The EPA is proposing several changes to its modeling guidance document Appendix W, including options to better model stable conditions and emissions from capped stacks, replacing its ARM with ARM2 for 1-hour NO2 modeling, adding a line source option to AERMOD, removing CALPUFF, and replacing CALINE3 models with AERMOD for transportation projects. The key changes address concerns about model overpredictions during low-wind conditions, allow modeling of capped and horizontal stacks subject to building downwash, use a revised ambient ratio method for 1-hour NO2 compliance demonstrations, add a buoyant line source option to AERMOD, remove CALPUFF due to maintenance concerns, and replace CALINE3 models with AERMOD which has

1. AIR DISPERSION
MODELING
USEPA REGULATORY PROPOSED
MODELING CHANGES
The EPA as part of the 11th
modeling conference has proposed
making several changes to Appendix W, which is the federal
modeling guidance document. Appendix W was last updated
in 2005. The current EPA proposed changes include:
• Options to address concerns regarding model over
predictions during stable/low-wind conditions.
• An option to better address emission releases from capped
and horizontal stacks subject to building downwash.
• ARM2 modeling of NOx emissions for compliance with the
1-hour NO2 standard.
• Addition of a buoyant line source option.
• The proposed removal of Calpuff
• The replacement of the Caline models with AERMOD for
transportation projects
These proposed changes are presented in more detail below.
MODEL OVER PREDICTIONS DURING
STABLE/LOW-WIND CONDITIONS
The Beta ADJ_U* option in AERMET is associated with the Bulk
Richardson Number (BULKRN) option. It has been modified to
include a more refined method for calculating THSTAR
extending its use to very stable/low wind conditions, based on
Luhar and Raynor (BLM, v132, 2009). EPA is proposing that the
ADJ_U* option (with or without BULKRN) be incorporated into
the regulatory version of AERMET.
AERMOD v15181 includes a new LowWind3 (LW3) non-
DFAULT/BETA option. The LowWind3 option increases the
minimum value of σv from 0.2 to 0.3 m/s, consistent with the
LowWind2 option, but eliminates upwind dispersion, consistent
with the LowWind1 option.
The LowWind3 option uses an “effective” σy value that replicates
the centerline concentration accounting for meander, but sets
concentrations to zero (0) for receptors more than 6σy off the
plume centerline, similar to the FASTALL option.
EPA has proposed that the LowWind3 option be incorporated
into the AERMOD regulatory version, while the LowWind1 and
LowWind2 options are still available for testing & evaluation
purposes.
The proposed Beta ADJ_U* option in AERMET and Low_Wind
option in AERMOD have been evaluated based on several
relevant field studies, including:
• The 1993 Cordero Rojo surface coal mine fugitive dust
study in eastern Wyoming using the 24-hour PM10
concentrations (using AERMOD v14134);
• The 1974 NOAA Oak Ridge, TN, tracer study for a low-level
release on the Oak Ridge peninsula with sampling arcs at
100m, 200m, and 400m, and wind speeds ranging from
0.15 to 0.73 m/s (10 of 11 cases < 0.5 m/s);. and
• The 1974 NOAA Idaho Falls, ID, tracer study for a low-level
release with sampling arcs at 100m, 200m, and 400m, and
wind speeds ranging from 0.75 to 1.93 m/s (4 of 11 cases <
1.0 m/s).
CAPPED AND HORIZONTAL STACKS
A Model Clearinghouse memorandum dated July 9, 1993,
provided recommendations for modeling capped and
horizontal stacks. The Clearinghouse procedure involved
setting the exit velocity (Vs) to 0.001 m/s and adjusting the stack
diameter (Ds) to maintain the actual flow rate and buoyancy of
the plume.
With the introduction of the PRIME algorithm for building
downwash, an issue arose. The PRIME numerical plume rise
algorithm uses the input Ds to define the initial radius of the
plume – use of a larger effective radius can alter results in
physically unrealistic ways. The AERMOD Implementation
Guide suggests using Vs=0.001m/s with actual Ds as an interim
solution.
