Summary of Comments and Responses on the September 1986
Supplemental Proposal to Revise the Guideline on Air Quality Models
April 1987
Source Receptor Analysis Branch
Monitoring and Data Analysis Division
Office of Air Quality Planning *ȣ Standards
Environmental Protection Agency
-------
Table of Contents
Summary of Comments and Responses on the September 1986 Supplemental Proposal
to Revise th*1 Guideline on Air Quality Models
Page
I. Introduction « 1
II. Rough Terrain Diffusion Model (RTDM) - Model Status 4
III. Rough Terrain Diffusion Model (RTDM) - Data Input 13
IV. Industrial Source Complex (ISC) Model 17
V. Offshore and Coastal Dispersion (OCD) Model 23
VI. AVACTA II Model 27
VII. References 28
Appendix A
Appendix B
-------
I. Introduction
The Guideline on Air Quality Models(Revised)^ was published in July
1986. The purpose of the guideline is to promote consistency in the use of
air quality models within the air management process. The guideline provides
model users with a common basis for estimating pollutant concentrations,
assessing control strategies and specifying emission limits.
During the public comment period on the above guideline, EPA received
requests to consider several additional new modeling techniques. However,
some developers did not submit those models to EPA until much later. Due to
insufficient time for public review, these models could not be included in
the revised guideline. In a separate regulatory proposal on September 9,
1986, EPA sought public comment on four additional models (51 FR 32180).
These four additions, if promulgated in a final rule-making notice, will
require modifications to the Guideline on Air Quality Models(Revised). EPA
has placed in Docket A-80-46 for public examination all the technical
support material received on these four models and draft changes to the
guideline text. The changes show where and how these four models would be
incorporated in the guideline should they be adopted. Written comments
were sought by October 9, 1986. However, due to requests from several
groups, the comment period was extended until December 9 (51 FR 37418).
Both Federal Register notices are contained in Appendix A of this document.
The purpose of this document is to summarize the comments received and to
present EPA responses to the major issues raised by the commenters on this
supplemental proposal.
-------
Summary of Comments
All comments received on this proposal are available from Docket
A-80-46 items V-D and V-H . Names of organizations that submitted written
comments as well as their assigned docket reference number are shown in
Appendix B of this document. Although a total of 31 items are listed, many
of the early submittals are only a request for an extension of the comment
period. The comments can be divided into the following separate categories:
- whether to use the Rough Terrain Diffusion Model (RTDM) as a third
level screening technique for complex terrain applications;
- what the meteorological input requirements of the RTDM model should be;
- whether to adopt the American Petroleum Institute's (API) modifications
to the Industrial Source Complex (ISC) model;
- whether to adopt the Offshore and Coastal Dispersion (OCD) model as
a preferred model for applications to sources located over water.
No comments were received on the proposal concerning the inclusion of AVACTA II
as an alternative model in Appendix B of the guideline (i.e., for use on a
case-by-case basis).
EPA Responses to Comments
The summary of responses is organized according to the above categories.
In some instances, summarizing the comments themselves required some inter-
pretation. Whenever possible the exact words used by the commenter were used
in the comment summary. However, in many instances it was necessary to re-
word or substantially condense the comments. In those Instances, every
effort was made to maintain the exact meaning of the commenter. Following
the summary of comments on each separate topic, the EPA response to these
comments is given. Where there are subtopics in the comment summary, the
-------
response is separated into subparagraphs. Guidance and editorial changes
associated with the resolution of these issues are incorporated in thp
appropriate sections of the guideline and are published as Supplement A
(1987) to the "Guideline on Air Quality Models (Revised)."
-------
II. Rough Terrain Diffusion Model (RTDM) - Model Status
Comment Summary (Approve RTOM)
The majority of commenters urged EPA to adopt its proposal on the use
of RTDM as a third-level screening model for use in rural complex terrain
applications because of superior performance over other existing models
such as the COMPLEX I model and because it is a step forward. Some of
these commenters also generally requested that EPA adopt the RTDM model as
a refined model (i.e., can be used on a generic basis with site-specific
data) for inclusion in Appendix A of the guideline because of technical
merit.
EPA Response
Based on the comments, EPA maintains its proposal to recommend RTDM
with specified default options as a third-level screening model for esti-
mating air quality impact from stationary point sources in rural complex
terrain; less sophisticated screening techniques remain available as an
initial or a second-level screen. However, EPA's review of available
studies demonstrates that RTDM, as a refined model with full on-site data,
can substantially underpredict concentrations. Since no new analysis was
presented by the commenters to alter this conclusion, EPA does not agree
with the commenter's proposal to adopt RTDM as a refined model in Appendix
A of the guideline at this time. RTDM with full on-site data may be used
as a refined model on a case-by-case basis by following the demonstration
criteria described in Section 3.2.2 of the guideline.
Comment Summary (Technical Changes)
While arguing for their position to adopt RTDM, the commenters also
suggested several technical changes:
- there should be an option to use the stack top wind speed for
determining plume rise and the plume height wind speed for determining dilution;
- since RTDM results 1n a very narrow plume width prediction, receptors
should be spaced more closely to unequivocally locate the maximum concentration;
-------
- EPA should recommend applying RTDM for any terrain above stack top
and not require dual modeling for terrain in between stack top and plume
centerline.
EPA Response
The RTDM model, when used as a third-level screen, is designed to
accept as input, measured winds at only one height. EPA recommends that
wind data input to RTDM should be based on fixed measurements at stack top
height. For stacks greater than 100m, the measurement height may be limited
to 100m in height relative to stack base. This recommendation is broadened
to include wind data representative of plume transport height where such
data are derived from measurements taken with remote sensing devices such
as SODAR. Note that RTDM already uses wind speed at stack top to calculate
the plume rise and the critical dividing streamline height, and the wind
speed at plume transport level to calculate dilution, as the commenter
desires. The default values for the wind speed profile exponents, shown in
Table 5-2 of the guideline revisions as IDILUT=1, are used in the model to
extrapolate the wind speed from the measured height to other heights. Addi-
tional guidance on the routine use of measured (site-specific) wind speed
profile exponents is not possible at this time due to limitations in the
availability of such data and lack of experience in their application
within the technical community. Where such data are available, they may be
considered on a case-by-case basis after consultation with the appropriate
EPA Regional Office.
The receptor spacing requirements for RTDM have been clarified in the
guideline text by Indicating the need for detailed review of the site's
topography, carefully exercised judgement, and consultation with the EPA
-------
Regional Office. Due to the site-specific nature of this evaluation, how-
ever, receptor spacing can not be precisely prescribed on a generic basis.
Because RTDM is considered here to be a complex terrain screening model,
its use in a situation where a receptor is on terrain above stack height
must remain consistent with guidance provided for other screening models.
In the absence of a refined model, EPA recommends bracketing the highest
concentrations by using both simple terrain and complex terrain models for
receptors between stack height and plume height. Please see Section 5.0
of the guideline^ for a more detailed explanation of this method. When a
refined model such as CTDM is implemented, this approach will be re-examined.
This point has been explained in the prior public comment summary document2
and is not an issue in this rulemaking.
Comment Summary (Do Not Approve RTDM)
One corronenter opposed EPA's proposal to even adopt RTDM as a screening
model because:
- RTDM does not address other phenomena where high concentrations
can occur (e.g., on the lee side of hills, or during fumigation and stagna-
tion conditions in deep valleys). The highly conservative nature of the
currently approved complex terrain screening models compensates to some
extent for not addressing any of these phenomena.
