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Evaluation of Addition of Terrain Treatment to the
RLINE Source Type in AERMOD
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EPA-454/R-23-012
October 2023
Evaluation of Addition of Terrain Treatment to the RLINE Source Type in AERMOD
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Air Quality Assessment Division
Research Triangle Park, NC
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Preface
This document provides details for the incorporation of terrain into the RLINE Source Type. The
RLINE source type is a unique implementation, therefore the incorporation of terrain could not exactly
mirror that of POINT, VOLUME, and AREA sources in AERMOD, however the methods in RLINE
with terrain followed those of other source types with terrain as closely as possible. Details include
model formulation, AERMOD code modification, evaluation of suggested code changes, and
comparison to other AERMOD source types which include terrain.
11
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Acknowledgements
The model code changes and analyses for this document were performed by WSP Environment &
Infrastructure, Inc under contract number GS-00F-314CA 68HERH20F0394. EPA thanks the efforts of
Michelle Snyder, Laura Kent, and Melissa Buechlein for this project. The Office of Air Quality
Planning and Standards also thanks David Heist and R. Chris Owen of EPA's Office of Research and
Development for their efforts in updating RLINE.
111
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Table of Contents
1.0 Introduction 1
2.0 Terrain Treatment in AERMOD 1
2.1 Calculation of Critical Height and Terrain Weighting Factor 1
2.2 Calculation of Total Concentration 4
3.0 Original RLINE Source Type 5
3.1 Wind speed calculation for RLINE source type 6
3.2 Original Calculation of Total Concentration for RLINE Source 7
4.0 Modification of RLINE Source Type Calculations to Include Terrain Treatment 7
4.1 Calculation of critical height and terrain weighting factor using RLINE's wind speed profile .. 7
4.2 Modified calculation of total concentration to account for terrain 7
5.0 Model Intercomparison Setup 8
5.1 Meteorology 8
5.2 Source Input 9
5.3 Receptors 10
6.0 Model Evaluation and Comparison: Results 11
6.1 Low Wind Speed - Stable 11
6.2 Moderate Wind Speed - Convective 14
6.3 High Wind Speed - Neutral 17
6.4 Yearlong Meteorology 20
7.0 Discussion and Conclusions 22
8.0 Future Work 23
9.0 References 23
iv
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List of Figures
Figure 1: AERMOD two plume approach. The total concentration predicted by AERMOD is the
weighted sum of the two plumes. Figure from (U.S. EPA, 2023) 2
Figure 2: Construction of the weighting factor between two-plumes of terrain in AERMOD. Figure from
(U.S. EPA, 2023) 2
Figure 3: Windrose containing the wind speed and direction information for the year of meteorological
data that came from Raleigh-Durham International Airport 9
Figure 4: Terrain elevation (Zelev) for all receptor grids and slope angles examined 11
Figure 5: Gradient plot of LINE, RLINE, and VOLUME source types for all terrain cases during the
stable hour 12
Figure 6: Spatial plot of the LINE, RLINE, and VOLUME source types for flat terrain during the stable
hour 13
Figure 7: Spatial plot of the LINE, RLINE, and VOLUME source types for ridge terrain during the
stable hour 14
Figure 8: Gradient plot of LINE, RLINE, and VOLUME source types for all terrain cases during a
convective hour 15
Figure 9: Spatial plot of the LINE, RLINE, and VOLUME source types for flat terrain during a
convective hour 16
Figure 10: Spatial plot of the LINE, RLINE, and VOLUME source types for ridge terrain during a
convective hour 17
Figure 11: Gradient plot of LINE, RLINE, and VOLUME source types for all terrain cases during the
neutral hour 18
Figure 12: Spatial plot of the LINE, RLINE, and VOLUME source types for flat terrain during the
neutral hour 19
Figure 13: Spatial plot of the LINE, RLINE, and VOLUME source types for ridge terrain during the
neutral hour 20
Figure 14: SRCTYPE LINE versus RLINE for v22112 and v23132 for the Flat and Ridge terrains along
X-axis 21
Figure 15: SRCTYPE VOLUME versus RLINE for v22112 and v23132 for the Flat and Ridge terrains
along X-axis 22
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List of Tables
Table 1: Comparison of the stability function equations used in AERMOD generally and in RLINE 6
Table 2: Subset of representative meteorology used in analysis 8
Table 3: Range of values from Raleigh-Durham International Airport during 2018 9
Table 4: AERMOD input source parameters for the different source types 10
Table 5: Receptor grid slope angles and terrain types 10
Table 6: Stable hour with low wind speeds from the representative meteorology 11
Table 7: Convective hour with moderate wind speeds from the representative meteorology 14
Table 8: Neutral hour with high wind speeds from the representative meteorology 17
vi
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1.0 Introduction
The RLINE model is a dispersion model originally developed by the US EPA's Office of Research and
Development. The model is based on a steady-state Gaussian formulation and is designed to simulate
line-type source emissions from near-surface releases. The RLINE source type was initially developed
for use in flat terrain, which ignored the localized elevation and hill height variations at source and
receptor locations. The restriction of the RLINE source to only flat terrain ignores complex terrain in the
calculation of dispersion. Currently, the treatment of terrain by the POINT, AREA, VOLUME, and
OPENPIT sources use the concept of the two-plume model based on (Venkatram et al., 2001). This
approach simulates one plume's concentration at the receptor height and the other plume's concentration
at the receptor height plus the terrain height and the composite of the two plumes accounts for the total
concentration from each source. The objective of this work is to incorporate terrain dispersion treatment
into the RLINE source type using the existing two-plume methodology in AERMOD. Implementation of
terrain treatment for the RLINE source was tested with a variety of meteorological data. When possible,
the behavior of the RLINE source incorporating terrain was compared to the other AERMOD source
types (AREA/LINE and VOLUME) to ensure similar results when considering terrain.
