United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park, NC 27711
ETA HiO/R 92
October 1992
EPA Screening Procedures for Estimating the
Air Quality Impact of Stationary Sources
Revised
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Screening Procedures for Estimating the
Air Quality Impact of Stationary Sources,
Revised
U.S. Environm,'•-*. • i;r,n Agency
Region 5,Libr^:y iF: :, .;
77 West Jackson !:; .•j.^vr-d 19th n
Chicago, IL 60604-3590 f'°°r
U.S. ENVmONMENTAL PROTECTION AGENCY
Office of Air and Radiation
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
October 1992
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DISCLAIMER
This report has been reviewed by the Office of Air Quality
Planning and Standards, EPA, and approved for publication.
Mention of trade names or commercial products is not intended to
constitute endorsement or recommendation for use.
The following trademarks appear in this document:
IBM* is a registered trademark of International
Business Machines Corporation.
11
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PREFACE
This document presents current EPA guidance on the use of screening procedures
to estimate the air quality impact of stationary sources. The document is an update and
revision of the original Volume 10 of the "Guidelines for Air Quality Maintenance
Planning and Analysis", and the later Volume 10 (Revised), and is intended to replace
Volume 1 OR as the standard screening procedures for regulatory modeling of stationary
sources.
Many of the short-term procedures, outlined in this document, have been
implemented in a computerized version in a model entitled SCREEN2. In previous
editions of this document, the SCREEN user's guide was contained within an appendix
to the document. As of this edition, the SCREEN2 user's guide and documentation is
provided as a separate document entitled "SCREEN2 Model User's Guide," EPA-450/4-
92-006. Software copies of SCREEN2 may be downloaded from the Office of Air
Quality Planning and Standards (OAQPS) Technical Transfer Network (TTN) Bulletin
Board System (BBS) via modem by dialing (919) 541-5742. The TTN BBS now serves
as the primary source of air dispersion models, replacing the User's Network for Applied
Modeling of Air Pollution (UNAMAP). Copies of SCREEN2 in diskette form may be
obtained from the National Technical. Information Service (NTIS), U.S. Department of
Commerce, 5285 Port Royal Road, Springfield, VA 22161.
111
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ACKNOWLEDGMENTS
Special credit and thanks are due Mr. Thomas E. Pierce, EPA-AREAL, for his
assistance with developing the FORTRAN code for the SCREEN model and for his
technical suggestions on improving the procedures. Credit is due Mr. Russell F. Lee, who
served as EPA Project Officer on the preparation of the original version of the Volume
10 procedures, and who continued to provide valuable technical assistance for this
document, and Mr. Laurence J. Budney, author of the revised version of Volume 10,
which served as a foundation for development of the current document. The author also
acknowledges those who reviewed the document and provided many valuable comments,
including the EPA Regional Modeling Contacts, several State meteorologists, and
meteorologists within EPA-OAQPS. Credit and thanks are due Mr. Roger W. Erode, for
developing the "Screening Procedures for Estimating the Air Quality Impact of Stationary
Sources, Draft for Public Comment" documentation for SCREEN. The project officer for
SCREEN2, Dennis G. Atkinson, is responsible for the first revision to this document. Mr.
Atkinson is on assignment from the National Oceanic and Atmospheric Administration,
U.S. Department of Commerce. Final thanks are due James L. Dicke and Joseph A.
Tikvart of EPA-OAQPS for their support and insight.
IV
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TABLE OF CONTENTS
Preface iii
Acknowledgments iv
List of Tables vi
List of Figures yii
List of Symbols ix
1. Introduction 1-1
2. Source Data 2-1
2.1 Emissions 2-1
2.2 Merged Parameters for Multiple Stacks 2-2
2.3 Topographic Considerations 2-3
2.4 Source Building Complex 2-4
3. Meteorological Data 3-1
3.1 Wind Speed and Direction 3-1
3.2 Stability 3-3
3.3 Mixing Height 3-5
3.4 Temperature 3-6
4. Estimating Source Impact on Air Quality 4-1
4.1 Simple Screening Procedure 4-2
4.2 Estimating Maximum Short-Term Concentrations 4-6
4.3 Short-term Concentrations at Specified Locations 4-17
4.4 Annual Average Concentrations 4-21
4.4.1 Annual Average Concentration at a Specified Location 4-21
4.4.2 Maximum Annual Average Concentration 4-24
4.5 Special Topics 4-25
4.5.1 Building Downwash 4-25
4.5.2 Plume Irnpaction on Elevated Terrain 4-28
4.5.3 Fumigation 4-30
4.5.4 Estimated Concentrations from Area Sources 4-35
4.5.5 Volume Sources 4-36
4.5.6 Contributions from Other Sources 4-37
4.5.7 Long Range Transport 4-41
5. References 5-1
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LIST OF TABLES
Table
3-1 Wind Profile Exponent as a Function of Atmospheric
Stability for Rural and Urban Sites 3-2
3-2 Key to Stability Categories 3-4
4-1 Calculation Procedures to Use with Various Release Heights 4-11
4-2 Stability-Wind Speed Combinations that are Considered
in Estimating Annual Average Concentrations 4-23
4-3 Wind Speed Intervals Used by the National Climatic Data
Center (NCDC) for Joint Frequency Distributions of Wind
Speed, Wind Direction and Stability 4-23
4-4 Downwind Distance to the Maximum Ground-Level Concentration
for Inversion Break-up Fumigation as a Function of Stack
Height and Plume Height 4-32
4-5 Downwind Distance to the Maximum Ground-level Concentration
for Shoreline Fumigation as a Function of Stack Height and
Plume Height 4-34
4-6 Summary of Suggested Procedures for Estimating Initial Lateral
(ayo) and Vertical Dimensions (om) for Single Volume Sources . . . 4-37
VI
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LIST OF FIGURES
Figure Page
4-1 Maximum %u/Q as a Function of Plume Height, H
(for use only with the simple screening procedure) 4-44
4-2 Downwind Distance to Maximum Concentration and
Maximum %u/Q as a Function of Stability Class and
Plume Height (m); Rural Terrain 4-45
4-3 Downwind Distance to Maximum Concentration and
Maximum %u/Q as a Function of Stability Class and
Plume Height (m); Urban Terrain 4-46
4-4 Stability Class A, Rural Terrain; %u/Q Versus Distance
for Various Plume Heights (H), Assuming Very Restrictive
Mixing Heights (L) 4-47
•
4-5 Stability Class B, Rural Terrain; %u/Q Versus Distance
for Various Plume Heights (H), Assuming Very Restrictive
Mixing Heights (L) 4-48
4-6 Stability Class C, Rural Terrain; xu/Q Versus Distance
for Various Plume Heights (H), Assuming Very Restrictive
Mixing Heights (L) 4-49
4-7 Stability Class D, Rural Terrain; %u/Q Versus Distance
for Various Plume Heights (H), Assuming Very Restrictive
Mixing Heights (L) 4-50
4-8 Stability Class E, Rural Terrain; %u/Q Versus Distance
for Various Plume Heights (H), Assuming Very Restrictive
Mixing Heights (L) 4-51
4-9 Stability Class F, Rural Terrain; %u/Q Versus Distance
for Various Plume Heights (H), Assuming Very Restrictive
Mixing Heights (L) 4-52
4-10 Stability Classes A and B, Urban Terrain; %u/Q Versus
Distance for Various Plume Heights (H), Assuming
Very Restrictive Mixing Heights (L) 4-53
4-11 Stability Class C, Urban Terrain; %u/Q Versus Distance
for Various Plume Heights (H), Assuming Very Restrictive
Mixing Heights (L) 4-54
vn
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LIST OF FIGURES (CONT.)
Figure Page
4-12 Stability Class D, Urban Terrain; %u/Q Versus Distance
for Various Plume Heights (H), Assuming Very Restrictive
Mixing Heights (L) .................................. 4-55
4-13 Stability Class E, Urban Terrain; %u/Q Versus Distance
for Various Plume Heights (H), Assuming Very Restrictive
Mixing Heights (L) .................................. 4-56
4-14 Vertical Dispersion Parameter (a^ as a Function of
Downwind Distance and Stability Class; Rural Terrain ......... 4-57
4-15 Isopleths of Mean Annual Afternoon Mixing Heights ......... 4-58
4-16 Isopleths of Mean Annual Morning Mixing Heights .......... 4-59
4-17 24-hour %/Q Versus Downwind Distance, Obtained
from the Valley Model ..... . . . ....................... 4-60
4-18 Horizontal Dispersion Parameter (a,) as a Function of
Downwind Distance and Stability Class; Rural Terrain ......... 4-61
4-19 Maximum xu/Q as a Function of Downwind Distance and Plume
Height (H), Assuming a Mixing Height of 500m; D Stability .... 4-62
4-20 Maximum %u/Q as a Function of Downwind Distance and Plume
Height (H); E Stability ............................ ... 4-63
Vlll
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LIST OF SYMBOLS
Symbol Definition
A Parameter used in building cavity calculations and TTBL height factor
Ap Cross-sectional area of building normal to the wind (m2)
B Parameter used in building cavity calculations
C Contribution to pollutant concentration (g/m3)
Fb Buoyancy flux parameter (m4/s3)
H Total heat release rate from flare (cal/s)
L Alongwind horizontal building dimension (length, m)
L,, Lesser of building height or maximum projected width (m)
M Merged stack parameter
Q Pollutant emission rate (g/s)
QH Sensible heat release rate from flare (cal/s)
R Net rate of sensible heating by the sun (67 cal/m2/s)
S Length of side of square area source (m)
Ta Ambient temperature (K)
Ts . Stack gas exit temperature (K)
V Stack gas volume flow rate (m3/s)
W Crosswind horizontal building dimension (width, m)
cp Specific heat of air at constant pressure (0.24 cal/gK)
ds Stack inside diameter (m)
f Frequency of occurrence of a wind speed and stability category combination
g Acceleration due to gravity (9.806 m/s2)
h Height of release above terrain (h = hs - ht, m)
ix
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LIST OF SYMBOLS (CONT.)
Symbol Definition
hb Building height (m)
he Plume (or effective stack) height (m)
h; Height of the top of the plume (he + 2az, m)
h, Physical stack height (m)
hT Height of the Thermal Internal Boundary Layer (TIBL) (m)
ht Height of terrain above stack base (m)
hM Effective stack release height for flare (m)
he' Plume height modified for stack tip downwash (m)
m Multiplicative factor to account for effects of limited mixing
p Wind speed power law profile exponent
r Factor to adjust 1-hour concentration to longer averaging time
t,,, Time required for inversion break-up to extend from stack top to top
of plume (s)
u Wind speed (m/s)
Uc Critical wind speed (m/s)
•
us Wind speed at stack height (m/s)
Uj Wind speed at a height of Zt (m/s)
u. Friction velocity (m/s)
u10 Wind speed at a height of 10m (m/s)
uAh Normalized plume rise (m2/s)
vs Stack gas exit velocity (m/s)
x Downwind distance (m)
Downwind distance to maximum ground-level concentration (m)
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LIST OF SYMBOLS (CONT.)
Symbol Definition
x, Length of cavity recirculation region (m)
xs Distance from source to shoreline (m)
Xy Virtual point source distance (m)
z( Mixing height (m)
z,,, Mechanically driven mixing height (m)
Ah Plume rise (m)
AG/Az Potential temperature gradient with height (K/m)
Ax Length of side of urban area (m)
K pi (= 3.14159)
Gy Horizontal (lateral) dispersion parameter (m)
ay Initial horizontal dispersion parameter for area source (m)
az Vertical dispersion parameter (m)
XB Concentration contributions from other (background) sources (g/m3)
Xf Maximum ground-level concentration due to fumigation (g/m3)
Xmax Maximum ground-level concentration (g/m3)
Xp Maximum concentration for period greater than 1 hour (g/m3)
Xi Maximum 1-hour ground-level concentration (g/m3)
X24 Maximum 24-hour ground-level concentration (g/m3)
X/Q Relative concentration (s/m3)
Xu/Q Normalized relative concentration (m'2)
XI
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1. INTRODUCTION
This document is an update and revision of an earlier guideline1-2 for applying
screening techniques to estimate the air quality impact of stationary sources. The
application of screening techniques is addressed in Section 4.2.1 of the Guideline on Air
Quality Models (Revised).3 The current document incorporates changes and additions to
the technical approach. The techniques are applicable to chemically stable, gaseous or
fine particulate pollutants. An important advantage of the current document is that the
single source, short-term techniques can be easily executed on an IBM* - PC (personal
computer) compatible microcomputer with at least 256K of RAM using the SCREEN2
computer code. As with the earlier versions, however, many of the techniques can be
applied with a pocket or desk calculator.
