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

-------
 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

-------
                      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

-------
                                 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

-------
                           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

-------
                            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

-------
                              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

-------
                              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

-------
                         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

-------
                              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

-------
                           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)

-------
                           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

-------
                               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

-------
     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

-------
quality analysts should be engaged. An air quality simulation model applied improperly



can lead to serious misjudgments regarding the source impact.
                                      1-3

-------
                               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

-------
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

-------
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

-------
     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

-------
                        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

-------
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

-------
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

-------
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

-------
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

-------
     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

-------
             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

-------
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

-------
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

-------
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

-------
     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

-------
     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

-------
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

-------
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

-------
     (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

-------
     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

-------
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

-------
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

-------
     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

-------
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

-------
      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

-------
     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

-------
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

-------
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

-------
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

-------
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

-------
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

-------
       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

-------
                                                         INSTRUCTIONS

  1.    REPORT NUMBER
       Insert the LPA report number as it appears on the cover of the publication.

  2.    LEAVE BLANK

  3.    RECIPIENTS ACCESSION NUMBER
       Reserved for use by each report recipient.

  4.    TITLE AND SUBTITLE
       Title should indicate clearly and briefly the subject coverage of the report, and be di>ptaycd prominently. Set subtitle, tl used, in smaller
       type or otherwise subordinate it to mam title. When a report  is prepared in more ili.in one volume, re-prat ihe pnm.iry tide, .uld volume
       number and include subtitle for the specific title.

  5.    REPORT DATE
       Each report shall carry a date indicating at least month and year.  Indicate the IUMS on \vhich it \\,is elected if ,ir. Jaic «j mm-,  dale <>/
       approval, date of preparation,  etc.}.

  6.    PERFORMING ORGANIZATION COOE
       Leave blank.

  7.    AUTHOR(S)
       Give name(s)  in conventional order (John K. Doe. J. Robert Do?, etc./.  Li.st author's aH'ilialion if it iltttcrs Ironi ihe performing organi-
       zation.

  8.    PERFORMING ORGANIZATION REPORT NUMBER
       Insert if performing organization wishes to assign this number.

  9.    PERFORMING ORGANIZATION NAME AND ADDRESS
       Give name, street, city, state, and ZIP code. List no more than two levels ol an organi/alional lurcauhy.

  10.  PROGRAM ELEMENT NUMBER
       Use the program element number under which the report wa.s prepared. Subordinate numbers ui.iy be included m parentheses.

  11.  CONTRACT/GRANT NUMBER
       Insert contract or grant number under which report was prepared.

  12.  SPONSORING AGENCY NAME AND ADDRESS
       Include ZIP code.

  13.  TYPE OF REPORT AND PERIOD COVERED
       Indicate interim final, etc., and if applicable, dates covered.

  14.  SPONSORING AGkNCY CODE
       Insert appropriate code.

  15.  SUPPLEMENTARY NOTES
       Enter information not included elsewhere but useful', such a.s.  Prepared m cooperation with. I raiislation ol. Presented ,ii lunlcieiKi <>i
       To be published in. Supersedes, Supplements, etc.                                           «

  16.  ABSTRACT
       Include a brief (200 words or  less) factual summary of the most significant information contained m the rcpori  II tlu- rcp«ni loniams a
       significant  bibliography or literature survey, mention it here.

  17.  KEY WORDS AND DOCUMENT ANALYSIS
       (a) DESCRIPTORS • Select from the Thesaurus of Engineering and Scientilie  Tenm the proper authori/.cd lerms that identity the major
       concept of the research and are sufficiently specific and precise, to be used a.s index entries lor cataloging.

       (b) IDENTIFIERS AND OPEN-ENDED TERMS -  Use identifiers for project names, code name.s. equipment designators, etc   Use- open-
       ended terms written in descriptor form for those subjects for  which no descriptor exists.

       (c) COSATI MELD GROUP - l-ield and group assignments are to be taken Irom the I965 ("OSAII Subject ( uicgory List. Since the ma-
       jority of documents are multidisciplinary in nature, the  Primary I ield/(iroup assignment*s) will be spcxila discipline, area ol human
       endeavor, or  type of physical  object. The application(s) will be cross-relerenced  with secondary I iehl/(,roup assignments that will lollo*
       the primary postingts).

   18.  DISTRIBUTION STATEMENT
       Denote releasability to the public or limitation for reasons other than security lor example "Release-  Cnlimiicd."  < Me anv av.ul.ihihu  
-------