2.0 Terrain Treatment in AERMOD
The AERMOD system uses multiple preprocessors before the computation of dispersion concentrations.
One preprocessor is a terrain preprocessor, AERMAP (U.S. EPA, 2018), that handles variability in
terrain heights in the modeling domain. AERMAP uses gridded terrain data, usually from national
databases, to determine source and receptor terrain heights and these heights are input into AERMOD.
These terrain heights are then used in the computation of a critical height and the weighting factor used
in computation of dispersion from the emission source at each receptor location. The following sections
describe the computation of the terrain heights, critical height and weighting factor, then finally the
computation of the dispersion concentration.
2.1 Calculation of Critical Height and Terrain Weighting Factor
The two-plume model utilized by AERMOD calculates the dispersion plume for two cases: a horizontal
terrain impacting plume and a terrain following plume illustrated in Figure 1 (U.S. EPA, 2022). Figure 2
represents how these two plumes are computed separately, then combined by the weighting factor,
which represents the proportion of plume mass in the horizontal impacting state.
1
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Figure 1: AERMOD two plume approach. The
total concentration predicted by AERMOD is
the weighted sum of the two plumes. Figure
from (U.S. EPA, 2023)
Figure 2: Construction of the weighting factor
between two-plumes of terrain in AERMOD.
Figure from (U.S. EPA, 2023)
2
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The dividing streamline assumes the air below the critical height tends to move around the terrain object
(the horizontal plume state, middle panel of Figure 1) and the air above the critical dividing streamline
will travel over the terrain object (the vertical terrain responding plume state, bottom panel of Figure 1).
Thus, AERMOD first computes the critical height, then divides the plume using the dividing streaming
concept.
The critical height, Hc is the height at which the parcel of air has just enough kinetic energy to reach the
receptor height. The calculation of the critical height, is given by Eq. 1, originally published as Eq. 7 in
(Venkatram et al., 2001):
\u2{Hc} = f (z-()N2(Od( (1)
He
Where,
u{Hc} is the wind speed at the critical height
z is the receptor height,
Hc is the critical height,
£ is the potential temperature,
JV(£) is the Brunt-Vaisala frequency which is defined as
Only the lowest height that satisfies Eq. 1 is necessary to show that there is sufficient kinetic energy to
maintain a streamline i.e., terrain-following. Eq. 1 from (Venkatram et al., 2001) defines Hc in relation
to the terrain following height at each receptor location.
AERMOD uses the hill height scale, hc from AERMAP to calculate the critical height, Hc. The hill
height scale is the height with the greatest influence on dispersion for each receptor. The calculation of
the critical height uses the hill height scale, as defined in Eq. 49 of the AERMOD Model Formulation
(U.S. EPA, 2023) document (MFD).
1 _ rhc (MFD Eq. 49)
iu2{Hc}= f N2Qic — z)dz
Z JHn
The fraction of mass below the critical height, p, can be calculated using the critical height as shown in
MFD Eq. 50 (U.S. EPA, 2023):
<±> =
S"c Cs {xr,yr,zr}dz (MFD Ecl- 50)
J0 Cs{xr,yr,zr}dz
Where,
Cs {xr,yr,zr} is the concentration of the plume in the absence of the hill for stable conditions,
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The plume weighing factor, /, can be calculated from (pp using Eq. 2. The value of
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b. Terrain following plume (receptor height = flagpole)
c. Combine using the terrain weighing fraction.
3. Computation of total concentration: combine the coherent plume and the meander plume using
the meander weighting fraction.
