United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-83-042 July 1983
&EPA Project Summary
Meteorological Factors in the
Formation of Regional Haze
James G. Edinger and Timothy F. Press
The purpose of this research project
was to determine the role of mete-
orological factors in the formation of
widespread areas of haze in the eastern
United States.
Three case studies were made: A
summer haze episode, an off-season
haze episode and a non-haze episode.
Results showed that over the course
of 2 or 3 days emissions from widely
separated sources such as St Louis,
Chicago, Cincinnati and Pittsburgh are
leafed together by vertical and horizontal
shears and mixing by daytime convec-
tion to form a dilution volume many
hundreds to well overathousand km in
extent and 2 or 3 km in depth. Almost
all stations reporting haze during an
episode were confined to this dilution
volume and most of these in that part
of the plume containing emissions that
were 2 or 3 days old.
The dilution volume associated with
the off-season episode was of about
the same magnitude as that of the
summer case, but was shallower and
horizontally more extensive. Both of
these 3-day haze volumes were much
smaller than the dilution volume as-
sociated with the non-haze case which
blanketed almost the entire eastern
United States.
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
Extensive areas of hazy air covering
major portions of the eastern United States
have become a frequent summertime oc-
currence during the last few decades. This
regional haze, though well described by
satellite observations and the conventional
surface synoptic network, is not as well
understood as the smaller, meso-scale
visibility blight associated with plumes
from individual point or area sources such
as power plants and urban complexes. A
variety of models have been developed for
these meso-scale plumes. But as is
reported in a recent study by the National
Academy of Sciences, the modeling of
regional haze is not nearly as far advanced
The present study is directed toward
improving our understanding of the mete-
orological mechanisms involved in the
formation of such large volumes of more
or less uniformly polluted air. It is a
diagnostic enterprise. Its purpose is to
provide information useful for constructing
models, of the regional haze formation
process.
Air quality simulation models describ-
ing the long-range transport of air pollu-
tion have been devised. Usually they are
based on a requirement of mass balance.
The processes and mechanisms included
are: emission, chemical transformation,
physical removal, turbulent diffusion and
transport
The air motions used in these models
are observed atmospheric motions, not air
movement calculated from fundamental
physical principles. Typically they are the
observed wind averaged over the depth of
the polluted layer or the wind observed at
some standard level interior to the polluted
layer, often the 850 mb level. A certain
amount of observed detail in the wind field
is lost in the process of constructing
vertically averaged wind fields or in ac-
cepting the wind at any one level as being
representative of the vertically averaged
flow. These neglected (dispersive) motions
-------
are parameterized in the models by eddy
diffusion terms. Some air quality simulation
models have taken vertical wind shear
within the polluted layer into account for
travel times out to 24 hours. In the work
reported here the winds at all levels within
the polluted layer are used to determine
the long-range transport and dispersal of
pollution out to 3 day's travel from the
sources. It is done for a number of haze
episodes chosen from 30 years of record
(1948-1978). The purpose is to discover
the meteorological mechanisms associated
with the formation of these extensive
volumes of hazy air.
The analysis consists of constructing
the previous 3-day history of the hazy air. It
is assumed that the responsible pollutants
came from the major air pollution sources
in the eastern United States (large cities,
power plants, etc.). A 3-day dilution
volume is constructed for each major
source. These individual dilution volumes
overlap to form a conglomerate volume
which encompasses all of the air which in
3 days passed over the major sources.
Because of the overlap it may be assumed
that this large conglomerate volume would
also include air from any other source, of
whatever size, located in the area inter-
mediate in location to the major sources.
In reconstructing the extensive volume
of hazy air the major sources of emissions,
i.e. the large urban centers, are taken as
continuous point sources. Their nighttime
emissions are treated as plumes moving
with the surface winds as reported on the
3-hourly surface synoptic maps. Their
daytime emissions are assumed to mix
vertically by thermal convection through
the lowest 2 km of the atmosphere and to
move with the observed wind fields at
each level (50 mb intervals) within that
layer. The shearing motions and transla-
tion combined with the vertical mixing
produce a dilution volume associated with
the particular source. The effects of
horizontal motions on a scale smaller than
those described by the streamline analyses
are neglected.
The results of the investigation indicate
that the widespread volumes of hazy air
examined correspond closely with the
conglomerate dilution volume of the air
which during the previous three days
passed over the area that includes St
Louis, Chicago, Cincinnati and Pittsburgh
and points in between. They also show
that most of the haze is reported in air that
contains blended emissions that are 2 or 3
days old. Sources closer to the Atlantic
coast probably contribute to the haze but
because of the absence of data over the
adjacent ocean their influence could not
be determined.
Procedure
The meteorological analysis makes use
of the U.S. National Weather Service's
surface and upper air data. Horizontal
wind fields were constructed and analyzed
for the 950,900,850 and 800 mb levels.