The techniques described in this document can be used to evaluate the air quality
impact of sources pursuant to the requirements of the Clean Air Act,4 such as those
sources subject to the prevention of significant deterioration (PSD) regulation, addressed
in 40 CFR 52.21. The techniques can also be used, where appropriate, for new major or
minor sources or modifications subject to new source review regulations, and existing
sources of air pollutants, including toxic air pollutants. This document presents a
three-phase approach that is applicable to the air quality analysis:
Phase 1. Apply a simple screening procedure (Section 4.1) to determine if either
(1) the source clearly poses no air quality problem or (2) the potential
for an air quality problem exists.
Phase 2. If the simplified screening results indicate a potential threat to air
quality, further analysis is warranted, and the detailed screening (basic
modeling) procedures described in Sections 4.2 through 4.5 should be
applied.
Phase 3. If the detailed screening results or other factors indicate that a more
refined analysis is necessary, refer to the Guideline on Air Quality
Models (Revised).3
1-1
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The simple screening procedure (Phase 1) is applied to determine if the source poses
a potential threat to air quality. The purpose of first applying a simple screening
procedure is to conserve resources by eliminating from further analysis those sources that
clearly will not cause or contribute to ambient concentrations in excess of short-term air
quality standards or allowable concentration increments. A relatively large degree of
"conservatism" is incorporated in that screening procedure to provide reasonable assurance
that maximum concentrations will not be underestimated.
If the results of the simple screening procedure indicate a potential to exceed
allowable concentrations, then a detailed screening analysis is conducted (Phase 2). The
Phase 2 analysis will yield a somewhat conservative first approximation (albeit less
conservative than the simple screening estimate) of the source's maximum impact on air
quality. If the Phase 2 analysis indicates that the new source does not pose an air quality
problem, further modeling may not be necessary. However, there are situations in which
analysis beyond the scope of this document (Phase 3) may be required; for example when:
1. A more accurate estimate of the concentrations is needed (e.g., if the results
of the Phase 2 analysis indicate a potential air quality problem).
2. The source configuration is complex.
3. Emission rates are highly variable.
4. Pollutant dispersion is significantly affected by nearby terrain features or large
bodies of water.
In most of those situations, more refined analytical techniques, such as computer-based
dispersion models,3 can be of considerable help in estimating air quality impact.
In all cases, particularly for applications beyond the scope of this guideline, the
services of knowledgeable, well-trained air pollution meteorologists, engineers and air
1-2
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quality analysts should be engaged. An air quality simulation model applied improperly
can lead to serious misjudgments regarding the source impact.
1-3
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2. SOURCE DATA
In order to estimate the impact of a stationary point or area source on air quality,
certain characteristics of the source must be known. The following minimum information
should generally be available:
0 Pollutant emission rate;
0 Stack height for a point source and release height for an area source;
0 Stack gas temperature, stack inside diameter, and stack gas exit velocity (for
plume rise calculations);
0 Location of the point of emission with respect to surrounding topography, and
the character of that topography;
0 A detailed description of all structures in the vicinity of (or attached to) the
stack in question. (See the discussion of aerodynamic downwash in Section
4.5.1); and
0 Similar information from other significant sources in the vicinity of the subject
source (or air quality data or dispersion modeling results that demonstrate the
air quality impact of those sources).
At a minimum, impact estimates should be made with source characteristics
representative of the design capacity (100 percent load). In addition, the impacts should
be estimated based on source characteristics at loads of 50 percent and 75 percent of
design capacity, and the maximum impacts selected for comparison to the applicable air
quality standard. Refer to Section 9.1.2 in the Guideline on Air Quality Models
(Revised)3 for a further discussion of source data.
2.1 Emissions
The analysis of air quality impact requires that the emissions from each source be
fully and accurately characterized. If the pollutants are not emitted at a constant rate
(most are not), information should be obtained on how emissions vary with season, day
2-1
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of the week, and hour of the day. In most cases, emission rates vary with the source
production rate or rate of fuel consumption. For example, for a coal-fired power plant,
emissions are related to the kilowatt-hours of electricity produced, which is proportional
to the tonnage of coal used to produce the electricity. Fugitive emissions from an area
source are likely to vary with wind speed and both atmospheric and ground moisture
content. If pollutant emission data are not directly available, emissions can be estimated
from fuel consumption or production rates by multiplying the rates by appropriate
emission factors. Emission factors can be determined using three different methods. They
are listed below in decreasing order of confidence:
1. Stack-test results or other emission measurements from an identical or similar
source.
2. Material balance calculations based on engineering knowledge of the process.
3. Emission factors derived for similar sources or obtained from a compilation
by the U.S. Environmental Protection Agency.5
In cases where emissions are reduced by control equipment, the effectiveness of the
controls must be accounted for in the emissions analysis. The source operator should be
able to estimate control effectiveness in reducing emissions and how this effectiveness
varies with changes in plant operating conditions.
2.2 Merged Parameters for Multiple Stacks
Sources that emit the same pollutant from several stacks with similar parameters that
are within about 100m of each other may be analyzed by treating all of the emissions as
coming from a single representative stack. For each stack compute the parameter M:
M - ^IL . (2.D
Q
2-2
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where:
M = merged stack parameter which accounts for the relative influence of stack
height, plume rise, and emission rate on concentrations
h, = stack height (m)
V = (Ti/4) ds2 vs = stack gas volumetric flow rate (m3/s)
ds = inside stack diameter (m)
v, = stack gas exit velocity (m/s)
T, = stack gas exit temperature (K)
Q = pollutant emission rate (g/s)
The stack that has the lowest value of M is used as a "representative" stack. Then the
sum of the emissions from all stacks is assumed to be emitted from the representative
stack; i.e., the equivalent source is characterized by hSi, Vj, Tf and Q, where subscript 1
indicates the representative stack and Q = Qj + Q2 + • • • + On-
The parameters from dissimilar stacks should be merged with caution. For example,
if the stacks are located more than about 100m apart, or if stack heights, volumetric flow
rates, or stack gas exit temperatures differ by more than about 20 percent, the resulting
estimates of concentrations^due to the merged stack procedure may be unacceptably high.
2.3 Topographic Considerations
It is important to study the topography in the vicinity of the source being analyzed.
Topographic features, through their effects on plume behavior, will sometimes be a
significant factor in determining ambient ground-level pollutant concentrations. Important
features to note are the locations of large bodies of water, elevated terrain, valley config-
urations, and general terrain roughness in the vicinity of the source.
2-3
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Section 4.5.2 provides a screening technique for estimating ambient concentrations
due to plume impaction at receptors located on elevated terrain features above stack
height. The effects of elevated terrain below stack height can be accounted for in Sections
4.2 and 4.3. A screening technique for estimating concentrations under shoreline
fumigation conditions is presented in Section 4.5.3. Any other topographic considerations,
such as terrain-induced plume downwash and valley stagnation, are beyond the scope of
this guideline.
2.4 Source Building Complex
The downwash phenomenon caused by the aerodynamic turbulence induced by a
building may result in high ground-level concentrations in the vicinity of an emission
source. It is therefore important to characterize the height and width of structures nearby
the source. For purposes of these analyses, "nearby" includes structures within a distance
of five times the lesser of the height or width of the structure, but not greater than 0.8km
(0.5 mile).6 The screening procedure for building downwash is described in Section 4.5.1.
2-4
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3. METEOROLOGICAL DATA
The computational procedures given in Section 4 for estimating the impact of a
stationary source on air quality utilize information on the following meteorological
parameters:
0 Wind speed and direction
0 Stability class
0 Mixing height
0 Temperature
A discussion of each of these parameters follows.
3.1 Wind Speed and Direction
Wind speed and direction data are required to estimate short-term peak and
long-term average concentrations. The wind speed is used to determine (1) plume
dilution, and (2) the plume rise downwind of the stack. These factors, in turn, affect the
magnitude of and distance to the maximum ground-level concentration.
Most wind data are collected near ground level. The wind speed at stack height, us,
can be estimated from the following power law equation:
h p
us = u, (—] , (3.1)
zi
where:
us = the wind speed (m/s) at stack height, hs,
Uj = the wind speed at a reference height, z, (such as the anemometer height), and
p = the stability-related power law exponent from Table 3-1.
3-1
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Table 3-1. Wind Profile Exponent as a Function of Atmospheric Stability for Rural
and Urban Sites*
Stability Class
A
B
C
D
E
F
Rural Exponent
0.07
0.07
0.10
0.15
0.35
0.55
Urban Exponent
0.15
0.15
0.20
0.25
0.30
0.30
The power law equation may be used to adjust wind speeds over a height range from
f
about 10 to 300m. Adjustments to heights above 300m should be used with caution. For
release heights below 10m the reference wind speed should be used without adjustment.
For the procedures in Section 4 the reference height is assumed to be at 10m.
The wind direction is an approximation to the direction of transport of the plume.
The variability of the direction of transport over a period of time is a major factor in
estimating ground-level concentrations averaged over that time period.
Wind speed and direction data from National Weather Service (NWS), Air Weather
Service, and Naval Weather Service stations are available from the National Climatic Data
Center (NCDC), Federal Building, Asheville, NC [(704) 259-0682]. Wind data are often
also recorded at existing plant sites and at air quality monitoring sites. It is important that
the equipment used to record such data be properly designed, sited, and maintained to
record data that are reasonably representative of the direction and speed of the plume.
*The classification of a site as rural or urban should be based on one of the procedures
described in Section 8.2.8 of the Guideline on Air Quality Models (Revised).3
3-2
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Guidance on collection of on-site meteorological data is contained primarily in Reference
7, but also in References 3 and 8.
3.2 Stability
Stability categories, as depicted in Tables 3-1 and 3-2, are indicators of atmospheric
turbulence. The stability category at any given time will depend upon static stability
(related to the change in temperature with height), thermal turbulence (caused by heating
of the air at ground level), and mechanical turbulence (a function of wind speed and
surface roughness). It is generally estimated by a method given by Turner,9 which
requires information on solar elevation angle, cloud cover, cloud ceiling height, and wind
>
speed (see Table 3-2). Opaque cloud cover should be used if available, otherwise total
cloud cover may be used. The solar elevation angle is a function of the time of year and
the time of day, and is presented in charts in the Smithsonian Meteorological Tables.10
The hourly NWS observations include cloud cover, ceiling height, and wind speed. These
data are available from NCDC or the SCRAM BBS.* Methods for estimating
atmospheric stability categories from on-site data are presented in Reference 7. For
computation of seasonal and annual concentrations, a joint frequency distribution of
stability class, wind direction, and wind speed (stability wind rose) is needed. Such
distributions, called STAR summaries, can be obtained from NCDC for NWS stations.
"^Support (Tenter for .Regulatory Ajr Models Bjilletin Board System is a component of
the TTN (Technology Transfer Network) BBS maintained by OAQPS, accessible via
modem by dialing (919) 541-5742.
3-3
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Table 3-2. Key to Stability Categories*
Surface Wind
Speed at a Height
of 10m
(m/s)
<2
2-3
3-5
5-6
>6
Day
Incoming Solar Radiation (Insolation)
Strong
A
A-B
B
C
C
Moderate
A-B
B
B-C
C-D
D
Slight
B
C
C
D
D
Night**
Thinly Overcast
or :> 4/8 Low
Cloud Cover
F
E
D
D
D
< 3/8 Cloud
Cover
F
F
E
D
D
The neutral class (D) should be assumed for all overcast conditions during day or
night
Night is defined as the period from 1 hour before sunset to 1 hour after sunrise.