The order of the calculation for the POINT source is important as it shows that terrain is incorporated
before the impacts of meander are considered. Note, the VOLUME source calculations are a special
application of the POINT source calculation, so it would follow this same process. In addition, the
AREA (and LINE, a special case of the AREA calculation) follows the same general procedure but does
not include the meander and meander weighting components, so it would only have the computation of
the coherent plume (step 1). The goal of the current effort is to follow the same calculations and general
procedure, as closely as possible, for the RLINE source type calculations. This would need to involve
the entire 3-step process, since RLINE includes the meander weighting.
3.0 Original RLINE Source Type
The RLINE source type was developed to simulate line type source emissions by integrating point
sources along the line, and it can include the effects of barriers or depressed roadway segments. The
original RLINE source does not account for the effects of terrain when calculating the total
concentration, which can lead to over/under estimation of concentrations, depending on the source and
receptor orientation.
When RLINE calculates concentrations, it adjusts the coordinate system so that the source lies along the
Y-axis (perpendicular to the wind direction). The concentration of an RLINE source is found by
approximating the line as a series of point sources using a Gaussian plume formulation, and the
concentration at the receptor at (xr, yr, zr) can be expressed as a summation of the point sources along
the line, as shown in Eq. 3, originally published in (Snyder et al., 2013) as Eq. 10
Where,
C(xr, yr, zr) is the concentration at the receptor,
Y1 is the initial point of the RLINE source,
L is the length of the RLINE source,
dCpt is the contribution from an elemental point source.
The number of points necessary for convergence is variable and is a function of the distance from the
source line to the receptor.
C (pCy 13V i ^ Cp £
' 1
(3)
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3.1 Wind speed calculation for RLINE source type
The POINT, AREA, VOLUME, and OPENPIT sources calculate the critical height using 87 gridded
profile heights, 0 to 5000 meters, to calculate the effective wind speed. AERMOD calculates the vertical
wind profile using the Monin-Obukhov similarity theory, MFD Eq. 28 (U.S. EPA, 2023).
u,
U = ~k
u - u{7z0] [^-] for z < 7z0 (MFD Eq. 28)
ln (f) " ^ £} + ^ ©] f°T 7Z° " Z < Zl
u = u{zt} for z > zt
Where,
u is wind speed (m/s),
ut is surface friction velocity (m/s),
z0 is the surface roughness length (meters),
Zj is the mixing height (meters),
z is the height (meters),
L is the Monin-Obukhov length,
is the stability function for momentum which is defined in Table 1.
AERMOD interpolates the wind speed at the observed profile height by calculating the wind speed
using the similarity theory at the gridded profile height directly above and below the observed profile
height.
The RLINE source differs from the other AERMOD sources in the wind speed profile calculations.
There are two major differences to the wind speed calculations in RLINE compared to AERMOD. First,
RLINE follows the MFD Eq. 28 expect above the boundary layer height, z > Zj, where RLINE
continues to use the equation for 7z0 < z < zt. AERMOD keeps the wind speed constant above the
boundary layer, and RLINE continues to calculate an increasing wind speed. The second major
difference is that RLINE includes the displacement height in The changes to the calculation of
in RLINE and AERMOD are described in Table 1.
Table 1: Comparison of the stability function equations used in AERMOD generally and in RLINE
l^imnu-UT
\i:k\]()I)
ui.im:
Stable
''' tiJ
17 ('
17 h c J
/ —29 z„\
-17(l-e l j
Convcclivc
Mr)
A + *o\ •, n
2 log {—2^) + log [—2^) ~ 2 tan (*o) + 2
/ 16z\0'25
( 16 (z — dh)\0'25
ti f1+xz°\, i -i/v ^ , n
gl 2 J + log( 2 / (Xzo) + 2
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Where,
z0 is the surface roughness length (meters),
z is the height (meters),
L is the Monin-Obukhov length,
dh is the displacement height (meters).
3.2 Original Calculation of Total Concentration for RLINE Source
The original RLINE source order of the calculations are as follows, calculation of:
1. Concentrations from the coherent plume (receptor height = flagpole)
2. Concentrations from the meander plume (receptor height = flagpole)
3. Total concentration by combining meander and coherent plumes using the meander weighting
fraction.
4.0 Modification of RLINE Source Type Calculations to Include Terrain Treatment
The original RLINE source did not account for the effects of terrain when calculating the total
concentration, which can lead to over/under estimation of concentrations. In AERMOD terrain is
included by weighting the coherent and random portions of the plume, and when incorporating terrain
into RLINE the same methods were followed.
4.1 Calculation of critical height and terrain weighting factor using RLINE's wind speed profile
The RLINE source type incorporates terrain by first calculating the critical height and a terrain weighing
factor using the critical height. The computation of critical height depends on the wind speed as a
function of height. Again, RLINE has a wind speed profile slightly different from the other AERMOD
source types, as was described in Section 3.0. A wind speed profile is computed for the same heights as
the other AERMOD sources and is used instead of the AERMOD wind speed array in the computation
of critical height.
