Users of the U.S. EPA BPIP PRIME model have noticed occasional results, which are different from previous versions (BPIPPRM - Dated 04274).  The reason for such discrepancy is that the new version has two bug corrections.  Previously, BPIP-PRIME had two faulty assignments described in the Model Change Bulletin.  In the Model Change Bulletin #1 for the new BPIPPRM there is a description of a bug correction about one page before the end that reads as follows:

" (open quote)
The following two variables were erroneously equated and were changed from:
                MPADX(S,D) = PBL

                MPADY(S,D) = PBL

    to:

                MPADX(S,D) = XBADJ

                MPADY(S,D) = YBADJ
" (end quote)

The Model Change Bulletin (MCB) also describes a second bug correction (about 2/3 of the way through the MCB) that states as follows:

" (open quote)
An error was found where DPADY(S) was repeated twice instead of

    as DPADX(S, DPADY(S).  The code was changed from:

         +                    DPADY(S),DPADY(S)

      to:

         +                    DPADX(X),DPADY(S)

" (end quote).

Therefore, the numerical differences from the old to the new BPIP-PRIME is a result of two bug corrections, not of new bugs.

Fixed roof tanks: Model fixed roof tanks as a point (stack) source (representing the vent), which is usually in the center of the tank, and representing the tank itself as a building for downwash calculations.

Floating roof tanks: Model floating roof tanks as a circle of eight (or more) point sources, representing the tank itself as a building for downwash calculations. Distribute the total emissions equally among the circle of point sources.

All tanks: There is virtually no plume rise from tanks. Therefore, the stack parameters for the stack gas exit velocity and stack diameter should be set to near zero for the stacks representing the emissions. In addition, stack temperature should be set equal to the ambient temperature. This is done in ISCST3 and AERMOD by inputting a value of 0.0 for the stack gas temperature.

Note that it is very important for the diameter to be at or near zero. With low exit velocities and larger diameters, stack tip downwash will be calculated. Since all downwash effects are being calculated as building downwash, the additional stack tip downwash calculations would be inappropriate. Since the maximum stack tip downwash effect is to lower plume height by three stack diameters, a very small stack diameter effectively eliminates the stack tip downwash.

Extracted from: Proposed Guidance For Air Dispersion Modeling

The Terrain Grid Pathway (TG) should be used only when the Dry Depletion option in the Control Pathway is selected.

Dry depletion (removal) of the plume accounts for the loss of mass due to particles deposited on the surface from gravitational settling and dry deposition. When using the dry depletion option, the particle information (particle diameter, mass and density) must be specified in the Source Pathway.

Terrain Grid data is considered only in the calculation of dry depletion in elevated or complex terrain. The terrain grid data provided in the terrain grid file is used to integrate numerically the amount of plume that has been depleted along the path of the plume from the source to the receptor. If dry depletion is considered but the Terrain Grid Pathway is omitted, the model will interpolate linearly between the source base elevation and the receptor elevation.

In all other cases when no dry depletion is calculated the model will neglect the terrain grid data provided in Terrain Grid Pathway.

At present, the U.S. EPA AERMOD model does not have the capability to take advantage of hyper threading and dual processors. Dual processors allow you to run several cases simultaneously, handle larger work loads, and complete project runs in less time than a single processor.

If you have a dual processor computer, your AERMOD poject will still only run as though you have a single processor. However, you would still have the advantage of being able to carry out other unrelated work without further slowing down your AERMOD run.

This issue is currently being taken into consideration and it is possible that a parallel processor version of the AERMOD model will be developed in the future by the U.S. EPA.

At some point, you may wish to convert concentrations in micrograms per cubic meter (µg/m³) to concentrations in parts per million (ppm) or parts per billion (ppb).  For example, the ISC and AERMOD models generate output in µg/m³ while you may need results in ppm.

This conversion can be achieved using the following equations:

C_ppb = C_µg/m3 x 24.45/MW

C_ppm = C_µg/m3 x 0.02445/MW

where

C        = Concentration of the contaminant
MW    = Molecular weight of contaminant
24.45 = Constant representing the volume, in liters, of one               mole of a gas at standard temperature (25°C) and               pressure (1 atmosphere).

