United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park, NC 27711
EPA-454/R-93-055
October 1993
AIR
& EPA
AN EVALUATION OF A SOLAR RADIATION/DELTA-T METHOD
FOR ESTIMATING PASQUILL-GIFFORD (P-G) STABILITY CATEGORIES
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EPA-454/R-93-055
O
An Evaluation of a Solar Radiation/Delta-T Method
for Estimating Pasquill-Gifford (P-G) Stability Categories
October 1993 U.S. Environncr.':! ' . -Action Asency
Region 5, Library •;;-•.-j>!)
77 West Jackson Boulevard, 12th Floor
Chicago, IL 60604-3590
U. S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air Quality Planning and Standards
Technical Support Division
Research Triangle Park, NC 27711
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ACKNOWLEDGEMENTS
Special credit and thanks are due Dr. Desmond Bailey and
Mr. John Irwin for their technical assistance and advice
through all phases of the project, from meteorological
data acquisition and processing through testing and
development of the SRDT methodology. Thanks are offered
Mr. Gerry Moss and Mr. Pete Eckhoff for FORTRAN
programming support and in meteorological data retrieval
and reformatting. Thanks are offered to Mr. Russ Lee
and Mr. Roger Erode, Pacific Environmental Services for
their consultation on ISC2 runs. Special appreciation
is due Mr. Jim Paumier, Pacific Environmental Services
and Mr. Rob Wilson, EPA Region 10 for their scrupulous
peer review of this report.
DISCLAIMER
This report has been reviewed by the Office of Air
Quality Planning and Standards, EPA, and approved for
publication. Mention of trade names or commercial
products is not intended to constitute endorsement or
recommendation for use.
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PREFACE
In this report a comparison is made of two different
methods for estimating the hourly Pasquill-Gifford
stability categories required for the current generation
of regulatory dispersion models. The effects of
utilizing the two different methods (referred to as
Turner and SRDT in this report) in regulatory
applications of a Gaussian dispersion model, ISC2, is
also evaluated. A fundamental feature of the SRDT
method is the use of on-site meteorological data.
The Environmental Protection Agency must conduct a
formal and public review before the Agency can recommend
replacement of the Turner method for estimating
stability categories with the SRDT method. This report
is being released to establish a basis for review of the
consequences resulting from use of SRDT-derived
stability categories in routine dispersion modeling of
air pollution impacts.
111
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CONTENTS
Section Page
Acknowledgements ii
Preface iii
Contents iv
Figures v
Tables vi
1. Introduction 1
2. Rationale 3
3. Methods 6
3.1 Data Selection 6
3.2 Approach 6
4. Stability Comparison Results 9
4.1 Composite Results 9
4.2 Results for the Kincaid Site 11
4.3 Results for the Longview Site 14
4.4 Results for the Bloomington Site 15
4.5 Results for a AT Interval Other than 2-10m 15
4.6 Discussion 18
5. Results from Dispersion Modeling 22
5.1 Results for the Bloomington Site 22
5.2 Results for the Kincaid Site 23
5.3 Results for the Longview Site 23
5.4 Discussion 23
5.5 Analysis of Computed Mixing Heights 24
6. Summary and Conclusions 27
7. References 28
Appendix A Results of Randomization Analysis A-l
Appendix B Results from Gaussian Dispersion Modeling B-l
iv
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FIGURES
Number Page
4-1 Stability classification plot for composite data using key
described in Table 4-1; 2-10m AT values (19,540 valid hours) . . 13
4-2 Stability classification plot for Kincaid, IL data using key
described in Table 4-1; 2-10m AT (2916 valid hours; 83% of
the period) 13
4-3 Stability classification plot for Longview, WA data using key
described in Table 4-1; 2-10m AT values (8187 valid hours;
94% of the period) 17
4-4 Stability classification plot for Bloomington, IN data using
key described in Table 4-1; 2-10m AT values (8437 valid hours;
89% of the period) 17
4-5 Stability classification plot for Kincaid, IL data using key
described in Table 4-1 except AT values are from 10-50m (2917
valid hours; 83% of the period) 20
4-6 Stability classification plot for Longview, WA data using key
described in Table 4-1 except AT values are from 10-50m (8187
valid hours; 94% of the period) 20
B-l Mean and median mixing height by hour of the day for
Bloomington, IN site; 2-10m AT B-7
B-2 Mean and median mixing height by hour of the day for
Kincaid, IL site; 2-10m AT B-8
B-3 Mean and median mixing height by hour of the day for
Kincaid, IL site; 10-50m AT B-9
B-4 Mean and median mixing height by hour of the day for
Longview, WA site; 2-10m AT B-10
B-5 Mean and median mixing height by hour of the day for
Longview, WA site; 10-50m AT B-ll
v
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TABLES
Number Page
2-1 -Comparison of incoming solar radiation (insolation)
classifications 4
2-2 Conceptual matrix for insolation-based key to Pasquill-Gifford
(P-G) stability categories 4
3-1 Selected on-site meteorological data bases for the SRDT evaluation 7
4-1 Insolation-based key to Pasquill-Gifford (P-G) stability
categories derived from composite data from three sites
(19,540 valid hours) 9
4-2 Comparison of hourly stability classification via Turner versus
SRDT for composite data from all three sites using key described
in Table 4-1; AT values are from 2-10m 10
4-3 Joint frequency distribution matrix for all SRDT stability
categories appearing in Table 4-2 10
4-4 Stability classification results for composite data from all
three sites using key described in Table 4-1 and AT values
from 2-10m (19,540 valid hours) 12
4-5 Stability classification results for Kincaid, IL data using
key described in Table 4-1 and AT values from 2-10m (2616
valid hours; 83% of the period) 12
4-6 Stability classification results for Longview, WA data using
key described in Table 4-1 and AT values from 2-1Om (8187
valid hours; 94% of the period) 16
4-7 Stability classification results for Bloomington, IN data
using key described in Table 4-1 and AT values from 2-10m
(8437 valid hours; 89% of the period) 16
4-8 Stability classification results for Kincaid, IL data using
key described in Table 4-1 except AT values are from 10-50m
(2917 valid hours; 83% of the period) 19
4-9 Stability classification results for Longview, WA data using
key described in Table 4-1 except AT values are from 10-50m
(8187 valid hours; 94% of the period) 19
A-l Comparison of hourly stability categories via Turner versus
SRDT for random subsets of the composite data. . . . V . . . . A-3
B-l Design concentration ratios derived from ISC2ST for
Bloomington, IN site; 2-10m AT B-2
B-2 Design concentration ratios derived from ISC2ST for Kincaid,
IL site; 2-10m AT B-3
B-3 Design concentration ratios derived from ISC2ST for Kincaid,
IL site; 10-50m AT B-4
B-4 Design concentration ratios derived from ISC2ST for Longview,
WA site; 2-10m AT B-5
B-5 Design concentration ratios derived from ISC2ST for Longview,
WA site; 10-50m AT B-6
VI
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1. INTRODUCTION
The Guideline on Air Quality Models (Revised)" (EPA, 1986) recommends and
ranks four alternative schemes for estimating the Pasquill-Gifford (P-G)
stability category (Pasquill, 1961; Gifford, 1961) from on-site meteorological
measurements. The highest ranking is given to Turner's method (Turner, 1964)
which uses on-site wind speed coupled with observations of cloud cover and
ceiling height. However, obtaining the data necessary to implement Turner's
method requires a full time on-site observer, and may be impractical for use on
a routine basis in many circumstances.
At the Fourth Conference on Air Quality Modeling, October 1988 (EPA, 1990) ,
public concerns were presented for a practical alternative to the Turner method
for estimating P-G stability categories. A real need was expressed for a
method that did not require labor intensive data collection (e.g., hourly human
observation of clouds), i.e., one based exclusively on simple on-site
meteorological instrumentation. On February 13, 1991, EPA issued a notice of
proposed rulemaking to further augment the Guideline via Supplement B (56 FR
5900). Supplement B (Draft) included a new method for estimating the
P-G stability category. In this new method, on-site meteorological
measurements (10m wind speed in combination with solar radiation during the day
and temperature difference, AT, at night), are used in lieu of cloud cover and
ceiling height for determining the P-G stability category. The proposed method
was adapted from Bowen et al. (1983) and is herein referred to as the solar
radiation/delta-T (SRDT) method. Public comments presented at the Fifth
Conference on Air Quality Modeling, March 1991 (EPA, 1993) regarding the SRDT
method focused on two key issues: 1) development of the proposed SRDT method
was based on data from only one site (i.e., Kincaid, IL) for a limited time
period (i.e., 21 weeks during spring/summer); and 2) the method accommodated
AT measurements made only at the 2-lOm interval, whereas AT had been measured
at other intervals by many sources.
To address these concerns, an attempt was made to acquire several data
bases from diverse geographical areas. In addition, on-site AT measurements
"Hereinafter, the "Guideline"
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from other height intervals were considered for evaluation, as available.
