United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 2771
Research and Development
EPA-600/S3-83-023a&b June 198c
Project Summary
1978 Houston Oxidant
Modeling Study Volumes I and II
J. R Martinez, F. L Ludwig, C. Maxwell, and K. C. Nitz
During the period 15 September
through 12 October 1978, the U.S.
Environmental Protection Agency (EPA)
conducted a special program that mon-
itored air quality and meteorology in
the Houston, Texas area. The objec-
tives of the program were to obtain a
comprehensive data base suitable for
use with photochemical air-quality sim-
ulation models and to provide a de-
tailed body of data that can be used to
investigate Houston's air quality in
general and photochemical oxidants
(HOMS Study) and aerosols (HACS
Study) in particular.
The objectives of this contract were
(a) to evaluate the suitability of the
1978 Houston data base for photo-
chemical modeling application, (b) to
analyze spatial and temporal patterns
of pollutant concentrations, (c) to ar-
chive data in a manner suitable for use
with air quality simulation models, (d)
to analyze and characterize the quality
of the gaseous pollutant measurements,
and (e) to use the data to evaluate the
performance of the EKMA photochem-
ical model. Results from the data eval-
uation and archiving work are reported
in athree-volume report. (Results from
the EKMA evaluation work are reported
in a separate report)
The data evaluation effort is reported
in Volumes I and II, summarized here,
and the data quality characterization
effort is reported in Volume III.
This Project Summary was developed
by EPA's Environmental Sciences Re-
search Laboratory, Research Triangle
Park NC. to announce key findings of
the research project that is fully doc-
umented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
During the period 15 September to 12
October 1978, the U.S. Environmental
Protection Agency (EPA) conducted an
intensive program that monitored air qual-
ity and meteorology in the Houston, Texas
area. The purpose of the program was
twofold:
• To obtain a comprehensive, high
quality data base suitable for use
with photochemical air-quality sim-
ulation models.
• To provide a detailed body of aero-
metric data that can be used to
investigate Houston's air quality in
general, and photochemical oxidants
and aerosols in particular.
In keeping with these objectives, the
monitoring program consisted of two
complementary parts known respectively
as the Houston Oxidant Modeling Study
(HOMS) and the Houston Aerosol Charac-
terization Study (HACS). The data col-
lected during the program were subse-
quently forwarded to SRI, where the mea-
surements were assembled into a unified
data base and subjected to various anal-
yses, the results of which are described in
this report Included in this effort is a
comparison of the HOMS data to those
obtained in previous Houston studies, the
most comprehensive of which is the
Houston Area Oxidants Study (HAOS)
conducted in the summer of 1977. The
HAOS aerometric data have been analyzed
by SRI investigators, and some of their
results will be compared with those ob-
tained in the present investigation.
More specifically, the objectives of this
project are:
• To evaluate the suitability of the
1978 Houston data base for use
with photochemical air quality sim-
ulation models.
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• To analyze pollutant concentration
and meteorological data.
• To assemble a data base.
The results of this projectare reported in
two volumes. This volume. Volume I, de-
scribes the evaluation and analysis of the
data, and Volume II contains a guide to the
data base. A companion report will be
published later that will examine the qual-
ity of the measurements. Also, in accord-
ance with the project's scope of work, a
paper describing preliminary results was
presented at the Specialty Conference on
Ozone/Oxidants sponsored by the Air
Pollution Control Association (APCA) that
was held in Houston in October 1979. The
paper has been included in the conference
proceedings published by APCA.
Evaluation of HOMS Data Base
The HOMS monitoring program pro-
duced data that describe in detail the
spatial and temporal variations of ozone,
because the ozone monitoring network
was relatively dense. Consequently, there
is a rich ozone data base that can be used
for testing the ability of photochemical
models to predict ozone.
Photochemical models require specifi-
cation of the initial and boundary con-
centrations of hydrocarbons and NOX. In
this respect, the HOMS coverage was not
as complete as for ozone. The disposition
of the monitoring stations left gaps in the
southwestern half of the modeling region
for both hydrocarbons and NOX. This can
create problems in specifying initial con-
ditions since our analysis, which used
empirical orthogonal functions, showed
that there are small-scale variations in the
spatial patterns of hydrocarbons and NOx-
Since the small-scale changes are not well
defined in large areas of the modeling
region, the use of interpolation to estimate
initial conditions can yield poor approx-
imations of actual concentrations.
