xvEPA
                                United States
                                Environmental Protection
                                Agency
                                 Environmental Sciences Research  -""?,,
                                 Laboratory                   "W\
                                 Research Triangle Park NC 27711    ~/lj \
                                Research and Development
                                EPA-600/S3-82-043 Sept. 1982
Project  Summary
                                Evaluation  of  Four
                                Urban-Scale  Photochemical
                                Air  Quality Simulation  Models
                                Jack H. Shreffler and Kenneth L. Schere
                                  This research was initiated with the
                                intent of determining the accuracy of
                                four photochemical air quality
                                simulation models using data from the
                                Regional Air Pollution Study in St.
                                Louis. The models  evaluated in this
                                report are: The Photochemical Box
                                Model  (PBM), The Lagrangian
                                Photochemical  Model (LPM), The
                                Urban Airshed Model (UAM), and The
                                Livermore Regional  Air Quality Model
                                (LIRAQ). Emphasis  is directed at the
                                ability of the models to reproduce the
                                maximum 1-hour ozone concentra-
                                tions observed on  10 days selected
                                from nearly two years of data. The
                                PBM, LPM. and UAM have  been
                                tested  successfully and  show
                                potential as air quality management
                                tools. LIRAQ does not show potential
                                as  a  model  for  general  use,
                                irrespective of its  accuracy,  which
                                was impossible to judge at this time.
                                For  the three  other models, the
                                standard deviations of the differences
                                between observed ozone maxima and
                                predicted concentrations at the same
                                place and  time tend to be  large,
                                ranging 0.04 to 0.1 ppm for maxima
                                of  0.19  to  0.26 ppm.  Possible
                                resolution of this high variability might
                                improve performance of the  latter
                                three models.
                                  This Project Summary was devel-
                                oped by EPA's Environmental Sci-
                                ences Research Laboratory, Research
                                Triangle Park, NC.  to announce key
                                findings of the research project that is
                                fully documented in a separate report
                                 of the same title (see Project Report
                                 ordering information at back).

                                 Introduction
                                  The Regional Air Pollution Study
                                 (RAPS) was conducted in the St. Louis
                                 region over the period 1974 to 1977.
                                 RAPS was designed to  provide  a
                                 comprehensive  data set to test and
                                 evaluate numerical air quality
                                 simulation models for an urban area.
                                 While the RAPS field measurements
                                 were in progress, EPA surveyed the
                                 available, state-of-the-art,  photo-
                                 chemical  air  quality simulations
                                 models,  and  selected  three for
                                 evaluation.  Since  no existing model
                                 embodied  a  simple box-model
                                 approach, a box model was constructed
                                 by EPA. The four models evaluated by
                                 EPA  and  described herein are  as
                                 follows:

                                 Photochemical Box Model (PBM) -  a
                                 single-cell Eulerian model constructed
                                 by EPA.

                                 Lagrangian Photochemical Model (LPM)
                                 - a multi-level parcel model developed
                                 by  Environmental  Research  and
                                 Technology, Inc.

                                 Livermore Regional Air Quality Model -
                                 (LIRAQ) a single-level  Eulerian  grid
                                 model  developed by Lawrence
                                 Livermore Laboratory.

                                 Urban Airshed Model (UAM) - a multi-
                                 level, Eulerian grid model developed by
                                 Systems Applications, Inc.

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  The final report describes the results
of  simulations for  10  days when
maximum measured 03 concentrations
were 0.19 ppm to 0.26 ppm in St. Louis.
Generally, these days exhibit stagnation
conditions with little cloud cover and
represent situations conducive to the
production of photochemical  oxidant
from local emissions.
  The models were tested extensively,
and obvious errors or deficiencies were
corrected. However, no effort was made
to adjust or tune the model predictions
to observed concentration values. The
approach to the evaluation was to use
off-the-shelf  models,  much as they
would be used in a regulatory situation.
Even  so, great  care was taken  in
preparation of data  sets and model
executions.  Although  model
assumptions varied, the  authors were
careful to use data in similar manners in
all models. Data preparation and actual
execution  of  the  models  was
accomplished solely at EPA. The goal of
the evaluation was to provide a fair and
objective determination of the accuracy
of a set of photochemical models when
tested in an operational mode against a
comprehensive urban data base.
  An   integral  part  of  RAPS,  was
establishment of  a network  of  25
surface stations in and around St. Louis,
known as the Regional Air Monitoring
System (RAMS). The RAMS  stations
continually monitores various meteoro-
logical variables as  well as ambient
concentrations of  pollutant  gases,
providing observational data for com-
parison with model predictions.