Finally, a consequence analysis was needed to document the effect on design
concentration ratios if the new method is implemented. This report, presented
in seven sections, documents the SRDT evaluation with data from several sites,
and the consequence analysis of using the method in regulatory modeling
applications. Section 2 of this report presents the rationale behind the
Turner and SRDT methods for determining P-G stability categories. Section 3 is
a discussion of the methodology used in the analysis. Section 4 presents and
discusses the results of the stability classification comparison. Section 5
presents the results from employing the SRDT method in Gaussian dispersion
modeling. Section 6 provides a summary and conclusions, and references are
listed in Section 7. Appendix A contains results from the randomization
procedure used to ascertain the robustness of the SRDT method. Appendix B
contains tabulated results of design concentration ratios obtained via Gaussian
dispersion modeling.
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2. RATIONALE
Turbulence, which drives dispersion within the mixed layer of the
atmosphere, is a result of thermal and mechanical processes. The P-G stability
classification method parameterized these processes using observations of wind
speed and subjective estimates of incoming solar radiation (insolation).
Turner (1964) provided an objective means for implementing the P-G method using
routine airport observations available from the National Weather Service (NWS).
Stability class, using Turner's method, is a function of wind speed, the
insolation class (objectively determined based on the sun's position in the
sky), cloud cover, and ceiling height.
Uncertainty in the P-G method arises, in part, from the subjectivity in the
classification of insolation. For example, as indicated in Table 2-1, Pasquill
(1961) defined strong insolation as: "... sunny, midday, midsummer conditions
in England." Based on measurements at Kew Observatory in England, these
conditions correspond to insolation values of about 700 Wm"2, (Chandler, 1965;
Ludwig and Dabberdt, 1972, 1976). Similarly, Pasquill's definition of slight
insolation: "... sunny, midday, midwinter conditions in England" corresponds to
insolation values of about 420 Wm"2.
Insolation flux intensity varies diurnally, seasonally, and spatially.
There can be significant microscale influences on the amount of insolation
received at the ground surface. The intensity and spectral composition of the
insolation are also highly influenced by the amount and type of cloud cover
(Miller, 1981) . Objective methods for classifying insolation (Table 2-1)
include those of Turner (1964), Ludwig and Dabberdt (1972), Smith (1972) and
Bowen et al. (1983). Turner's method requires calculation of the solar
elevation angle based on location and time. The other methods require either
estimates or on-site measurements of insolation. Strong insolation is equated
with solar elevations exceeding 60 degrees (Turner, 1964) and insolation values
exceeding 560 Wm'2 (Ludwig and Dabberdt, 1976) to 700 Wm'2 (Bowen et al., 1983) .
Slight insolation is equated with solar elevations between 15 and 35 degrees
(Turner, 1964) and insolation values less than 280 Wm'2 (Ludwig and Dabberdt,
1976) to 350 Wm'2 (Bowen et al., 1983).
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Table 2-1. Comparison of incoming solar radiation (insolation) classifications.
Source
Pasquill, 1961
Chandler, 1965
Insolation (Wm~2)
Turner, 1964
Solar elevation
(degrees)
Ludwig and Dabberdt, 1972
Insolation (Wrrr2)
Smith, 1972
Insolation (Wm~2)
Bowen et al . , 1983
Insolation (Wm~2)
Strong
sunny , mi dday ,
midsummer
conditions in
England
700"
>60
>560
>600
>700
Moderate Slight Weak
sunny, midday,
midwinter
conditions in
England
420"
35 - 60 15 - 35 <15
280 - 560 <280
300 - 600 <300
350 - 700 <350
" Measurements made at Kew Observatory for conditions corresponding to
Pasquill's definitions.
Table 2-2. Conceptual matrix for insolation-based key to Pasquill-Gifford
(P-G) stability categories.
DAYTIME
Wind
Speed
(ms-1)
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A desire to retain the basic rationale of Turner's method was an important
consideration in the selection of objective procedures for use with on-site
data. Table 2-2 shows the structural matrix conceived for the insolation-based
P-G stability classification procedure. As explained later, the specific SRDT
"cutpoints" (limits for on-site meteorological parameters, i.e., u, - u«, E, -
E3, ATL/ used to estimate stability categories) were derived empirically. The
SRDT method is based on a development by Bowen et al. (1983), with
modifications as necessary to retain as much as possible of the structure and
behavior of Turner's method as implemented in the EPA meteorological
preprocessors for regulatory models (EPA, 1986). The first modification was to
replace Bowen's method for determining nighttime (insolation less than 35 Wm'2)
with the procedure which is based on calculations of sunrise and sunset. This
modification was necessary to maintain consistency in the SRDT method and that
used in EPA's meteorological preprocessors. Another modification was to
include an additional daytime insolation class (
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3. METHODS
3.1 Data Selection
A search produced 10 potentially suitable data bases. Of these, 3 were
selected as best meeting the requirements for this evaluation (Table 3-1). The
requirements for individual data bases included the following attributes: 1)
hourly average values for 2-10m temperature difference," 10m wind speed and
direction, and total solar radiation; 2) available cloud cover and ceiling
height data from a nearby, representative NWS station; 3) a continuous
monitoring record of sufficient length (preferably, at least one full year);
4) on-site meteorological monitoring having been done in accordance with EPA
guidance (EPA, 1987a); and 5) on-site meteorological data having been quality
assured. There was also a desire to acquire data bases that, in the aggregate,
were geographically within the contiguous United States.
3.2 Approach
As mentioned above, the method with which to compare the SRDT system is
that prescribed by Turner (1964), as implemented in the Meteorological
Processor for Regulatory Models (MPRM)(Irwin et al., 1988), hereafter referred
to as the "Turner method". The Turner method uses on-site wind speed coupled
with cloud cover and ceiling height observed on-site. Because on-site data for
cloud cover were unavailable, surface observations from a nearby, represen-
tative NWS station were used as surrogates. Accordingly, on-site data bases
were carefully selected (see Section 3.1) to ensure the integrity of their use
with surrogate NWS data. Determination of P-G stability categories was made
using MPRM (Version 1.3), configured to implement the Turner method.
Consistent procedures were used for all sites. For stabilities determined
using either the Turner method (via MPRM6) or the SRDT method, "smoothing"
(i.e., disallowing stability to change by more than one class per hour) was
disabled in this evaluation in an effort to make a direct comparison of the
stability categories generated by both methods. In all evaluations, quality
control measures were implemented to ensure that only data valid for joint
"Other intervals were also of interest.
bA special version of MPRM 1.3 (MPRMRUFF) was configured to output "rough"
hourly stability classes.
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Table 3-1. Selected on-site meteorological data bases for the SRDT evaluation.
AT Height Interval (m)
Source Location
NWS Station
Distance"
Period Insolation 2-10
Other
EPRI" Kincaid, IL
Springfield, IL
-25 4/80 - 8/80
Yes
Yes 10-50, 10-100
ENSR° Longview, WA
Portland, OR
-55 1/91 - 12/91
Yes
Yes
2-50
ENSR Bloomington, IN Indianapolis, IN
-70
7/91 - 7/92
Yes
Yes
"Kilometers from nearest NWS station.
""Electric Power Research Institute; data base used in original SRDT evaluation by EPA (see Section 1.0) .
"ENSR, Inc.; data were collected at a pulp and paper mill operated by Weyerhauser, Inc.
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stability comparison were used. All other data were bypassed but otherwise
accounted for (in all comparisons, valid data ranged from 83 to 94 percent).
Once the requisite P-G stability categories were determined via MPRM for
each site, the SRDT system was applied. The SRDT system uses on-site wind
speed, total solar radiation (daytime) and temperature difference (AT)
(nighttime). The temperature differences were measured with reliable
thermocouple systems. Outpoints for the solar radiation and AT parameters were
derived iteratively to obtain optimal fits for the entire time period at each
site. The observed range of direct solar radiation intensities reported for
contiguous U.S. locations (Miller, 1981) was investigated in an effort to
develop a daytime scale that would be geographically robust. Initial
evaluations indicated some site-to-site variations in the derived cutpoints.
Therefore, it was decided to pool the data from all three sites to determine
cutpoints from the composite data set. The occurrence of residuals (category
differences) on an hourly basis was minimized; attention was paid to the
distribution of those residuals by category and a systematic effort was
employed in the choice of cutpoints to evenly allocate those residuals across
all stability categories in an attempt to make the system as robust as
possible. These cutpoints were then applied to each individual data base to
assess site-specific residuals in the behavior of the SRDT method.
As a further effort to investigate the sensitivity of the results, the
composite data were randomly stratified into two complementary (mutually
exclusive) subsets; hourly records for which information was valid for joint
stability classification were randomly sorted into two bins. Records from each
bin were then used independently to evaluate the SRDT method. Cutpoints
determined for the pooled data were applied individually to each bin. This
approach allowed for an assessment of the sensitivity of the SRDT results to
the specific data employed in the analyses. Thus, for the composite data base,
results could be assessed as random fluctuations over many iterations.
Finally, a consequence analysis showing effects on design concentration
ratios was performed using three hypothetical sources, a hypothetical receptor
array on flat terrain, and a suitable Gaussian dispersion model (ISC2; see
Section 5).
