SCREEN3 Model User's Guide July 1995

U.S. ENVIRONMENTAL PROTECTION AGENCY

Office of Air Quality Planning and Standards, Technical Support Division

Research Triangle Park, North Carolina 27711

(Pilat edited in October 2006 for U of Washington CEE 490 web page)

I. Introduction

1.1 Overview of SCREEN3 User's Guide (The SCREEN inside of TSCREEN is slightly different).

Note that for the SCREEN model, there is the"Screening Procedures for Estimating the Air Quality Impact of Stationary Sources" which is available at the EPA's web site at http://www.epa.gov/scram001/guidance_permit.htm ). A PDF copy is available at: http://www.engin.umich.edu/class/aoss563/EPA_Models/DOCS/

1.2 Purpose of SCREEN

The SCREEN model was developed to provide an easy-to-use method of obtaining pollutant concentration estimates based on the screening procedures document. By taking advantage of the availability and use of personal computers, the SCREEN model makes screening calculations accessible to a wide range of users. The SCREEN model does not need meteorolical data as it assumes the worst case weather conditions.

1.4 What will SCREEN do

SCREEN runs interactively on the PC, meaning that the program asks the user a series of questions in order to obtain the necessary input data, and to determine which options to exercise. SCREEN can perform all of the single source, short‑term calculations in the screening procedures document, including estimating maximum ground‑level concentrations and the distance to the maximum, incorporating the effects of building downwash on the maximum concentrations for both the near wake and far wake regions ), estimating concentrations in the cavity recirculation zone, estimating concentrations due to inversion break‑up and shoreline fumigation, and determining plume rise for flare releases. The model can incorporate the effects of simple elevated terrain on maximum concentrations, and can also estimate 24‑hour average concentrations due to plume impaction in complex terrain using the VALLEY model 24‑hour screening procedure. Simple area sources can be modeled with SCREEN using a numerical integration approach. The SCREEN model can also be used to model the effects of simple volume sources using a virtual point source procedure. The area and volume source algorithms are described in Volume II of the “Industrial Source Complex” (ISC) model user's guide. The SCREEN model can also calculate the maximum concentration at any number of user‑specified distances in flat or elevated simple terrain, including distances out to 100 km for long‑range transport.

1.5 What will SCREEN not do

SCREEN can not explicitly determine maximum downwind air pollutant concentrations from multiple sources, except for the procedure to handle multiple nearby stacks by merging emissions into a single "representative" stack. With the exception of the 24‑hour estimate for complex terrain impacts, the results from SCREEN are estimated maximum 1‑hour concentrations. To handle longer period averages, the screening procedures document contains recommended adjustment factors to estimate concentrations out to 24 hours from the maximum 1‑hour value. .

1.6 How will SCREEN results compare to hand calculations from the document

The SCREEN model is based on the same modeling assumptions that are incorporated into the screening procedures and nomographs, and for many sources the results will be very comparable, with estimated maximum concentrations differing by less than about 5% across a range of source characteristics. For some sources, particularly taller sources with greater stack gas buoyancy, the differences in estimated concentrations will be larger, with the hand calculation exceeding the SCREEN model result by as much as 25 %.

SCREEN examines a full range of meteorological conditions, including all atmospheric stability classes and wind speeds to find maximum impacts, whereas to keep the hand calculations tractable only a subset of meteorological conditions (stability classes A, C, and E or F) likely to contribute to the maximum concentration are examined. The use of a full set of meteorological conditions is required in SCREEN because maximum concentrations are also given as a function of distance, and because A, C, and E or F stability may not be controlling for sources with building downwash (not included in the hand calculations). SCREEN explicitly calculates the effects of multiple reflections of the plume off the elevated inversion and off the ground when calculating concentrations under limited mixing conditions. To account for these reflections, the hand calculation screening procedure increases the calculated maximum concentrations for A stability by a factor ranging from 1.0 to 2.0. The factor is intended to be a conservative estimate of the increase due to limited mixing, and may be slightly higher (about 5 to 10 %) than the increase obtained from SCREEN using the multiple reflections, depending on the source. Also, SCREEN handles the near neutral/high wind speed case by examining a range of wind speeds for stability class C and selecting the maximum. In contrast, the hand calculations are based on the maximum concentration estimated using stability class C with a calculated critical wind speed and a 10 meter wind speed of 10 m/s. This difference should result in differences in maximum concentrations of less than about 5 % for those sources where the near neutral/high wind speed case is controlling.