- EPA's Complex Terrain Dispersion Model (CTDM) is now at an advanced
stage of development;
- both the Westvaco and Widows Creek data bases show a substantial
number of instances in which the concentrations at specific monitoring
stations were underpredicted by RTDM, as proposed. RTDM has not been suf-
ficiently tested to ensure protection of the ambient air quality standards
and it 1s possible that the model was "tuned" to perform better on the
Westvaco data base.
EPA Response
EPA agrees that the RTDM model does not address all meteorological
phenomena 1n complex terrain. That 1s why EPA has recommended the use of
-------
RTDM as a screening technique until research produces a refined model. The
goal of the modeling guidance is to ensure that the best possible scientific
procedures are implemented for operational use. From the information pre-
sented to EPA through the public comment and review process, it is apparent
that RTDM as proposed by EPA, with assigned predetermined values irrespective
of site-specific conditions, is more accurate, while still providing con-
servative estimates. It is also based on better scientific theory^ than
existing complex terrain screening models.
The commenter did not provide credible scientific data to sustain the
argument that a model must be as highly conservative to encompass concentra-
tions produced by all meteorological phenomena such as lee side effects,
fumigation, and stagnation. Existing screening models were never intended
to address these phenomena nor do they have the necessary "physics" to do
so. The commenter appears to be arguing for disallowing the use of RTDM
because it is less conservative and may result in less stringent emission
limits than those provided by current models. EPA has anticipated the need
for improved complex terrain models and has conducted over the last five years
the Complex Terrain Model Development program3,4. The goal of this research
program is to develop reliable atmospheric dispersion models that are appli-
cable to large pollutant sources located in complex terrain. RTDM, which deals
with stable plume impaction, is a step in this direction. Research programs to
consider other phenomena such as lee side effects and stagnation are underway o(
are being considered by EPA's Office of Research and Development and by others.
Until this research can fully address how to model these phenomena, the use
of RTDM with specified default parameters as proposed is the best available
-------
method. To fill a void in knowledge, EPA is encouraging the placement of
monitors at locations where these other phenomena are suspected to cause a
violation of the NAAQS (e.g., potential lee side effects downwind of the
Lovett Generating Station operated by Orange & Rockland Utilities).
Although the CTDM model is in an advanced stage of development, approval
of RTDM as proposed should not be affected. RTDM is not competing with CTDM.
Adopting RTDM presents an opportunity to improve EPA's present screening
techniques for modeling in complex terrain. The CTDM model, which is still
in a research and development mode, is a more physically realistic model.
Once CTDM is fully operational, experience in its use, data requirements,
etc. will still be needed. When this experience is gained, EPA may propose
CTDM as a refined air quality model. Nevertheless, there will still be a
need for a reasonably accurate, yet simple screening model that yields con-
servative concentration estimates, which is the role filled by the RTDM
model.
The various tests of the RTDM model have been summarized and referenced
in the EPA proposal (51 FR 32180) and the reports describing the details of
these tests have been placed in Docket A-80-46. EPA believes these tests
provide an adequate demonstration of this model's ability, with specified
default parameters, to provide sufficiently accurate but consistently con-
servative concentration estimates. Rather than limit the issue of accuracy
to a receptor-by-receptor basis as suggested by the commenter, EPA's position
has always been that the performance of any model, whether for complex ter-
rain or some other application, should be based on the ability to predict
highest concentrations on a network-wide basis. EPA has found that under
and overprediction varies from site to site. This has been demonstrated
8
-------
for even the widely used flat terrain model MPTER. In applying models to
evaluate attainment of short-term deterministic ambient standards or PSD
increments, EPA has always examined whether the highest, second-highest value
in the network, not the value at a specific monitoring site1 is within the
NAAQS limits. For this type of application RTDM is conservative.
The possibility of "tuning" the model was raised by EPA as a cautionary
remark to the model developer at the time RTDM was being developed. Sub-
sequent scientific review^ by the American Meteorological Society-EPA Steering
Committee on the performance of eight complex terrain dispersion models con-
curred that the RTDM model was a clear choice. Additional evaluation of the
performance of this model was made by one of the commenters on this proposal
(Docket Item V-D-19). This evaluation also showed that this model is more
accurate than the COMPLEX I screening model.
Comment Summary (Model Performance)
One commenter provided data to show that when applied to the Widows
Creek plant, RTDM in the specified default mode was more accurate than
EPA's COMPLEX I model. RTDM also provides conservative estimates for 10 of
12 possible combinations of available meteorological data. The commenter's
suggestions for mitigating the two situations with underestimated concentra-
tions include requiring a multiyear (e.g., 3 years) analysis or multiplying
the predicted concentrations by a correction factor (e.g., 1.2). The need to
use a correction factor does not diminish RTDM's superiority over COMPLEX I.
EPA Response
EPA appreciates the commenter's data on the performance of the RTDM model
at the Widows Creek plant. Much useful information was obtained from this
analysis. However, because of the limited terrain height compared to plume
height at this facility, the analysis could not determine the performance of
the peak concentrations expected during plume impaction which would occur
when the plume Intercepts high terrain. When the highest, second-highest
-------
concentration is used in assessing potential violations of short term NAAQS,
RTDM's predicted concentrations are consistently closer to the corresponding
measurements than COMPLEX I regardless of which of the 12 meteorological
data sets are used as input. Meanwhile, the underpredictions for RTDM are
within the error margin associated with other Gaussian models. As stated
in the guideline^ (page 10-3), "errors in highest estimated concentrations of
+_ 10 to 40 percent are found to be typical, i.e., certainly well within the
often-quoted factor-of-two accuracy . . . ." On the other hand, substantial
overpredictions by RTDM, greater than this margin of error, occurred with most
of the meteorological data combinations that could have been selected.
The commenter's suggestions to apply "correction factors" to concentra-
tion estimates from RTDM, if only one year of data is used, is contrary to
EPA's modeling guidance (Please see Section 8.2.11 of the guideline1). EPA
is sympathetic to the commenter's suggestion to increase the period of data
record to 3 years if this improves confidence in the model prediction.
However, while urging up to 5 years of on-site data, the guideline does not
require using more than one year where a longer period of record for on-site
data 1s not available.
Comment Summary (Impact Assessment)
One commenter stated that the proper basis for assessment of the impact
of RTDM's approval 1s not current emission limits but rather the alternative
emission limits that would result from applying currently approved EPA rough
terrain models to sources whose limits are not now based on approved models.
EPA must compare the allowable emissions from such sources, when modeled with
currently approved rough terrain models, to the emissions allowed by RTDM in
order to fairly present the net Impact of approving RTDM. The commenter
claimed that there would be a greater total Increase 1n emissions than
calculated by EPA.
10
-------
EPA Response
EPA does not agree with the commenter's statement that emission limits
based on currently approved EPA complex terrain screening models should have
been used for all sources in evaluating the impact of the RTDM model. In
its analysis, EPA did consider all existing sources in complex terrain with
emission limits based on the Valley and COMPLEX I screening models. For
these sources current emission limits were compared with those that would
be derived from RTDM, as the commenter desired. However, many sources in
complex terrain have emission limits set on some basis other than the cur-
rently approved models. EPA also considered these sources and compared
their emission limits based on alternative modeling techniques with those
that would be based on RTDM. Since EPA does not plan to impose retroactively
the revised modeling guideline to change the emission limits for these latter
sources, the comparison with RTDM is appropriate and the EPA analysis is
valid. It would be wholly inappropriate to base a comparison on emission
limits derived from Valley or COMPLEX I where these models were not actually
used. Contrary to the commenter's claim, the analysis by EPA, placed in the
Docket, showed that the magnitude of the potential increases in national
emissions associated with the use of RTDM will not be appreciable.