4.2 Modified calculation of total concentration to account for terrain
The steps to calculate the concentration of RLINE source that incorporate terrain include, the calculation
of:
1. Computation of concentrations from the two coherent plumes:
a. Horizontal impacting plume (receptor height = flagpole + terrain)
b. Terrain following plume (receptor height = flagpole)
c. Combine using the terrain weighting fraction.
2. Computation of meander plume:
a. Horizontal impacting plume (receptor height = flagpole + terrain)
b. Terrain following plume (receptor height = flagpole)
c. Combine using the terrain weighing fraction.
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3. Computation of total concentration: combine the coherent plume and the meander plume using
the meander weighting fraction.
5.0 Model Intercomparison Setup
AERMOD was run with two sets of meteorological data and seven different receptor grids. The seven
receptor grids were designed to test the impacts of terrain at a variety of slope angles chosen based on
the thresholds for AERMAP (U.S. EPA, 2018). The smaller set of meteorological data was used for
most of the testing to examine impacts of a variety of stability conditions and wind speeds. The larger,
yearlong dataset included a multitude of wind directions, atmospheric conditions, and surface roughness
values, and was used to test the impacts of terrain on runtimes and in a wide range of realistic
meteorology conditions.
5.1 Meteorology
The AERMOD runs used a simple, representative meteorology, created from MAKEMET (U.S. EPA,
2022), which includes 22 hours covering 6 stability conditions and wind speeds ranged from 0.5 - 18.0
m/s. One run was done for a year of meteorological data which used data from Raleigh-Durham
International Airport (RDU) during the entire year of 2018. This captured a wide range of real
meteorological conditions and scenarios which were not shown in the representative meteorology
dataset.
5.1.1 Representative Meteorology
The influence of terrain on the total concentration at a receptor varies depending on the meteorological
conditions; therefore, various conditions were tested. Most of the model runs used a simple
representative meteorology created from MAKEMET. Winds are from 270 degrees in all hours, which
would blow along the positive x-axis for all source and receptor configurations. A range of surface
roughness values from 0.01m to 1.0m was used to generate 22 hours of representative meteorology;
however, only a subset was selected for specific presentation in this document.
The subset consists of three hours as shown in Table 2Error! Reference source not found.. The three
hours represented all atmospheric stability conditions and a range of wind speeds. The terrain algorithm
behaves differently in convective and stable conditions, as described in Section 0. These three hours
show variable response to the ridge terrain feature in each atmospheric stability condition and are
compared with the terrain response of the LINE and VOLUME source types in Section 6.0.
Table 2: Subset of representative meteorology used in analysis
l);ik-lillH-
\\ ind Spivd
(m/s)
Surl':Ki-
kdll^lllH'SS (111)
I
(lll/s)
\\
(lll/x)
Mi\in(;
1/1/2000 20:00:00
0.5
0.048
-9
24
13.4
6
2/17/2000 12:00:00
4
0.01
0.284
1.8
756
-7.4
1
2/26/2000 6:00:00
10
0.589
1.2
1041
-285.2
4
8
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5.1.2 One Year of Meteorology
The most recent year of pre-processed meteorological data from the North Carolina Department of
Environmental Quality (NCDEQ) for RDU was downloaded and used for this analysis
(https://www.deq.nc.gov/about/divisions/air-quality/air-quality-permitting/modeling-
meteorology/meteorological-data). The year of meteorological data from RDU was paired with
concurrent upper air data from the Piedmont Triad International Airport (GSO) in Greensboro, NC, and
the AERSURFACE options reflect "wet" conditions for 2018. This data set covered a wide range of
values which are described in Table 3. The total number of valid hours is 8714 of the possible 8760 for
2018, which represents 99.5% completeness. This large set of meteorological hours show variable
response to the ridge terrain feature and are compared with the terrain response of the LINE and
VOLUME source types in Section 6.0.
Table 3: Range of values from Raleigh-Durham International Airport during 2018
Wind Speed
(m/s)
Wind
Direction
(deg)
Surface
Roughness
(m)
U* (m/s)
W* (m/s)
Mixing Height
(m)
Monin-
Obukhov
Length (m)
PG
0.0-12.16
0-360
0.0170-
1.2860
0.017-1.98
0.010-1.894
5 - 4000
-8888.0-
8888.0
1-6
RDU Yearlong Meteorology
#
Wr
SW
Wind Speed (m/s)
¦ 0 s 2.6
¦ 2.6 s 5.2
5.2 S 7.8
¦ 7.8 s 10.4
¦ 10.4 £ 13
Figure 3: Windrose containing the wind speed and direction information for the year of meteorological
data that came from Raleigh-Durham International Airport
5.2 Source Input
The RLINE roadway source was centered at the origin and runs along the x=0 axis of the receptor grid
with a width of 3.6 meters and length of 600 meters. Equivalent LINE and VOLUME sources were
9
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generated for direct comparison with the RLINE source type. Source parameters for each source type are
shown in Table 4, where each source type will have a total emission of 216 g/s.