Because three-dimensional effects are
so important in the formation and trans-
port of ozone, we focused our attention on
the 1 9 days when aircraft data were avail-
able. About half were judged to have
reasonable good data for defining initial
conditions. Data were deemed suitable for
specifying boundary conditions at the up-
wind edge of the region on 13 of these
days. Regarding boundary conditions we
recommend that future programs include
aircraft flights that circumnavigate the
modeling region, since this would enhance
the model's ability to specify accurate
boundary conditions. Several of the HOMS
flight paths ranged far from the modeling
region, and consequently the data obtained
are not very useful for purposes of model-
ing conditions in the urban area itself.
Aldehydes are an important input in
modern photochemical models but they
are seldom measured. Total aldehydes
and formaldehyde were monitored during
the HOMS on a limited basis. Although the
data obtained are useful, the amount is
insufficient to permit a detailed character-
ization of temporal and spatial fluctua-
tions. Future monitoring programs should
include more extensive efforts to measure
aldehydes. Specifically, it would be pre-
ferable to measure aldehydes for an ex-
tended period at a few well-chosen loca-
tions than to monitor for short periods at
several sites, as was done in the HOMS.
The 0600-0900 (CDT) MMHC/NOX
ratio is an important descriptor of ozone-
precursor conditions. Both NMHC and
NOx were measured concurrently at only
six of the monitoring sites and the data
capture rate at four of the sites was rather
low, so the ratio could be estimated for
only 30 of the maximum possible 108
site-days at those four sites. The two
remaining sites had a combined data cap-
ture rate of 45 of a possible 54 site-days.
Thus, the analysis of patterns of NMHC/
NOX ratios in the HOMS is limited by the
relatively small amount of data. Incidental-
ly, the HAOS data are similarly afflicted,
having captured about 26 percent of the
possible NMHC/NOX ratios at five sites.
The number of monitoring sites where
meteorological parameters were measured
varied between one and five during the 19
days of aircraft operations. Such coverage
is very sparse, and may be inadequate for
modeling applications. To compensate for
the sparseness of the meteorological data,
mass-conserving interpolation methods
should be used to generate the wind field
that is an input of photochemical air-
quality models. The need for a good wind-
field generator for modeling studies is
also suggested by the case studies of
particular days. These show that wind-
shear effects can be important determi-
nants of ozone levels. Although current
models treat such effects, they can do so
only if the proper wind inputs are pro-
vided.
The HOMS coincided with a period
when the Houston area recorded some of
its highest ozone levels ever. This is for-
tunate in the sense that the air quality data
can be used to model worst-case or near-
worst-case conditions, which of course
are of interest for regulatory reasons.
Despite the high ozone levels, both PAN
and NOz concentrations were relatively
low, a result that confirms the findings
previously reported for the HAOS. Tf
HOMS analysis showed that the highe
N02 levels occurred in downtov.
Houston and in locations west and nortl
west of downtown. PAN was measured.
one rural location (Sheldon, Site 18) ar
one suburban site (Jackrabbit, Site 2C
Mean PAN values do not differ very muc
at these two sites, but the maximum PA
at Jackrabbit is more than twice that i
Sheldon.
Detailed hydrocarbon analyses wer
conducted at 10 sites located in industric
urban, suburban, and rural areas. Th
analysis of 11 selected compounds forth
period 0800-0900 showed that ethan
and propane are relatively uniformly di:
tributed. Acetylene and propylene cor
centrations have similar, nonuniform sp<
tial distributions. Isopentane and isobi
tane exhibit considerable spatial fluctu;
tions, as do toluene and the xylene;
Benzene shows a dichotomous spatu
pattern: industrial, urban, and suburba
sites have similar mean concentration
that are significantly higher than for site
on the fringes of the urban envelope. IN
butane follows a comparable pattern, e>
cept that the industrial site clusters wit
the outlying locations.