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4. STABILITY COMPARISON RESULTS
4.1 Composite Results
For the pooled analysis from the three data bases (i.e., Kincaid, Longview,
and Bloomington), 19,540 hours (89.6% of those potentially available for 909
days) were valid for making the joint comparison of stability classes. As
indicated in Table 4-1, the optimum outpoints for solar radiation were 925,
675, and 175 Wm"2. Daytime wind speed cutpoints were 2.0, 3.0, 5.0, and 6.0 ms"1;
those for nighttime wind speed were 2.0 and 2.5 ms"1. Using these cutpoints,
comparison of hourly stability categories for both methods showed reasonable
agreement (Table 4-2). The joint frequency distribution of hourly stability
categories modulated via the SRDT method was examined (Table 4-3). Of most
interest was the discrimination made at night as a function of wind speed and
AT. Most of the category "sorting skill" is being made on the basis of wind
speed, with AT adding a refinement. The weak discrimination seen with
nighttime AT has been observed by others (Bowen et al., 1983; Bowen and Pamp,
1994). To check this phenomenon, the nominal value for the AT cutpoint (ATL)
was varied iteratively from 0.0, -0.01, -0.02, -0.03, +0.01, +0.02, and +0.03.
No systematic improvement was seen over that using ATL = 0.0, the value employed
Table 4-1. Insolation-based key to Pasquill-Gifford (P-G) stability categories
based on composite data from three sites.
Wind
Speed"
(ms-1)
<2.0
2.0 -* 3.0
3.0 -» 5.0
5.0 -> 6.0
>6.0
DAYTIME"
Solar Radiation (Wm"2)
>925
A
A
B
C
C
925 -» 675
A
B
B
C
D
675 -» 175 <175
B
C
C
D
D
D
D
D
D
D
NIGHTTIME"
Wind
(ms-1)
<2.0
2.0 -» 2.5
^2.5
2 -10m AT (°Cm-')
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Table 4-2. Comparison of hourly stability classification via Turner versus
SRDT for composite data from all three sites using key described
in Table 4-1; AT values are from 2-10m.
SRDT
A
B
C
Dday
Anight
E
F
TOTAL
%"
P-G Stability Categories as Estimated via Turner
A B C D^ D^ E F&G
108
118
31
4
0
0
0
261
41.4
160
1230
429
244
0
0
0
2063
59.6
3
393
970
1037
0
0
0
2403
40.4
2
252
996
3787
0
0
0
5037
75.2
0
0
0
0
2085
659
892
3636
57.3
0
0
0
0
905
250
55
1210
20.7
0
0
0
0
651
662
3617
4930
TOTAL
273
1993
2426
5072
3641
1571
4564
19540
73.4
Percent coincidence of the hourly stability categories based on the
distribution derived via Turner (see Section 3.2).
Table 4-3. Joint frequency distribution matrix for all SRDT stability
categories appearing in Table 4-2.b
ws
<2.0
2.0-3.0
3.0-5.0
5.0 - 6.0
>6.0
TOTAL
SOLAR RADIATION (Wnf2)
>925
25
50
72
12
3
162
925-675
198
341
493
114
86
1232
675-175 <175
1087
826
1471
401
349
4134
1676
925
1156
246
233
4236
AT/AZ (°Cm-1)
WS
<0.0 2:0.0
<2.0
2.0 - 2.5
>2.5
549
188
773
1510
4564
1022
2680
8266 I 19540
b The composite data valid for comparison comprised 9764 daytime hours and
9776 nighttime hours.
10
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in the SRDT method examined by Bowen et al. (1983). It was therefore decided
to retain the cutpoint at 0.00Cm"'. The nighttime wind speed cutpoints were
likewise varied iteratively over reasonable values. The proximity of the
chosen cutpoints (0.5 ms'1 apart) was necessary to get the required sorting
skill in concert with AT, and no alternative AT cutpoint (ATL) allowed the two
nighttime wind speed cutpoints to be any further apart than 0.5 ms"1.
Overall, the stability classifications for the two methods coincided for
62% of the hours, and were within one category for 89% of the hours (Table
4-4; Fig. 4-1). Absolute residuals (|A|) expressed as a percentage of hours
allocated to each category, were analyzed by stability category, and by day
versus night. Across all categories, the mean residual was less than one
percent. The mean absolute residual was greater for nighttime hours than for
daytime hours. As indicated in Table 4-2, however, the coincidence of
categories by both methods varied as a function of stability category. The
greatest coincidence (75%) occurred with daytime D's, while the least (21%)
occurred with E's. As these results were considered to be optimum for the
pooled data, the cutpoints were applied to the individual sites to assess their
residuals.
The results of the randomization analysis are presented in Appendix A. The
SRDT method was not seen to be sensitive to random variations in the data. The
results for complementary subsets of the pooled data were virtually identical.
4.2 Results for the Kincaid Site
The first data base examined" was from the Electric Power Research
Institute (EPRI) Plume Model Validation and Development Program (PMVDP) for the
plains site, Kincaid, Illinois. The meteorological monitoring site is located
in central Illinois; the surrounding terrain is flat and uniform (z0 - 10cm).
The site and its environs have been extensively described elsewhere (EPRI,
1983). Though meteorological measurements were made from March through
November, 1980, data for a 21-week period (7 April - 31 August 1980) were
considered to be of highest quality. On-site measurements of interest included
those of 10m wind speed and direction, 2-10m, 10-50m and 10-100m AT, and total
"These data were used in the analysis for EPA's initial proposal to adopt SRDT.
11
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Table 4-4. Stability classification results for composite data from all three
sites using key described in Table 4-1 and AT values from 2-10m
(19,540 valid hours).
STABILITY
A
B
C
D*,
Anight
E
F
Turner (%)
261 (1.3)
2063 (10.6)
2403 (12.3)
5037 (25.8)
3636 (18.6)
1210 (6.2)
4930 (25.2)
SRDT (%)
273 (1.4)
1993 (10.2)
2426 (12.4)
5072 (26.0)
3641 (18.6)
1571 (8.0)
4564 (23.4)
|A| (%)
(0.1)
(0.4)
(0.1)
(0.2)
^
(0.0)
(1.8)
(1.8)
-
Mean (%)
(0.193)
(1.20)
(0.62)
Hourly coincidence of stability categories: 61.7%
Hourly categories ± one class: 89.4%
Table 4-5. Stability classification results for Kincaid, IL data using key
described in Table 4-1 and AT values from 2-10m (2916 valid hours;
83% of the period).
STABILITY
A
B
C
Dday
Anight
E
F
Turner (%)
61 (2.1)
376 (12.9)
497 (17.0)
618 (21.2)
322 (11.0)
237 (8.1)
805 (27.6)
SRDT (%)
42 (1.4)
301 (10.3)
432 (14.8)
777 126.6)
611 (21.0)
257 (8.8)
496 (17.0)
|A| (%)
(0.7)
(2.6)
(2.2)
(5.4)
-
(10.0)
(0.7)
(10.6)
-
Mean (%)
- -
(2.7)
(7.1)
(4.6)
Hourly coincidence of stability categories: 56.4%
Hourly categories ± one class: 89.7%
12
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3D
(J
C
O>
U
U
o
Figure 4-1.
B C D DCnite} E
P-G Stabi I i ty Category
Stability classification plot for composite data using key
described in Table 4-1; 2-10m AT values (19,540 valid hours)
3D
P-G Stabi I 1ty Category
Figure 4-2. Stability classification plot for Kincaid, IL data using key
described in Table 4-1; 2-10m AT values (2916 valid hours;
83% of the period).
13
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solar radiation. In addition to the on-site data, concurrent surface obser-
vations from the NWS station at Springfield, IL (WBAN #93822), about 25km
northwest of the site, were obtained.
For the Kincaid data base, 2616 hours (83% of those potentially available
for 147 days) were valid for making the joint comparison of stability classes.
Overall, the stability classifications for the two methods coincided for 56% of
the hours, and were within one category for 90% of the hours (Table 4-5; Fig.
4-2). Unstable and E categories were comparable. The SRDT method
overrepresented the nighttime D category and underrepresented the F category.
4.3 Results for the Longview Site
ENSR Consulting and Engineering provided the next data base. The
meteorological monitoring site is located in Longview, in southwest Washington,
along the Columbia River approximately 65km inland from the Pacific Ocean.
While the terrain within the local proximity of the city of Longview is
relatively flat (z0 - 10cm), the terrain immediately across the Columbia River
in Oregon and just outside the Longview city limits in Washington ascends
quickly into a series of ridges and hills. The city itself (and the monitoring
site) is approximately 5m above mean sea level (msl) due to its low-lying
position along the Columbia River. Terrain extends 60m above msl within 3km of
the monitoring site. Data for calendar year 1991 were available for this
analysis and were collected and quality assured according to EPA guidance (EPA,
1987a), as well as ENSR's own internal standard operating procedures. On-site
measurements of interest included those of 10m wind speed and direction, 2-10m
and 2-50m AT, and total solar radiation. In addition to the on-site data,
surface observations from the NWS station at Portland, OR (WBAN #24229), about
65km southeast of the site, were obtained. The topographical setting for the
site, unique among the three sites examined in this analysis, is such that
local micrometeorologial effects are possible. Preliminary analyses of
nighttime wind speeds indicated that the site is influenced by nighttime
drainage flows, resulting in relatively low (33% si ms"1; 62% s2 ms'1;
u =1.9 ms"1) and uniform velocities.
For the Longview data base, 8187 hours (94% of those potentially available
for 365 days) were valid for making the joint comparison of stability classes.
14
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Overall, the stability classifications for the two methods coincided for 58% of
the hours, and were within one category for 85% of the hours (Table 4-6; Fig.