Appropriate insolation categories may be determined through the use of sky cover
and solar elevation information as follows:
Sky Cover
(Opaque or Total)
4/8 or Less or Any
Amount of High Thin
Clouds
5/8 to 7/8 Middle
Clouds (7000 to
16,000ft base)
5/8 to 7/8 Low Clouds
(less than 7000ft base)
Solar Elevation Angle
>60°
Strong
Moderate
Slight
Solar Elevation Angle
< 60° but > 35°
Moderate
Slight
Slight
Solar Elevation Angle
< 35° but > 15°
Slight
Slight
Slight
»
3-4
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3.3 Mixing Height
The mixing height is the distance above the ground to which relatively unrestricted
vertical mixing occurs in the atmosphere. When the mixing height is low (but still above
plume height) ambient ground-level concentrations will be relatively high because the
pollutants are prevented from dispersing upward. For estimating long-term average
concentrations, it is generally adequate to use an annual-average mixing height rather than
daily values.
•
Mixing height data are generally derived from surface temperatures and from upper
air soundings which are made at selected NWS stations. The procedure used to determine
mixing heights is one developed by Holz worth.11 Tabulations and summaries of mixing
height data can be obtained from NCDC.
For the purposes of calculations made in Section 4.2 and for use in the SCREEN2
model, a mechanically driven mixing height is estimated to provide a lower limit to the
mixing height used during neutral and unstable conditions. The mechanical mixing height
is calculated from:12
. 0.2)
where:
u* = friction velocity (m/s)
f = Coriolis parameter (9.374 x 10'V1 at 40° latitude)
Using a log-linear vertical profile for the wind speed, and assuming a surface roughness
length of about 0.3m, u. may be estimated from the 10m wind speed, u10, as
u* = 0.1 u,0
Substituting for u, in (3.2) yields
3-5
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Zm = 320 uw (3.3)
If the plume height is calculated to be above the mixing height determined
from Equation 3.3, then the mixing height is set at 1m above the plume height for
conservatism in SCREEN2.
3.4 Temperature
Ambient air temperature must be known hi order to calculate the amount of rise of
•
a buoyant plume. Plume rise is proportional to a fractional power of the temperature
difference between the stack gases and the ambient air (see Section 4.2). Ambient
temperature data are collected hourly at NWS stations, and are available from NCDC or
from the SCRAM BBS. For the procedures in Section 4, a default value of 293K is used
for ambient temperature if no data are available.
3-6
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4. ESTIMATING SOURCE IMPACT ON AIR QUALITY
A three-phase approach, as discussed in the introduction, is recommended for
estimating the air quality impact of a stationary source:*
Phase 1. Simple screening analysis
Phase 2. Detailed screening (basic modeling) analysis
Phase 3. Refined modeling analysis
The Phase 3 analysis is beyond the scope of this guideline, and the user is referred to the
Guideline on Air Quality Models (Revised).3 This section presents the simple screening
procedure (Section 4.1) and the detailed screening procedures (Sections 4.2 through 4.5).
All of the procedures, with the partial exception of the procedures in Sections 4.5.2 and
4.5.3, are based upon the bi-variate Gaussian dispersion model assumptions described in
the Workbook of Atmospheric Dispersion Estimates.9 A consistent set of units (meters,
grams, seconds) is used throughout:
Distance (m)
Pollutant Emission Rate (g/s)
Pollutant Concentration (g/m3)
Wind Speed (m/s)
To convert pollutant concentration to micrograms per cubic meter (pg/m3) for comparison
with air quality standards, multiply the value in g/m3 by 1 x 106.
. *The techniques described in this section can be used, where appropriate, to evaluate
sources subject to the prevention of significant deterioration regulations (PSD - addressed
in 40 CFR 52.21), new major or minor sources subject to new source review regulations,
and existing sources of air pollutants, including toxic air pollutants.
4-1
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4.1 Simple Screening Procedure
The simple screening procedure is the "first phase" that is recommended when
assessing the ah- quality impact of a new point source. The purpose of this screening
procedure is to eliminate from further analysis those sources that clearly will not cause
or contribute to ambient concentrations in excess of short-term air quality standards.
The scope of the procedure is confined to elevated point sources with plume heights
of 10 to 300m, and concentration averaging times of 1-hour to annual. The procedure is
particularly useful for sources where the short-term air quality standards are "controlling";
i.e., in cases where meeting the short-term standards provides good assurance of meeting
the annual standard for that pollutant. Elevated point sources (i.e., sources for which the
emission points are well above ground level) are often in that category, particularly when
they are isolated from other sources.
When applying the screening procedure to elevated point sources, the following
assumptions must apply:
1. No aerodynamic downwash of the effluent plume by nearby buildings occurs.
(Refer to Section 4.5.1 to determine if building downwash is a potential
problem.)
2. The plume does not impact on elevated terrain. (Refer to Section 4.5.2 to
determine if elevated terrain above stack height may be impacted.)
If the potential for building downwash exists, then SCREEN2 should be used to
estimate air quality impact and the simple screening procedure is not applicable.
If the potential for plume impaction on elevated terrain exists, then the calculation
procedure described in the indicated section should also be applied, and the higher
concentration from the terrain impaction procedure and the simple screening procedure
4-2
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should be selected to estimate the maximum ground-level concentration. The effects of
elevated terrain below stack height should also be accounted for by reducing the computed
plume heights by the maximum terrain height above stack base.
The screening procedure utilizes the Gaussian dispersion equation to estimate the
maximum 1 -hour ground-level concentration for the source in question (Computations 1-6
below). To obtain concentrations for other averaging times up to annual, multiply the
1-hour value by an appropriate factor (Computation 7). Then account for background
concentrations (Computation 8) to obtain a total concentration estimate. That estimate is
then used, in conjunction with any elevated terrain estimates, to determine if further
analysis of the source impact is warranted (Computation 9):
Step 1. Estimate the normalized plume rise (uAh) that is applicable to the source
during neutral and unstable atmospheric conditions. (Stable atmospheric conditions are
not treated explicitly since this simple screening procedure does not apply to stack heights
less than 10m or cases with terrain intercepts.) First, compute the buoyancy flux
parameter, Fb:
T-T
3.12V [ s a]
T
4-3
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where:
g = acceleration due to gravity (9.806 m/s2)
v, = stack gas exit velocity (m/s)*
d, = stack inside diameter (m)
Ts = stack gas exit temperature (K)*
T, = ambient air temperature (K) (If no ambient temperature data are available,
assume that Ta = 293K.)
V = (71/4^,^, = actual stack gas volume flow rate (m3/s)
Normalized plume rise (uAh) is then given by:
uAh = 21.4Fb3/4 when Fb <55 m4/s3 (4.2)
uAh = 38.7Fb3/5 when Fb >55 m4/s3
Step 2. Divide the uAh value obtained from Equation 4.2 by each of five wind
speeds (u = 1.0, 2.0, 3.0, 5.0 and 10 m/s) to estimate the actual plume rise (Ah) for each
wind speed:
uAh = (uAh)/u
Step 3. Compute the plume height (r^) that will occur during each wind speed by
adding the respective plume rises to the stack height (hs):
he = hs + Ah
If the effects of elevated terrain below stack height are to be accounted for, then reduce
each plume height by the maximum terrain height above stack base.
*If stack gas temperature or exit velocity data are unavailable, they may be
approximated from guidelines that yield typical values for those parameters for existing
sources.13
4-4
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Step 4. For each plume height computed in (3), estimate a %u/Q value from Figure
4-1.14
Step 5. Divide each %u/Q value by the respective wind speed to determine the
corresponding %/Q values:
Step 6. Multiply the maximum %u/Q value obtained in (5) by the emission rate Q
(g/s), and incorporate a factor of 2 margin of safety, to obtain the maximum 1-hpur
ground-level concentration %j (g/m3) due to emissions from the stack in question:
Xi = 2£ l-g-J
The margin of safety is incorporated in the screening procedure to account for the
potential inaccuracy of concentration estimates obtained through calculations of this type.
If more than one stack is being considered, and the procedure for merging
parameters for multiple stacks is not applicable (Section 2.2), (1) through (6) must be
applied for each stack separately. The maximum values (%j) found for each stack are then
added together to estimate the total maximum 1-hour concentration.
Step 7. To obtain a concentration estimate (%p) for an averaging time greater than
one hour, multiply the 1-hour value by an appropriate factor, r:*
Xp = r Xi
*See the discussion in Step 5 of Section 4.2 which addresses multiplication factors for
averaging times longer than 1-hour.
4-5
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Step 8. Next, contributions from other sources (XB) should be taken into account,
yielding the final screening procedure concentration estimate Xm« (g/m3):
Xnuz = %p "*" XB-
Procedures on estimating concentrations due to other sources are provided in Section
4.5.5.
Step 9. Based on the estimate of X™, and (if applicable) estimate of concentrations
due to terrain impaction problems, determine if further analysis of the source is warranted.
If any of the estimated concentrations exceeds the air quality level of concern (e.g., an air
quality standard), proceed to Section 4.2 for further analysis. If the concentrations are
below the level of concern, the source can be safely assumed to pose no threat to that air
quality level, and no further analysis is necessary.
4.2 Estimating Maximum Short-Term Concentrations
The basic modeling procedures described in the remainder of this document
comprise the recommended "second phase" (or detailed screening) that may be used in
assessing air quality impacts. The procedures are intended for application in those cases
where the simple screening procedure (first phase) indicates a potential air quality
problem. As with the first phase (simple screening) analysis in Section 4.1, if elevated
terrain above stack height occurs within 50km of the source, then the procedure in Section
4.5.2 should be applied in addition to the procedures in this section. The highest
concentration from all applicable procedures should then be selected to estimate the
maximum ground-level concentration. Even if the plume is not likely to impact on
elevated terrain, the user should account for the effects of elevated terrain below stack
height. If the terrain is relatively uniform around the source, then a procedure to account
4-6
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for terrain effects is to reduce the computed plume height, hg (for all stabilities), by the
maximum terrain elevation above stack base within a 50km radius from the source. The
adjusted plume height can then be used in conjunction with the "flat terrain" procedures
described in this section.
If there are only a few isolated terrain features in otherwise flat terrain, then the flat
terrain estimates from this section should be expanded to include the procedures of Section
4.3 applied to the locations with elevated terrain. For the additional calculations the
computed plume height, h,. should be reduced by the terrain height above stack base
corresponding to the specific terrain features. The procedures in this section can be
applied without the aid of a Computer (a pocket or desk calculator will suffice). However,
they are subject to the same limitations as the simple screening procedure, i.e., no building
downwash occurs (see Section 4.5.1), no terrain impaction occurs (Section 4.5.2), and
plume heights do not exceed 300m. An alternative approach is to use the SCREEN2
computer code that has been made available by EPA for use on an IBM* - PC compatible
microcomputer with at least 256K of RAM. The SCREEN2 code replaces the PTPLU,
PTMAX, and PTDIS codes previously used in conjunction with Volume 10R2 and the
original SCREEN model. It is applicable to all of the procedures contained in this section
and Section 4.3, but also includes calculations for the special cases of building downwash,
fumigation, elevated terrain, area sources and long range transport described in Section
4.5. Complete documentation on the use of these procedures is provided in the SCREEN2
Model User's Guide.
This section (4.2) presents the basic procedures for estimating maximum short-term
concentrations for specific meteorological situations. If building downwash occurs (see
Section 4.5.1), then SCREEN2 must be used in lieu of these procedures. In Steps 1-3,
4-7
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plume rise15J6J7 and a critical wind speed are computed. In Step 4, maximum 1-hour
concentrations are estimated. In Step 5, the 1-hour concentrations are used to estimate
concentrations for averaging times up to 1 year. Contributions from other sources are
accounted for in Step 6.
Step 1. Estimate the normalized plume rise (uAh) that is applicable to the source
during neutral and unstable atmospheric conditions. First, compute the buoyancy flux
term, Fb, using Equation 4.1 (repeated here for convenience):
a , T-T . T-T
Fb = Lvtf [ s a] = 3.12V [ ' *]
(4.1), 4 ^ ^
where:
g = acceleration due to gravity (9.806 m/s2)
vs = stack gas exit velocity (m/s)*
ds = stack inside diameter (m)
T, = stack gas exit temperature (K)*
Ta = ambient air temperature (K) (If no ambient temperature data are available,
assume that Ta = 293K.)
