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                                 United States
                                 Environmental Protection
                                 Agency
                                  Environmental Sciences Research ^*
                                  Laboratory                   '.
                                  Research Triangle Park NC 27711
                                 Research and Development
                                  EPA-600/S3-82-078  Oct. 1982
Project Summary
                                 Estimating  Concentrations
                                 Downwind  from an
                                 Instantaneous  Puff  Release
                                 William B. Petersen
                                   This document provides an  ap-
                                 proach to estimating concentrations
                                 downwind of an  instantaneous puff
                                 release.  Dispersion  of the puff is
                                 described by the Gaussian puff equa-
                                 tion, using the dispersion parameters
                                 presented in Meteorology and Atomic
                                 Energy. The  primary purpose of  this
                                 work is to provide estimates of the
                                 instantaneous peak concentration or
                                 average  concentration through the
                                 use of simple equations and nomo-
                                 grams. Example  problems  are pro-
                                 vided to  demonstrate the use of the
                                 equations and nomograms. A com-
                                 puterized Gaussian puff model is also
                                 presented.
                                   This Project Summary was devel-
                                 oped by EPA's Environmental Sciences
                                 Research Laboratory.  Research  Tri-
                                 angle 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
                                   This document  is provided m re-
                                 sponse to the need for making disper-
                                 sion estimates of accidental releases of
                                 hazardous chemicals. Every year there
                                 are news stories of industnal-or trans-
                                 portation-related spills requiring the
                                 evacuation of nearby residents. The
                                 main objective of this work is to provide
                                 the air pollution  specialist  a meth-
                                 odology for estimating peak concen-
                                 trations downwind  of an instantaneous
                                 release.
                                   The equations,  nomograms,  and
                                  methodology described in  this docu-
                                  ment were not designed for the fireman
                                  at the site of an accidental spill. He will
                                  not have the time or the technical back-
                                  ground to use  the equations  and
                                  nomograms properly. The  factors af-
                                  fecting his decision to order an evacua-
                                  tion will be based primarily on the size of
                                  the spill, and the nature and toxicity of
                                  the substance. However, material provided
                                  in this document would be  valuable to
                                  the air pollution specialist who is
                                  assisting the fire department in estab-
                                  lishing procedures to determine toxic
                                  corridors. The dispersion  algorithms
                                  described would provide the basis for a
                                  model catering to the specific needs of
                                  the user  A computer  algorithm de-
                                  scribed in this document could also be
                                  adapted for on-site use at  a chemical
                                  plant using real time meteorology to
                                  predict downwind concentrations.

                                  Approach
                                   The document is composed of three
                                  major sections. The first part of Section
                                  2 describes a technique for estimating
                                  peak  concentrations of instantaneous
                                  puff releases as a function of atmos-
                                  pheric stability and downwind distance.
                                  This discussion was designed so  that
                                  within minutes after a spill, estimates of
                                  peak concentration could be provided to
                                  the agencies directly concerned with
                                  the immediate health and welfare of the
                                  public. These initial estimates could be
                                  provided without the aid of a computer.
                                  The remainder of Section 2 is concerned
                                                                     * US GOVERNMENT PRINTING OFFICE 1982-559-017/0856

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  with estimating average concentrations
  and exposure. Section 3 provides a
  discussion of  a generalized Gaussian
  puff equation  with the same assump-
  tions and limitations  of the approach
  described in Section 2. The equations in
  Section 3 are  the basis for a computer
  model described in Section 4.
    The modeling effort is significantly
  simplified if only the magnitudes of the
  concentrations must be  predicted,
  without  regard to exactly  where the
  concentrations will occur. If concentra-
  tion estimates  are to be made at specific
  points, it is important to account for puff
  trajectory; and,  because methods for
  determining the trajectory of the puff
  contain  deficiencies,  estimating con-
  centrations  at  point locations is  very
  difficult.  Releases  are seldom point
  sources, but are more typically small
  area  sources.  Small  area  source re-
  leases can be  modeled to some extent
  by using initial dispersion parameters.
  The initial  horizontal dispersion is
  typically calculated by dividing the initial
  horizontal dimension of the area source
  by 4.3. This method will give reasonable
  concentration  estimates at downwind
  distances greater than about five times
  the horizontal  dimension of the source.
  As with the lateral dispersion, the initial
  vertical  dispersion  parameter is  typi-
  cally  evaluated by  dividing  the initial
  height of the source by 2.15.
    Many sources are  not truly instan-
  taneous. However, if the travel time is
  long compared to the release time, then
  the puff model can be used to estimate
  concentrations. If the travel time to a
  receptor is on the same order or smaller
  than  the release time, the Gaussian
  plume model  can be  used.  Where the
  release time is finite  but smaller than
       the travel time, the instantaneous puff
       model will provide a worst-case esti-
       mate of peak concentrations for  the
       same total release. It is difficult to
       evaluate the effect on average concen-
       trations over a given sampling time for a
       finite release using simple  models,
       since the  puff  is  distorted  in  the
       downwind direction.
       Recommendations
        The model estimates are appropriate
       for travel times where the atmosphere
       can be assumed to be steady state. A
       number of factors affect the length of
       time  the atmosphere  will remain
       approximately  steady: season  of the
       year, latitude, synoptic conditions, and
       local meteorology  such as sea  breeze
       and terrain flows.  During  periods  of
transition  (such as sunrise or sunset),
the stability, mixing height, wind profile,
wind  speed,  and  wind direction are
likely  to change;  therefore, particular
care should be exercised in character-
izing the atmosphere during these time
periods. For situations where the travel
time of the puff is greater than an hour,
the factors which affect the dispersion
and trajectory of the  puff should be
incorporated in the  concentratioh
estimates.
  The  Gaussian  instantaneous puff
model is applicable for neutrally buoy-
ant nonreactive releases. The dis-
persion of a  small negatively buoyant
release can be modeled using the puff
equation,  provided the material mixes
rapidly with the ambient  air  and
disperses like a  neutrally buoyant
source.
         The  EPA author William B. Peterson is with the Environmental Sciences
           Research Laboratory, Research Triangle Park, NC 27711.
         The complete report, entitled "Estimating Concentrations Downwind from an
           Instantaneous Puff Release," (Order No. PB  82-261  959; Cost:  $9.00,
           subject to change) will be available only frorh:
                 National Technical Information Service
                 5285 Port Royal Road
                 Springfield, VA 22161
                 Telephone: 703-487-4650
         The EPA Project Officer cay? be contacted at:
                 Environmental fdfences Research Laboratory
                 U. S'Environmental Protection Agency
                 Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
                 Postage and
                 Fees Paid
                 Environmental
                 Protection
                 Agency
                 EPA 335
Official Business
Penalty for Private Use $300
           PS    0000329

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