4-3). Unstable categories generally compared better, while (as with Kincaid)
some disparity occurred for the nighttime D category.
4.4 Results for the Bloomington Site
ENSR Consulting and Engineering also provided the third data base. The
meteorological monitoring site, equipped with a 10m tower, is located in a
rural area with slightly rough terrain (z0 - 25cm) about 70km south of
Indianapolis, near Bloomington, IN. To compensate for several days of missing
data due to frequent lightning-caused outages, data for the 13-month period
July 1991 - July 1992 were provided. These data were collected and quality
assured according to the provisions of EPA guidance (EPA, 1987b), and ENSR's
own internal standard operating procedures. On-site measurements of interest
included those of 10m wind speed and direction, 2-10m AT, and total solar
radiation. In addition to the on-site data, concurrent surface observations
from the NWS station at Indianapolis, IN (WBAN #93819) were obtained.
For the Bloomington data base, 8437 hours (89% of those potentially
available for 397 days) were valid for making the joint comparison of stability
classes. Overall, the stability classifications for the two methods coincided
for 67% of the hours, and were within one category for 94% of the hours (Table
4-7; Fig. 4-4). As with Kincaid, but to a lesser extent, the SRDT method
underrepresented the F stability category.
4.5 Results for a AT Interval Other than 2-10m
There was interest to investigate the performance of the SRDT method for AT
values measured at intervals above 10m. On-site meteorological data collected
at Kincaid and Longview afforded just such an opportunity, as they included 10-
50m AT values. As a group, the 10-50m AT values at both sites were less
variable in absolute magnitude and less frequently in the isothermal/positive
range as compared with their 2-10m counterparts.' Therefore, it was
anticipated that a AT cutpoint of slightly less than 0.0 "Cm'1 (e.g., -0.01 or
"For Kincaid, 95% of the 2-10m AT values were in the isothermal/positive range,
versus 79% of those from 10-50m. Likewise, for Longview, 89% of the 2-10m AT
values were in the isothermal/positive range, versus 46% of those from 10-50m.
15
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Table 4-6. Stability classification results for Longview, WA data using key
described in Table 4-1 and AT values from 2-10m (8187 valid hours;
94% of the period).
STABILITY
A
B
C
Dday
Dnigfc
E
F
Turner (%)
81 (1.0)
880 (10.7)
829 (10.1)
2153 (26.3)
1682 (20.6)
478 (5.8)
2084 (25.4)
SRDT {%)
175 (2.1)
993 (12.1)
776 (9.5)
1999 (24.4)
1268 (15.5)
714 (8.7)
2262 (27.6)
|A| (%)
(1.2)
(1.4)
(0.6)
(1.9)
— »
(5.0)
(2.9)
(2.2)
-
Mean (%)
(1.3)
(3.4)
(2.2)
Hourly coincidence of stability categories: 58.0%
Hourly categories ± one class: 85.0%
Table 4-7. Stability classification results for Bloomington, IN data using key
described in Table 4-1 and AT values from 2-10m (8437 valid hours;
89% of the period).
STABILITY
A
B
C
D*y
Dnieto
E
F
Turner (%)
119 (1.4)
807 (9.6)
1077 (12.8)
2266 (26.9)
1632 (19.3)
495 (5.9)
2041 (24.2)
SRDT (%)
56 (0.7)
699 (8.3)
1218 (14.4)
2296 (27.2)
1762 (20.9)
600 (7.1)
1806 (21.4)
|A| (%)
(0.7)
(1.3)
(1.6)
(0.3)
-
(1-6)
(1.2)
(2.8)
-*
Mean (%)
-
(1.0)
(1.9)
(1.4)
Hourly coincidence of stability categories: 66.9%
Hourly categories ± one class: 93.5%
16
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30
P-G Stabi I ity Category
Figure 4-3. Stability classification plot for Longview, WA data using key
described in Table 4-1; 2-10m AT values (8187 valid hours;
94% of the period).
P-G Stab I I ity Category
Figure 4-4. Stability classification plot for Bloomington, IN data using key
described in Table 4-1; 2-10m AT values (8437 valid hours;
89% of the period).
17
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-0.02) would have shown greater skill for the 10-50m AT's. Iteratively, it was
found that a value of -0.01 "Cm"1 seemed to produce only marginally better
results than those using 0.0 °Cm''. For Kincaid, the nighttime mean absolute
residual (|A|) was 6.9% for ATL = -0.01 and -0.02 (versus 7.9% for ATL = 0.0).
For Longview, |A| was 3.9% for ATL = -0.01 and -0.02 (versus 4.3% for ATL =
0.0). Temperature difference offers only a minor refinement to the
determination of stability category. Therefore, it was decided to employ a
0.0 "Cm"1 cutpoint, regardless of measurement height interval.
The results for Kincaid (Table 4-8; Fig. 4-5) were similar to those using
the 2-10m AT's; the mean residual for nighttime stability categories was about
7 percent. For the Longview site (Table 4-9; Fig. 4-6), the mean residual for
nighttime stability categories was about 4 percent, slightly greater (4.3%)
than that using the 2-10m AT's (3.4%). The analyses for both sites serve to
show that stability categories can be as reasonably determined using AT values
measured above 10m using the same AT criteria as for 2-10m. Indeed, analyses
with more data bases may better support the notion that use of a AT cutpoint
somewhat less that 0.0 °Cm'! affords better classification skill for intervals
above 10m. However, for practicality in implementing the SRDT method it was
considered that 0.0 "Cm"1 was reasonable, particularly given site-to-site
variability seen among the data bases used here.
4.6 Discussion
Of interest in illustrating how the mechanics of the SRDT method work, it
may be noted that the D stability category at night occurred for about 12 to 15
percent fewer hours at the Longview site than those examined at the Bloomington
and Kincaid site, respectively. This difference is primarily explainable by
the nighttime wind regime. At the Longview site, nighttime winds s2 ms~' (a
requirement for stable classification for any lapse rate) occurred 11 to 25
percent more frequently than did those at Bloomington and Kincaid, respec-
tively. It is thought that such a regime at the Longview site is due to
micrometeorological effects (see Section 4.3).
Also, as indicated by data from the Kincaid and Longview sites, prevalence
of nighttime D categories increased for the 10-50m AT data compared to their
18
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Table 4-8. Stability classification results for Kincaid, XL data using key
described in Table 4-1 except AT values are from 10-50m (2917 valid
hours; 83% of the period).
STABILITY
A
B
C
D*y
Dnigh.
E
F
Turner (%)
61 (2.1)
376 (12.9)
497 (17.0)
618 (21.2)
323 (11.1)
237 (8.1)
805 (27.6)
SRDT (%)
42 (1.4)
301 (10.3)
432 (14.8)
777 (26.6)
631 (21.6)
277 (9.5)
457 (15.7)
|A| (%)
(0.7)
(2.6)
(2.2)
(5.4)
-
(10.5)
(1.4)
(11.9)
-
Mean (%)
(2.7)
(7.9)
(5.0)
Hourly coincidence of stability categories: 56.1%
Hourly categories ± one class: 90.5%
Table 4-9. Stability classification results for Longview, WA data using key
described in Table 4-1 except AT values are from 10-50m (8187 valid
hours; 94% of the period).
STABILITY
A
B
C
EV
Anight
E
F
Turner (%)
81 (1.0)
880 (10.7)
829 (10.1)
2153 (26.3)
1682 (20.5)
478 (5.8)
2084 (25.4)
SRDT (%)
175 (2.1)
993 (12.1)
776 (9.5)
1999 .(24.4)
1553 (19.0)
1011 (12.3)
1680 (20.5)
|A| (%)
(1.2)
(1.4)
(0.6)
(1.9)
-
(1.5)
(6.5)
(4.9)
-
Mean (%)
- _
(1.3)
(4.3)
(2.6)
Hourly coincidence of stability categories: 56.6%
Hourly categories ± one class: 88.8%
19
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(D
U
CZ
OJ
13
(J
U
o
3D
25 -
2O —
15 —
10 -
5 -
P-G Stab I I ity Category
Figure 4-5. Stability classification plot for Kincaid, IL data using key
described in Table 4-1 except AT values are from 10-50m (2917
valid hours; 83% of the period).
O)
U
b
U
U
o
30
25 -
2O —
10 -
5 -
P-G Stabi I ity Category
A
Figure 4-6. Stability classification plot for Longview, WA data using key
described in Table 4-1 except AT values are from 10-50m (8187
valid hours; 94% of the period).
20
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2-10m AT counterparts at both sites. The increase is due to the less frequent
occurrence of isothermal/positive lapse rates seen in the upper boundary layer
(Section 4.5), and the increase seen is roughly proportional to the decrease in
occurrence of such lapse rates.