For example:

100 µg/m³ of SO₂, which has a molecular weight of 64, converted to ppm:

C_ppm = 100 x 0.02445/64

         = 0.0382 ppm = 38.2 ppb

ISC-AERMOD View is capable of performing this conversion for you: simply go to Tools | Concentration Converter, and select µg/m³ to ppm.  All you need to do is specify the molecular weight (MW) in grams per mole of the specific pollutant you are modeling and click convert.  This will generate a new plotfile with concentrations in ppm.

When performing deposition calculations using the ISC or AERMOD models, “averaging period” can be a confusing term, and it turns out to be a misnomer in this circumstance.

When an averaging period is selected for concentration calculations, the output value is the average of the hourly concentrations within that period.   However, when an averaging period is selected for a deposition calculation, the output value is in fact the sum of the amount deposited (accumulated) each hour, for every hour within the period.  Hence, for deposition, the averaging period is not a true averaging period.

To further clarify, for concentration, the maximum value for an averaging period will decrease and the averaging period increases, as the average of several values cannot exceed the maximum.  On the other hand, the maximum value for a deposition "averaging period" will increase as the averaging period increases, as it is the total amount deposited over the period (which increases as the period increases).  For example,

Averaging Time	1 hr	3 hrs	24 hrs
Concentration in µg/m3	70	39	15
Deposition in g/m2	.005	.012	.045

For a more technical description of how dry deposition is calculated, please refer to the ISCST3 User’s Guide at: http://www.weblakes.com/ISCVOL2/133.htm

When running the U.S. EPA ISCST3 model, you should be aware that there is a limit to the number of plotfiles that can be generated by this model.  The current EPA-compiled version of ISCST3 allows for a maximum of 30 plotfiles to be created during any given run.

This limit is a result of how the model was compiled.  Compared to AERMOD and ISC-PRIME, ISCST3 was compiled with a 16-bit compiler (see chart below):

U.S. EPA Model	Dated	Compiler	Date Compiled
ISCST3.EXE	02035	Lahey Fortran 16-bit	Feb. 04, 2002
ISC3P.EXE	04269	Visual Fortran Win 32	Aug. 26, 2004
AERMOD.EXE	04300	Visual Fortran Win 32	Apr. 14, 2005

If you attempt to run an ISCST3 project with more than 30 plotfiles specified, you may encounter an error similar to the following:

“OU E500 5360 OUPLOT:Fatal Error Occurs Opening the Data File of PLTFL000”

where “PLTFL000” refers to the type of file and file unit number that is causing the error.  This fatal error will potentially occur for each plotfile that is in excess of the 30-plotfile limit.

ISC-AERMOD View users that are currently in maintenance can visit the update site at http://support.lakes-environmental.com/updates.html to download a recompiled version of ISCST3.EXE that is capable of generating a greater number of plotfiles.

The U.S. EPA ISC and AERMOD models calculate period and annual averages by summing the total number of hourly concentrations in the period and dividing by the number of hours (minus calm hours).  This is referred to as calms processing.

A calm hour is defined as an hour of met data during which the average wind speed is less than 1m/s.  During these conditions the measurement precision of wind speed and wind direction is unacceptable; therefore, the calms processing routine sets concentrations to zero for calm hours, and short term averages are calculated according to the EPA’s calms policy.  For example, if five hours of data are run for a pollutant concentration, in which hour 2 is a calm hour, a simple average would result in a lower concentration due to the inclusion of the calm hour’s zero value.  However, calms processing, excludes zero concentration from the calm hour yielding a more reasonable result.

Hours	Concentration (µg/m3)
1	0.76733
2	0
3	0.70298
4	0.65836
5	0.73312
Simple Average	0.15347
Calms Processing	0.19183

For multi-year runs with the annual option, the model assumes that complete years of meteorological data are available.  Annual averages, calculated using calms processing, are then summed and divided by the number of years in the run.

When preparing your air dispersion modeling project using one of the U.S. EPA air dispersion models - ISCST3, ISC-PRIME or AERMOD (the models) - you may be required to model source specific parameters that change over time.  For example, such parameters can be source emissions, stack gas exit temperature or stack gas exit velocity.  This can be accomplished by specifying an Hourly Emission File.