Table 4: AERMOD input source parameters for the different source types.
So 11 I'll'
Number ill' Siiunvs
(ill)
ri> (in)
Length (in)
\\ iillli (in)
Release 1 leiglil (in)
lllllissiiui rale
rline
1
N/A
600
3.6
0.1 g / nr/s
LINE
2.0
1.3
0.1 g/m2/o
VOLUME
167
1.674
N/A
N/A
1.2934 g/s
5.3 Receptors
A dense spatial grid of receptors (-500 m - 500 m) with 10 m spacing in the x and y directions was used
for all cases, for a total of 10,200 receptor locations. Rather than using AERMAP, a similar process was
used to determine the terrain and hill heights for all receptor locations in the spatial grid.
Four types of receptor grids were used for analysis to fully resolve the impacts of terrain at different
elevation gradients. Table 5 describes the different combination of terrain types and elevation profiles
which were used to make the receptor grids.
Table 5: Receptor grid slope angles and terrain types
Terrain Tvpe
Slope Angle
Upslope Starts
Peak
Downslope Ends
Max Height
Flat
None
N/A
N/A
N/A
0.0 m
Ridge
5-degrees
x= 110m
x = 250 m
x = 390 m
12.5 m
10-degrees
24.69 m
30-degrees
80.83 m
Figure 4 spatially represents all terrain and angle combinations, colored by terrain elevation. In the
ridge terrain example, there is a clear line of elevated terrains.
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Comparison of AERMOD Concentration
and Source LINE
Data was queried by rule: Source == "LINE"
FLAT 0 degrees
Ridge 10 degrees
Ridge 05 degrees
Ridge 30 degrees
Zele
I
-500 -250
250 500 -500 -250
250 500
Figure 4: Terrain elevation (Zelev) for all receptor grids and slope angles examined.
6.0 Model Evaluation and Comparison: Results
The RLINE terrain implementation is compared with the VOLUME and LINE sources, which both
incorporate terrain and can approximate a line/roadway type source equivalent to an RLINE source.
During comparison, note the LINE source type does not include meander like the VOLUME and RLINE
sources; therefore, only the downwind concentrations are examined. Also, consider that during low wind
conditions the LINE source type may overpredict compared to VOLUME and RLINE due to the lack of
a low wind meander plume.
6.1 Low Wind Speed - Stable
The stable hour from the 22 hours of representative meteorology that was used for discussion of the
RLINE source with terrain is shown in Table 6. During stable atmospheric conditions, the terrain
weighting factor will vary between % and 1 as described in Section 2.1. The resulting concentrations in
this section will fully represent the impacts of terrain.
Table 6: Stable hour with low wind speeds from the representative meteorology
Datetime
Wind Speed
(m/s)
Surface
Roughness (m)
u*
(in/s)
W*
(m/s)
Mixing
Height (m)
Monin-Obukhov
Length (m)
PG
1/1/2000 20:00:00
0.5
0.01
0.048
-9
24
13.4
6
n
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The gradient plots in Figure 5 shows the concentration for each model and terrain type along Y = 0 m
(which is the center of the source and terrain feature) and X > -25 m (to remove upwind concentrations
from analysis). In all terrain cases, RLINE concentrations are lower than the LINE and VOLUME
source concentrations. The RLINE source with ridge terrain feature has a larger decrease in
concentrations in response to the terrain than the LINE or VOLUME sources. The RLINE source type
was affected further upwind and downwind of the terrain feature than LINE or VOLUME. There is also
a noticeable difference in the peak concentration for each source type. This is due to a few factors: the
VOLUME source has an exclusion zone, so not all volume sources contribute to the concentration at
receptors closest to the source roadway source; the AREA/LINE source does not include meander
treatment, so when receptors are within the source width only the portion of the source upwind of the
receptor contributes to the receptor concentration. The discrepancy in peaks is particularly noticeable for
the receptor at 0 m.
Comparison of AERMOD Concentration
Datetime 2000-01-01 20:00:00,
and Met Case zO 0.01; Bowen ratio 0.8; Albedo 0.15; Wind speed 0.5 m/s
Data was queried by rule: Ycoord == 0 & Xcoord > -25 & Version == "v23132_dev"
Figure 5: Gradient plot of LINE, RLINE, and VOLUME source types for all terrain cases during the
stable hour
The case with no terrain and a flat elevation profile was used as a baseline to test if the incorporation of
terrain into RLINE had impacts that were unexpected and identify the differences in concentrations
between the three source types.
The spatial plots shown in Figure 6 show the difference in the spread of concentrations in the LINE
source type compared to RLINE and VOLUME. Along the x-axis the RLINE and LINE concentrations
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are similar, and the VOLUME concentrations are significantly lower and decrease more quickly with an
increased downwind distance.