Comment Summary (Miscellaneous)
The model developer recommended that RTDM (Version 3.2) supercede the
previous Version 3.1 which contained two programming errors.
One commenter urged EPA to make RTDM, as well as all other EPA recommended
models, publicly available through the National Technical Information Service
(NTIS).
11
-------
EPA Response
To correct the programming errors identified by the model developer, EPA
agrees to substitute Version 3.2 of RTDM for Version 3.1 in this rulemaking.
EPA has tested Version 3.2 of RTDM that was submitted by this developer and
has found the changes (to the earlier Version 3.1) to be correct, necessary
and with little impact on maximum estimated concentrations.
EPA agrees with the commenter views to make readily available to the
public all EPA recommended models. EPA has notified the RTDM model devel-
oper of EPA's intent to release RTDM through NTIS for public distribution
in a manner similar to other models recommended for regulatory applications.
(See Docket Items VI-I-1, VI-D-1).
Conclusion
EPA reaffirms its proposal to modify the guideline to list RTDM as a
third level screening model for estimating air quality impact from stationary
point sources in rural complex terrain. Users who may wish not to commit
the additional resources necessary to use RTDM as a third-level screening
model are not required to do so; the existing initial or a second-level
screening technique remains available for use. Since it has been demon-
strated that RTDM with full on-site data can underpredict concentrations
substantially, EPA does not recommend RTDM as a refined model at this time;
however, it remains an available option for users, subject to case-by-case
approval.
12
-------
III. Rough Terrain Diffusion Model (RTDM) - Data Input
Comment Summary (Use of Remote Sensing Devices)
Commenters stated that specifying stack top wind measurements pre-
cludes the use of SODAR [which can not provide reliable measurements at
lower heights, e.g., 50m or less], but can provide reliable wind measure-
ments at heights typical of plume transport.
EPA Response
EPA does not intend to preclude the use of remote sensing devices
(e.g., SODAR) to directly measure wind speed and direction at plume transport
height, provided that the necessary data quality assurance and recovery rate
requirements are met. Section 5.2.1.4 of the guideline^- is being revised to
explicitly state that SODAR may be used to measure winds as indicated here.
Comment Summary (On-site Data)
Some commenters stated that EPA should require little, if any, on-site
data because RTDM is only a screening model. Some suggested that EPA should
allow the use of NWS data exclusively because using these data, RTDM is still
more accurate than COMPLEX I.
EPA Response
EPA does not agree with the commenter's statement that RTOM should not
require any on-site data because it is a screening model. The Guideline on
Air Quality Models (Revised), on page 5-4 that was subject to prior public
comment, states that the input of on-site (site-specific) data for the pre-
sent second-level complex terrain screening models is preferred. It has al-
ways been EPA's intention that on-site data be used in these models, e.g.,
COMPLEX I. Allowing the use of off-site data with the third-level screening
technique, RTDM, would be inconsistent with this guidance. Since EPA did not
seek comments on the use of off-site data for RTDM, the Agency does not consider
this matter an issue in the present rulemaking.
13
-------
Comment Summary (Wind Input)
One commenter suggested that 10m winds may be measured on-site, but
no additional on-site data (such as stability and temperature) should be
required. Another commenter stated that it is not necessary to limit the
use of RTDM (as a screen) to situations in which only upper level [100m]
stack top on-site data are available and EPA should not '•equine these data.
While use of stack top winds may be preferable, and while it may be appro-
priate for EPA to ask RTDM users to gather such data, other wind data [as
low as the 10m level] can be used if necessary without having a significant
adverse effect on the model's performance. Specifically, RTDM has been
tested with both surface level (10m) on-site data, and even NWS data, and
it still provides conservative predictions that are closer to observed
concentrations than the predictions provided by COMPLEX I. They further
added that, as a minimum, the use of on-site surface level data should be
allowed on a 2-year interim basis until upper level on-site wind data are
available in order to apply RTDM either as a screening or as a refined model.
Other commenters, however, supported EPA's position to require stack top
wind data.
EPA Response
EPA has examined all the arguments presented by the commenters and
continues to recommend that for input to RTDM winds should be based on fixed
measurements at stack top height. For stacks greater than 100m, the measure-
ment height may be limited to 100m in height relative to stack base. This
recommendation is broadened to include wind data representative of plume
transport height where such data are derived from measurements taken with
remote sensing devices such as SODAR. EPA's rationale is that the best data
from the scientific point of view should be input to RTDM. This rationale
is augmented further in the response to comments below.
EPA's recommendation to use measurements representative of wind flow at
stack top is consistent with the prevailing scientific opinion that use of
stack top winds 1s superior in complex terrain, as the commenter acknowl-
edges, and that in complex terrain these winds can not be estimated accurately
14
-------
from surface level measurements. The use of stack top winds is also consist-
ent with present modeling guidance given in Section 9.3.3.2 of the guideline'-
which was subject to an earlier rulemaking.
Although the sensitivity analysis to determine the performance of the
RTDM model when used with different wind measurement levels as presented by
one of the commenters is very interesting, a serious limitation exists. As
pointed out earlier in Section II, the limited height of terrain at the Widows
Creek facility precluded comparisons (between model predictions and observed
concentrations) during conditions when the plume centerline actually impinges
on high terrain; a condition that must be considered when determining the de-
sign concentration. Based on the limited analysis presented by the commentersi
EPA cannot conclude that more conservative estimates are always obtained when
lower level winds (than at stack top height) are used as input to RTDM.
EPA's rationale for its recommendations is that the scientific integrity
of this model is enhanced with the input of these winds. EPA has made known
its position on wind data since July 1985 and thus there has been ample time
to plan for, if not complete much of, the data collection process. (See
Docket Item IV-E-3). For these reasons, the need for an interim period,
during which the best scientific data are exempt from use in RTDM, is not
technically justified.
Nevertheless, the Agency does not wish arbitrarily to preclude any source
from using RTDM for lack of necessary data input. Thus, the Agency encourages
source owners to allow for time necessary to (1) gather input data needed for
new models, (2) execute the model to determine the emission limit; and (3) sub'
m1t the documentation for rulemaking action. Since EPA 1s Interested in the
most scientifically credible analysis, the Agency will work with the source
to develop reasonable schedules for collecting and using these data in RTDM.
15
-------
Comment Summary (Miscellaneous)
Several commenters also requested clarification between the data
input requirements stated in the proposed text and the "10 meter (default)"
statement for anemometer height in the proposed table.
EPA Response
The commenters1 suggestions to clarify the data requirements in the
text and table have been implemented. In addition, the variable ITIPD has
been set to 1 (i.e., is used), as recommended by the model developer, to
correct an error in the original EPA proposal (see Docket Item IV-I-21).
Conclusion
EPA reaffirms its recommendation that, for input to RTDM as a third-
level screening model, winds should be measured at stack top height. For
stacks greater than 100m, the measurement height may be limited to 100m
in height relative to stack base. Appropriate allowance for the use of
measurements from remote sensing devices is also provided. EPA's rationale
is that the best data from the scientific point of view (i.e., winds repre-
sentative of conditions at stack top height) should be input to this model
to improve confidence in the predicted concentrations.
16
-------
IV. Industrial Source Complex (ISC) Model
Comment Summary (Do Not Support EPA's Proposal)
For a variety of reasons, several commenters did not support the
modification to the building downwash algorithm in the ISC model as
exactly proposed by EPA. Some claimed the EPA proposal to select the
worst of two estimates (from the API version and the original ISC model)
was inconsistent with the API approach. Some claimed that this proposed
approach has no physical basis and is based on model-to-model comparisons.