The SCREEN model results also include the effects of buoyancy-induced dispersion, which are not accounted for by the hand calculations (except for fumigation). The inclusion of buoyancy-induced dispersion in SCREEN may either increase or decrease the estimated concentrations, depending on the source and distance. For sources with plume heights below the 300 meter limit of the hand calculations, the effect of buoyancy-induced dispersion on estimated maximum concentrations will usually be less than about ± 10 %. For elevated sources with relatively large stack gas buoyancy, the inclusion of buoyancy-induced dispersion may be expected to decrease the estimated maximum concentration by as much as 25 %.

For a point source, the user will be asked to provide the following inputs:

Point Source Inputs

Emission rate (g/s)

Stack height (m)

Stack inside diameter (m)

Stack gas exit velocity (m/s) or flow rate (ACFM = actual cubic ft/minute; or m³/s)

Stack gas temperature (K)

Ambient temperature (K) (use default of 293K if not known)

Receptor height above ground (may be used to define flagpole receptors) (m)

Urban/rural option (U = urban, R = rural)

SCREEN allows for the selection of urban or rural plume dispersion coefficients. The urban dispersion option is selected by entering a 'U' (lower or upper case) in column 1, while the rural dispersion option is selected by entering an 'R' (upper or lower case).

The simple elevated terrain option would be used if the terrain rises above the stack base elevation but is less than the height of the physical stack.

2.4.1 Building Downwash Option

Following the basic input of source characteristics, SCREEN will first ask if building downwash is to be considered, and if so, asks for the building height, minimum horizontal dimension, and maximum horizontal dimension in meters. The downwash screening procedure assumes that the building can be approximated by a simple rectangular box. Wake effects are included in any calculations made using the automated distance array or discrete distance options. Cavity calculations are made for two building orientations ‑ first with the minimum horizontal building dimension alongwind, and second with the maximum horizontal dimension alongwind. The cavity calculations are summarized at the end of the distance-dependent calculations.

2.4.2 Complex Terrain Option

The complex terrain option of SCREEN allows the user to estimate impacts for cases where terrain elevations exceed stack height. If the user elects this option, then SCREEN will calculate and print out a final stable plume height and distance to final rise for the VALLEY model 24‑hour screening technique. This technique assumes stability class F (E for urban) and a stack height wind speed of 2.5 m/s.

For complex terrain, maximum air pollutant concentrations are expected to occur for plume impaction on the elevated terrain under stable conditions. The user is therefore instructed to enter minimum distances and terrain heights for which impaction is likely, given the plume height calculated, and taking into account complex terrain closer than the distance to final rise. If the plume is at or below the terrain height for the distance entered, then SCREEN will make a 24‑hour concentration estimate using the VALLEY screening technique.

If the terrain is above stack height but below the plume centerline height for the downwind distance entered, then SCREEN will make a VALLEY 24‑hour estimate (assuming E or F and 2.5 m/s), and also estimate the maximum concentration across a full range of meteorological conditions using simple terrain procedures with terrain "chopped off" at physical stack height. The higher of the two estimates is selected as controlling for that distance and terrain height (both estimates are printed out for comparison). The simple terrain estimate is adjusted to represent a 24‑hour average by multiplying by a factor of 0.4, while the VALLEY 24‑hour estimate incorporates the 0.25 factor used in the VALLEY model.

Calculations continue for each terrain height/distance combination entered until a terrain height of zero is entered. It should be noted that SCREEN will not consider building downwash effects in either the VALLEY or the simple terrain component of the complex terrain screening procedure, even if the building downwash option is selected. In the complex terrain option, SCREEN uses a receptor height above ground of 0.0m (i.e. no flagpole receptors) even if a non‑zero value of the receptor height is entered.

2.4.3 Simple Elevated or Flat Terrain Option

The user is given the option in SCREEN of modeling either simple elevated terrain, where terrain heights exceed stack base but are below stack height, or simple flat terrain, where terrain heights are assumed not to exceed stack base elevation. If the user elects not to use the option for simple terrain screening with terrain above stack base, then flat terrain is assumed and the terrain height is assigned a value of zero. If the simple elevated terrain option is used, SCREEN will prompt the user to enter a terrain height above stack base. If terrain heights above physical stack height are entered by the user for this option, they are chopped off at the physical stack height.