V = (7t/4)ds2vs = actual stack gas volume flow rate (m3/s)
Normalized plume rise (uAh) is then given by:
uAh = 21.4Fb3/4 when Fb <55 m4/s3
(4.2)
uAh = 38.7Fb3/5 when Fb >55 m4/s3
If the emissions are from a flare, then the normalized plume rise and an effective
release height may be determined with the following procedure:
*If stack gas temperature or exit velocity data are unavailable, they may be
approximated from guidelines that yield typical values for those parameters for existing
sources.13
4-8
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(a) Calculate the total heat release rate, H (cal/s), of the flared gas based on the heat
content and the gas consumption rate.
(b) Calculate the buoyancy flux term, Fb, for the flare:*
Fb = 1.66 x ia5 x H (4.3)
(c) Calculate the normalized plume rise (uAh) from Equation 4.2.
(d) Calculate the vertical height of the flame, h, (m), assuming the flame is tilted 45°
from the vertical:19
hf = 4.56 x 1C'3 x H°'478 (4.4)
(e) Calculate an effective release height for the tip of the flame:
h* = h, + hf.
Use hse in place of hs along with the value of uAh calculated from (c) in determining
plume heights in the following procedures.
Step 2. Estimate the critical wind speed (uc) applicable to the source during neutral
and near-neutral atmospheric conditions. The critical wind speed is a function of two
opposing effects that occur with increasing wind speed; namely, increased dilution of the
effluent as it leaves the stack (which tends to decrease the maximum impact on
ground-level concentration) and suppression of plume rise (tending to increase the
impact). The wind speed at which the interaction of those opposing effects results in the
highest ground-level concentration is the critical wind speed.
gQ
'This formula was derived from: Fb = — (Eqn. 4.20, Briggs15), assuming
Ta = 293K, p = 1205 g/m3, and cp = 0.24 cal/gK, and that the sensible heat release rate,
QH = (0.45)H.18
4-9
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The critical wind speed can be estimated through the following approximation:
(4.5)
Assume that the value of uc from Equation 4.5 corresponds to the stack height wind
speed. If the value of uc calculated from Equation 4.5 is less than 1.0 m/s, then use uc =
1.0 m/s. If the value of uc calculated from Equation 4.5 is greater than 15.0 m/s, then use
uc = 15.0 m/s.
Step 3. Stable atmospheric conditions may be critical if the emission height is less
than 50m. The stable case plume rise (Ah) should be estimated as follows:
p T 1/3
A/z = 2.6 [ * " ] (4.6)
The value A6/Az is the change in potential temperature with height. A
value of 0.035 K/m for F stability should be used for both urban and rural sites. The
classification criteria of a site as rural or urban should be based on one of the procedures
described in Section 8.2.8 of the Guideline on Air Quality Models (Revised).3
Step 4. Estimate maximum 1-hour concentrations that will occur during various
dispersion situations. First, using Table 4-1 as a guide, determine the dispersion situations
and corresponding calculation procedures applicable to the source being considered. Then
apply the applicable calculation procedures, which are described on the following pages,
in order to estimate maximum 1-hour concentrations. Then proceed to Step 5.
As discussed earlier and as noted in Table 4-1, the hand calculation procedures
presented in this step are limited by certain assumptions, namely that no building
downwash occurs (Section 4.5.1), no terrain impaction occurs (Section 4.5.2), and that
4-10
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Table 4-1. Calculation Procedures to Use with Various Release Heights
Height of Release Above Terrain, h
Applicable Calculation Procedures
h >_50m
(a) Unstable / Limited Mixing
(b) Near-neutral / High Wind
10 _< h < 50m
(a) Unstable / Limited Mixing
(b) Near-neutral / High Wind
(c) Stable
h < 10m and Ground Level Sources
(b) Near-neutral / High Wind
(c) Stable
NOTE;*
If hs < hb + 1.5L,,, refer to Section 4.5.1 on building downwash and use SCREEN2.
If elevated terrain above stack height occurs within 50km, refer to Section 4.5.2.
If fumigation is potentially a problem (e.g., for rural sources with hs >_ 10m), refer to
Section 4.5.3.
If the plume height, he = hs + (uAh/uJ is greater than 300m, then the procedures in this
section are not applicable (i.e., SCREEN2 may be used without this restriction).
*h = hs - ht
hs = physical stack height
h, = terrain height above stack base
hb = height of nearby structure
Lt, = lesser of height or maximum projected width of nearby structure
4-11
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plume heights are below 300m. For cases involving building downwash or plume heights
above 300m, SCREEN2 should be used. Documentation for these procedures is provided
in SCREEN2 Model User's Guide.
Procedure (a): Unstable/Limited Mixing
During very unstable conditions, the plume from a stack will be mixed to ground
level relatively close to the source, resulting in high short-term concentrations. These
concentrations can be significantly increased when the unstable conditions occur in
conjunction with a limited mixing condition. Limited mixing (also called plume trapping)
occurs when a stable layer aloft limits the vertical mixing of the plume. The highest
concentrations occur when the mixing height is at or slightly above the plume height.
Calculation Procedure:
1. Compute the plume height, he, that will occur during A stability and 10m wind
speeds of 1 and 3 m/s. Adjust the wind speeds from 10m to stack height using Equation
3.1 and the exponent for stability class A. Use the uAh value computed in Step 1.
h = h + MA/l
' ' ~
= hs + Ah
If vs < 1.5us, account for stack tip downwash as follows:
' ht = ht + A/I + 2[-JL - 1.5]^ (4.7)
***
If elevated terrain is to be accounted for, then reduce the computed plume height for each
wind speed by the maximum terrain elevation above stack base.
4-12
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2. For both wind speeds considered in (1), determine the maximum
1-hour xu/Q using the curve for stability A on Figure 4-2 (rural)9 or
A-B on Figure 4-3 (urban).20
3. Compute the maximum 1-hour concentration, %„ for both cases using:
Xl = mQ *Z2. , (4.8)
where m is a conservative factor to account for the increase in concentration expected due
to reflections of the plume off the top of the mixed layer. The value of m depends on the
plume height as follows:*
m = 2.0 for 290m < he
m = 1.8 for 270m < he < 290m
m = 1.5 for 210m < he < 270m
m = 1.2 for 180m < he < 210m
m = 1.1 for 160m < he < 180m
m = 1.0 for he< 160m
Select the highest concentration computed.
Procedure (b): Near-neutral/High Wind
Some buoyant plumes will have their greatest impact on ground-level concentrations
during neutral or near-neutral conditions, often in conjunction with high wind speeds.
Calculation procedure:
1. Compute the plume height, he, that will occur during C stability with a stack
height wind speed of us = uc, the value of the critical wind speed computed in Step 2. If
uc < 10 m/s, then also compute the plume height that will occur during C stability with
*The values of m are based on an assumed minimum daytime mixing height of about
320m (see Section 3.3).
4-13
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a 10m wind speed of 10 m/s. Adjust the 10 m/s wind speed from 10m to stack height
using Equation 3.1 and the exponent for stability class C. Use the uAh value computed
in Step 1:
uAh
If vf < 1.5ut, account for stack tip downwash using Equation 4.7. If elevated terrain is to
be accounted for, then reduce the computed plume height for each wind speed by the
maximum terrain elevation above stack base.
2. For the wind speed(s) considered in (1), determine the maximum 1-hour %u/Q
using the curve for stability C on Figure 4-2 (rural)9 or Figure 4.3 (urban).20
3. Compute the maximum 1-hour concentration %t for each case using:
and select the highest concentration computed.
Procedure (c): Stable
Low-level sources (i.e., sources with stack heights less than about 50m) sometimes
produce the highest concentrations during stable atmospheric conditions. Under such
conditions, the plume's vertical spread is severely restricted and horizontal spreading is
also reduced. This results in what is called a fanning plume.
Calculation procedures:
A. For low-level sources with some plume rise, calculate the concentration as
follows:
4-14
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1. Compute the plume height (1O that will occur during F stability (for rural
cases) and 10m wind speeds of 1, 3, and 4 m/s,* or E stability (for urban cases) and 10m
wind speeds of 1, 3, and 5 m/s. Adjust the wind speeds from 10m to stack height, using
Equation 3.1 and the appropriate exponent. Use the stable plume rise (Ah) computed
from Equation 4.6 in Step 3:
he = hs + Ah
If vs < 1.5us, account for stack tip downwash using Equation 4.7. If elevated terrain is
to be accounted for, then reduce the computed plume height for each wind speed by the
maximum terrain elevation above stack base.
2. For each wind speed and stability considered in (1), find the maximum 1-hour
XU/Q from Figure 4-2 (rural)9 or 4-3 (urban).20 Compute the maximum 1-hour
concentration for each case, using
x«/<2
x, = Q
Us
and select the highest concentration computed.
B. For low-level sources with no plume rise (he = hs), find the maximum 1-hour
xu/Q from Figure 4-2 (rural case - assume F stability) or 4-3 (urban case - assume E
stability). Compute the maximum 1-hour concentration, assuming a 10m wind speed of
1 m/s. Adjust the wind speed from 10m to stack height using Equation 3.1 and the
appropriate exponent.
„ _ n XU/Q
*Refer to the discussion on worst case meteorological conditions in the SCREEN2
User's Guide for an explanation of the use of F stability with a 4 m/s wind speed.
4-15
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Step 5. Obtain concentration estimates for the averaging times of concern. The
maximum 1-hour concentration (%i) is the highest of the concentrations estimated in Step
4, Procedures (a) - (c). For averaging times greater than 1-hour, the maximum
concentration will generally be less than the 1-hour value. The following discussion
describes how the maximum 1-hour value may be used to make an estimate of maximum
concentrations for longer averaging times.
The ratio between a longer-term maximum concentration and a 1-hour
maximum will depend upon the duration of the longer averaging time, source
characteristics, local climatology and topography, and the meteorological conditions
associated with the 1-hour maximum. Because of the many ways in which such factors
interact, it is not practical to categorize all situations that will typically result in any
specified ratio between the longer-term and 1-hour maxima. Therefore, ratios are
presented here for a "general case" and the user is given some flexibility to adjust those
ratios to represent more closely any particular point source application where actual
meteorological data are used. To obtain the estimated maximum concentration for a 3-,
8-, 24-hour or annual averaging time, multiply the 1-hour maximum (%t) by the indicated
factor:
Averaging Time Multiplying Factor
3 hours 0.9 (±0.1)
8 hours 0.7 (±0.2)
24 hours 0.4 (±0.2)
Annual 0.08 (±0.02)
The numbers in parentheses are recommended limits to which one may diverge
from the multiplying factors representing the general case. For example,
if aerodynamic downwash or terrain is a problem at the facility, or if the
4-16
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emission height is very low, it may be necessary to increase the factors (within the limits
specified in parentheses). On the other hand, if the stack is relatively tall and there are
no terrain or downwash problems, it may be appropriate to decrease the factors.
Agreement should be reached with the Regional Office prior to modifying the factors.
The multiplying factors listed above are based upon general experience with
elevated point sources. The factors are only intended as a rough guide for estimating
maximum concentrations for averaging times greater than one hour. A degree of
conservatism is incorporated in the factors to provide reasonable assurance that maximum
concentrations for 3-, 8-, 24-hour and annual values will not be underestimated.
Step 6. Add the expected contribution from other sources to the concentration
estimated in Step 5. Concentrations due to other sources can be estimated from measured
data, or by computing the effect of existing sources on air quality in the area being
studied. Procedures for estimating such concentrations are given in Section 4.5.5. At this
point in the analysis, a first approximation of maximum short-term ambient concentrations
(source impact plus contributions from other sources) has been obtained. If concentrations
at specified locations, long-term concentrations, or other special topics must be addressed,
refer to applicable portions of Sections 4.3 to 4.5.