When ISC predicts higher values than the API version, it is because ISC
assumes downwash under stable conditions and low wind speeds, an unlikely
event based on field data and literature review. In addition, the version
of the ISC model proposed by EPA contained some features that were not
included in the version of ISC proposed by API (i.e., stack tip downwash,
buoyancy-induced dispersion, final plume rise and different wind profile
exponents).
EPA Response
The refinements in the treatment of building wake effects in the ISC model
as proposed by API are: (1) reduced plume rise due to initial plume dilution;
(2) enhanced plume spread as a linear function of the effective plume height;
and (3) specification of building dimensions as a function of wind direction.
EPA agreed that all three refinements would improve the realism of the ISC
model. Most of the controversy in this rulemaking is related to the best
method to implement the second refinement as explained next.
In the present ISC model, the effect of building wake on plume spread
occurs abruptly for effective stack heights less than Good Engineering Practice
(GEP) formula height. That is, for any plume height from a stack with greater
than GEP formula height, the building wake is assumed to have no effect on
plume spread, but for a plume height from a stack with less than formula height
the full wake effect is assumed. Wind tunnel studies6 have suggested as a
refinement that it 1s Important to compute plume enhancement (the vertical and
17
-------
crosswind spreading of the plume due to building wakes) as a continuous
function of stack height. Based on this information, API proposed a linear
"decay" function. Although there is uncertainty about the correct shape of
this "decay" factor, EPA felt that the API approach was reasonable and
proposed it.
After further analysis of the data provided by API, EPA found that most
of these data were collected at sources with small stack height to building
height ratios (Hs/Hg) of approximately 1,5. A change in the shape of the
linear decay factor could have a modest effect on concentrations for these
sources, but a substantial effect on sources with HS/HR > 1.5. Since it was
uncertain how accurately the API linear treatment works for these latter
cases, EPA originally proposed to select the highest of two estimates (from
the API version and the original ISC model) in order to avoid arbitrarily
selecting lower concentrations. EPA's concern about the potential under-
prediction of the proposed API version stems from the apparent systematic
tendency for the present ISC model to underestimate downwash concentrations.7
Thus, for large sources, which usually have HS/HB > 1.5, the misapplication
of the API results would result in a further, more serious, underestimation
of potential impact during downwash conditions. The intent of EPA's original
proposal can be accomplished by limiting the use of the API modifications
to sources with HS/HB <. 1.5 while requiring sources with HS/HB > 1.5 to use
the ISC model in UNAMAP Version 6, as described in more detail later in this
section.
EPA's analysis has also Indicated that API downwash modifications perform
1n a similar manner to ISC for sources with HS/HB <. 1.5 during stable, low
wind speed conditions contrary to the commenter's assertion. This analysis
1s shown in Docket Item VI-E-3.
18
-------
The API modifications were adopted on a different version of the ISC
model than that originally proposed by API because of timing. API explored
the possibility of making these refinements during the Third Conference on
Air Quality Modeling in January 19852. However, documentation of these
refinements was not submitted to EPA until early 1986. By that time, the
ISC model was already in the process of being changed, from UNAMAP Version
5 to Version 6, also a result of public comment at the third modeling con-
ference. Thus, EPA could only propose the API downwash algorithm in con-
junction with the new UNAMAP Version 6 of ISC. One commenter has subsequently
incorporated the API modifications into the ISC model in UNAMAP Version 6
and identified this approach as ISC-6MOD. This approach is discussed in
detail at the end of this section.
Comment Summary (Support EPA's Proposal But Require More Tests)
Several commenters stated that the API data do not provide EPA with
sufficient basis to approve the inclusion of the API modifications as
proposed by EPA. The API tests are limited and are not applicable to
elevated receptors, a wide range of potential critical meteorological
conditions, and stack height (Hs) to building height (Hjj) ratios greater
than 1.3. Moreover, the use of standard deviation of wind direction
fluctuations (sigma-A) to estimate lateral dispersion (sigma-y) was shown
to improve the performance of the API version of the ISC model and should
have been proposed by EPA. One commenter stated a concern for the applica-
tion of the API modifications in the urban mode of the ISC model (where the
McElroy-Pooler (M-P) dispersion coefficients are employed) until a rigorous
evaluation indicates this to be appropriate.
EPA Response
EPA has already limited the application of the ISC model to receptors
on terrain below stack height and it is unnecessary to further limit the
API downwash modification to elevated receptors as the commenter suggests.
19
-------
Building downwash usually occurs close to the point of release and it is
unlikely that any significant differences in terrain height exist within
such short distances from the point of release.
EPA agrees with the commenter that the performance of the API modifications
to ISC under all meteorological conditions has not been addressed by API.
However, the meteorological conditions are adequately accounted for by the
API test data for the application being proposed here.
EPA agrees with the commenter that the API modifications to the ISC
model appear to be limited to certain stack height to building height ratios.
EPA's review of the data shows this ratio extends up to about 1.5. Where
the HS/HB ratio is greater than 1.5, EPA believes that the data presented
to date do not support the API modification or, in turn, any changes to the
current procedure in the ISC model in UNAMAP Version 6 for these ratios.
EPA does not recommend the use of sigma-A to estimate sigma-y for a
couple of reasons. First, the relationship between sigma-A and sigma-y is
site-specific and no generic guidance is available for applicability to
other locations. Second, the use of sigma-A to estimate sigma-y implies
that another variable is needed to determine vertical dispersion or sigma-z.
This so called "split sigma" approach is not recommended by EPA.2
EPA disagrees with the commenter's suggestion to limit the application
of the API modifications of the ISC model to the rural version of that
model. The API modifications are an attempt to provide more realistic con-
centration estimates during building downwash conditions. The downwash
phenomenon is a function of building geometry with respect to the wind. The
commenter presented no data and EPA has no reason to believe that the down-
wash phenomenon occurs differently in rural than in urban environments.
20
-------
In addition, for the far-wake region concentrations converge to values that
would occur in the absence of the building. Therefore, it is technically
sound to assume that the downwash algorithm proposed by API can be used in
rural as well as urban settings. The adequacy of the M-P dispersion
coefficients themselves was addressed previously by EPA2 and is therefore
not an issue in this rulemaking.
If EPA were to accept the commenter's conjecture that using the API
modifications is inappropriate in urban settings, sources located in urban
areas and with HS/HB < 1.5 would be required to use the present ISC model.
This creates a problem because the better API downwash technique, acknowledged
by both EPA and the model developer, could not be applied to these sources.
The commenter did not present for EPA's consideration an alternative approach
for these urban sources. Thus, EPA rejects the commenters conjecture and has
decided to accept the application of the API modifications for both rural and
urban settings 1n the ISC model.
Comment Summary (Alternative Approach: ISC-6MOD)
The model developer submitted an alternative approach which incorporates
in principle the ideas contained in the API proposal to modify the downwash
algorithm but minimizes the differences with the ISC UNAMAP Version 6 by
incorporating many of the new features of the ISC model except for stack tip
downwash and buoyancy-induced dispersion. The commenter stated that the two
features not Included are implicitly accounted for in the API scheme to treat
downwash. The commenter referred to this proposed version as ISC-6MOD which
is most similar in concept to the model originally proposed by API and does
not result 1n significantly different maximum concentrations.