The simple elevated terrain screening procedure assumes that the plume elevation above sea level is not affected by the elevated terrain. Concentration estimates are made by reducing the calculated plume height by the user‑supplied terrain height above stack base. Neither the plume height nor terrain height are allowed to go below zero.

2.4.4 Choice of Meteorology (Not available in TSCREEN)

For simple elevated or flat terrain screening, the SCREEN user will be given the option of selecting from three choices of meteorology: (1) full meteorology (all stability classes and wind speeds); (2) specifying a single stability class; or (3) specifying a single stability class and wind speed. Generally, the full meteorology option should be selected. The other two options were originally included for testing purposes only, but may be useful when particular meteorological conditions are of concern.

2.4.5 Automated Distance Array Option

The automated distance array option of SCREEN gives the user the option of using a pre‑selected array of 50 distances ranging from 100m out to 50 km. Increments of 100m are used out to 3,000m, with 500m increments from 3,000m to 10 km, 5 km increments from 10 km to 30 km, and 10 km increments out to 50 km. When using the automated distance array, SCREEN prompts the user for a minimum and maximum distance to use, which should be input in free format, i.e., separated by a comma or a space. SCREEN then calculates the maximum concentration across a range of meteorological conditions for the minimum distance given ( 1 meter), and then for each distance in the array larger than the minimum and less than or equal to the maximum.

2.4.6 Discrete Distance Option

The discrete distance option of SCREEN allows the user to input specific distances. Any number of distances ( 1 meter) can be input by the user and the maximum concentration for each distance will be calculated. The user will always be given this option whether or not the automated distance array option is used. The option is terminated by entering a distance of zero (0). SCREEN will accept distances out to 100 km for long‑range transport estimates with the discrete distance option. However, for distances greater than 50 km, SCREEN sets the minimum 10 meter wind speed at 2 m/s to avoid unrealistic transport times.

2.6 Area Source

The third source type option in SCREEN is for area sources, which is selected by entering 'A' or 'a' for source type. The area source algorithm in SCREEN is based on a numerical integration approach, and allows for the area source to be approximated by a rectangular area. The inputs requested for area sources are as follows:

Area Source Inputs

Emission rate [g/(s-m²)]

Source release height (m)

Length of larger side of the rectangular area (m)

Length of smaller side of the rectangular area (m)

Receptor height above ground (m)

Urban/rural option (U = urban, R = rural)

Note that the emission rate for area sources is input as an emission rate per unit area in units of g/(s-m²). These units are consistent with EPA's ISCST model.

Because the air pollutant concentration at a particular distance downwind from a rectangular area is dependent on the orientation of the area relative to the wind direction, the SCREEN model provides the user with two options for treating wind direction. The first option, which should be used for most applications of SCREEN, is for the model to search through a range of wind directions to find the maximum concentration. The range of directions used in the search is determined from a set of look-up tables based on the aspect ratio of the area source, the stability category, and the downwind distance. The SCREEN model also provides the user an option to specify a wind direction orientation relative to the long axis of the rectangular area. The second option may be used to estimate the concentration at a particular receptor location relative to the area. The output table for area sources includes the wind direction associated with the maximum concentration at each distance.

The user has the same options for handling distances and the same choices of meteorology as described above for point sources, but no complex terrain, elevated simple terrain, building downwash, or fumigation calculations are made for area sources. Distances are measured from the center of the square area. .

2.7 Volume Source

The fourth source type option in SCREEN is for volume sources, which is selected by entering 'V' or 'v' for source type. The volume source algorithm is based on a virtual point source approach, and may be used for non-buoyant sources whose emissions occupy some initial volume. The inputs requested for volume sources are as follows:

Volume Source Inputs

Emission rate (g/s)

Source release height (m)

Initial lateral dimension of volume (m)

Initial vertical dimension of volume (m)

Receptor height above ground (m)

Urban/rural option (U = urban, R = rural)

3. Technical Description

Most of the techniques used in the SCREEN model are based on assumptions and methods common to other EPA dispersion models . For the sake of brevity, lengthy technical descriptions that are available elsewhere are not duplicated here.

3.1 Basic Concepts of Dispersion Modeling

SCREEN uses a Gaussian plume model that incorporates source-related factors and meteorological factors to estimate pollutant concentration from continuous sources. It is assumed that the pollutant does not undergo any chemical reactions, and that no other removal processes, such as wet or dry deposition, act on the plume during its transport from the source. The Gaussian model equations and the interactions of the source‑related and meteorological factors are described in Volume II of the ISC user's guide (EPA, 1995b), and in the Workbook of Atmospheric Dispersion Estimates (Turner, 1970).