4.3 Short-Term Concentrations at Specified Locations
In Section 4.2, maximum concentrations are generally estimated without specific
attention to the location(s) of the receptor(s). In some cases, however, it is particularly
important to estimate the impact of a source on air quality in specified (e.g., critical)
areas. For example, there may be nearby locations at which high pollutant concentrations
already occur due to other sources, and where a relatively small addition to ambient
4-17
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concentrations might cause ambient standards to be exceeded. Another example would
be where an isolated terrain feature occurs in otherwise flat terrain, and concentrations at
the elevated terrain location may exceed those estimated for flat terrain. These procedures
assume that no building downwash occurs (Section 4.5.1), no terrain impaction occurs
(Section 4.5.2), and that plume heights do not exceed 300m.
Each of the sources affecting a given location can be expected to produce its greatest
impact during certain meteorological conditions. The composite maximum concentration
at that location due to the interaction of all the sources may occur under different
meteorological conditions than those which produce the highest impact from any one
source. Thus, the analysis of this problem can be difficult, and may require substantial
use of high-speed computers. Despite the potential complexity of the problem, some
preliminary calculations can be made that will at least indicate whether or not a more
detailed study is needed. For example, if the preliminary analysis indicates that the
estimated concentrations are near or above the air quality standards of concern, a more
detailed analysis will probably be required.
Calculation procedure:*
Step 1. Compute the normalized plume rise (uAh) for neutral and unstable
conditions, utilizing the procedure described in Step 1 of Section 4.2.
Step 2. Compute the plume rise, Ah, that will occur during C stability (to represent
neutral and unstable conditions) with 10m wind speeds of 1, 3, 5,10, and 20 m/s. Adjust
*If SCREEN2 is used, refer to the discrete distance option described in the SCREEN2
Model User's Guide.
4-18
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the wind speeds from 10m to stack height using Equation 3.1 and the exponent for
stability class C.
A/i =
Step 3. Compute the plume height (hj that will occur during each wind speed by
adding the respective plume rises to the stack height (h,):
he = hs + Ah
If vs < 1.5 us, account for stack tip downwash using Equation 4.7. If elevated terrain is
to be accounted for, then reduce the computed plume height for each wind speed by the
terrain elevation above stack base for the specified location.
Step 4. For each stability class-wind speed combination listed below, at the
downwind distance of the "specified location," determine the %u/Q value from Figures
4-4 through 4-7 (rural) or Figures 4-10 through 4-12 (urban) for non-stable conditions.
Note that in those figures (see the captions) very restrictive mixing heights are assumed,
resulting in trapping of the entire plume within a shallow layer.
Stability Class 10m Wind Speed (m/s)
A " 1,3.
B 1, 3, 5
C 1, 3, 5, 10
D 1, 3, 5, 10, 20
Step 5. (If the physical stack height is greater than 50m and flat terrain is being
assumed, Steps 5 and 6 may be skipped.) Compute plume heights (hj that will occur for
stability class E and 10m wind speeds of 1, 3, and 5 m/s, and for stability class F (rural
4-19
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sources only) and 10m wind speeds of 1 and 3 and 4 m/s.* Adjust the wind speeds from
10m to stack height using Equation 3.1 and the appropriate exponent. Use the stable
plume rise (Ah) computed from Equation 4.6 in Step 3 of Section 4.2:
he = hs + Ah.
If vs < 1.5us, account for stack tip downwash using Equation 4.7. If elevated terrain is
to be accounted for, then reduce the computed plume height for each case by the terrain
elevation above stack base for the specified location.
Step 6. For each stability class-wind speed combination considered in Step 5, at
the downwind distance of the specified location, determine a xu/Q value from Figures 4-8
and 4-9 (or Figure 4-13 for the urban case).
Step 7. For each xu/Q value obtained in Step 4 (and Step 6 if applicable), compute
X/Q:
~Q = ~
Step 8. Select the largest x/Q and multiply by the source emission rate (g/s) to
obtain a 1-hour concentration value (g/m3):
Y = o (2L)
A) 1C, \ -~/
max
Step 9. To estimate concentrations for averaging time greater than 1-hour, refer
to the averaging time procedure described earlier (Step 5 of Section 4.2). To account for
contributions from other sources, see Section 4.5.5.
*Refer to the discussion on worst case meteorological conditions in the SCREEN2
Model User's Guide for an explanation of the use of F stability with a 4m/s wind speed.
4-20
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4.4 Annual Average Concentrations
This section presents procedures for estimating annual average ambient
concentrations caused by a single point source. The procedure for estimating the annual
concentration at a specified location is presented first, followed by a suggestion of how
that procedure can be expanded to estimate the overall maximum annual concentration
(regardless of location). The procedures assume that the emissions are continuous and at
a constant rate. The data required are emission rate, stack height, stack gas volume flow
rate (or diameter and exit velocity), stack gas temperature, average afternoon mixing
height, and a representative stability wind rose.* Refer to Sections 2 and 3 for a
discussion of such data.
4.4.1 Annual Average Concentration at a Specified Location
Calculation procedure:
Step 1. (Applicable to stability categories A through D). Using the procedure
described in Step 1 of Section 4.2 (Equations 4.1 and 4.2) obtain a normalized plume rise
value, uAh.
Step 2. (Applicable to stability categories E and F). Use Equation 4.6 from Step
3 of Section 4.2 to estimate the plume rise (Ah) as a function of wind speed for both
stable categories (E and F) using values of A0/Az = 0.02 K/m for category E and A8/Az
= 0.035 K/m for category F.
'The stability wind rose is a joint frequency distribution of wind speed, wind direction
and atmospheric stability for a given locality. Stability wind roses for many locations are
available from the National Climatic Data Center, Asheville, North Carolina.
4-21
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Step 3. Compute plume rise (Ah) for each stability-wind speed category in Table
4-2 by (1) substituting the corresponding wind speed for u in the appropriate equations
referenced in Step 1 or 2 above and (2) solving the equation for Ah. The wind speeds
listed in Table 4-2 are derived from the wind speed intervals used by NCDC (Table 4-3)
in specifying stability-wind roses. The wind speeds may be adjusted from 10m to stack
height using Equation 3.1.
Step 4. Compute plume height (h,.) for each stability-wind speed category in Table
4-2 by adding the physical stack height (h,) to each of the plume rise values computed in
Step 3:
he = h, + Ah
Step 5. Estimate the contribution to the annual average concentration at the
specified location for each of the stability-wind speed categories in Table 4-2. First,
determine the vertical dispersion coefficient (aj for each stability class for the downwind
distance (x) between the source and the specified location, using Figure 4-14. (Note: For
urban F stability cases, use the az for stability E.) Next, determine the mixing height (Zj)
applicable to each stability class. For stabilities A to D, use the average afternoon mixing
height for the area (Figure 4-15). For urban stability E use the average morning mixing
height (Figure 4-16). For rural stabilities E and F, mixing height is not applicable. Then,
use that information as follows: for all stability-wind conditions when the plume height
(hj is greater than the mixing height (Zj), assume a zero contribution to the annual
concentration at the specified location. For each condition when az £ O.SZj and for all
rural stability E and F cases, apply the following equation9 to estimate the contribution
C (g/m3):
4-22
-------
Table 4-2. Stability-Wind Speed Combinations That Are Considered in Estimating
Annual Average Concentrations
Atmospheric
Stability Categories
A
B
C
D
E
F
Wind Speed (m/s)
1.5
*
*
*
*
*
*
2.5
*
*
*
*
*
*
4.5
*
*
*
*
7
*
*
9.5
*
*
12.5
*
* It is only necessary to consider the stability-wind speed conditions marked with an
asterisk.
Table 4-3. Wind Speed Intervals Used by the National Climatic Data Center (NCDC)
for Joint Frequency Distributions of Wind Speed, Wind Direction and
Stability
Class
1
2
3
4
5
6
Speed Interval
m/s knots
0 to 1.8
1.8 to 3.3
3.3 to 5.4
5.4 to 8.5
8.5 to 11.0
> 11.0
Oto3
4 to 6
7 to 10
11 to 16
17 to 21
>21
Representative Wind
Speed (m/s)
1.5
2.5
4.5
7.0
9.5
12.5
4-23
-------
C = I-] exp [-1 fl (4.9)
ar u x 2 az
For each condition during which az > O.SZj, the following equation9 is applied:
C - (4.10)
Z. U X
In equations 4.9 and 4.10:
Q = pollutant emission rate (g/s)
u = wind speed (m/s)
f = frequency of occurrence of the particular wind speed-stability combination
(obtained from the stability-wind rose (STAR) summary available from NCDC)
for the wind direction of concern. Only consider the wind speed-stability
combinations for the wind direction that will bring the plume closest to the
specified location.
Step 6. Sum the contributions (C) computed in Step 5 to estimate the annual
average concentration at the specified location.
4.4.2 Maximum Annual Average Concentration
To estimate the overall maximum annual average concentration (the maximum
concentration regardless of location) follow the procedure for the annual average
concentration at a specified location, repeating the procedure for each of several receptor
distances, and for all directions. Because of the large number of calculations required, it
is recommended that a computer model such as ISCLT2 be used.21
4-24
-------
4.5 Special Topics
4.5.1 Building Downwash
In some cases, the aerodynamic turbulence induced by a nearby building will cause
a pollutant emitted from an elevated source to be mixed rapidly toward the ground
(downwash), resulting in higher ground-level concentration immediately to the lee of the
building than would otherwise occur. Thus, when assessing the impact of a source on air
quality, the possibility of downwash problems should be investigated. For purposes of
these analyses, "nearby" includes structures within a distance of five times the lesser of
the height or width of the structure, but not greater than 0.8km (0.5 mile).6 If downwash
is found to be a potential problem, its effect on air quality should be estimated. Also
when Good Engineering Practice (GEP) analysis indicates that a stack is less than the GEP
height, the following screening procedures should be applied to assess the potential air
quality impact. The best approach to determine if downwash will be a problem at a
proposed facility is to conduct observations of effluent behavior at a similar facility. If
this is not feasible, and if the facility has a simple configuration (e.g., a stack adjacent or
attached to a single rectangular building), a simple rule-of-thumb22 may be applied to
determine the stack height (hs) necessary to avoid downwash problems:
hs^hb+ 1.5 L,,, (4.11)
where hb is building height and l^ is the lesser of either building height or maximum
projected building width. In other words, if the stack height is equal to or greater than
hb + 1.5 Lj,, downwash is unlikely to be a problem.
If there is more than one stack at a given facility, the above rule must be
successively applied to each stack. If more than one building is involved the rule must
be successively applied to each building. Tiered structures and groups of structures should
4-25
-------
be treated according to Reference 6. For relatively complex source configurations the rule
may not be applicable, particularly when the building shapes are much different from the
simple rectangular building for which the above equation was derived. For these cases,
refined modeling techniques3 or a wind tunnel study is recommended.
If it is determined that the potential for downwash exists, then SCREEN2 should
be used to estimate the maximum ground-level pollutant concentrations that occur as a
result of the downwash. The building downwash screening procedure is divided into the
following two major areas of concern:
A. Cavity Region, and
B. Wake Region
Generally, downwash has its greatest impact when the effluent is caught in the cavity
region. However, the cavity may not extend beyond the plant boundary and, in some
instances, impacts in the wake region may exceed impacts in the cavity region. Therefore,
impacts in both regions must be considered if downwash is potentially a problem.
When SCREEN2 is run for building downwash calculations, the program prompts
the user for the building height, the minimum horizontal building dimension, and the
maximum horizontal building dimension.
A. Cavity Region
The cavity calculations are made using methods described by Hosker.23 Cavity
calculations are based on the determination of a critical (i.e., minimum) wind speed
required to cause entrainment of the plume in the cavity (defined as being when the plume
centerline height equals the cavity height). Two cavity calculations are made, the first
using the minimum horizontal dimension alongwind, and the second using the maximum
horizontal dimension alongwind. The SCREEN2 output provides the cavity concentration,
4-26
-------
cavity length (measured from the lee side of the building), cavity height and critical wind
speed for each orientation. The highest concentration value that potentially affects
ambient air should be used as the maximum 1-hour cavity concentration for the source.
A more detailed description of the cavity effects screening procedure is contained in the
SCREEN2 Model User's Guide. For situations significantly different from the worst case,
and for complex source configurations, a more detailed analysis is required.24-25 If this
estimate proves unacceptable, one may also wish to consider a field study or fluid
modeling demonstration to show maintenance of the NAAQS (National Ambient Air
Quality Standard) or PSD increments within the cavity. If such options are pursued, prior
agreement on the study plan and methodology should be reached with the Regional Office.