Results and Discussion
  The Photochemical Box Model (PBM),
a   single-cell  Eulerian  air  quality
simulation  model,  simulates  the
transport and chemical transformation
of air pollutants in smog-prone urban
atmospheres.  The model's domain is set
in  a  variable  volume,  well-mixed
reacting  cell  where the physical and
chemical processes responsible for the
generation of ozone  (03)  by  its
hydrocarbon (HC) and oxides of nitrogen
(NOx) precursors  are mathematically
created. To apply the model to the St.
Louis RAPS data base, the horizontal
scale of the single cell was 20 km and
the vertical  scale  was  time-varying,
proportional to the depth  of the mixed
layer. The model domain was centered
on  downtown St.  Louis in  such  a
manner  that   the  20 x  20  km area
encompassed  most  of  the  major
emissions sources on either side of the
 Mississippi River .  Uniform distribution
 of  source  emissions  was  assumed
 across the surface  of the cell. Twelve of
 the RAMS surface monitoring stations
 were  located within  the  cell's
 boundaries.
  The following statistics summarize
the differences  between the  observed
hourly maximum concentration of 03 for
each day and the PBM. prediction at the
same  time.  The  concentrations  are
averages over the PBM domain.
      AC  = Obs. - Pred (ppm)

      AC  = -0.033

  s.d.(AC) = 0.041

   |AC|  =  0.039
  The  evaluation   of   the  PBM is
 continuing  in  EPA's  Meteorology
 Division. This analysis is the first major
 step toward a thorough understanding
 of model performance. Initial evidence
 shows that the model is a useful tool in
 assessing the  urban air quality  for
 photochemically  reactive pollutants,
 especially in stagnation conditions. The
 PBM  is relatively simple to use and its
 data requirements are far less stringent
 than  most other numerical air quality
 simulation models. The areas of further
 study that should be pursued include: (a)
 the hysteresis problem during advection
 conditions, (b) the relationship between
 the average O3  concentration observed
 within  the  model domain  and  the
 maximum O3 level  observed at a single
 station, (c) the  continued testing  and
 refinement  of  the  chemical  kinetic
 mechanism within  the PBM, and (d) the
 sensitivity of the model to variations in
 selected parameters such as initial and
 uoundary  concentrations, initial  cell
 depth, emissions, wind speed, and solar
 radiation.
   The  Lagrangian  Photochemical
 Model  (LPM)  was developed  by
 Environmental Research  and
 Technology, Inc. and adapted for use
 with  the RAPS data base. (The LPM is
 essentially identical to the general-use
 model  named  ELSTAR).  The  LPM
 considers a portion of the atmosphere
 as an identifiable  parcel which can be
 tracked from  early morning  to  late
 afternoon. As the parcel moves over the
 various emission  sources,  pollutants
 are assimilated, vertically mixed, and
 subjected to photochemical reactions in
 the presence of solar radiation.
 The following statistics summarize the
 differences   between  the  observed
hourly maximum 03 concentration and
the LPM prediction at the same time and
place.
      AC  = Obs - Pred (ppm)

      A~C = -0.004

   s.d.(AC) = 0.11

      |AC| = 0.080
  The LPM has shown promise as an
effective  tool  to  understand  03
production in an  urban region. The
model  is  relatively   easy  to  use,
inexpensive  to execute, and  seems
immune to  various execution  errors
which tend  to arise unexpectedly in
complex computations  of  this  sort.
Areas of further research should incude
the following: (a) the importance of the
initial conditions versus the emissions
accumulated  in  the  parcel,  (b)
reasonable   methods  to  include
horizontal diffusion, (c)  the vertical
oiffusivity as it relates to unrealistic
build-up of pollutants at ground level.
  The Livermore Regional Air Quality
Model  (LIRAQ)  was  developed  at
Lawrence Livermore Laboratory (LLL)
under funding of the National Science
Foundation. The model is an Eulerian
grid  type  and  was  specifically
constructed for use in the San Francisco
Bay area,  where the Bay Area Quality
Management District  (BAAQMD)
retains  an  interest  in  LIRAQ
applications.  Reflecting the  originally
intended region of study, the model has
provision  for  generating  mass-
consistent  wind  fields  in  complex
terrain  but allows  only one grid cell
vertically. In 1975, EPA entered into an
Interagency Agreement (IAG) with LLL
to have  LIRAQ adapted for use with the
RAPS data base in  St.  Louis. Because
LIRAQ uses special features of the CDC
7600 computing system, adapting it to
run on  the EPA's UNIVAC  1110 was
deemed impractical. Therefore, LIRAQ
was  transferred   to  the  Lawrence
Berkely  Laboratory (LBL) computing
facility where EPA  could have  direct
access to the model.
  The  effort  to   evaluate LIRAQ
performa nee was beset with difficulties,
and EPA is considering eliminating the
model from  its evaluation  program.
With  limited testing, LIRAQ did not
produce significant  concentrations of
ozone,  suggesting  errors in the
emission  and/or chemistry module.
Presently, three days (195,  226, and   |
275 of 1976) have been tested on the
LBL  system  by EPA.  The  Day 275