EPA Response
EPA believes that the data and rationale presented by the commenter
support the ISC-6MOD approach, which 1s based on the ISC model in UNAMAP
Version 6 but does not Include stack tip downwash and buoyancy-Induced
dispersion. However, data presented 1n support of applying ISC-6MOD show
21
-------
that use of these modifications is limited to Hs/Hg less than 1.5, especially
in terms of the higher observed concentrations which are of regulatory con-
cern. Where the. Hs/HB ratio is greater than 1.5, EPA believes that the data
presented to date do not support the ISC-6MOD approach; the basic downwash
approach in ISC UNAMAP Version 6 should continue to be used.
Conclusion
EPA agrees with the alternative approach presented by the model
developer and believes that it represents the best scientific method avail-
able at this time. EPA will revise its modeling guidance and modify the
ISC model in UNAMAP Version 6 to include the ISC-6MOD downwash algorithm in
the regulatory default option switch (ISW(28)=1) for sources with HS/HB £
1.5. However, for sources with Hs/Hg > 1.5, there is no scientific basis to
change the ISC model. Since the basic building downwash algorithm proposed
by API and the performance improvements remain essentially intact with
ISC-6MOD, and since the existing ISC model will continue to be used when
these modifications are inapplicable, EPA believes that the alternative
approach effectively accomplishes the same goal as the original proposal
and does not constitute a significant change. Thus, a re-proposal of the
modified downwash algorithm OSC-6MOD is not necessary.
22
-------
V. Offshore and Coastal Dispersion (OCD) Model
Comment Summary
Commenters were generally favorable on the proposal to adopt the OCD
model as a refined model in the guideline. However, one stated that the
OCD model should remain in Appendix B and not be designated as an EPA
preferred model until the following technical issues are resolved:
- the OCD model appears to be continually undergoing substantial
revisions and it is uncertain whether current model evaluation summaries
are applicable for the version of OCD currently in use.
- detailed model evaluations of OCD using tracer data demonstrate
significant underpredictions of peak measured concentrations.
EPA Response
The OCD model was proposed as a preferred or Appendix A model because
it is a unique approach needed to fill a void in the existing regulatory
program. The version of the OCD model recommended by EPA is designated by
the Department of Interior, Minerals Management Service (MMS) as Version
3.0 (Rev 85329). EPA has asked and received confirmation from the MMS that
the recommended version of this model was indeed used in the evaluation
studies. (Please see Docket Item VI-D-2.) From the list of corrections
shown by the commenter (Docket Item V-D-17, attachment E-l), it appears
that these changes were needed to eliminate minor coding inconsistencies
in the program. If this model is substantially revised in the future, EPA
Intends to ask the MMS to re-do the evaluation summaries and use additional
data bases.
The OCD model as recommended by EPA was evaluated by the MMS using
three data bases (Docket Item V-D-9). A review of the model evaluation
results shows no consistent tendency for underpredictions for the ten
highest ranked concentrations. From a regulatory point of view, these are
among the most important statistics since model estimates should demonstrate
23
-------
compliance with the national standards, which are not to be exceeded more
than once per year. Naturally, EPA has some concern about the ability of
the OCD to predict the highest concentration. However, here also, the
underprediction of about 16-30% for two of the data bases are in contrast
with the overprediction of about 85% in the third data base. EPA examined
the example referred to by the commenter, and it appears that the commenter
is referring to examples where a hybrid OCD model, not proposed, is used.
This should not be confused with the version of OCD addressed in this rule-
making. Even so, the underprediction cited does not appear to be significant
(i.e., 10-20 percent) and is within the error margin associated with other
Gaussian models.
Comment Summary (Miscellaneous)
One commenter raised several other objections to the use of the OCD Model;
- over water dispersion characterized by OCD is much greater than that
characterized by MPTER. This results in an inconsistency in the EPA modeling
guideline. Dispersion over water has been demonstrated in numerous studies
to be less than dispersion over land.
- the OCD model can accept monthly climatological values of certain
parameters in lieu of measured hourly values. It is inappropriate for
such important and widely varying parameters to be specified as a monthly
average.
- the EPA model summary implies that the OCD model may be combined with
another model and used for assessing air quality impact on complex terrain.
Another commenter questioned if the experiments to validate OCD were
influenced by high altitude transport of onshore emissions.
EPA Response
The characterization of dispersion over water Is an Integral part of
this model and cannot be evaluated separately. EPA does not require that
24
-------
all models remain completely consistent when these models are designed for
different applications. The OCD model is not recommended by EPA for over
land applications and should not be compared with the MPTER model.
The studies (referenced earlier) show very little difference between
OCD's preferred and default methods for parameterizing offshore stability
and determining the hierarchy of meteorological data. Generally, EPA
prefers the use of available site-specific data rather than default values
wherever possible. The MMS agrees that hourly data should be used and a
preference for such data has been included in the modeling guidance.
Combining OCO with any other complex terrain model, such as COMPLEX I,
cannot be approved on a generic basis. All hybrid models should be reviewed
on a case-by-case basis. EPA will modify the model summary in Appendix A
of the guideline to eliminate any possible confusion on this issue.
The three OCD model validation studies (referenced on page 23), relied
on tracer gas measurements and, therefore, are not influenced by high altitude
transport of onshore emissions.
Conclusion
EPA has concluded to adopt OCD as a preferred model to be listed in
Appendix A of the guideline because:
1. The performance of the OCD model is judged by EPA to be acceptable.
There is no other competing model to which the performance of OCD can be
compared and relative accuracy quantified.
2. OCD 1s a unique OP pioneering approach to specific analysis problems.
3. OCD has been used for regulatory application 1n the past and its
selection results 1n a minimum disruption of regulatory programs.
4. OCD is widely released through NTIS.
25
-------
5. The use of OCD has resulted in public familiarity with this model.
6. This model has met the solicitation requirements outlined in
45 FR 20157^ including the practicality of the model, based on technical merit,
for use in ongoing regulatory programs.
26
-------
VI. AVACTA II Model
There was no comment on the proposal to include this model as an
alternative model in Appendix B of the Guideline on Air Quality Models
(Revised). Models listed in that category may be considered for use on
a case-by-case basis as described on page 3-8 of the modeling guideline.
Thus, EPA will adopt this model as proposed.
27
-------
VII. References
1. Environmental Protection Agency, 1986. Guideline on Air Quality Models
(Revised). EPA Publication No. EPA 450/2-78-027R. U. S. Environmental
Protection Agency, Research Triangle Park, NC (NTIS No. PB 86-245248).
2. Environmental Protection Agency, 1986. Summary of Comments and Responses
on the December 1984 Proposed Revisions to the Guideline on Air Quality
Models. U. S. Environmental Protection Agency, Research Triangle Park,
NC. [Docket Item IV-G-26]
3. Lavery, R. F., B. R. Greene, B. A. Egan, and F. A. Schiermeier, 1983.
The EPA Complex Terrain Model Development Program. Preprints, 6th
Symposium on Turbulence and Diffusion, March 15-22, 1983. Boston,
Massachusetts. American Meteorological Society, Boston.
4. Schiermeier, F. A., T. F. Lavery, D. G. Strimaitis, A. Venkatram,
B. R. Green, and B. A. Egan, 1983. EPA Model Development for Stable
Impingement on Elevated Terrain Obstacles. Proceedings, 14th Interna-
tional Technical Meeting on Air Pollution Modeling and Its Applications,
Copenhagen, Denmark.
5. White, F. D., Ed., and J. K. Ching, R. L. Dennis and W. H. Snyder,
1985. Summary of Complex Terrain Model Evaluation. EPA Publication No.
EPA 600/3-85-060. U. S. Environmental Protection Agency, Research Triangle
Park, NC. (NTIS No. PB 85-236891).