For the analysis of stability comparisons, Tables 4-4 through 4-9 emphasize
the overall comparability of the frequency of occurrence of stability
categories in the aggregate, i.e., without regard to hourly correspondence. As
explained in Section 3.2, a systematic effort was employed to evenly allocate
residuals on an hourly basis across all stability categories. Detailed hourly
correspondence of stability categories was analyzed for all comparisons but
reported (in matrix format) only in Table 4-2 for the pooled data. For 2-10m
AT measurements, hourly correspondence of stability categories ranged from 56
to 58 percent for three data bases analyzed; categories were within one class
for 85 to 94 percent of the hours examined. For 10-50m AT measurements, hourly
correspondence of stability categories ranged from 56 to 57 percent for two
data bases analyzed; categories were within one class for 89 to 91 percent of
the hours examined. As indicated in the matrix for the pooled data (Table 4-2),
infrequently the corresponding categories differed by two classes or more. An
important point to remember in viewing these results is that having a stability
category that differs by no more than one class most of the time on an hourly
basis can still result in quite different design concentrations as different
wind speeds, directions and mixing heights are being linked with those
stability categories.
These results suggest use of a nighttime AT cutpoint value of 0.0 ''Cm'1 is
robust enough to accommodate a range of height intervals, provided attention is
given to proper siting of temperature sensors so as to effectively characterize
the boundary layer. Consistent with probe placement guidance (EPA, 1987a), the
lower temperature sensor should be least in the range of order of 20z0 - 100z0,
but never less than 1m, above the ground surface (Irwin et al., 1985) . The
upper sensor should be of the order 5 times the lower sensor height. These
criteria ensure that the lower instrument level is within the surface layer and
that a reasonable separation is maintained between the two measurement levels.
Stronger temperature gradients are expected in the lower atmosphere. Hence, as
the distance above ground increases for the lower measurement level, the
separation distance accordingly should increase.
21
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5. RESULTS FROM DISPERSION MODELING
To ascertain the possible effect of the new SRDT stability classification
method on design concentrations, a sensitivity analysis was performed on the
ratios of design concentrations (XSRDr/XTuror) . The Industrial Source Complex
(ISC2) model was used to compute concentration values for averaging times of
1-hour, 3-hour, 24-hour, and the entire period modeled. ISC2 provides both the
high first high (H1H) and the high second high (H2H) concentrations. Three
stationary point sources" of heights 35m, 100m, and 200m, respectively, were
used in these analyses. These same sources have been used in past modeling
evaluations to assess the impact of revisions to regulatory air quality models
(Lee et al., 1979). Receptors were arranged in a polar grid network with 36
radials and 180 sites on 5 concentric rings at 800m, 2000m, 4000m, 7000m, and
15000m, respectively; the sources were placed at the origin. Flat terrain was
assumed and the model was run in the RURAL mode. Hours with on-site wind speed
less than 0.5 ms"1 were treated as calms, and the option 'MSGPRO', which allows
processing of missing hours,b was set 'ON'. For analyses of 24-hour concen-
trations, days with more than 6 missing hours were omitted. The daytime
morning mixing height is determined daily within the meteorological processor
based on the stability category just before sunrise. The maximum afternoon
mixing height (ziKnax) was preset to 2500m. This configuration conferred a
measure of consistency between the ISC2 runs. The tabulated results for all
ISC2 runs are presented in Appendix B.
5.1 Results for the Bloomington Site
At the Bloomington, IN site, where the AT interval was 2-10m, the composite
mean ratio (across 4 averaging times, 3 source heights, and both concentration
types; 24 values) was 1.06 (median was also 1.06), with a range (R) of 0.85 -
1.24 (Table B-l); the geometric mean ratio was 1.05. For the H1H concen-
trations (12 values), the mean ratio was 1.07 (R = 0.97 - 1.16), with a median
of 1.05; the geometric mean was 1.07. For the H2H concentrations (12 values),
"For the 35m stack, parameters were: Q8 = 100 gs"1, T, = 432K, v. = 11.7 ms"1 d, = 2.4m
For the 100m stack, parameters were: Q. = 100 gs'1, T. = 416K, v, = 18.8 ms , d, = 4.6m
For the 200m stack, parameters were: Q, = 100 gs"1, T, = 425K, v, = 26.5 ms"1, d, = 5.6m
bSuch hours are also processed as calms.
22
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the mean ratio was 1.05 (R = 0.85 - 1.24), with a median of 1.07; the geometric
mean was 1.04.
5.2 Results for the Kincaid Site
For analyses done with 2-10m AT data, the composite mean ratio was 1.06
(median was 1.08), with a range of 0.75 - 1.62 (Table B-2); the geometric mean
ratio was 1.05. For the H1H concentrations, the mean ratio was 1.09 (R = 0.77
- 1.62), with a median of 1.12; the geometric mean was 1.08. For the H2H
concentrations, the mean ratio was 1.04 (R = 0.75 - 1.41), with a median of
1.05; the geometric mean was 1.01.
5.3 Results for the Longview Site
For analyses done with 2-10m AT data, the composite mean ratio was 1.24
(median was 1.20), with a range of 1.00 - 1.70 (Table B-4); the geometric mean
ratio was 1.22. For the H1H concentrations, the mean ratio was 1.25 (R =
1.00 - 1.70), with a median of 1.20; the geometric mean was 1.23. For the H2H
concentrations, the mean ratio was 1.24 (R = 1.00 - 1.62), with a median of
1.19; the geometric mean was 1.22.
5.4 Discussion
As noted in the above results (as well as in the footnotes for Tables B-l
to B-5), for each site and'AT interval a composite mean ratio was computed
based on 24 values. Mean ratios were also computed for the H1H and H2H
concentrations based on 12 values for each. Likewise, the geometric mean and
standard deviation were also computed and reported. Because the design
concentration ratios are considered to be approximately log-normally
distributed, the latter statistics probably better characterize the ratio
distribution. Formal hypothesis testing is impractical as strict independence
among the ratios cannot be assumed. For example, a concentration-value that
figures into a ratio for one averaging time may also figure into a ratio for a
longer averaging time at the same site.
The ratios at both the Kincaid and Bloomington sites do not appear to be
significantly different than 1.0. Nor is there sufficient evidence to warrant
the conclusion that design concentrations predicted via SRDT-derived
stabilities are from a different population than those predicted via Turner-
derived stabilities. While the same relationship does not appear to be the
case with the Longview data, neither can meaningful confidence bounds be
23
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established. As explained in Section 4.6, there may be some site-specific
features at the Longview site that result in some unusual influences. Taken
on the whole, the range of concentration differences seen among the three sites
is of the order expected for site to site differences and professional
judgement should be used in viewing the modeling results presented.
5.5 Analysis of Computed Mixing Heights
For ratios 21.15, special attention was given to the influence of computed
mixing height values for the averaging times involved. Such values are
themselves determined by the occurrence of stability category, and can be
highly influential in the model predicted concentrations. Thus, values for
estimated mixing heights in the short term, as well as their long term
distribution pattern, was of interest in assessing the consequence of the SRDT
method on concentration values; one must interpret such concentrations in the
context of the associated mixing heights. The behavior of the computed mixing
heights was investigated using simple statistics, and the diurnal patterns are
depicted in Figures B-l to B-5. In these Figures, the mean (zt) (Figs. B-la to
B-5a) and median (Figs. B-lb to B-5b) mixing height was determined by hour of
the day. For convenience, only daytime hours were analyzed.
At the Bloomington site, on three occasions the ratio was al.15. The
associated mixing heights were not seen to have been influential in the
prediction of the higher ambient concentration via SRDT-derived stability
categories. While the period averages were consistently higher via SRDT, the
period averaged daytime mixing heights via Turner (2150m; median = 2500m) were
lower than those computed via SRDT (2170m; median = 2500m). Therefore, mean
mixing height does not adequately explain the high concentrations for the
period.
At the Kincaid site, on 8 occasions the ratio was si.15. The associated
mixing heights were seen to be increasing the ratio in half of these instances,
though the pattern appeared to be random. The period averages were higher via
SRDT only for the 35m stack. As with the Bloomington site, the period averaged
daytime mixing heights via Turner (2040m; median = 2500m) were lower than those
computed via SRDT (2130m; median = 2500m). When stability category was
estimated using the 10-50m AT data, the results were virtually identical to
those found with the 2-10m AT data (compare Tables B-2 and B-3).
24
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At the Longview site, on 14 occasions the ratio was 21.15. In some of
these cases, differences in early morning daytime mixing heights seemed to
account for the differences seen in concentration values. This was especially
true for the 200m source. As with the Kincaid site, period averages were
consistently higher via SRDT. However, the period averaged daytime mixing
heights via Turner (2150m; median = 2500m) were greater than those computed via
SRDT (2130m; median = 2500m). When stability category was estimated using the
10-50m AT data, the results were virtually identical to those found with the
2-10m AT data (compare Tables B-4 and B-5). The period averaged daytime mixing
heights via Turner using the 10-50m AT data (2470m; median = 2500m) were also
greater than those computed via SRDT (2150m; median = 2500m). For this site,
the mean mixing height may at least partially explain the high concentrations
for the period.
In general, at all sites and for both AT intervals, the mean mixing height
at or just after sunrise computed by MPRM is greater with SRDT-derived
stabilities (z^^j.) than with those derived via Turner (z,_ri(mCT.) stabilities
(Figs. B-la to B-5a) . Whereas, except for at Kincaid, mean afternoon" z^^j.
seems to be lower than zi_Ttmier .
This disparate behavior in the time series of computed mixing heights is an
inherent trait of the mixing height algorithm as implemented in MPRM and can,
in part, be traced directly to how mixing heights are determined for daytime
hours having neutral stability. In these cases, the algorithm interpolates in
time by one of two different algorithms, depending on the estimated stability
just before sunrise. This may result in spurious increases and decreases in
the time series of early morning daytime mixing height values.