B. Wake Region
Wake effects screening can also be performed with SCREEN2. SCREEN2 uses the
downwash procedures contained in the User's Guide for the Industrial Source Complex
(ISC2) Dispersion Models21 and applies them to the full range of meteorological
conditions described in the SCREEN2 Model User's Guide. SCREEN2 accounts for
downwash effects within the "near" wake region (out to ten times the lesser of the
building height or projected building width, lOLb), and also accounts for the effects of
enhanced dispersion of the plume within the "far" wake region (beyond lOLb). The same
building dimensions as described above for the cavity calculations are used, and
SCREEN2 calculates the maximum projected width from the values input for the
minimum and maximum horizontal dimensions. The wake effects procedures are
described in more detail in the ISC2 manual.
4-27
-------
4.5.2 Plume Impaction on Elevated Terrain
There is growing acceptance of the hypothesis that greater concentrations can occur
on elevated than on flat terrain in the vicinity of an elevated source.* That is particularly
true when the terrain extends well above the effective plume height. A procedure is
presented here to (1) determine whether or not an elevated plume may impact on elevated
terrain and, (2) estimate the maximum 24-hour concentration if terrain impaction is likely.
The procedure is based largely upon the 24-hour mode of the EPA Valley Model.26 A
similar procedure that accounts for terrain heights above plume height using the Valley
Model, and compares results from the Valley Model to simple terrain calculations for
terrain between stack height and plume height, is included in the SCREEN2 program. A
concentration estimate obtained through the procedure in this section will likely be
somewhat greater than provided by the Valley Model or by the SCREEN2 program,
primarily due to the relatively conservative plume height that is used in Step 1:
Step 1. Determine if the plume is likely to impact on elevated terrain in the
vicinity of the source:
(1) Compute one-half the plume rise that can be expected during F
stability and a stack height wind speed (us) of 2.5 m/s. (The reason for using only
one-half the normally computed plume rise is to provide a margin of safety in determining
both if the plume may intercept terrain and the resulting ground-level concentration. This
assumption is necessary because actual plume heights will be lower with higher stack
height wind speeds, and because impacts on intervening terrain above stack height but
below the full plume height might otherwise be missed.)
*An exception may be certain flat terrain situations where building downwash is a
problem (See Section 4.5.1).
4-28
-------
FT lf3
2'6 11J
A/i =
Refer to Steps 1 and 3 of Section 4.2 for a definition of terms.
(2) Compute a conservative plume height (hj by adding the physical stack
height (h,) to Ah:
he = h, + Ah
(3) Determine if any terrain features in the vicinity of the source are as
high as h,.. If so, proceed with Step 2. If that is not the case, the plume is not likely to
intercept terrain, and Step 2 is not applicable.*
Step 2. Estimate the maximum 24-hour ground-level concentration on elevated
terrain in the vicinity of the source:
(1) Using a topographic map, determine the distance from the source to
the nearest ground-level location at the height h,..
(2) Using Figure 4-17 and the distance determined in (1), estimate a
24-hour %/Q value.
(3) Multiply the (x/Q)24 value by the emission rate Q (g/s) to estimate the
•
maximum 24-hour concentration, %24, due to plume impaction on elevated terrain:
X* - G [£l
kJ 24
*Even if the plume is not likely to impact on elevated terrain (and for all concentration
averaging times of concern) the user should account for the effects of elevated terrain on
maximum concentrations. A procedure to account for elevated terrain below stack height
is described in Section 4.2 and consists of reducing the computed plume height, h^ (for
all stabilities), by the elevation difference between stack base and location of the
receptor(s) in question. The adjusted plume heights can then be used in conjunction with
the "flat-terrain" modeling procedures described earlier.
4-29
-------
4.5.3 Fumigation
Fumigation occurs when a plume that was originally emitted into a stable layer is
mixed rapidly to ground-level when unstable air below the plume reaches plume level.
Fumigation can cause very high ground-level concentrations.27 Typical situations in which
fumigation occurs are:
1. Breaking up of the nocturnal radiation inversion by solar warming of the
ground surface;
2. Shoreline fumigation caused by advection of pollutants from a stable marine
environment to an unstable inland environment; and
3. Advection of pollutants from a stable rural environment to a turbulent urban
environment.
The following procedure can be used for estimating concentrations due to inversion
break-up and shoreline fumigation in rural areas. Sources located within 3km of a large
body of water should be evaluated for shoreline fumigation. Procedures for estimating
concentrations during the third type, rural/urban, are beyond the scope of this document.
Calculation procedures:
Step 1. Compute the plume height (hj that will occur during F stability and a stack
height wind speed of 2.5 m/s:
he = hs + Ah
To obtain a value for Ah, use the procedure described in Step 3 of Section 4.2 with u =
2.5 m/s. If vs < 1.5us, account for stack tip downwash using Equation 4.7.
Step 2. Estimate the downwind distance to maximum ground-level concentration
using (a) for inversion break-up and (b) for shoreline fumigation.
(a) For inversion break-up fumigation, use Table 4-4 (derived from Equation
(5.5) of Turner's Workbook)9 to estimate the downwind distance at which the maximum
fumigation concentration is expected to occur, which is based on the time required for the
4-30
-------
mixed layer to develop from the top of the stack to the top of the plume. If this distance
is less than about 2km, then fumigation concentrations are not likely to exceed the limited
mixing concentrations estimated in Step 4, Procedure (a), of Section 4.2, and may be
ignored.
(b) For shoreline fumigation, the maximum fumigation concentration is
expected to occur where the top of the stable plume intercepts the top of the thermal
internal boundary layer (TIBL). The distance to this location, measured from the
shoreline, may be estimated from Table 4-5. The distances in Table 4-5 are based on the
assumption of a parabolic TIBL shape.28 Subtract the distance from the source to the
shoreline from the value in Table 4-5 in order to obtain the downwind distance to the
maximum from the source. If the distance obtained is less than 0.2km, then the shoreline
fumigation screening procedure should not be applied since the plume/TIBL interaction
may be influenced by transitional plume rise effects.
Step 3. At the distance estimated in (2), determine the value of cry from Figure 4-18
and of az from Figure 4-14 for F stability. Since the effects of buoyancy-induced
dispersion (BID) have been incorporated in the distances determined in (2) above, it is
recommended that the values for ay and az be adjusted for BID effects as follows:
a' =
3.5
(4.13)
°: =
a2
1 3.5
where Ah is the plume rise determined in (1) above. The maximum fumigation estimate,
particularly for shoreline fumigation, is sensitive to the inclusion of BID since it effects
the distance to the maximum as well as the actual concentration calculation.
4-31
-------
Table 4-4. Downwind Distance (km) to the Maximum Ground Level Concentration for Inversion Break-up Fumigation
as a Function of Stack Height (h,) and Plume Height (hj*
hs
10
20
30
40
50
60
70
80
90
100
125
150
175
200
225
250
275
300
Plume Height, h.
<60
(<2)
<<2)
«2)
(<2)
(<2)
.
.
.
.
.
.
.
-
.
.
.
.
-
60
2.6
2.3
«2)
(<2)
(<2)
«2)
.
-
.
.
-
-
-
.
-
.
-
-
70
3.6
3.3
2.9
2.5
2.0
«2)
(<2)
-
-
.
-
-
-
-
-
.
-
-
80
4.7
4.3
3.9
3.5
3.1
2.5
(<2)
(<2)
-
.
-
-
-
.
-
.
.
-
90
5.9
5.5
5.1
4.7
4.2
3.7
3.1
2.4
(<2).
.
-
-
-
-
-
-
-
-
100
7.2
6.8
6.4
5.9
5.4
4.9
4.3
3.6
2.9
«2)
-
-
-
-
-
-
-
-
125
11
11
10
9.5
9.0
8.4
7.7
7.1
6.3
5.5
3.2
-
-
-
-
-
-
-
150
16
15
14
14
13
12
12
11
10
9.4
7.2
4.5
-
-
-
-
-
-
175
20
20
19
19
18
17
16
16
15
14
12
9.0
5.9
-
-
.
.
-
200
26
25
24
24
23
22
21
21
20
19
17
14
11
7.5
-
-
-
-
225
32
31
30
30
29
28
27
26
25
24
22
19
16
13
9.1
-
-
-
250
38
38
37
36
35
34.
33
32
31
30
28
25
22
18
15
11
.
-
275
46
45
44
43
42
41
40
39
38
37
34
31
28
24
21
17
13
-
300
53
52
51
50
49
48
47
46
45
44
41
37
34
31
27
23
19
14
*Assume Stability Class F and Wind Speed = 2.5 m/s.
-------
Step 4. Compute the maximum fumigation concentration (%f), using the following
equation:9
For the inversion break-up case, the concentration x/ can be expected to persist for about
30 to 90 minutes. For shoreline fumigation, the high ground-level concentrations can
persist as long as the stable onshore flow persists, up to several hours, although the
location may shift as the direction of the onshore flow shifts.
Step 5. If the estimated fumigation concentration, Xr» is less than the maximum
1-hour concentration, XL estimated from Step 4 of Section 4.2, then the effects of
fumigation may be ignored. If the estimated fumigation concentration exceeds the
maximum 1-hour concentration estimated from Step 4 of Section 4.2, then the effect of
fumigation on longer averaging periods may be accounted for as follows. The value of
X used with the multiplying factors in Step 5 (Section 4.2) should be adjusted using a
weighted average of Xi and xf, assuming that Xr persists for 90 minutes. The weighted
average should be calculated as follows:
Averaging Time Adjustment of Y for Fumigation
3 hours Y
X. — y
8 hours ' = 13Xl
16
24 hours Y ' - 15Xl +
Xl 16
4-33
-------
Table 4-5. Downwind Distance (km) to the Maximum Ground Level Concentration for Shoreline Fumigation
as a Function of Stack Height (hs) and Plume Height (hj*
hs
10
20
30
40
50
60
70
80
%
100
125
150
175
200
225
250
275
300
Plume Height, h,,
<60
(<0.2)
(<0.2)
(<0.2)
(<0.2)
(<0.2)
-
-
-
-
-
-
-
-
-
-
-
-
-
60
0.22
(<0.2)
(<0.2)
(<0.2)
(<0.2)
(<0.7)
-
-
-
-
-
-
-
-
-
-
- .
-
70
0.31
0.28
0.25
0.22
(<0.2)
(<0.2)
(<0.2)
-
-
-
-
-
-
-
-
-
-
-
80
0.42
0.38
0.34
0.31
0.28
0.25
0.23
0.22
-
-
-
-
-
-
-
-
-
-
90
0.54
0.49
0.45
0.41
0.38
0.34
0.31
0.29
0.28
-
-
-
-
-
-
-
-
-
100
0.67
0.62
0.58
0.53
0.49
0.45
0.42
0.39
0.36
0.35
-
-
-
-
-
-
-
-
125
1.1
1.0
0.96
0.90
0.85
0.79
0.75
0.70
0.66
0.62
0.57
-
-
-
-
-
-
-
150
1.6
1.5
1.4
1.4
1.3
1.2
1.2
1.1
1.1
1.0
0.89
0.85
-
-
-
-
.
-
175
2.2
2.1
2.0
1.9
1.8
1.8
1.7
1.6
1.5
1.5
1.3
1.2
1.2
-
.
-
.
-
200
2.9
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2.1
1.9
1.7
1.6
1.6
.
-
.
-
225
3.6
3.5
3.4
3.3
3.2
3.1
3.0
2.9
2.8
2.7
2.5
2.3
2.2
2.0
2.0
.
.
-
250
4.5
4.4
4.2
4.1
4.0
3.9
3.8
3.7
3.6
3.5
3.2
3.0
2.8
2.6
2.5
2.5
.
-
275
5.4
5.3
5.2
5.0
4.9
4.8
4.7
4.5
4.4
4.3
4.0
3.8
3.5
3.3
3.2
3.0
3.0
-
300
6.5
6.3
6.2
6.0
5.9
5.8
5.6
5.5
5.4
5.2
4.9
4.6
4.4
4.1
3.9
3.7
3.6
3.6
'Assume Stability Class F and Wind Speed = 2.5 m/s.