6. Huber, A. H., and W. H. Snyder, 1982. Wind Tunnel Investigation of the
Effects of a Rectangular-Shaped Building on Dispersion of Effluents from
Short Adjacent Stacks. Atmos. Environ, 176:2837-2448.
7. Environmental Protection Agency, 1981. An Evaluation Study for the
Industrial Source Complex Dispersion Model. EPA Publication No.
EPA 450/4-81-002. U. S. Environmental Protection Agency, Research
Triangle Park, NC. (NTIS No. PB 81-176539).
28
-------
Appendix A
-------
32180 Federal Register / Vol. 61. No. 174 / Tuesday. September 8. 1886 f Proposed Rules
ENVIRONMENTAL PROTECTION
AGENCY
40 CFR Parts 51 and 52
[AH-FRL-3030-4, Dock* No. A-40-46]
Requirement* for Preparation,
Adoption, and Submlttal of
Imptcnwntation Plan*
*O8NCY: Environmental Protection
Agency {EPA}.
ACTON: Supplemental notice »f
Proposed rulemaking. _
SUMMARY: EPA is issuing a supplement
to the notice of proposed rulemaking
wat was published an December 7, 1984
[48 FR 48018]. Today's notice proposes
to include (1) addition of a specific
version of the Rough Terrain Dispersion
Model (RTDM) as a screening model, (2)
modification of the downwash algorithm
In the Industrial Source Complex (ISC)
model, (3) addition of the Offshore and
Coastal Dispersion (OCD) model to
EPA's list of preferred models, and (4)
addition of the AVACTA II model as an
alternative model in the "Guideline on
Air Quality Models. Revised." EPA 450/
2--78-027R. The revised guideline lists
the air quality models required to
estimate air quality impact for sources
of air pollutants which appear at 40 CFR
51<24 and 5&21. The purpose of the
Proposed changes is to augment the
guidance in response to a substantial
Dumber of public comments urging the
Agency to do so. EPA is soliciting public
comments on these four proposed
•Ganges only.
°*Tta: The period for comment on these
Proposed changes doses October 9,
1886.
Written comments should
{* submitted to: Central Docket Section
W.E-131). U.S. Environmental Protection
Agency, 401 M Street, SW., Washington,
°C 20460, Attention: Docket A-80-48.
Copies of reports referenced herein, aa
W«U as all public comments an
maintained at Docket A-40-46. The
Docket is available for public inspection
*ad copying between 8:00 ajn. and 4.-00
Pja- Monday through Friday, at the
*?dr*«« above. A reasonable fee may be
^fgad for copying. A copy of the
Proposed revised pages to the guideline
*?y be obtained from the Docket or the
ow.
TACT:
L0t«ph A. Tikvart Chief, Source
***Ptor Analysis Branch, Office of Air
^ty Planning and Standards, US.
{^ironmental Protection Agency,
fifuarch Triangle Park. North Carolina
if7!!; telephone (019) 541-5561 or Jawad
% Tomna, telephone (010) 541-5681.
SUmiMCMTAMV
Background
In March I960, EPA issued a Notice
soliciting air quality models developed
outside the Agency for potential
inclusion in the planned revisions to the
Guideline on Air Quality Models [45 FR
20157]. EPA received nearly 30 air
quality models from private model
developers. These were reviewed for
technical feasibility and for utility to
potential users, hi addition to a review
by EPA for technical merit
documentation, validation, and coding, a
submitted model is open to public
review and comment
On December 7.1084 [49 FR 48018],
EPA proposed amendments to its
regulations concerning air quality
models and announced that it would
hold a public hearing on these proposed
amendments. On December 20,1S84 (49
FR 49484], EPA announced the Third
Conference on Air Quality Modeling to
provide a forum for public review. A
transcript of all oral comments received
at the conference, as well as a record of
all written comments, is maintained in
Docket A-80-48. EPA is in the final
stages of the rulemaking process on the
revisions to the Guideline on Air Quality
Models and will announce these
revisions in a separate Federal Register
notice.
During the public comment period.
EPA received requests to consider
several additional new modeling
techniques. However, the developers did
not submit those models to EPA until
much later. Due to insufficient time for
public review, these models could not be
included hi the revised guideline. In this
separate regulatory proposal, EPA is
seeking public comment on four
additional models as described below.
These four additions, when
promulgated, will require modifications
to the "Guideline on Air Quality Models,
Revised." A copy of those pages
proposed for revision is also available in
Docket A-80-48, Item IV-J-2 for public
review and comment
Proposed Action
A. Rough Terrain Diffution Mods!
Th* "Guideline on Air Quality
Models, Revised" describes two level*
of sophistication of models: Screening
and refined. Screening models provide a
conservative estimate of the air quality
infpact of a source. The purpose of
screening models is to eliminate from
further consideration those sources that
clearly will not causa or contribute to
ambient concentrations in excess of
either the National Ambient Air Quality
Standards (NAAQS) or the allowable
prevention of significant deterioration
(PSD) concentration increments. Refined
models consist of those analytical
techniques that provide more detailed
treatment of physical and chemical
atmospheric processes, require more
detailed and precise input data,' and
provide more specialized concentration
estimates. As a result, they provide a
more accurate estimate of source impact
and the effectiveness of control
strategies.
Due to a lack of scientifically sound
and proven techniques, the guideline
does not recommend any refined air
quality model for determining pollutant
concentrations from sources in
"complex terrain" areas (where the
height of the terrain exceeds the height
of the source being modeled]. Such a
model would apply in situations where
the impaction of effluent plumes on
terrain at elevations equal to or greater
than plume centerline during stable
atmospheric conditions are believed to
cause air quality problems. In the
• absence of an approved case-specific,
refined, complex terrain model, the
guideline specifies screening models in
two categories, depending on the level
of sophistication and the amount of
Input data required. The Valley
Screening Technique is recommended
for first level screening with an assumed
worst case; COMPLEX I and the
SHORTZ/LONGZ models are
recommended with hourly measured
meteorological data^as second level
screening techniques for rural and urban
applications, respectively.
Because there is perceived to be an
immediate need, EPA received
numerous public comments suggesting
that it place a high priority on
accelerating the development of a
refined complex terrain model based on
current research and/or recommend
such a model based on review of
candidate models. Many commenters,
notably from the utility industry,
proposed the Rough Terrain Diffusion
Model (RTDM) as a preferred, refined
model that can fill this need. One of the
technical improvements in RTDM is the
ability to determine the critical
streamline, La., whether a plume will
likely flow over, around or impact on a
terrain obstacle. Further discussion on
this model is contained in Docket Items
IV-D-22 and 27: IV-H-21; IV-K-8 and
11: and IV-B-1.
EPA recognizes the efforts of many
groups to develop a refined model for
complex terrain applications and. EPA
has been conducting, over the last five
years, the Complex Terrain Model
Development program as documented in
the Fourth Milestone Report [EPA/800/
-------
Federal Register / Vol. 51. No. 174 / Tuesday. September 9. 1986 / Proposed Rules
32181
3-64/110). While thi* program ii a
tignificant Hep toward the solution of a
complicated problem, completion of the
research project is not expected before
late 1986. In addition. EPA has also been
evaluating the performance of eight
currently available complex terrain
models that were submitted as a result
of 45 PR 20137. The first available data
base developed under this program.