Comparison of AERMOD Concentration
Datetime 2000-01-01 20:00:00,
and Hill Shape FLAT
Data was queried by rule: avgconc > 2 & Version = "v23132_dev"
1 -hour
AERMOD Concentration
I 500000
I
400000
300000
200000
100000
Figure 6: Spatial plot of the LINE, RLINE, and VOLUME source types for flat terrain during the stable
hour
One of the terrain shapes, the ridge, was run for three slope angles, 5-, 10-, and 30-degrees. The spatial
plots showing the concentrations for the three source types, Figure 7, show a similar result to the flat
terrain case. RLINE concentrations downwind of the terrain feature are slightly higher than those for the
LINE and the VOLUME sources. The area of low concentration over the terrain feature is slightly wider
for the RLINE source than the other two source types. The overall shape of the concentration changes is
the same between the three sources.
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Comparison of AERMOD Concentration
Datetime 2000-01-01 20:00:00,
and Hill Shape Ridge
Data was queried by rule: avgconc > 2 & Version == "v23132_dev"
250-
-250-
-500-
o
o
o
>-
-250-
-500-
1-hour
AERMOD Concentration
I
500000
400000
300000
200000
100000
-250 0
Xcoord
500 -500
Figure 7: Spatial plot of the LINE, RLINE, and VOLUME source types for ridge terrain during the
stable hour
6.2 Moderate Wind Speed - Convective
The convective hour highlighted for discussion is shown in Table 7. Recall from Section 2.1, during
convective conditions the terrain weighting factor is set to 'A as the plume is assumed to be entirely
above the critical dividing streamline height.
Table 7: Convective
lour with moderate wind speeds from the representative meteorology
Datetime
Wind Speed
(m/s)
Surface
Roughness (m)
u*
(m/s)
W*
(m/s)
Mixing
Height (m)
Monin-Obukhov
Length (m)
PG
2/17/2000 12:00:00
4
0.01
0.284
1.8
756
-7.4
1
14
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The gradient plots in Figure 8 show the concentration for each model and terrain type. Each figure is
queried to Y = 0 to highlight the center of the source and terrain feature, and X > -25 m to remove
upwind concentrations from analysis. In the flat terrain case, RLINE concentrations are slightly below
the LINE and VOLUME source concentrations. The results from the variety of terrain cases continued
this trend in concentrations.
Comparison of AERMOD Concentration
Datetime 2000-02-17 12:00:00,
and Met Case zO 0.01; Bowen ratio 0.8; Albedo 0.15; Wind speed 4 m/s
Data was queried by rule: Ycoord == 0 & Xcoord > -25 & Version == "v23132_dev"
FLAT
l\
4
k
1 v
JV
Ridge
HillAngle
¦ 0
• 05
A 10
~ 30
HillAngle
Hill Angle
10
30
Figure 8: Gradient plot of LINE, RLINE, and VOLUME source types for all terrain cases during a
convective hour
The case with no terrain and a flat elevation profile was used as a baseline to test if the incorporation of
terrain into RLINE had impacts that were unexpected and was used as a baseline for the differences in
concentrations between the three source types.
The spatial plots shown in Figure 9 show the difference in the spread of concentrations in the LINE
source type compared to RLINE and VOLUME. However, along the x-axis, the concentrations are
similar, VOLUME and LINE appear to have higher concentrations away from the source, which is
centered on (0,0). The concentrations for the RLINE source type decrease more quickly with increased
downwind distance at all points along the source when compared to the VOLUME and LINE sources.
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Comparison of AERMOD Concentration
Datetime 2000-02-17 12:00:00,
and Hill Shape FLAT
Data was queried by rule: avgconc >2 & Version == "v23132_dev"
Figure 9: Spatial plot of the LINE, RLINE, and VOLUME source types for flat terrain during a
convective hour
The ridge terrain shape was run for three slope angles, 5-, 10-, and 30-degrees. The spatial plots showing
the concentrations for the three source types, Figure 10, show a similar result to the flat terrain case. The
RLINE source has a sharper decrease in concentrations with increased downwind distance and an
overall slightly lower concentrations than the VOLUME or LINE sources.
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Comparison of AERMOD Concentration
Datetime 2000-03-08 07:00:00,
and Hill Shape Ridge
Data was queried by rule: avgconc >2& Version == "v23132_dev"
•500
500
Lll>
1
IE
RL
1
NE
VOL
1
UME
1
1
J
1
1
|
1
1
1
1
1-hour
AERMOD Concentration
I
100000
50000
-500 -250
250 500 -500 -250 0 250
Xcoord
500 -500 -250
Figure 10: Spatial plot of the LINE, RLINE, and VOLUME source types for ridge terrain during a
convective hour
6.3 High Wind Speed - Neutral
One neutral hour from the 22 hours of representative meteorology was used for discussion of the RLINE
source with terrain. During neutral stability conditions the terrain weighting factor is set to V2 as the
plume is assumed to be entirely above the critical dividing streamline height.