-------
The adjusted value, 3^' , should then be used with the multiplying factors in Step 5 of
Section 4.2.
4.5.4 Estimated Concentrations from Area Sources
The SCREEN2 area source algorithm is based on the equation for a finite line
segment source. The current version of the Industrial Source Complex (ISC2)21 model
also incorporates this method of calculating downwind concentrations from area sources.
This algorithm requires that the area source be square in shape. That is, the length of one
side of the square is input to the program. Areas which have irregular shapes can be
simulated by dividing the area source into multiple squares that approximate the geometry
of the area source. The centerline ground-level concentration at a downwind distance x
(measured from the downwind edge of the area source) is given by:
QAKx. x
erf (___) , (4.15)
J2ua Jzi^a
" s z ' y
where:
QA = area source emission rate (mass per unit area per unit time)
K = a scaling coefficient -to convert calculated concentrations to desired units
(default value of 1 x 106 for Q in g/m2s and concentration in pg/m3)
x0 = length of the side of the area source (m)
4-35
-------
It is recommended that, if the separation between an area source and a receptor is less
than one length of the side of the area source x^ then the area source should be
subdivided into smaller area sources.
Estimate maximum short-term (1-hour) concentrations by following the procedure
for point sources outlined in Step 4 of Section 4.2, assuming no plume rise, Ah = 0. Do
not use the multiplying factors in Step 5 of Section 4.2 to correct for averaging times
greater than 1 hour. Concentrations close to an area source will not vary as much as those
•
for point sources in response to varying wind directions, and the meteorological conditions
which are likely to give maximum 1-hour concentrations (Procedures (b) and (c) of
Section 4.2) can persist for several hours. Therefore it is recommended that the maximum
1-hour concentration be conservatively assumed to apply for averaging periods out to
24 hours.
4.5.5 Volume Sources
SCREEN2 uses a virtual point source algorithm to model the effects of volume
sources. Therefore, the Gaussian equation is used to calculate concentrations produced
by volume source emissions. This method for calculating volume sources is also used by
the Industrial Source Complex (ISC2)21 model. If the volume source is elevated, the user
assigns the effective emissions height he. The user also assigns initial lateral (ayo) and
vertical (a^) dimensions for the volume source. Lateral (Xy) and vertical (xz) virtual
distances are added to the actual downwind distance x for the ay and az calculations. The
virtual distances are calculated from solutions to the sigma equations as is done for point
sources with building downwash.
4-36
-------
The volume source option is used primarily to simulate the effects of non-buoyant
emissions from sources such as building roof vents. Table 4-6 below summarizes the
general procedures suggested for estimating initial lateral (ayo) and vertical (a^,)
dimensions for a single volume source. There are two types of volume sources: (1)
surface-based sources, which may also be modeled as area sources, and (2) elevated
sources.
Table 4-6. Summary of Suggested Procedures for Estimating Initial Lateral (ayo) and
Vertical Dimensions (a^) for Single Volume Sources
Description of Source
Surface-based source (he ~ 0)
Elevated source (he > 0) on
or adjacent to a building
Elevated source (he > 0) not
on or adjacent to a building
Initial Dimension
Lateral (avo) Vertical (aro)
side length divided by 4.3
ii
tt
vertical dimension
divided by 2.15
building height divided
by 2.15
vertical dimension
divided by 4.3
4.5.6 Contributions from Other Sources
To assess the significance of the air quality impact of a proposed source, the
impact of nearby sources and "background" must be specifically determined. (Background
includes those concentrations due to natural sources, and distant or unspecified man-made
sources.) The impact of the proposed source can be separately estimated, applying the
techniques presented elsewhere in Section 4, and then superimposed upon the impact of
the nearby sources and background to determine total concentrations in the vicinity of the
4-37
-------
proposed source. This section addresses the estimation of concentrations due to nearby
sources and background. Three situations are considered:
A. A proposed source relatively isolated from other sources.
B. A proposed source in the vicinity of a few other sources.
C. A proposed source in the vicinity of an urban area or other large number of
sources.
It must be noted that in all references to air quality monitoring in the following
discussion, it is assumed that the source in question is not yet operating. If the source is
emitting pollutants during the period of air quality data collection, care must be taken not
to use monitoring data influenced by the impact of the source. Additional guidance on
determining background concentrations is provided in Section 9.2 of the Guideline on Air
Quality Models (Revised).3
A. Relatively Isolated Proposed Source
A proposed source may be considered to be isolated if it is expected that background
will be the only other significant contributor to ambient pollutant concentrations in its
vicinity. In that case, it is recommended that air quality data from monitors in the vicinity
of the proposed source be used to estimate the background concentrations. If monitoring
data are not available from the vicinity of the source, use data from a "regional" site; i.e.,
a site that characterizes air quality across a broad area, including that in which the source
is located. Annual average concentrations should be relatively easy to determine from
available air quality data. For averaging times of about 24 hours or less, meteorology
should be accounted for; i.e., the combined source / background concentration must be
calculated for several meteorological conditions to ensure that the maximum total
concentration is determined.
4-38
-------
B. Proposed Source in the Vicinity of a Few Other Sources
If there already are a few sources in the vicinity of the proposed facility, the air
quality impact of these sources should be accounted for. As long as the number of nearby
sources is relatively small, the recommended procedure is to use (1) air quality monitoring
data to estimate background concentrations and (2) dispersion modeling to estimate
concentrations due to the nearby source(s). Then superimpose those estimates to
determine total concentrations in the vicinity of the proposed source.
To estimate background concentrations, follow the same basic procedure as in the
case of an isolated source. In this case, however, there is one added complication. Wind
direction must be accounted for in order to single out the air quality data that represent
background only (i.e., data that are not affected by contributions from nearby sources).
Concentrations due to the nearby sources will normally be best determined through
dispersion modeling. The modeling techniques presented in this guideline may be used.
The user should model each source separately to estimate concentrations due to each
source during various meteorological conditions and at an array of receptor locations (e.g.,
see Sections 4.3. and 4.4.1) where interactions between the effluents of the proposed
source and the nearby sources can occur. Significant locations include (1) the area of
expected maximum impact of the proposed source, (2) the area of maximum impact of
the nearby sources, and (3) the area where all sources will combine to cause maximum
impact. It may be necessary to identify those locations through a trial and error analysis.
4-39
-------
C. Proposed Source Within an Urban Area or in the Vicinity of a Large Number of
Sources
For more than a very small number of nearby sources, it may be impractical to
model each source separately. Two possible alternatives for estimating ambient
concentrations due to the other sources are to use air quality monitoring data or a
multisource dispersion model. If data from a comprehensive air monitoring network are
available, it may be possible to rely entirely on the measured data. The data should be
adequate to permit a reliable assessment of maximum concentrations, particularly in (1)
the area of expected maximum impact of the proposed source, (2) the area of maximum
impact of the existing sources and (3) the area where all sources will combine to cause
maximum impact. In some cases, the available air quality monitor data will only be
adequate to estimate general area-wide background concentrations. In such cases, there
is no choice but to use dispersion modeling to estimate concentrations due to the nearby
sources. If possible, a multisource dispersion model should be used. The ISCLT model
can be applied for long-term concentration estimates, and the MPTER or ISCST model
for short-term estimates (MPTER can handle up to 250 point sources but cannot handle
building downwash effects). If it is not feasible to apply a multisource model, and there
is a considerable number of nearby sources, a rough estimate of maximum concentrations
due to those sources can be made by arbitrarily grouping the sources into an area source
through the following equation.29 (The estimate is primarily applicable to receptor
locations near the center of the area source, defined below, although it may be considered
a reasonable first-approximation for any location within the area):
C = 18Q , (4.17)
u
4-40
-------
where:
C = maximum short term (1-24 hours) contribution to ground-level concentrations
from the area source (g/m3)
Q = average emission rate (g/m2/s) within the area defined by Ax
u = assumed average wind speed (m/s) for the averaging time of concern (use
2 m/s if no data are available)
Ax = length (m) of one side of the smallest square area that will contain the nearby
sources, ignoring relatively small outlying sources or any source that is
considerably removed from the other sources.
The best results will be obtained with the above equation when emissions are uniformly
distributed over the defined area. Any large point sources in the vicinity should be
modeled separately, and the estimated concentrations manually superimposed upon that
computed for the area source. Because this is an area source approximation, the
adjustment factors for averaging times greater than one hour should not be used.
4.5.7 Long Range Transport
In certain instances it will be necessary to estimate the air quality impact of a
proposed source at locations beyond its vicinity (beyond roughly 30 - 50km). To estimate
seasonal or annual average concentrations (out to about 100km) the procedures of Section
4.4 provide a rough estimate. The procedures are limited to plume heights greater than
50m, and should not be applied beyond 100km. For short-term estimates (concentration
averaging times up to about 24 hours) beyond the vicinity of the source and out to 100km
downwind, the following procedure is recommended. The procedure accounts for the
meteorological situations with the greatest persistence that are likely to result in the
highest concentrations at large distances, i.e., neutral/high wind conditions (Steps 1-4) and
stable conditions (Steps 5-7):
4-41
-------
Step 1. Estimate the normalized plume rise (uAh) applicable to neutral and
unstable atmospheric conditions. Use the procedure described in Step 1 of Section 4.2.
Step 2. Compute plume height, h,., that will occur during D stability with a 10m
wind speed of 5 m/s. Adjust the wind speed from 10m to stack height, using Equation
3.1 and the exponent for stability class D:
L L
*• " *• +
Step 3. Using Figure 4-19, obtain a %u/Q value for the desired downwind distance
(D stability case). (If the plume height is greater than 300m, then the value corresponding
to he = 300m may be used for conservatism.)
Step 4. Compute the maximum 1-hour D stability concentration, x^, using the
Xu/Q value obtained in Step 3:
x - O
m« ^ — ~
s
For Q, substitute the source emission rate (g/s), and use the value of u$ determined in Step 2.
Step 5. Compute the plume height h,. = h, + Ah that will occur during E stability
with a 10m wind speed of 2 m/s. Adjust the wind speed from 10m to stack height using
Equation 3.1 and the exponent for stability class E. Use the stable plume rise (Ah)
computed from Equation 4.6 in Step 3 of Section 4.2:
he = hs -i- Ah
Step 6. From Figure 4-20, obtain a %u/Q value for the same distance considered
in Step 3 above. (If the plume height is greater than 300m, then the value corresponding
to he = 300m may be used for conservatism).
4-42
-------
Step 7. Compute the maximum 1-hour E stability concentration, x,,,,
using the %u/Q value obtained in Step 6:
where u, was determined in Step 5.
Step 8. Select the higher of the Xm« values computed in Steps 4 and 7. The
selected value represents the highest 1-hour concentration likely to occur at the specified
distance.
Step 9. To estimate concentrations for averaging times up to annual, multiply the
1-hour value by the factors presented .hi Step 5 of Section 4.2.
4-43
-------
10-3
T 1 I t I III
10-
10-6
I I I I 1 II I
I I I I I I I I
10
20
50 100 200
PLUME HEIGHT, m
500
1000
Rgure 4-1. Maximum %u/Q as a function of plume height, H (for use only with the
simple screening procedure).
4-44
-------
01
2 5 10-4
MAXIMUM xu/Q. m 2
10'
3 2
10'*
Figure 4-2. Downwind distance to maximum concentration and maximum %u/Q as a function of stability class for rural
terrain.17 Plume heights (m) are indicated on the curves.
4-45
-------
1fl-5 2 5
MAXIMUM xu/Q.m'2
Figure 4-3. Downwind distance to maximum concentration and maximum xu/Q as a
function of stability class for urban terrain. Plume heights (m) are
indicated on the curves.
4-46
-------
1C'3
10-*
12 S 10
DOWNWIND DISTANCE, km
20
50
100
Figure 4-4. Stability class A, rural terrain; %u/Q vs. distance for various plume
heights (H), assuming very restrictive mixing heights (L); L = 50m for
H < 50m; L = H for H > 50m.
4-47
-------
0.2
0.5
12 S 10
DOWNWIND DISTANCE, km
20
50
100
Figure 4-5. Stability class B, rural terrain; %u/Q vs. distance for various plume
heights (H), assuming very restrictive mixing heights (L); L = 50m for
H < 50m; L = H for H > 50m.