Mobile sources were found to be re
sponsible for acetylene and propylene a
several locations, including downtowi
Houston, which was expected. Howevei
evidence of stationary sources contribute!
by propylene was also found at a numbe
of sites. Benzene was found to follow th
same pattern as propylene. At all the site
examined, toluene and the xylenes ap
peared to receive substantial contribu
tions from stationary sources. The same i
true for isopentane and the two butanes
The HOMS and HAOS hydrocarbon dat
were compared and found to be general!1
consistent in the pattern of concentration
and hydrocarbon/acetylene ratios. How
ever, the benzene/toluene ratio in thi
HOMS data was different from that in thi
HAOS measurements. The indications an
that the benzene/toluene ratio in thi
HAOS may not be correct.
The 0600-0900 detailed hydrocarboi
data for the HOMS were compared witl
similar data from Denver and southen
California. Denver's hydrocarbon mixtun
was found to be similar to that in suburbai
Houston. The mixtures m downtown Hous
ton and Los Angeles were also found to bi
similar except for toluene and the xylenes
for which levels in the Los Angeles are;
were considerably higher than in Houston
In general, the southern California dafo
differ most markedly from Houston's H
the higher content of aromatics, whicl
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appear to be mainly due to stationary
sources.
The 0600-0900 n-butane/propylene
ratio is used as an indicator of the reactivity
of the hydrocarbon mixture in some air-
quality models. For this reason, we com-
pared these ratios in Houston, Denver, and
southern California. The mean and median
ratios at four of five sites in southern
California were somewhat lower than in
Houston or Denver, which suggests that
the mixture may be slightly more reactive.
In particular, the ratio in downtown Hous-
ton is higher than in downtown Los
Angeles, but is similar to the ratio at Long
Beach.
The analysis of the aldehyde data showed
that formaldehyde ranged from 1 to 28
ppb, and total aldehydes from 4 to 40 ppb.
The highest aldehyde levels were associ-
ated with elevated ozone concentrations.
However, although formaldehyde exceeded
20 ppb only when maximum daily ozone
was at least 100 ppb, the converse is not
generally true.
Examination of 0600-0900 (CDT)
NMHC/NOx ratio showed that they ranged
from 1 to 210, with six ratios having
values exceeding 100. Ninety-two per-
cent of the ratios did not exceed 85. The
median ratio for the area was 14, which is
similar to the median ratio of 12 found in
the HAOS. The distributions of the HOMS
and HAOS ratios were compared and
found to diverge substantially at the high
end. The divergence is caused by the
presence of several very high values in the
HOMS data, ratios that are high because
the NOX was very low, not because NMHC
was high.
The spatial patterns of NMHC, NOX, and
ozone were analyzed using empirical ortho-
gonal functions. The analysis showed that
the spatial distribution of ozone has a high
degree of uniformity, and is characterized
by large-scale features. By contrast, the
spatial distributions of NMHC and NOX are
less uniform and show evidence of small-
scale influences. These results imply that
the ozone-monitoring network need not
be as dense as it was, but that the number
of stations monitoring NMHC and NOX
probably should be more numerous to
capture the small-scale features. For NMHC
and NOX, these results imply that it may be
inaccurate to estimate initial conditions for
a grid-type model by interpolation or extra-
polation based on the monitoring network.
Several days were identified that may be
of interest for modeling applications. An
ozone episode occurring from 1 through 5
October 1 978 seemed particularly inter-
esting. Case studies of potential modeling
days were performed for 20 and 29 Sep-
tember and 1 and 2 October. The case
studies showed the influence of condi-
tions aloft on ground-level concentrations.
It is clear from these and other studies that
the processes affecting the formation and
transport of ozone are very three-dimen-
sional. Thus, if modeling efforts are to
succeed, it is important that the models be
provided with accurate well-resolved initial
and boundary conditions. Moreover, the
performance of the models should be
evaluated at both the ground-level stations
and at points aloft for which measure-
ments exist.
J. R. Martinez, F. L. Ludwig. C. Maxwell, and K. C. Nitzare with SRI International,
Menlo Park, CA 94025.
K. Demerjian and B. Dimitriades are the EPA Project Officers (see below).
This Project Summary covers Volumes I and II of three volumes of the complete
report, entitled "1978 Houston Oxidant Modeling Study."
"Volume I. Data Evaluation and Analysis," (Order No. PB 83-194 191; Cost:
$14.50, subject to change)
"Volume II. Data-Base Guide," (Order No. PB 83-194 209; Cost: $11.50,
subject to change)
The above reports are available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officers can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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