At the Bloomington site, the early morning ~z,_SKDT is considerably higher
than zi_Txnier . This is mainly due to the greater number of nighttime hours with D
stability via SRDT. In mid-morning this pattern reverses, followed by
convergence at about 1400 hours. At the Kincaid site, the early morning z,_S{DT
is also higher than zl_Tlimer, again due to a greater number of nighttime hours
having D stability via SRDT. After a mid-morning convergence, z^^^ again
exceeds z{_Tltnifr, followed by convergence at about 1400 hours. Note that 'Z^^T is
"I.e., from about 0900 - 1400 hours.
25
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never lower than z(_Ttma. . At Longview, the pattern is not quite so predictable/
explainable as that for the other sites. For the 2-10m AT data, the early
morning z,.Tmtr is initially higher than z.-.^oj-, due to a greater number of
nighttime hours having D stability via Turner. By 0600 hours, though, the
pattern reverses. Following a mid-morning convergence, there is another
reversal, with zt.Tmfr again higher than ~zi.SRDT; by 1400 hours the z,.'s converge.
For the 10-50m AT data, the pattern is the same as that for the 2-10m AT data
except that the early morning ~zi.stDT is higher than ~z,_Txmer, even though there
were still 8 percent more nighttime hours with D stability via Turner! Perhaps
some of the aforementioned micrometeorological influences (Section 4.6) were
operating in some more complex way for the computation of mixing heights at
this site.
The occurrence of a larger early morning mixing height is largely related
to the prevalence of nighttime D stability category (specifically, its
occurrence the hour before sunrise). This relationship was borne out in all
mixing height analyses except for that using the 10-50m AT data at the Longview
site (Figure B-5a), where the relationship was reversed. The mixing height
analyses serve to illustrate how the prediction of ambient concentrations is
affected by mixing heights', which themselves exhibit complex patterns due to
the influence of stability categories and their occurrence relative to the time
of sunrise. Because of the nonlinear linkage between the occurrence of
stability category and predicted concentration via factors such as mixing
height, care should be exercised in interpreting the stability comparison
results and those from the dispersion modeling (see discussion at end of
Section 4.6).
Thus, in the comparisons made in this evaluation, apparent disparities in
mixing heights, which may indeed result in the prediction of significant
concentration values, are as likely as not an artifact of the computational
system. Though it was possible, it was not deemed prudent to "factor" the
mixing height influence out because it would not have emulated the complete
computational system as it is employed for making model predictions in
regulatory applications.
26
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6. SUMMARY AND CONCLUSIONS
Turner's method for estimating the P-G stability categories provides a
practical procedure for the routine implementation of Gaussian dispersion
models if representative cloud observations are available. The proposed SRDT
method uses on-site meteorological data without the need for such cloud
observations, while retaining the basic structure and rationale of Turner's
method.
A comparative analysis was performed using on-site data from three sites:
Kincaid, IL (7 April - 31 August 1980), Longview, WA (January - December 1991),
and Bloomington, IN (July 1991 - July 1992). Meteorological data included 10m
wind speed, total solar radiation, and temperature difference (AT). All three
sites had AT data from 2-10m; 10-50m AT data were available from Kincaid and
Longview. Valid observations from all three sites were pooled to yield a
composite data base of 19,540 hours. The SRDT method was developed empirically
to emulate the results obtained using the Turner stability estimation scheme.
Through iterations, optimum "cutpoints" (meteorological parameter limits) were
determined, initially using only the 2-10m AT data. For the pooled data,
stability categories estimated by both methods coincided for 62 percent of the
hours, and were within one class for 89 percent. Using the same cutpoints for
each site gave comparable results. In an effort to evaluate the system for AT
intervals other than 2-10m, the 10-50m AT data were evaluated for two sites.
Using the same optimum cutpoints determined for the 2-10m AT data produced
virtually the same distribution of stability categories.
Using ISC2, an analysis of the effects on design concentration ratios,
XSROT/^Tumef was completed for three hypothetical sources. For three sites, the
ratios averaged 1.06 to 1.24 across four averaging times, three source heights,
and two concentration types. Computed mixing heights were examined to help
understand their influence in the model-predicted concentrations.
27
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7. REFERENCES
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Chandler, T.J., 1965. The Climate of London. Hutchinson and Co., Ltd.;
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Institute, Palo Alto, CA.
Gifford, F.A. Jr., 1961. Use of routine meteorological observations for
estimating atmospheric dispersion. Nuclear Safety 2(4): 47-57.
Irwin, J.S., S.E. Gryning, A.A.M. Holtslag and B. Sivertsen, 1985. Atmospheric
Dispersion Modeling Based on Boundary Layer Parameterization. EPA-600/
3-85-056. U.S. Environmental Protection Agency, Research Triangle Park,
NC.
Irwin, J.S., J.O. Paumier and R.W. Erode, 1988. Meteorological Processor for
Regulatory Models (MPRM 1.2) User's Guide. EPA-600/3-88-043R. U.S.
Environmental Protection Agency, Research Triangle Park, NC.
Lee, R.F., J.A. Tikvart, J.L. Dicke and R.W. Fisher, 1979. The effect of
revised dispersion parameters on concentration estimates. Proceedings,
Fourth Symposium on Turbulence, Diffusion, and Air Pollution, American
Meteorological Society, Boston, MA; pp. 70-74.
Ludwig, F.L. and W.F. Dabberdt, 1972. Evaluation of the APRAC-1A urban
diffusion model for carbon monoxide. Final report, Coordinating Research
Council and EPA Contract CAPA-3-68 (1-69), Stanford Research Institute,
Menlo Park, CA; 147 pp. (NTIS No. PB 210819)
28
-------
Ludwig, F.L. and W.F. Dabberdt, 1976. Comparison of two practical stability
classification schemes in an urban application. Journal of Applied
Meteorology, 15: 1172-1176.
Miller, D.H., 1981. Energy at the Surface of the Earth. Volume 27.
International Geophysics Series. Academic Press; 516pp.
Pasquill, F., 1961. The estimation of the dispersion of windborne material.
Meteorological Magazine, 90: 33-49.
Smith, F.B., 1972. A scheme for estimating the vertical dispersion of plumes
from a source near ground level. Proc. Third Meeting Expert Panel on Air
Pollution Modeling. NATO Committee on Challenges of Modern Society,
XVII-1 to XVII-14.
Turner, D.B., 1964. A diffusion model for an urban area. Journal of Applied
Meteorology, 3: 83-91.
29
-------
Appendix A
Results of Randomization Analysis
A-l
-------
As discussed in the Section 3.2, the composite data were randomly split
into two complementary subsets and the same stability classification and
comparison applied to each. Results of the stability calculations for the two
methods are presented in Table A-l. The results shown are representative of
what was seen throughout all analyses performed. Different seed values" would
result in different cases being selected for the two bins (i.e., Bin 0 and
Bin 1). The results shown indicate only minor differences in the comparison
statistics between the two stability estimation methods.
For Bin 0, valid data for use in joint stability calculations were
available for 9834 out of 10970 hours (89.6 percent) randomly selected of the
909-day period. For Bin 1, valid data were available for 9706 out of 10846
hours (89.4 percent) so selected. For Bin 0, the stability classifications for
the two methods coincided for 62 percent of the hours and were within one
category for 89 percent of the hours. The unstable category was the same,
while the neutral category decreased slightly and the stable category increased
slightly. The mean absolute residual (see Section 4.1) was 0.88% over all
categories; for daytime categories it was 0.40%, while for nighttime categories
it was 1.5%. For Bin 1, the stability classifications for the two methods also
coincided for 62 percent of the hours and were within one category for 89
percent of the hours. Stable and unstable categories decreased slightly, while
the neutral category increased slightly. The mean absolute residual was 0.82%
over all categories; for daytime categories it was 0.45%, while for nighttime
categories it was 1.3%. The frequency distributions of stability categories
for the two methods (both bins) are displayed in Table A-l.
"Randomization was accomplished using a random number generator. The unique
assignment of a subset to one bin versus that assigned to the other is
controlled by the seed value.
