the solar day.
In this manner, the possibility of high shear of such a
plume during nocturnal transport is also avoided.
6. CONCLUSION
Before the release of Models-3/CMAQ in 1998, the
initial operational versions (lOVs) of PDM and PinG will
be tested with multi-plume data of the SOS/Nashville
Ozone Field Study of 1995. Major areas of anticipated
future improvements of PinG include:
1. Higher vertical resolution of the plume to at least
match that of MM5 and CTM, both of which include
about seven layers from ground up to 800 mb (~ 2
km). Such an improvement will greatly improve the
vertical distribution of surface emissions as well as
pave the way for better treatment of non-MEPSE
elevated major point-source emissions;
2. Inclusion of aqueous-phase chemistry (same as in
CTM);
3. Inclusion of an aerosol module (same as in CTM);
4. Improved treatment of SGS plume-plume
interactions.
Acknowledgements/Disclaimer: This work is supported jointly
by TVA (under Contract TV-88817V to UAH) and by EPA (under
Contract DW64937190 to TVA). This paper has been reviewed in
accordance with EPA's peer and administrative review policies
and approved for presentation and publication. Mention of trade
names or commercial products does not constitute endorsement or
recommendation for use.
7. REFERENCES
Byun, D., et al., 1998: Description of Models-3
Community Multiscale Air Quality (CMAQ) modeling
system. 10th Joint AMS-AWMA Conf. on Air Poll.
Met., Phoenix AZ, 11-16 Jan. 1998.
EPA, 1994: National Air Quality and Emissions Trends
Report for 1993, EPA 454/R-94-026, OAQPS, U.S.
EPA, Res. Triangle Park, NC .
Gillani, N.V. and W.E. Wilson, 1980: Formation and
transport of ozone and aerosols in power plant
plumes. Annals N. Y. Acad. Sci. 338, 276-296.
Gillani, N.V., S. Kohli and W.E. Wilson, 1981: Gas-to-
particle conversion of sulfur in power plant plumes: I.
Parameterization of the gas-phase conversion rate
in moderately polluted ambient conditions. Atmos.
Environ. 15, 2293-2313.
Gillani, N.V., 1986: Ozone formation in pollutant plumes:
Development and application of a reactive plume
model with arbitrary crosswind resolution. EPA-
600/S3-86-051, U.S. EPA, Res. Triangle Park, NC.
Gillani, N.V. and J.E. Pleim, 1996: Subgrid-scale
features of anthropogenic emissions of VOC and
NOx in the context of regional Eulerian models.
Atmos. Environ. 30(12), 2043-2059.
Gillani, N.V., et al., 1997: Relative production of ozone
and nitrates in urban and rural power plant plumes:
I. Composite results of ten days of field measure-
ments. Submitted, J. Geophys. Res.
Kumar, N. and A.G. Russell, 1996: Development of a
computationally efficient reactive subgrid-scale
plume model. J. Geophys. Res. 101, 16737-16744.
Morris, R.E., et al., 1992: Overview of the variable-gid
UAM-V. 85th Annual Meeting, AWMA, Kansas City.
Myer, T., et al., 1996: The implementation of a plume-in-
grid module in SAQM. Report SYSAPP-96/06,
Systems Applications Int., San Rafael, CA.
Seigneur, C., et al., 1983: On the treatment of point-
source emissions in urban air quality modeling.
Atmos. Environ. 17(9), 1655-1676.
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TECHNICAL REPORT DATA
1. REPORT NO.
PA/600/A-97/083
2.
4. TITLE AND SUBTITLE
The Plume-in-Grid Treatment of Major Elevated Point-
Source Emissions in Models-3
S.REPORT DATE
6.PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Noor V. Gillani1, Arastoo Biazar1, Yu-Ling Wu1, James
Godowitch2, Jason Ching2, and Robert Imhoff3
8.PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
*Univ. Of Alabatna-Huntsville, Huntsville, KL
^ame as Block 12
3Tennessee Valley Authority, Muscle Shoals, AL
10.PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency
Office of Research and Development
National Exposure Research Laboratory
Research Triangle Park, NC 27711
13.TYPE OF REPORT AND PERIOD COVERED
Preprint, FY-98
14. SPONSORING AGENCY CODE
EPA/600/9
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The plume-in-grid modeling approach for treating the subgrid scale dispersion,
transport, and chemistry of pollutants contained in plumes released from major point
source stacks is described. A plume dynamics model provides the plume rise height,
plume position, and the horizontal and vertical plume dispersions in the subgrid
scale phase when the pollutants are to be simulated by the Lagrangian reactive plume
module. The Lagrangian plume algorithm is embedded in the Eulerian chemistry-
transport grid model and operates simultaneously. A coupling exists between the
models as the grid concentrations provide boundary conditions for entrainment to the
subgrid plume sections, and a feedback ocurrs when the plume size and chemical
criteria indicate a plume's pollutants are to be transferred to the grid system for
further simulation. The methods for simulating the plume processes are also
presented and the input/output data involved with these two components of the
Models3 plume-in-grid approach are discussed.
17.
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