Table 8: Neutral hour with high wind speeds from the representative meteorology
Datetime
Wind Speed
(m/s)
Surface
Roughness (m)
u*
(m/s)
W*
(m/s)
Mixing
Height (m)
Monin-Obukhov
Length (m)
PG
2/26/2000 6:00:00
10
0.01
0.589
1.2
1041
-285.2
4
The gradient plots in Figure J1 shows the concentration for each model and terrain type along Y = 0
which is the center of the source and terrain feature and where X > -25 m to remove upwind
concentrations from analysis. In the flat terrain case RLINE concentrations are between the LINE and
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VOLUME source concentrations. The results from the variety of terrain cases continued this trend in
concentrations. The RLINE source with ridge terrain feature and a 30-degree slope angle has a larger
decrease in concentrations in response to the terrain than the LINE or VOLUME sources.
Comparison of AERMOD Concentration
Datetime 2000-02-26 06:00:00,
and Met Case zO 0.01; Bowen ratio 0.8; Albedo 0.15; Wind speed 10 m/s
Data was queried by rule: Ycoord == 0 & Xcoord > -25 & Version == "v23l32_dev"
Figure 11: Gradient plot of LINE, RLINE, and VOLUME source types for all terrain cases during the
neutral hour
The case with no terrain and a flat elevation profile was used as a baseline to test if the incorporation of terrain
into RLINE had impacts that were not expected and identify the differences in concentrations between the three
source types.
The spatial plots shown in Figure 12 show the difference in the spread of concentrations in the LINE source type
compared to RLINE and VOLUME. Along the x-axis the concentrations are similar between the three sources.
The concentrations for the RLINE source type decrease more quickly with increased downwind distance at all
points along the source when compared to the VOLUME and LINE sources.
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Comparison of AERMOD Concentration
Datetime 2000-02-26 06:00:00,
and Hill Shape FLAT
Data was queried by rule: avgconc > 2 & Version == "v23132_dev"
1 -hour
AERMOD Concentration
| 12500
10000
7500
5000
2500
500 -500 -250 0 250 500 -500 -250 0 250
Xcoord
Figure 12: Spatial plot of the L INE, RLINE, and VOLUME source types for flat terrain during the
neutral hour
One of the terrain shapes, the ridge was rnn for three slope angles, 5-, 10-, and 30-degrees. The spatial
plots showing the concentrations for the three source types, Figure 13, show a similar result to the flat
terrain case. The RLINE source has a sharper decrease in concentrations with increased downwind
distance and overall, slightly lower concentrations than the VOLUME or LINE sources.
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Comparison of AERMOD Concentration
Datetime 2000-02-26 06:00:00,
and Hill Shape Ridge
Data was queried by rule: avgconc > 2 & Version == "v23132_dev"
LINE
RLINE
VOLUME
5 o
1-hour
AERMOD Concentration
I
10000
5000
-500 -250
250 500 -500
-250 0 250
Xcoord
500 -500 -250
250 500
Figure 13: Spatial plot of the LINE, RLINE, and VOLUME source types for ridge terrain during the
neutral hour
6.4 Yearlong Meteorology
In addition to the representative meteorology, one year of meteorology collected from RDU was used to
gain a better understanding of the runtimes associated with the new algorithms and to see the impacts on
the model in more diverse meteorological condition. A summary of the meteorological conditions can be
found in Table 3. The results of these runs can be found in Figure 14. Overall, the RLINE concentrations
are slightly lower than the LINE concentrations. This lower RLINE concentration is due to the treatment
of meander, as some of the emissions mass is shifted into the meander plume.
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Comparison of AERMOD Concentration
and SRCGROUP ALL
Data was queried by rule: Ycoord == 0
Figure 14: SRCTYPE LINE versus RLINE for v22112 and v23132 for the Flat and Ridge terrains along
X-axis.
RLINE concentrations from v22112 (without RLINE terrain) v23132 (including RLINE terrain) are
compared to the VOLUME source in Figure 15. The results of this intercomparison showed that during
stable conditions concentrations were typically lower for small concentrations when accounting for
terrain, but higher for the highest concentrations when accounting for terrain. However, concentrations
are nearly identical during neutral and slightly higher in convective conditions.
Outliers in Figures 14 and 15, where RLINE concentrations are greater than a factor of two higher than
the LINE and VOLUME sources are where receptors are located very close to the source. A similar
observation was seen in the gradient plots (for e.g., Figure 11), where the RLINE peak was higher than
the LINE and VOLUME sources due to exclusion of part of the source or exclusion zones for receptors
very close to the source.