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 simulation produced an error in LIRAQ
 execution of unknown origin. On Days
 195 and 226, little ozone was generated
 by the model,  and  nitrogen  oxides
 seemed  to totally  dominate  the
 photochemical system. Even if current
 problems  are resolved, there are cost-
 benefit considerations centered on the
 adequacy  of the  model's theoretical
 framework  (single vertical cell), cost,
 and inaccessibility for general use. The
 LIRAQ conversion was initiated with the
 intent of having a complete spectrum of
 state-of-art models in existence in 1975
 for testing by EPA. Reconsiderations of
 its eventual utility have  made  LIRAQ
 less than attractive for further research.
 Since  LIRAQ  is  tied to a specific
 computer  system, its prospects for use
 in  a  general regulatory  setting - the
 ultimate goal of the model evaluation
 program - are extremely dim.
   The Urban Airshed Model (UAM) is a
 three-dimensional (3-D)   grid-type  or
 Eulerian,   photochemical  air  quality
 simulation  model   developed  by
 Systems Applications, Inc. (SAI) of San
 Rafael,  California.  The structure of the
 model consists of a lattice array of cells,
 arranged  so that  the total  volume
 represents an  urban  domain and  in
 which  the  physical  and  chemical
 processes  responsible for photochem-
 ical  smog are  mathematically
 simulated. The horizontal dimensions of
 each cell are constant but the heights of
 the  cells   vary  throughout  a  model
 simulation according to changes in the
 depth of the mixed layer. To apply the
 model to the St. Louis RAPS data base,
 the area modeled was 60 km wide and
 80 km long. Each individual cell was 4
 km on a horizontal side. Vertically, there
 were four  layers of cells  in total; the
 bottom two layers simulated the mixed
 layer and  the top two represented the
 region  immediately above the  mixed
 layer. The domain of the UAM was
 centered just west of downtown St.
 Louis  and  included  the  entire
 metropolitan area.
  The  following statistics summarize
the differences  between the observed
 hourly maximum 03 concentration and
the UAM  prediction at the same time
and place.

      AC = Obs - Pred (ppm)

      AC  = -0.074

  s.d.(AC) = 0.033

     | AC)  = 0.074
  The   evaluation  of  the  UAM  is
continuing at the Meteorology Division
of EPA, and additional test days may be
chosen to  add to those already done.
From the work performed to this point, it
is clear that the potential use of a grid-
type model such as the UAM is great,
although the complexity of the model
often makes solving the problems which
arise more difficult. Areas of further
study that should be pursued include: (a)
the  adequacy  of  the  wind-field
methodology, and (b) the sensitivity of
the  model to  variations  in  selected
parameters such as initial and boundary
concentrations, emissions, wind fields,
and solar radiation.


Conclusions

  The  evaluations  of  urban  photo-
chemical air quality  simulation models
presented in this report represent the
first  comprehensive effort to examine
model  performance  under  controlled
conditions. Great care  was taken in
establishing the same data  base  and
impartial running procedures for all
models.  The following  points briefly
summarize the  general conclusions
emerging from the study.


  1.  The PBM model performs the best
     in near-stagnation conditions.

  2.  The LPM fixed-box formulation for
     the air parcel needs modification.
     Unrealistically high ozone predic-
     tions result under certain circum-
     stances.
  3.   LIRAQ lacks potential as a model
      for general-use for a variety of
      reasons and might be phased out
      of the  evaluation program.  No
      decision was made on  use by
      specialized  groups for specific
      locales.
  4.   The  UAM tends to consistently
      underpredict ozone, the  effect
      possibly results  from  spurious
      numerical diffusion.
  5.   For all  models,  the user should
      have  a  strong  scientific back-
      ground and be extremely careful
      in  implementing  air  quality
      simulations.

  6.   For all models, there is substantial
      variabilty between specific 1 -hour
      predicted and observed concen-
      trations at a particular location.
      Decision  makers  in regulatory
      agencies should be cognizant of
      this variability.
  Using the results described in this
report and the possible model problems
they elucidate, a final investigation of
the structure of each  model should be
carried out. If any further problems are
identified, with workable solutions, they
should be corrected.  Model  modifica-
tions  should  be  based  on  sound
scientific judgements and not solely on
a  desire to  improve the results. A
decision should be made on whether to
pursue further development and testing
in the special case of LIRAQ.  When
model changes are final, another set of
runs should be made and analyzed. Up
to 10 additional days from the RAPS
could be included in the final  analyses.
  TheEPA authorsJackH. Shreftier (also the EPA Project Officer, see below) and
    Kenneth L. S chore are with the Environmental Sciences Research Laboratory,
    Research Triangle Park. NC 27711.
  The complete report, entitled "Evaluation of Four Urban-Scale Photochemical
    Air Quality Simulation Models." (Order No.  PB 82-239 278; Cost: $16.50,
    subject to change) will be available only from:
          National Technical Information Service
          5285 Port Royal Road
          Springfield, VA 22161
          Telephone: 703-487-4650
  The EPA Project Officer can be contacted at:
          Environmental Sciences Research Laboratory
          U. S. Environmental Protection Agency
          Research Triangle Park, NC 27711
                                                                                       t, US GOVERNMENT PRINTING OFFICE: 1M2 -559-017/0806

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