United States
Environmental Protection
Environmental Sciences Research
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).
  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
   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.

    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-
  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
  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

 appear  to  be mainly due to stationary
  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
  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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