A-2
-------
Table A-l. Comparison of hourly stability categories via Turner versus SRDT for random subsets of the
composite data (see page A-4 for notes).a
TESTOPL*
Turner
A B C D^ D^ E F&G TOTAL
A 45 72
B 62 627
C 18 235
D^ 2 128
SRDT D —
EDO
F 0 0
TOTAL 127 1062
STABILITY CLASS
A
B
C
D
E
F
UNSTABLE
NEUTRAL
STABLE
i i
202 127
489 517
488 1875
0 0
0 0
0 0
1180 2520
0
0
0
0
1079
359
446
1884
0 0
0 0
0 0
0 1
427 324
134 325
29 1822
590 2471
119
1018
1259
2493
1830
818
2297
9834
FREQUENCY (%)d
Turner SRDT
1.3
10.8
12.0
44.8
6.0
25.1
24.1
44.9
31.1
1.2
10.4
12.8
44.0
8.3
23.4
24.4
44.0
31.7
TEST1PU
Turner
A B C D.. D^, E F&G
A 63
B 56
C 13
D^ 2
SRDT
D^ 0
E 0
F 0
TOTAL 134
88
603
194
116
0
0
0
1001
STABILITY CLASS
A
B
C
D
E
F
UNSTABLE
NEUTRAL
STABLE
2 1
191 125
481 479
549 1912
0 0
0 0
0 0
1223 2517
0
0
0
0
1006
300
446
1752
0 0
0 0
0 0
0 0
478 327
116 337
26 1795
620 2459
FREQUENCY (%)e
TOTAL
154
975
1167
2579
1811
753
2267
9706
Turner SRDT
1.4
10.3
12.6
44.0
6.4
25.3
24.3
44.0
31.7
1.6
10.0
12.0
45.2
7.8
23.4
23.7
45.2
31.1
A-3
-------
Notes for Table A-l
a) On-site data are pooled from all three sites (see Section 3.2).
b) This analysis was done using TESTOPL, which selects records randomly assigned an index of 0 from
pooled data sets. Of 21816 records read from the 3 raw meteorological input files, 10846 were ignored
while 10970 were randomly selected for processing. Of those selected, 1136 were rejected for missing
data. These included 649 with flags for invalid P-G stabilities and 1106 with flags for missing on-
site data, including:
706 with flags for 10m wind speed;
682 with flags for total solar radiation; and
265 with flags for 2-10m AT/AZ measurements.
Thus, the Turner/SRDT comparison matrix is based on 9834 valid records (hours), or 89.6% of the records
randomly selected with initial seed value: 1500. Randomly selected were 5679 daytime hours and 5291
nighttime hours; processed were 4889 daytime hours and 4945 nighttime hours.
c) This analysis was done using TEST1PL, which selects records randomly assigned an index of 1 from
pooled data sets. Of 21816 records read from the 3 raw meteorological input files, 10970 were ignored
while 10846 were randomly selected for processing. Of those selected, 1140 were rejected for missing
data. These included 646 with flags for invalid P-G stabilities and 1122 with flags for missing on-
site data, including:
715 with flags for 10m wind speed;
699 with flags for total solar radiation; and
264 with flags for 2-10m AT/AZ measurements.
Thus, the Turner/SRDT comparison matrix is based on 9706 valid records (hours), or 89.5% of the records
randomly selected with initial seed value: 1500. Randomly selected were 5686 daytime hours and 5160
nighttime hours; processed were 4875 daytime hours and 4831 nighttime hours.
d) Using TESTOPL, the stability classifications for the two methods coincided for 61.7% of the hours, and
were within one category for 89.4% of the hours (with P-G categories F and G via Turner combined).
e) Using TEST1PL, the stability classifications for the two methods coincided for 61.6% of the hours, and
were within one category for 89.4% of the hours (with P-G categories F and G via Turner combined).
A-4
-------
Appendix B
Results of Gaussian Dispersion Modeling:
A Consequence Analysis
B-l
-------
Table B-l. Design concentration ratios derived from ISC2ST for Bloomington,
IN site; 2-10m AT (see Section 5.1).
Sourceb
Single
35m
Stack
Single
100m
Stack
Single
200m
Stack
Ambient Concentration" via ISC2ST (/*gnf3)
Avg. Time Turner0 SRDTd /„ Rat/*°" ,
3 ^XsRHT/ATunier'
1-hour
3 -hour
24 -hour
Period
(8437 hours)
1-hour
3 -hour
24 -hour
Period
(8437 hours)
1-hour
3 -hour
24 -hour
Period
(8437 hours)
245.7
232.9
217.3
198.4
76.1
67.9
4.38
4.12
55.9
55.3
34.2
31.1
7.98
6.49
0.349
0.348
30.2
24.3
15.4
12.0
2.93
2.60
0.104
0.097
238.1
233.7
225.3
214.2
75.7
67.9
4.90
4.64
56.8
47.3
35.5
33.7
9.26
8.02
0.391
0.376
30.7
25.9
17.7
11.2
3.24
2.65
0.110
0.105
0.97
1.00
1.04
1.08
1.00
1.00
1.12
1.13
1.02
0.85
1.04
1.08
1.16
1.24
1.12
1.08
1.02
1.06
1.15
0.94
1.10
1.02
1.06
1.08
"For each averaging time, high 1st high (HlH) concentration appears above
dotted line; high 2nd high (H2H) concentration appears below dotted line.
bSee text, Section 5.0, for description of all source and receptor parameters
used in the dispersion model runs.
"Hourly mixing heights are computed based on Turner-derived stabilities.
dHourly mixing heights are computed based on SRDT-derived stabilities.
'Statistical analysis (see Section 5.1): median
s
For all 24 values:
For 12 HlH concentration ratios only:
For 12 H2H concentration ratios only:
1.06 1.06 0.08 1.05 1.08
1.05 1.07 0.06 1.07 1.06
1.07 1.05 0.10 1.04 1.10
B-2
-------
Table B-2. Design concentration ratios derived from ISC2ST for Kincaid, IL
site; 2-10m AT (see Section 5.2).
Source11
Single
35m
Stack
Single
100m
Stack
Single
200m
Stack
Ambient Concentration" via ISC2ST (pgm3)
Avq. Time Turner0 SRDTd /„ Ra5i°e \
^XSRDT/ ATun*r>
1-hour
3 -hour
24 -hour
Period
(2842 hours)
1-hour
3 -hour
24 -hour
Period
(2842 hours)
1-hour
3 - hour
24 -hour
Period
(2842 hours)
254.4
233.5
201.9
186.4
60. 8f
53.5
5.86
5.25
61.8
54.7
37.2
27.8
9.21
7.43
0.649
0.649
26.3
24.6
12.3
11.1
3.00
2.44
0.225
0.221
236.7
236.1
234.5
194.4
69.8
56.5
6.43
5.96
61.8
37.0
36.6
30.6
10.6
7.48
0.567
0.563
32.2
30.1
19.9
15.7
3.65
2.70
0.172
0.166
0.93
1.01
1.16
1.04
1.15
1.06
1.10
1.13
1.00
0.68
0.99
1.10
1.15
1.01
0.87
0.87
1.22
1.22
1.62
1.41
1.22
1.11
0.77
0.75
"For each averaging time, high 1st high (H1H) concentration appears above
dotted line; high 2nd high (H2H) concentration appears below dotted line.
bSee text, Section 5.0, for description of all source and receptor parameters
used in the dispersion model runs.
"Hourly mixing heights are computed based on Turner-derived stabilities.
dHourly mixing heights are computed based on SRDT-derived stabilities.
"Statistical analysis (see Section 5.2): median
s
For all 24 values:
For 12 H1H concentration ratios only:
For 12 H2H concentration ratios only:
fOne hour was missing in the computation of this concentration.
1.08 1.07 0.21 1.05 1.22
1.12 1.10 0.22 1.08 1.21
1.05 1.03 0.20 1.01 1.22
B-3
-------
Table B-3. Design concentration ratios derived from ISC2ST for Kincaid, IL
site; 10-50m AT (see Section 5.2).
Sourceb
Single
35m
Stack
Single
100m
Stack
Single
200m
Stack
Ambient Concentration" via ISC2ST (ngm*)
Avg. Time Turner0 SRDT" ,v Ra5i°C ^
>XSRDT/ ATunBr'
1-hour
3 - hour
24 -hour
Period
(2842 hours)
1 - hour
3 - hour
24 -hour
Period
(2842 hours)
1-hour
3 -hour
24 -hour
Period
(2842 hours)
254.4
233.5
201.9
186.4
60. 8f
53.5
5.86
5.25
61.8
54.7
37.2
27.8
9.21
7.43
0.649
0.649
26.3
24.6
12.3
11.1
3.00
2.44
0.225
0.221
236.7
236.1
234.5
194.4
69.8
56.5
6.43
5.96
61.8
37.0
36.6
30.6
10.6
7.48
0.567
0.563
32.2
30.1
19.9
15.7
3.65
2.70
0.172
0.166
0.93
1.01
1.16
1.04
1.15
1.06
1.10
1.14
1.00
0.68
0.99
1.10
1.15
1.01
0.87
0.87
1.22
1.22
1.62
1.41
1.22
1.11
0.77
0.75
"For each averaging time, high 1st high (H1H) concentration appears above
dotted line; high 2nd high (H2H) concentration appears below dotted line.
bSee text, Section 5.0, for description of all source and receptor parameters
used in the dispersion model runs.
"Hourly mixing heights are computed based on Turner-derived stabilities.
dHourly mixing heights are computed based on SRDT-derived stabilities.
"Statistical analysis (see Section 5.2): median
B
1.08
1.12
1.05
For all 24 values:
For 12 H1H concentration ratios only:
For 12 H2H concentration ratios only:
fOne hour was missing in the computation of this concentration.
B-4
1.07 0.21 1.05 1.22
1.10 0.22 1.08 1.21
1.03 0.20 1.01 1.22
-------
Table B-4. Design concentration ratios derived from ISC2ST for Longview, WA
site; 2-10m AT (see Section 5.3).