3.3 Plume Rise for Point Sources

The use of the methods of Briggs to estimate plume rise are discussed in detail in Section 1.1.4 of Volume II of the ISC user's guide. These methods are also incorporated in the SCREEN model.

3.6 Building Downwash

3.6.1 Cavity Recirculation Region

The cavity calculations are a revision of the procedure described in the Regional Workshops on Air Quality Modeling Summary Report, Appendix C (EPA, 1983), and are based largely on results published by Hosker (1984).

If non‑zero building dimensions are input to SCREEN for either point or flare releases, then cavity calculations will be made as follows. The cavity height, h_c (m), is estimated based on an equation from Hosker (1984):

Using the plume height based on momentum rise at two building heights downwind, including stack tip downwash, a critical (i.e., minimum) stack height wind speed is calculated that will just put the plume into the cavity (defined by plume centerline height = cavity height). The critical wind speed is then adjusted from stack height to 10 meter using a power law with an exponent of 0.2 to represent neutral conditions (no attempt is made to differentiate between urban or rural sites or different stability classes). If the critical wind speed (adjusted to 10 meters) is less than or equal to 20 m/s, then a cavity concentration is calculated, otherwise the cavity concentration is assumed to be zero. Concentrations within the cavity, X_c, are estimated by the an approximation.

The equations for cavity height, concentration and cavity length are all sensitive to building orientation through the terms L, W and A_p. Therefore, the entire cavity procedure is performed for 2 orientations, first with the minimum horizontal dimension alongwind and second with the maximum horizontal dimension alongwind. For screening purposes, this is thought to give reasonable bounds on the cavity estimates. The first case will maximize the cavity height, and therefore minimize the critical wind speed. However, the A_p term will also be larger and will tend to reduce concentrations. The highest concentration that potentially effects ambient air should be used as the controlling value for the cavity procedure.

3.6.2 Wake Region

The calculations for the building wake region are based on the ISC model. The wake effects are divided into two regions, one referred to as the "near wake" extending from 3L_b to 10L_b (L_b is the lesser of the building height, h_b, and maximum projected width), and the other as the "far wake" for distances greater than 10L_b. For the SCREEN model, the maximum projected width is calculated from the input minimum and maximum horizontal dimensions as (L² + W²)^0.5. The remainder of the building wake calculations in SCREEN are based on the ISC user's guide.

It should be noted that, unlike the cavity calculation, the comparison of plume height (due to wind momentum rise at two building heights) to wake height to determine if wake effects apply does not include stack tip downwash. This is done for consistency with the ISC model.

3.8 Complex Terrain 24-hour Screen

The SCREEN model also contains the option to calculate maximum 24‑hour concentrations for terrain elevations above stack height. A final plume height and distance to final rise are calculated based on the VALLEY model screening technique (Burt, 1977) assuming conditions of F stability (E for urban) and a stack height wind speed of 2.5 m/s. Stack tip downwash is incorporated in the plume rise calculation.

The user then inputs a terrain height and a distance (m) for the nearest terrain feature likely to experience plume impaction, taking into account complex terrain closer than the distance to final plume rise. If the plume height is at or below the terrain height for the distance entered, then SCREEN will make a 24‑hour average concentration estimate using the VALLEY screening technique. If the terrain is above stack height but below plume centerline height, then SCREEN will make a VALLEY 24‑hour estimate (assuming F or E and 2.5 m/s), and also estimate the maximum concentration across a full range of meteorological conditions using simple terrain procedures with terrain "chopped off" at physical stack height, and select the higher air pollutant concentration. Calculations continue until a terrain height of zero is entered. For the VALLEY model air pollutant concentration, SCREEN will calculate a sector-averaged ground-level air pollutant concentration with the plume centerline height (h_e) as the larger of 10.0m or the difference between plume height and terrain height.

Note that for screening purposes, concentrations are not attenuated for terrain heights above plume height. The dispersion parameter, _ze, incorporates the effects of buoyancy induced dispersion. For the simple terrain calculation SCREEN examines concentrations for the full range of meteorological conditions and selects the highest ground level concentration. Plume heights are reduced by the chopped off terrain height for the simple terrain calculation. To adjust the concentrations to 24‑hour averages, the VALLEY screening value is multiplied by 0.25, as done in the VALLEY model, and the simple terrain value is multiplied by the 0.4 factor.