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Comparison of AERMOD Concentration
and SRCGROUP ALL
Data was queried by rule: Ycoord == 0
FLAT
Ridge
Hill_Angle
¦ 0
• 05
~ 10
~ 30
winds
I
200000 400000
Figure 15: SRCTYPE VOLUME versus RLINE for v22112 and v23132 for the Flat and Ridge terrains
along X-axis.
7.0 Discussion and Conclusions
The RLINE source type was developed for use in flat terrain, which ignored the localized elevation and
hill height variations at source and receptor locations and the complex terrain in the calculation of
dispersion. The addition of terrain into RLINE mimicked the implementation of terrain in the VOLUME
and LINE sources as closely as possible. RLINE was developed as an independent source which was
incorporated into AERMOD. There are multiple other differences between RLINE formulation and the
other source types currently in the model, though it should be noted that there are differences in
formulations between these other source types (e.g., the AREA source does not account for meander or
only the POINT source can model plume rise). There are two differences that can have an impact on the
implementation of terrain: 1) RLINE uses different dispersion curves and 2) RLINE estimates the
transport windspeed as a function of height slightly different than the other AERMOD sources. Since the
terrain weighting factor uses critical height, which is impacted by the wind speed profile, there will
inherently be small differences in the RLINE terrain processing from the other AERMOD sources,
though no more so than for dispersion without terrain considerations. Modifications to the calculation of
critical height were made since RLINE generates a wind table which uses displacement height in the
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calculation of the vertical profile of wind speed, rather than a displacement height of 0 as is done in
other AERMOD source types.
The incorporation of terrain into RLINE concentration calculations was tested with seven receptor grids,
two meteorological datasets, and compared against two source types in AERMOD which include terrain.
The analysis detailed in this document highlighted three hours of the 22 representative meteorology
hours which covered three stability conditions and three wind speeds. During convective and neutral
conditions, the terrain weighting factor is set to V2 for all source types and the results of the model
analysis showed RLINE concentrations that were either between the VOLUME and LINE source or
slightly below their concentrations. The RLINE source did have a larger response to terrain than either
source, especially for the 30-degree slope. As noted above, some of the underlying differences in the
RLINE formulation (dispersion curves and transport wind speeds, in addition to the lack of meander for
the LINE source) will inherently lead to differences between the three model approaches.
The terrain weighting factor varies as a function of the wind speed and temperature gradient during a
stable atmosphere and the results of the analysis during this meteorology hour showed the largest
impacts of terrain on all three source types. RLINE again had slightly lower concentrations than either
LINE or VOLUME sources at the terrain feature. RLINE also had a larger response to terrain for all
slope angles.
A yearlong meteorological dataset was also used to gain a better understanding of the runtimes
associated with the new algorithms and to see the impacts on the model in more diverse meteorological
conditions. The yearlong model test did not reveal any potential issues and supports the conclusion that
terrain processing has been successfully added to the RLINE source, consistent with the other
AERMOD sources.
Overall, the new algorithms to incorporate terrain into RLINE appear consistent with the response for
the other source types.
8.0 Future Work
There is still work that could be done to improve and further test the implementation of terrain into the
RLINE source. As discussed in the previous section, due to the inclusion of calculating the direct and
meander plume twice for each point along the integration line, there is a dramatic increase in runtime of
the RLINE source compared to the LINE source. Finally, there is further testing that could be explored
to ensure that the RLINE source is performing as expected. RLINE with terrain could be tested with
deposition, MAXDCONT, NO2 chemistry methods, and event processing.
9.0 References
Snyder, M.G., Venkatram, A., Heist, D.K., Perry, S.G., Petersen, W.B., Isakov, V., 2013. RLINE: A line
source dispersion model for near-surface releases. Atmos Environ 77, 748-756.
https://doi.ore/10.1016/i .atmosenv.201'< 0 0 I
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U.S EPA, 2023: AERMOD Model Formulation Document. EPA-454/B-23-010. U.S. Environmental
Protection Agency, Research Triangle Park, North Carolina.
U. S. EPA, 2018: User's Guide for the AERMOD Terrain Preprocessor (AERMAP). EPA-454/B-18-
004. U.S. Environmental Protection Agency, Research Triangle Park, North Carolina.
U.S. EPA, 2022. AERSCREEN User's Guide. EPA-454/B-21-005. U.S. Environmental Protection
Agency, Research Triangle Park, North Carolina.
Venkatram, A., Brode, R., Cimorelli, A., Lee, R., Paine, R., Perry, S., Peters, W., Weil, J., Wilson, R.,
2001. A complex terrain dispersion model for regulatory applications, Atmospheric Environment.
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United States Office of Air Quality Planning and Standards Publication No. EPA-454/R-23-012
Environmental Protection Air Quality Assessment Division October 2023
Agency Research Triangle Park, NC
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