4-48
-------
10-3
0.2
0.5
12 5 10
DOWNWIND DISTANCE, km
20
SO 100
Figure 4-6. Stability class C, rural terrain; %u/Q vs. distance for various plume
heights (H), assuming very restrictive mixing heights (L); L = 50m for
H < 50m; L = H for H > 50m.
4-49
-------
II I I I III
10-1
12 S 10
DOWNWIND DISTANCE, km
100
Figure 4-7. Stability class D, rural terrain; %u/Q vs. distance for various plume
heights (H), assuming very restrictive mixing heights (L); L = 50m for
H < 50m; L = H for H > 50m.
4-50
-------
10-2
0.2
0.5
12 5 10
DOWNWIND DISTANCE, km
20
50 100
Figure 4-8. Stability class E, rural terrain; %u/Q vs. distance for various plume
heights (H), assuming very restrictive mixing heights (L); L = 50m for
H < 50m; L = H for H > 50m.
4-51
-------
10-2 —
0.1 O.Z
o.s
1 Z S 10
DOWNWIND DISTANCE, km
SO 100
Figure 4-9. Stability class F, rural terrain; %u/Q vs. distance for various plume
heights (H), assuming very restrictive mixing heights (L); L = 50m for
H < 50m; L = H for H > 50m.
4-52
-------
0.2
0.5
2 5 10
DOWNWIND DISTANCE, km
20
50
100
Figure 4-10. Stability classes A and B, urban terrain; %u/Q vs. distance for various
plume heights (H), assuming very restrictive mixing heights (L); L =
50m for H < 50m; L = H for H > 50m.
4-53
-------
0.1 0.2
0.5 1 2 5 10 20
DOWNWIND DISTANCE, km
50 100
Figure 4-11. Stability class C, urban terrain; %u/Q vs. distance for various plume
heights (H), assuming very restrictive mixing heights (L); L = 50m for
H < 50m; L = H for H > 50m.
4-54
-------
0.2
0.5
2 5 10
DOWNWIND DISTANCE, km
20
50
100
Figure 4-12. Stability class D, urban terrain; %u/Q vs. distance for various plume
heights (H), assuming very restrictive mixing heights (L); L = 50m for
H < 50m; L = H for H > 50m.
4-55
-------
12 5 10
DOWNWIND DISTANCE, km
20
SO 100
Figure 4-13. Stability class E, urban terrain; %u/Q vs. distance for various plume
heights (H), assuming very restrictive mixing heights (L); L = 50m for
H < 50m; L = H for H > 50m.
4-56
-------
5,000
2.000
1.000
500
200
e
i 100
SO
20
10
0.1 0.2
0.5 1 2 5 10
DOWNWIND DISTANCE, km
20
50 100
Figure 4-14. Vertical dispersion parameter (aj as a function of downwind
distance and stability class; rural terrain.
4-57
-------
Figure 4-15. Isopleths (hundredths of meters) of mean annual afternoon mixing heights.
4-58
-------
Figure 4-16. Isopleths (hundredths of meters) of mean annual morning mixing heights.
4-59
-------
I «MNI
,0-5
1 I | I ill
s
10-6
10-7
10-*
0.1 0.2
0.5 1 2 5
DOWNWIND DISTANCE, km
10
20
50 100
Figure 4-17. 24-hour %/Q vs. downwind distance, obtained from the Valley model.
Assumptions include: stability class F, wind speed = 2.5 m/s, and
plume height 10m above terrain.
4-60
-------
10,000
5,000
2,000
1.0QQ
50
Xxx x
S ' ' / / *
- 200
I
100
/ /
' '
50
20
10
0.1 0.2
0.5 1 2 5 10
DOWNWIND DISTANCE, km
20
50 100
Figure 4-'18. Horizontal dispersion parameter (ory) as a function of downwind
distance and stability class; rural terrain.
4-61
-------
20
SO
DOWNWIND DISTANCE, km
100
Figure 4-19. Maximum %u/Q as a function of downwind distance and plume height
(H), assuming a mixing height of 500m; D stability.
4-62
-------
20 SO
DOWNWIND DISTANCE, km
100
Figure 4-20. Maximum %u/Q as a function of downwind distance and plume height
(H); E stability.
4-63
-------
5. REFERENCES
1. U.S. Environmental Protection Agency, September 1974. Guidelines for Air Quality
Maintenance Planning and Analysis, Volume 10: Reviewing New Stationary
Sources. EPA-450/4-74-011 (OAQPS Number 1.2-029), Research Triangle Park,
NC 27711.
2. U.S. Environmental Protection Agency, October 1977. Guidelines for Air Quality
Maintenance Planning and Analysis, Volume 10 (Revised): Procedures for
Evaluating Air Quality Impact of New Stationary Sources. EPA-450/4-77-001
(OAQPS Number 1.2-029R), Research Triangle Park, NC 27711.
3. U.S. Environmental Protection Agency, 1986. Guideline on Air Quality Models
(Revised) (1987). EPA-450/2-78-027R. U.S. Environmental Protection Agency.
Research Triangle Park, NC 27711.
4. U.S. Congress, August 1977. Clean Air Act Amendments of 1977 - Public Law
95-95, Section 302 (j).
U.S. Congress, November 1990. Clean Air Act Amendments of 1990 - Public Law
101-549.
5. U.S. Environmental Protection Agency, 1985. Compilation of Air Pollution
Emission Factors, Volume I: Stationary Point and Area Sources. Publication No.
AP-42, Fourth Edition, Research Triangle Park, NC 27711.
6. U.S. Environmental Protection Agency, 1985. Guideline for Determination of Good
Engineering Practice Stack Height (Technical Support Document for the Stack
Height Regulations), (Revised), EPA-450/1-80-023R. U.S. Environmental
Protection Agency, Research Triangle Park, NC 27711.
7. U.S. Environmental Protection Agency, 1987. On-site Meteorological Program
Guidance for Regulatory Modeling Applications, EPA-450/8-87-013. U.S.
Environmental Protection Agency. Research Triangle Park, NC 27711.
8. U.S. Environmental Protection Agency, 1987. Ambient Monitoring Guidelines
for Prevention of Significant Deterioration (PSD). EPA-450/4-87-007. U.S.
Environmental Protection Agency, Research Triangle Park, NC 27711.
9. Turner, D.B., 1970. Workbook of Atmospheric Dispersion Estimates. Revised,
Sixth printing, Jan. 1973. Office of Air Programs Publication No. AP-26. U.S.
Environmental Protection Agency. U.S. Government Printing Office, Washington,
D.C. 20402.
10. List, R.J., 1966. Smithsonian Meteorological Tables. Sixth Revised Edition (Third
Reprint). Smithsonian Institution, Washington, D.C.
5-1
-------
11. Holzworth, G.C., 1972. Mixing Heights, Wind Speeds, and Potential for Urban Air
Pollution Throughout the Contiguous United States. Office of Air Programs
Publication No. AP-101, U.S. Environmental Protection Agency. U.S. Government
Printing Office, Washington, D.C. 20402.
12. Randerson, D., 1984. Atmospheric Boundary Layer. In Atmospheric Science and
Power Production. Randerson, D. (ed.), DOE/TIC-27601, U.S. Department of
Energy, Washington, D.C.
13. U.S. Environmental Protection Agency, 1971. Exhaust Gases from Combustion and
Industrial Sources, APTD-0805. Pub. No. PB203-861, NTIS, Springfield, VA
22151.
14. Turner, D.B. and E.L. Martinez, 1973. A Simple Screening Technique for
Estimating the Impact of a Point Source of Air Pollution Relative to the Air Quality
Standards. (NOAA manuscript) U.S. Environmental Protection Agency, Research
Triangle Park, NC 27711.
15. Briggs, G.A., 1969. Plume Rise. USAEC Critical Review Series TID-25075,
National Technical Information Service, Springfield, VA 22151.
16. Briggs, G.A., 1971. Some Recent Analyses of Plume Rise Observation Pages
1029-1032 of the Proceedings of the Second International Clean Air Congress,
edited by H.M. Englund and W.T. Berry. Academic Press, NY.
17. Briggs, G.A., 1975. Plume Rise Predictions. In: Lectures on Air Pollution and
Environmental Impact Analysis, D.A. Haugen, ed. American Meteorological
Society, Boston, MA; pp. 59-111.
18. Leahey, D.M. and MJ.E. Davies, 1984. Observations of Plume Rise from Sour Gas
Flares. Atmospheric Environment, 18: 917-922.
19. Beychok, M., 1979. Fundamentals of Stack Gas Dispersion. Irvine, CA.
*
20. McElroy, J.L. and Pooler, F., December 1968. St. Louis Dispersion Study, Volume
II - Analyses. AP-53, National Air Pollution Control Administration, Arlington, VA
22203.
21. U.S. Environmental Protection Agency, 1992. User's Guide for the Industrial
Source Complex (ISC2) Dispersion Models, EPA-450/4-92-008a. U.S.
Environmental Protection Agency, Research Triangle Park, NC 27711.
22. Snyder, W.H. and R.E. Lawson, Jr., 1976. Determination of a Necessary Height for
a Stack Close to a Building—A Wind Tunnel Study. Atmospheric Environment,
10: 683-691.
23. Hosker, R.P., 1984. Flow and Diffusion Near Obstacles. In: Atmospheric Science
and Power Production. Randerson, D. (ed.), DOE/TIC-27601, U.S. Department of
Energy, Washington, D.C.
5-2
-------
24. Huber, A.H. and W.H. Snyder, October 1976. Building Wake Effects on Short
Stack Effluents. Preprint volume: Third Symposium on Atmospheric Turbulence,
Diffusion and Air Quality. Pubh'shed by American Meteorological Society, Boston,
MA; pp. 235-242.
25. Huber, A.H., 1977. Incorporating Building/Terrain Wake Effects on Stack
Effluents. Preprint volume: AMS-APCA Joint Conference on Applications of Air
Pollution Meteorology, November 29 - December 2, 1977, Salt Lake City, UT.
26. Burt, E.W., September 1977. Valley Model User's Guide. EPA-450/277-018.
U.S. Environmental Protection Agency, Research Triangle Park, NC 27711.
27. Lyons, W.A. and H.S. Cole, 1973. Fumigation and Plume Trapping on the Shores
of Lake Michigan During Stable Onshore Flow. J. of Applied Meteorology,
12: 494-510.
28. Stunder, M. and S. SethuRaman, 1986. A Statistical Evaluation of Coastal Point
Source Dispersion Models. Atmospheric Environment, 20: 301-315.
29. Hanna, S.R. 1971. A Simple Method of Calculating Dispersion from Urban Area
Sources. Journal of the Air Pollution Control Association, 12: 774-777.
5-3
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA - 459/R-92-019
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Screening Procedures for Estimating the Air
Quality Htpact of Stationary Sources, Revised
5. REPORT DATE
October 1992
6. PERFORMING ORGANIZATION CODE
'. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
WING ORGANIZATION NAME AND AOORESS
10. PROGRAM ELEMENT NO.
11 CONTRACT/GRANT NO
12. SPONSORING AGENCY NAME AND ADDRESS
Office of Air Quality Planning and Standards
U. S. Environmental Protection Agency
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
This document presents current EPA guidance on the use of the revised screening
procedures for estimating the air quality impact of stationary sources. The original
version of this document (EPA-450/4-88-010) was a draft for public conment which has
subsequently been included as part of the Guideline on Air Quality Models. SCREEN2
technical support is provided herein. Major changes in this version of SCREEN2 are
the finite line segment method for area sources, addition of wind speeds in the wind
speed-stability matrix for calculating concentrations, and the inclusion a single
volume source option.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COSAT1 Field, G
jfOup
Air Pollution
Atmospheric Diffusion
Atmospheric Models
Meteorology
New Source Review
ISU'
rEMENT
19. SECURITY CLASS (Tins Report/
None
21 NO OF PAGES
91
Unlimited
20. SECURITY CLASS
None
22. PRICE
EPA Form 2220-1 (R«v. 4-77) PREVIOUS EDITION is OBSOLETE
-------
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