The Hourly Emission File can be created using Microsoft Excel and converted into a text file format for use by the models.  Each record or line of the Hourly Emission File must include the keyword (SO HOUREMIS), followed by the Year, Month, Day, Hour, Source ID, Emission Rate (g/s or user units).  For point sources, the record must also include stack gas exit temperature (K), and stack gas exit velocity (m/s).

The following is an example of what the Hourly Emission Files should look like:

SO HOUREMIS 86  1  1  1  STACK1  145.0  283  9.4
SO HOUREMIS 86  1  1  1  STACK2  150.0  284  9.0
SO HOUREMIS 86  1  1  2  STACK1      0.0  285  9.4
SO HOUREMIS 86  1  1  2  STACK2  120.0      0  8.0
SO HOUREMIS 86  1  1  3  STACK1  120.0      0  9.4
SO HOUREMIS 86  1  1  3  STACK2      0.0   -20  9.4


Hourly Emission File Tips:

1. Only one hourly emission file can be specified for each run.



2. The data must include the exact same dates/time as are included in the meteorological input files.  Failing to have the date/hour match will cause the model run to fail.



3. Source IDs must correspond to the source IDs defined in the SO Pathway and be in the same order.



4. The models will use the stack release height and stack inside diameter defined in the Source Pathway, but will use the emission rate, exit temperature and exit velocity from the hourly emission file.  If these parameters are missing in the file, then the models will consider the missing values as zero.



5. An undocumented behavior of the current version of the models occurs when a project is run using the entire year of met data but the hourly emission file and the met data file do not cover the same period (e.g., met data runs from Jan 1 to Dec 31 and the hourly emission file runs from Jan 1 to Jun 30).  Provided that both files begin at the same date/time, the models will still run; however, the models will use the parameters found in the last record of the hourly emission file throughout the rest of the met period.  For this reason, it is very important that your hourly emission file match the met file specified in your project.


In the AERMOD model, this can be verified by using the AERMOD Model Debugging Output Option (CO DEBUGOPT MODEL) specified in the Control Pathway.  CAUTION: the generated debugging file can be very large!  It is recommended that, when using this option, you limit your project run to a few days in order to inspect the intermediate calculations contained in this file.


6. Gas Exit Temperature Tips:
   • A gas exit temperature equal to 0.0 K for any particular hour indicates to the models that the plume is being released at the ambient temperature specified in the met file.
   • A negative gas exit temperature indicates to the models that the plume has an exit temperature that exceeds the ambient temperature by a fixed amount.  For example, if the ambient temperature in the met data file is 265.9 K for a particular hour and -20 was specified in the hourly emission file for that hour than the models will read the gas exit temperature as 265.9 + 20 = 285.9 K.

When calculating building downwash using the US EPA BPIP and BPIPPRM* programs, you may encounter the need to model a building with a sloped roof.  As BPIP is only capable of modeling buildings with flat roofs, you will need to approximate the roof.

It is always advisable to check with your regulator to learn how such buildings should be modeled, but two possibilities are given below:

1. Single Tier (Conservative)
   
   Creating a building with a flat roof at the height of the sloped roof’s peak will give you a larger building profile, therefore giving you a conservative estimate of the building downwash.
   
   



2. Multiple Tiers (More Realistic)
   
   By creating a building with a series of tiers that decrease in size, you can approximate a sloped roof.  The more tiers you use in this approximation, the more realistic it will be.
   
   

*Building Profile Input Program (BPIP) and Building Profile Input Program for PRIME (BPIPPRM) are US EPA programs that calculate downwash values for input into the ISCST3 and AERMOD air dispersion models.

It is our continuous effort to improve CALPUFF View for any changes in the latest CALPUFF modeling system.  At present, CALPUFF View (version 1.5) is compatible to the BETA CALPUFF modeling system version 5.711 (and 5.711a).  The BETA version contains new features and many important bug fixes from the official version.  The latest BETA version is available for download from Earth Tech.  CALPUFF View users can import the BETA version through “File | Preferences…”   Not doing so, however, may cause CALPUFF to stop.

Symptom:
To identify the above problem, check the CALPUFF list file for the following message:
(HTMINBC)                  Default:   500.     ! HTMINBC = 500.0 !