Source1"
Single
^Sm
jjiii
Stack
GJ»-.^1x-v
oingie
100m
1 V/Vslll
Stack
Single
200m
^\j\jiii
Stack
Ambi
Avg . Time
3 - hour
Period
(8187 hours)
Period
(8187 hours)
Period
(8187 hours)
ant Concentratia
Turner"
234.4
228.8
196.8
189.6
82.7
58.5
7.24
6.14
55.9
55.8
34.3
29.6
7.01
6.11
0.613
0.575
30.2
26.3
17.8
10.6
2.56
2.39
0.195
0.187
n" via ISC2ST (/i
SRDTd
236.3
235.5
217.7
211.5
82.7
58.5
9.31
7.34
55.9
55.8
34.3
33.1
8.58
7.76
0.962
0.835
36.0
32.5
23.7
17.1
4.34
2.88
0.306
0.301
gnf5)
Ratio"
(XSRDT/XTUTOI-)
1.01
1.03
1.11
1.12
1.00
1.00
1.29
1.18
1.00
1.00
1.00
1.12
1.22
1.27
1.57
1.45
1.19
1.24
1.33
1.62
1.70
1.21
1.57
1.61
"For each averaging time, high 1st high (H1H) concentration appears above
dotted line; high 2nd high (H2H) concentration appears below dotted line.
bSee text, Section 5.0, for description of all source and receptor parameters
used in the dispersion model runs.
"Hourly mixing heights are computed based on Turner-derived stabilities.
dHourly mixing heights are computed based on SRDT-derived stabilities.
"Statistical analysis (see Section 5.3): median X
s
For all 24 values:
For 12 H1H concentration ratios only:
For 12 H2H concentration ratios only:
1.20 1.24 0.23 1.22 1.19
1.20 1.25 0.25 1.23 1.21
1.19 1.24 0.22 1.22 1.18
B-5
-------
Table B-5. Design concentration ratios derived from ISC2ST for Longview, WA
site; 10-50m AT (see Section 5.3).
Source1"
Single
35m
Stack
Single
100m
Stack
Single
200m
Stack
Ambient Concentration" via ISC2ST (ngm3)
Avg. Time Turner* SKDT* (XWxL)
1-hour
3 -hour
24 -hour
Period
(8187 hours)
1-hour
3 -hour
24 -hour
Period
(8187 hours)
1-hour
3 -hour
24 -hour
Period
(8187 hours)
234.4
228.8
196.8
189.6
82.7
58.5
7.24
6.14
55.9
55.8
34.3
29.6
7.01
6.11
0.613
0.575
30.2
26.3
17.8
10.6
2.56
2.39
0.195
0.187
236.3
235.5
217.7
211.5
82.7
58.5
9.32
7.35
55.9
55.8
34.3
33.1
8.58
7.78
0.963
0.835
36.0
32.5
23.7
17.1
4.34
2.88
0.306
0.301
1.01
1.03
1.11
1.12
1.00
1.00
1.29
1.20
1.00
1.00
1.00
1.12
1.22
1.27
1.57
1.45
1.19
1.24
1.33
1.62
1.70
1.21
1.57
1.61
"For each averaging time, high 1st high (H1H) concentration appears above
dotted line; high 2nd high (H2H) concentration appears below dotted line.
bSee text, Section 5.0, for description of all source and receptor parameters
used in the dispersion model runs.
"Hourly mixing heights are computed based on Turner-derived stabilities.
dHourly mixing heights are computed based on SRDT-derived stabilities.
'Statistical analysis (see Section 5.3): median
s
For all 24 values:
For 12 H1H concentration ratios only:
For 12 H2H concentration ratios only:
sc
1.20 1.24 0.23 1.23 1.19
1.20 1.25 0.25 1.23 1.21
1.19 1.24 0.22 1.22 1.18
B-6
-------
3, ODD
2, 5DO
2, ODD
, 500
J ODD
500
7 B
10 11 12 13 14 15 16 17 18 19 20
HOUR CDay-time on|
Figure B-la. Mean mixing height by hour of the day for Bloomington, IN site;
2-10m AT (see Table B-l).
N
3, ODD
2,50O -
2,000 —
1J5OO —
000 —
5OO —
8 9 10 11 12 13 14 15 16 17 18 19 20
HOUR CDaytime only^
Figure B-lb. Median mixing height by hour of the day for Bloomington, IN
site; 2-10m AT (see Table B-l).
B-7
-------
N
2_ 5DO
2^-400
2, 2OO
2, ODD
, BOO
J BDO
, 4DO
2DO
ODD
5 6 7 8 9 ID 11 12 13 14 15 16 17 18 19
HOUR C Day-time only]}
Figure B-2a. Mean mixing height by hour of the day for Kincaid, XL site;
2-10m AT (see Table B-2).
Kl
3, ODD
2, 500
^ ODD
J 5OO
000
500
-O
J_
J
L
5 B 7 8 9 1O 11 12 13 14 15 16 17 18 19
HOUR CDaytime only}
Figure B-2b. Median mixing height by hour of the day for Kincaid, IL site;
2-10m AT (see Table B-2).
B-8
-------
N
2, EDO
2, 4OO
2, 2OD
2, ODD
,800 —
, BOO -
1,400 —
1,200 —
000
_L
_L
_L
_L
J_
_L
J_
_L
_L
5 6 7 B 9 1O 11 12 13 14 15 16 17 18 19
HOUR CDaytime only}
Figure B-3a. Mean mixing height by hour of the day for Kincaid, IL site;
10-50m AT (see Table B-3).
-e
M
3, OOO
2, 500
2, OOO
, 500
, OOO
500 —
_L
J_
J_
_L
_L
9 1O 11 12 13 14 15 16 17 18
HOUR CDaytime onlyD
19
Figure B-3b. Median mixing height by hour of the day for Kincaid, IL site;
10-50m AT (see Table B-3).
B-9
-------
N
3J ODD
2. 500
2, ODD
, 5OO
-1 „ ODD
500
J I
SRDT Turner
\ | I I
5 B 7 B 9 1O 11 12 13 14 15 16 17 18 19
HOUR CDaytime on Iy^
Figure B-4a. Mean mixing height by hour of the day for Longview, WA site;
2-10m AT (see Table B-4).
3, OOO
2,5OO -
2,OOO —
_y 1_ 5OO
M
1,000 —
5OO — -
SRDT Turner-
e
5 B 7 8 9 1O 11 12 13
15 16 17 18 19
HOUR CDaytime only^
Figure B-4b. Median mixing height by hour of the day for Longview, WA site;
2-10m AT (see Table B-4).
B-10
-------
N
2, 6DO
^ -4DO
2, 2DD
2, OOO
, BDD
_ BOO
4DO
1 _ 2OO
J L
J L
J L
5 6 7 8 9 1O 11 12 13 14 15 16 17 18 19
HOUR CDaytime on Iy}
Figure B-5a. Mean mixing height by hour of the day for Longview, WA site;
10-50m AT (see Table B-5).
N
3, OOO
2, 500
2, OOO
j 5OO
, 000
500
J I
J I
5 6 7 8 9 1O 11 12 13 1-4 15 16 17 18 19
HOUR CDayt_ime only^i
Figure B-5b. Median mixing height by hour of the day for Longview, WA site;
10-50m AT (see Table B-5).
B-ll
-------
TECHNICAL REPORT DATA
(Please read Instructions on reverse before completing)
1. REPORT NO.
EPA-454/R-93-055
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Evaluation of a Solar Radiation/Delta-T Method for Estimating
Pasquill-Gifford (P-G) Stability Categories
5. REPORT DATE
October 1993
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
C. Thomas Coulter
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Technical Support Division
Research Triangle Park, NC 27711
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This publication documents the effort made to develop and evaluate a new methodology for
estimating stability category using on-site meteorological data that can be automatically collected and
logged, e.g., wind speed and solar radiation during daytime and temperature difference (AT) at night.
The new method (Solar Radiation/Delta-T, SRDT) uses 5 wind speed classes and 4 insolation classes
during daytime, and 3 wind speed classes and 2 AT classes during nighttime. To fulfill the objectives of
the evaluation three on-site meteorological data bases were obtained: Kincaid, IL (4/80 - 8/80),
Longview, WA (1/91 - 12/91), and Bloomington, IN (7/91 - 7/92). The data were pooled to yield
19,540 valid hours. Using the composite data, stability classification criteria were determined iteratively
for the SRDT method. Stability categories via both methods were rigorously compared for all valid
hours. Overall, stability categories coincided for 62 % of the hours examined, and were ± 1 class for
89 % of the hours. The same criteria were then applied to each of the three sites individually to assess
site to site variability. This variability was seen to be of the order of that seen within an individual site.
For two sites, the SRDT method was evaluated for AT values measured from an interval other than 2-
10m (i.e., 10-50m). The same methodology produced vurtually identical results. Finally, a dispersion
model (ISC2) was run to demonstrate the effect of the SRDT method on design concentration ratios
using meteorological data from all sites and for both AT measurement heights.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Atmospheric Dispersion Modeling
Meteorological Preprocessors
Atmospheric Stability Estimation
18. DISTRIBUTION STATEMENT
Release Unlimited
19. SECURITY CLASS (Report)
Unclassified
21. NO. OF PAGES
40
20. SECURITY CLASS (Page)
Unclassified
22. PRICE
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION IS OBSOLETE
U.S. Environme;V.:: ; '•cclion Agency
Region 5, Library •;; L-I2J)
77 West Jackson Boulevard, 12th Floor
Chicago, IL bOc.?'4"3,rOO
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