Cinder Cone Butie. and also the
Westvaco-Luke Milt data base were
used to compare model predictions with
observed air quality data. The
performance of these models was
evaluated using an extensive set of
statistical measures recommended by
the American Meteorological Society. Of
the eight models tested. RTDM showed
the best overall performance for both
data sets. For more detail see
"Evaluation of Complex Terrain Air
Quality Simulation Models." EPA-450/
RTDM requires an extensive set of
meteorological data input such as
horizontal and vertical turbulence
measurements and measured vertical
temperature profiles (delta-T). A survey
by the Utility Air Regulatory Croup
(UARG) has shown that this type of data
is not presently collected at moat power
plant facilities in complex terrain areas
(Docket Item FV-E-3| and model inputs
would therefore be restricted to more
routine meteorological measurements.
At the January 1965 Conference on Air
Quality Modeling. EPA expressed
concern about how well RTDM would
perform without the use of the more
sophisticated on-site meteorological
data. Also. EPA asked for more
information on how well RTDM would
perform relative to COMPLEX I for
distances beyond two kilometers from
the source. To respond to EPA's
concern* about the use of RTDM with
more routine data. UARG further
evaluated RTDM model performance
using the Cinder Cone Butte and
Westveco-Luke Mill data bases. Three
raeteoroiofical input seta wen used
First, for the full on-site caae,
meteorological Inputs Included on-cite
turbulence, wind profile and
temperature profile measurement*.
Second, for the default case, only on-site
temperature, wind speed, and wind
direction measurements were used as
model Inputs. All other parameters were
baaed on default values specified in the
model Third, for delta-T/defaull ease.
on-site temperature profiles were used.
in addition to other model Inputs being
the same as case two above. The results
indicated that RTDM only with default
input eats (caae 2 above) did not
underpredict the highest concentration*.
The other two (full on-site and deita-T/
default) did underpredict high
concentrations for some comparisons
with observed data. For more detail
refer to Docket Item IV-K-e.
In addition, UARG also evaluated
RTDM model performance using 2 years
of data from the Tennessee Valley
Authority's Widows Creek power plant.
located in northeastern Alabama where
the number of monitoring sites is
sufficient. A description of this
evaluation is contained in Docket Item
IV-K-6. The conclusion drawn from this
study is that for the primary averaging
times, there were instances of
underpredictions of "•"'nOT
concentrations while for the default data
input set the model overpredicted. The
delta-T/defauit caae was not tested.
Independent of these model
evaluation studies, a considerable
amount of high quality, research grade
data were being collected under the
EPA's continuing Complex Terrain
Model Development program at the
Tracy Power Plant east of Reno.
Nevada. This area is surrounded by
complex terrain in many directions. For
21 hours of data that had previously
undergone full data quality control
checks, UARG further tested RTDM. The
analysis showed that RTDM with full
on-site data, substantially
underpredicted the concentntiona. but
RTDM with default data almost exactly
predicted maximum concentration*. For
more detail see Docket Item IV-K-11.
These evaluations indicate that.
although RTDM may be technically
superior to other model*.
EPA's COMPLEX I screening model
when all the necessary on-«te data are
available, there is a potential for
underprediction*. In the default mode.
RTDM la not aa sophisticated but
provide* a margin of safety necessary to
ensure the protection of ambient air
quality standard*. In thi* mode. RTDM
can mote property be eharacterixed aa a
screening model If on-eite temperature,
stability, wind speed and direction data
are input to Una modal RTDM may be
classified aa a third lev*l screen— ame
sophisticated than the present second
lev*! screes, COMPLEX L yet not aa
sophisticated a* the model developers
hadaeeumed.
EPA la therefore seeking public
comment oa it* proposal to farther
modify the revised foiddtaM to Uat
«TDM « a third level screen for
estimating air quality impact from
stationary point aoorce* in rani
complex terrain. EPA believe* that thi*
proposal he* several good aapects. First
atoce It ha* been demonatratad that
RTDM with Ml on-aite data can
underpredict concentrations
substantially. EPA cannot recommend
RTDM as a refined model at this time-
EPA believes that it is more prudent to
await results from its Complex Terrain
Model Development program before
recommending a refined model. Second.
many users may not wish to commit the
additional resources necessary to use
RTDM as a third level screen, and EPA
believes they should not presently be
required to do so: less sophisticated
screening techniques (i.e.. more
conservative) remain available. Finally.
the RTDM model with full on-site data H
an available option for users, subject to
ease-by-case approval.
B. Modifications to the Industrial
Source Complex Model (ISC)
EPA received comments from the
American Petroleum Institute (API)
requesting the approval of a modified
version of the building downwash
algorithm in the Industrial Source
Complex (ISC) model. A current
shortcoming of ISC is that *e effect of
building wakes on plume dispersion
occurs abruptly for effective stack
heights less than Good Engineering
Practice (CEP). That ia. for any plume
height greater than either 2.5 building
height* or the building height plus 1-5
time* the lesser of the building height or
width, the building wake is assumed to
have no effect on dispersion. For a
plume height slightly less than CEP the
full wake effect is assumed This leads
to an unrealistic physical discontinuity
which ia not duplicated either in
physical simulation (e.g.. wind tunnel] or
by obaervation. For meteorological
condition* under which downwash doe*
not occur, the ISC model would be
unchanged The effect of the proposed
changes to ISC ia to replace the present
"full downwaah/no downwash" model
with one that predicts progressively-
increasing downwash with decreases to
stack heights at and below GEP. The
proposed modification* to ISC include:
(1) The enhancement of the dispersion
coefficients through the introduction of •
linearly decreasing decay factor a* a
function of plume height (at two building
height* downwind) above building
height instead of the current step .
function behavior at a •pedfic height: Pf
final phnnt rise reduced by initial plu0«
dilution instead of the current approacn
where final plume height is unaffected
by building downwash: (3) different
i building widths and heights for
every tan degree wind sector (4)
optional input of hourly stack
temperature, velocity and emission
rates. For more detailed discussion* ot
-------
32182
Federal Register / Vd. 51, No. 174 / Tuesday, September 9, 1986 / Proposed Rules
these changes, see Docket Items IV-D-
28 and IV-1-23.
The modifications to ISC were
evaluated by API with two sets ef field
data. The first set consists of tracer
experiments at two natural gas
compressor stations and the second
consists of one year of air quality data '•
collected at a «OOMW power plant
Model evaluation results indicated that
the short-term version of the existing
(unmodified) ISC model was
underpredicting maximum ground-level
concentrations, under moderate to high
wind speeds and neutral stability
conditions, by a factor of 2-4. The
proposed modifications improve the
physics of the model (i.e. do better than
the ISC model at matching the
meteorological conditions of the highest
observed concentrations), although the
results indicate a slight overproduction.
EPA proposes therefore to revise the
downwash algorithm for the ISC model
«the guideline by using the API
Codifications for applications to stacks
•horter than the Good Engineering
Practice (GEP) stack height EPA
recommends mis change for the
following reasons. First EPA's own
*odel evaluation program [An
evaluation Study for the Industrial
£>urce Complex Dispersion Model,
epA-450/4-81-002] has indicated an
*Pparent systematic tendency to
"aderestimate the concentrations
Ptoduced near the source by buoyant
•Jack emissions subject to building
**ke effects. APTs Aiding* support
;*ese conclusions and have provided
^aatructive help to develop realistic
••gorithnu to accurately predict ground
<«vel concentrations. Second, although
**»y other caonMBtan stated that the
|*p- model's current downwash
Ugorithm overpredicts ground level
P^centrations, this condusipn ha* not
°**n supported by a comparison of
measured versus predicted
£Offshan and Coattal Qitpenion
,EPA received comments from the
' Management Service (MMS)
requesting the inclusion of the Offshore
and Coastal Dispersion (OCD) model in
the revised guideline as a recommended
model for application to sources located
over water [Docket Item FV-D-8]. This
model has been approved for use by the
MMS in 50 FR12248. In subsequent
communications between EPA and
MMS, MMS has agreed that the
applies tion of this model should be
limited to estimating air quality impact
from offshore sources on onshore flat
•terrain areas. This model is not
recommended by EPA for use in air
quality impact asaesements for onshore
sources.