ERROR IN SUBR. READIN -- Error in input data --
Variable not found in variable dictionary
Variable: HTMINBC
Variable Dictionary: SYTDEP       MHFTSZ       JSUP         CONK1

Diagnosis:
The BETA CALPUFF version 5.711 (dated June 25, 2003) introduces new variables to support a new feature for modeling BC puffs (MBCON=2).  The official CALPUFF version 5.7 (dated April 2, 2003) does not recognize the new variable: HTMINBC, and therefore stops at that line.

Remedy:
Update to the BETA CALPUFF version.  For users who must rely on the official version, a patch is also available on request.

With the release of the Final US EPA-OSW HHRAP (2005), changes have been introduced to the way air modeling should be performed for risk assessment projects.  The most notable changes are the addition of gas dry deposition modeling and the use of a separate mercury vapor phase air modeling run.

If your risk assessment project includes the modeling of mercury, then two distinct vapor phase runs are necessary: one for mercury and one for the other compounds.  The gas dry deposition option in ISCST3 is a non-default modeling option which requires that you specify the parameter “TOXICS” under the MODELOPT keyword.  For the gas dry deposition option, you must include the gas dry deposition velocity parameter in the Control Pathway (GASDEPVD).

The HHRAP recommends the following deposition velocities:

• Deposition velocity = 0.0290 m/s for mercury (GASDEPVD 0.0290)
• Deposition velocity = 0.0050 m/s for organics, chlorine and HCl (GASDEPVD 0.0050)

See below an example of the parameters that should be included in the ISCST3 input file under the Control Pathway for the Mercury Vapor Phase run:

CO STARTING
 TITLEONE Air Modeling Run for Risk Assessment
 TITLETWO Mercury Vapor Phase
 MODELOPT CONC DEPOS DDEP WDEP DRYDPLT WETDPLT RURAL TOXICS
 GASDEPVD 0.0290
 AVERTIME 1 ANNUAL
 POLLUTID UNITY
 TERRHGTS ELEV
 RUNORNOT RUN
CO FINISHED

When conducting a risk assessment project (e.g. one following the 2005 US EPA OSW HHRAP), COPC emission rates for area sources should be specified in grams per second (g/s).  This may cause some confusion as the unitized emission rate specified for area sources in your ISCST3/AERMOD air dispersion modeling project would have been specified as an emission rate in grams per second per square meter [g/(s*m²)].

There are two methods that you can follow when specifying emission rates for area sources in your air dispersion modeling input files and in your risk assessment project.  These two methods are explained in more detail below.

Method 1:

In your ISCST3 or AERMOD air modeling input file, set the emission rate for your area source to 1 g/(s*m²).

For example:

• The area source you are modeling has an area of 500 m².
• Specify the emission rate for this area source in the ISCST3/AERMOD input file as 1 g/(s*m²).
• The total emission rate in g/s for the area source would be equal to 500 g/s [500 m² * 1 g/(s*m²)].

If you used Method 1 for the air modeling unitized emission rate, then in your risk assessment project, enter the area source COPC emission rate as if it was being specified in g/(s*m²).

For example:

• If the total emission of COPC A for the 500 m2 area source is 3.5 g/s, this equates to 0.007 g/(s*m²).
• Enter an emission rate of 0.007 g/s for COPC A for the area source.

Method 2:

In your ISCST3 or AERMOD air modeling input file, set an emission rate in g/(s*m²) such that the total emission rate of the source is 1 g/s.

For example:

• The Area source you are modeling has an area of 500 m².
• Specify the emission rate for this area source in the ISCST3/AERMOD input file as 0.002 g/(s*m²).
• The total emission rate in g/s for the area source would be equal to 1 g/s [500 m² * 0.002 g/(s*m²)].

If you used Method 2 for the air modeling unitized emission rate, then in your risk assessment project you must specify the emission rate for the COPC in g/s [Emission (g/(s*m²)) x Area of Source (m²)].

For example:

• If the total emission of COPC A for the 500 m² area source is 3.5 g/s (or 0.007 g/(s*m²)).
• Enter an emission rate of 3.5 g/s for COPC A for the area source.