The EPA is proposing a modeling change that will allow the use
of the POINTHOR & POINTCAP options to address capped and
horizontal stacks subject to building downwash. The EPA
proposed changes to the POINTHOR & POINTCAP options in
the presence of building downwash are as follows. For the

2. POINTHOR option with downwash, the exit velocity is assigned
as the initial horizontal velocity of the plume. For the POINTCAP
option with downwash, the initial plume radius is assigned as 2
Ds to account for an initial plume spread from the cap, and the
initial horizontal velocity of the plume is assigned as the initial
exit velocity divided by 4 to account for suppressed momentum
and buoyancy.
1-HOUR NO2 MODELING USING THE
AMBIENT RATIO METHOD (ARM) #2
The EPA is proposing to replace the existing ARM with a revised
Ambient Ratio Method 2 (ARM2). ARM introduced in AERMOD
version 13350 comes from an evaluation of the ratios of
NO2/NOx from the EPA’s Air Quality System (AQS) record of
ambient air quality data. ARM2 was developed by sorting all
the AQS data into bins of 10 ppb increments for NOx values less
than 200 ppb and into bins of 20 ppb for NOx in the range of
200-600 ppb. From each bin, the 98th percentile NO2/NOx
ratio was determined and finally, a sixth-order polynomial
regression was generated based on the 98th percentile ratios
from each bin to obtain the ARM2 equation, which is used to
compute a NO2/NOx ratio based on the total NOx levels.
If the total NOx from a Tier 1, total conversion analysis exceeds
the 150-200 ppb threshold outlined above, then the NO2/NOx
ISR of the primary source should be considered. If an adequate
demonstration can be provided that the primary source has
ISRs that are all below 0.2, then ARM2 should be appropriately
conservative for a Tier 2 analysis.
BUOYANT LINE SOURCES
Appendix W currently recommends the use of the Buoyant Line
and Point (BLP) model for buoyant line sources. The BLP model
uses outdated dispersion theory. The meteorological data is
prepared for input using PCRAMMET, which is incapable of
processing the current met data, including 1-minute ASOS data.
The BLP model also lacks processing options to support the
statistical forms of the 1-hour NO2, 1-hour SO2 and 24-hour
PM2.5 NAAQS.
The AERMOD model has been modified beginning with v15181
to add a BUOYLINE option to model buoyant line sources using
met data processed through the AERMET meteorological
processor. This AERMOD option means model results can be
output for direct comparison to the statistical forms of the 1-
hour NO2, 1-hour SO2 and 24-hour PM2.5 NAAQS.
SAYING GOOD-BYE TO CALPUFF
The EPA is proposing to remove the CALPUFF modeling system
as an EPA-preferred model for long-range transport. The
removal of CALPUFF is due to concerns about the management
and maintenance of the model code given the frequent change
in ownership since promulgation in the previous version of the
Guideline. In addition, CALPUFF is being removed to provide
the user community flexibility in estimating single-source
secondary pollutant impacts using more appropriate modeling
techniques, such as photochemical transport models.
REPLACING THE CALINE3 MODELS
WITH AERMOD FOR TRANSPORTATION
PROJECTS
At the time of Appendix W’s promulgation in 2005, there had
been no comparisons between AERMOD and CALINE3 with
sufficient merit to modify the status of CALINE3 as the preferred
model for mobile source (transportation) applications. Since
2005, EPA describes the model comparisons of AERMOD and
CALINE3 as warranting the removal of CALINE3 from the list in
Appendix A. Performance statistics for six models (CALINE3,
CALINE4, AERMOD-volume, AERMOD-area, ADMS, and RLINE)
for both Idaho and California field studies show the CALINE
models were the worst performing. When the focus was on the
top 25 concentrations that are most relevant in a performance
evaluation of regulatory models, AERMOD performed the best.
In addition to the evidence about model performance, CALINE3,
CAL3QHC, and CAL3QHCR have several limitations.
• Meteorological pre-processors for the CALINE3 models are
only available for older meteorological data sets. As a result,
newer, higher resolution meteorological data, that is more
representative of actual wind conditions cannot readily be
used.
• For CAL3QHCR, only 1 year of meteorological data can be
used in each model run. For refined PM10 and PM2.5
analyses, this requires multiple model runs to cover a 5-year
modeling period with resulting model output data from up
to 20 model runs that must be separately post-processed
to obtain the necessary results.
CONTACT INFORMATION
Patrick Ryan, Ph.D.
Phone: 319.626.5342
RyanPatrick@stanleygroup.com