Hie OCD model was evaluated by the
model developers in order to determine
whether its performance was better than
the original MMS model (CRSTER with
stability classes A and B collapsed into
C). Further discussion of this model is
contained in Docket Hems IV-D-9 and
rv-i-22. Aerometric data from three
separate offshore and coastal diffusion
experiments wen used in this
evaluation. Two versions of the OCD
model wen tested. One required
measurements of parameters
(turbulence intensities) that an not
made on a routine basis: the other
version was run without the turbulence
intensity data. The results indicated that
both versions of the OCD model
performed better than the original MMS
model Even without turbulence
intensity data, the OCD model is a good.
screening model for assnsiring air quality
impact from offshore sources under very
stable atmospheric conditions. When
these conditions an present the highest
onshore coocaotrationa an expected
from these offshore sources.
EPA is proposing to list the OCD
model as a preferred nodal in the
revised guideline, for estimating air
. quality impact from offshore sources on
onshore flat terrain areas. EPA's
recommendation of this action is based
on fan finiitnof that **"* materials
submitted in support of this model meet
the six requinmeats outlined by EPA in
Ha notice solidtinf sir quality models
developed outside the Agency in 46 FR
20157.
D.AVACTA 11 Model
EPA received a comment from the
model developer AeroVironment. Inc.
requesting the inclusion of the AVACTA
II model as an alternative model in
Appendix B of the revised guideline
[Docket Item IV-K-13J. Models listed in
Appendix B must meet the six criteria
outlined in 45 FR 20157 and may be
considered for use on a case-by-case
basis subject to a demonstration.
EPA has determined that the material
submitted in support of the model meets
the six criteria and is proposing to list
this model as an alternative model in the
revised guideline.
Classification
The revisions being proposed herein
to the previous proposal will not change
the conclusions regarding Executive
Order 12291. Regulatory Flexibility Act
Economic Impact Assessment or
Paperwork Reduction Act which were
previously stated. Consequently this
action is not considered major under
E.0. 12291.
List of Subjects
40CFRPart51
Administrative practice and
-procedure. Air pollution control
Intergovernmental relations. Reporting
and recordkeeping requirements. Ozone,
Sulfur oxides. Nitrogen dioxide. Lead.
Particulata matter, Hydrocarbons.
40CFRPart52
Air pollution central Ozone, Sulfur
oxides. Nitrogen dioxide. Lead.
Authority: Sees. 185(e) and 320. aeon Air,
Act 42 U.&C. 747SU). 7B20.
Dattd: August ft. 19B6.
DoaR.Oay.
AuutantAAniniftratorforAirmd
Radiation.
JPR Doc W-194B7 Filed 9-4-8* 849 Ul]
-------
37418 Federal Register / Vol. 51. No. 204 / Wednesday. October 22. 1966 / Proposed Rules
ENVIRONMENTAL PROTECTION
AGENCY
40 CFR Parts 51 and 52
[AH-FHL-3097-3, Docket No. A-80-46]
Requirements for Preparation,
Adoption, and Sufamlttal of
Implementation Plans
AGENCY: Environmental Protection
Agency (EPA).
ACTION: Proposed rule: extension of
comment pehod.
notice extends the time
period in which to file comments in
response to the Supplemental Notice of
Proposed RuJemaking in Central Docket
No. A-80-46 (9/9/86. 51 fit 32180]
concerning the proposed tndmton of
four air quality models into the
"Guideline on Air Quality Models
(Revised)," EPA 450/2-78-027R. On
September 29. Hunton and Williams on
behalf of the Utility Air Regulatory
Group requested that thfroominent
period be extended n evder to provide
more time to evaluate and teat the
proposed ISC modifications. On October
1. the Natural Reaoafcea Defense
Council requested that the comment
period be extended in order to provide
more time to exaaxiaa end review the
documents submitted to the Docket
conceraiiig the ISC and RTQM aodela.
Other pntetttial enmmmian hawe also
expressed «a ifiietsat ia additioB«l Uae
to comment. As a result, as noted below,
the public comment pehod has been
extended to December 9,1966.
DATE: Comments may now be Hied on or
before December 9.1986.
ADDRESS: Written comments should
continue to be submitted to: Central
Docket Section (LE-131). U.S.
Environmental Protection Agency. 401 M
Street SW., Washington. DC 20406,
Attention: Docket A-80-46.
FOB FURTHER INFORMATION CONTACT:
Joseph ATikvart, Chief. Source
Receptor Analysis Branch. Office of Air
Quality Planning and Standards. UJ5.
Environmental Protection Agency.
Research Triangle Park, NC 27711:
telephone (919) 541-5561 or Jawad S.
Touma. telephone (919) 541-5681.
SUPPLEMENTARY INFORMATION: The
following item was included in the
Docket but reference to it was
inadvertently omitted from the original
proposal On page 32180. column 3. at
the ead of the 2nd full paragraph add:
Docket Item IV-4-21. User's Guide to the
Rough Terrain Diffusion Model (RTDM),
(Rev. 3.W). September 1985.
Dated: October 10.1986.
Don K. Clay.
Acting Assistant Administrator far Air cad
Radiation.
(FR Doc. 86-23634 Filed 10-21-80; 8:45 am]
BOUNO CODE (MMO-M
-------
Appendix B
-------
Docket
Number
V-D-1
V-D-2
V-D-3
V-D-4
V-D-5
V-D-6
V-D-7
V-D-8
V-0-9
V-D-10
V-0-11
V-D-12
V-D-13
V-D-14
V-D-15
V-D-16
V-D-17
V-D-18
V-D-19
V-D-20
V-D-21
V-D-22
V-D-23
List of Commenters Appearing in Docket A-80-46
Commenter
Hunton & Williams
Tennessee Valley Authority
US EPA (OAQPS)
Pennsylvania Power & Light Co.
Hammermill Paper Co
Natural Resources Defense Council
Conoco, Inc.
Aluminum Co. of America
Westvaco
State of Maryland, Office of
Environmental Programs
Middle South Services, Inc.
County of Santa Barbara, CA
Illinois Power Co.
U S Department of Interior,
Minerals Managment Service
County of Santa Barbara, CA
TRC Environmental Consultants, Inc.
County of Santa Barbara, CA
General Public Utilities Corp.
Tennessee Valley Authority
Pennsylvania Dept. of Env. Resources
Potomac Electric Power Co.
Natural Resources Defense Council
Proctor & Gamble Co.
Date of
Document
9-29-86
10-06-86
9-29-86
9-26-86
9-29-86
10-01-86
10-01-86
10-08-86
10-06-86
10-06-86
10-07-86
10-03-86
10-03-86
10-21-86
10-15-86
12-01-86
12-04-86
12-02-86
12-08-86
12-04-86
12-08-86
12-09-86
12-09-86
-------
Docket Date of
Number Commenter Document
V-D-24 Environmental Research & Technology 12-08-86
V-D-25 Hunton & Williams 12-09-86
V-D-26 Boston Edison 12-05-86
V-D-27 American Petroleum Institute 12-09-86
V-D-28 Middle South Services, Inc. 12-08-86
Late Arrivals:
V-H-1 US Department of Agriculture, 12-17-86
Forest Service
V-H-2 Hunton & Williams 01-08-87
V-H-3 Hunton & Williams 02-02-87
------- |