Figure 1 is an example. Surface flow was
determined from the photostatic records
of the U.S. National Weather Service's 3-
hrly surface synoptic maps. The vertical
profiles of temperature and humidity were
generated by computer graphics.
The dilution volume developed during
the first 24 hrs for any one source is
constructed as follows. The initial instant
is taken as 00 GCT. During the subsequent
twelve hours, nighttime, a plume is laid
down at or near ground level. Its location
and configuration is determined by con-
structing a streakline for 12 GCT from the
3-hrly surface wind maps. It is assumed
that vertical mixing motions are absent
during this twelve hour interval.
During the subsequent twelve hour in-
terval, 12 to 00 GCT, daytime, mixing
motions distribute the emissions vertically
through the layer from the surface up to
the 800 mb level. It is assumed that at a
time not far removed from 12 GCT the
surface plume develops vertically into a
curtain which is subject to winds at all
levels up to 800 mb. Shears in the wind
subsequently carry the emissions at various
levels off in different directions, deforming
and tilting the curtain. Streaklines con-
structed at each level for the time 24 hours
after emission began depict the location of
this deformed curtain. Vertical mixing
motions from the surface up to 800 mb
transform the curtain into a volume, its
horizontal projection being the area over
which the emissions have been spread
during the 24 hour interval.
The volume which contains the first
day's emissions from the source in question
continues to grow during the next day due
to shearing motion in the vertical and
another 12 hours of vertical mixing, and is
transported (usually) away from the source.
To determine the shape of the dilution
volume and its location at the end of the
second day, trajectories(24 to48hr) origi-
nating at significant points around the
perimeter of the 24 hr volume at each level
are constructed. The end points of these
Figure 1. Streamline map, 00 GCT 25 February 1973 at 850 mb level.
-------
trajectories define the surface of a volume,
the horizontal projection of which is the
area covered by the dilution volume at the
end of the second day.
To construct the dilution volume that
contains not only emissions during the
first 24 hours but emissions during the
second day as well, one needs to construct
the dilution volume for air moving over the
source during the second day. This volume
when added to the other constitutes the
two day polluted wake of the source (its
48 hr dilution volume). In the case studies
carried out these procedures were repeated
for a third day's emissions so that the
dilution volume containing three day's
emissions from a given source were
obtained.
The selection of the 800 mb level as the
upper limit of the vertical stirring and
mixing followed unsuccessful efforts to
define the top of the mixing layer by
inspecting the plotted radiosonde tem-
perature and humidity profiles at all stations
in the eastern United States at both the00
and 12 GCT times. Careful analysis of the
profiles together with the associated sur-
face weather maps suggested that during
typical summer haze episodes in the eastern
United States there is no extensive daytime
stable layer in the lowest few kilometers
placing an upper limit on corrective mixing.
To the contrary reports of towering cumulus
frequently appear here and there within
the hazy volume suggesting that the mixing
in some places reaches and exceeds the
500 mb level. The upper limit to the
vertical stirring and mixing is at best ill-
defined and irregular. In all of the case
studies the top of the stirred layer was
assumed to be at the 800 mb level. In one
case, in an attempt to improve the fit
between the constructed haze volume and
the actual haze observations the level was
raised to 700 mb with some improvement
of the fit with the observations.
To select the major sources of emissions
in the northeastern United States the sta-
tistics appearing in EPA reports (1978)
were used. In preliminary case studies
nine major urban industrial areas were
chosen to represent the sources of emis-
sions. Each was treated as a point source.
Several considerations resulted in the re-
vision downward in the number of these
sources to just four: St Louis, Chicago,
Cincinnati and Pittsburgh. They were: (1)
preliminary results suggested that it was
emissions two or three days old that were
the major contributors to the haze and
some of the emissions from the cities
along the Atlantic coast moved into the
data void over the ocean in less than three
days preventing the completion of the
construction of their dilution volumes and
(2) the two and three day dilution volumes
from individual point sources overlapped
each other to such a large extent that
including sources more closely spaced
than the four chosen would have only a
small effect on the size of their combined
dilution volume.
Results
Four haze episodes were selected for
analysis from the historical record, 1948
to 1978. In addition, one non-haze episode
which was meteorologically similar to the
haze cases was examined.
Figures 2,3, and 4 show the results of
the study of an off-season (not summer)
haze episode, OOGT 1 March 1973.
Figure 2 shows the 00 GCT 1 March
1973 location of one-day-old emissions
(released during the last 24 hrs) from St
Louis, Chicago, Cincinnati and Pittsburgh.
The black dots indicate stations reporting
haze at 00 GCT I March 1973. Figure 3
gives the location at 00 GCT I March 1973
of two-day-old emissions (released between
24 and 48 hrs ago), and Figure 4 gives the
location of the corresponding three-day-
old emissions (released between 48 and
72 hrs ago).
Examination of these figures and others
not reproduced have shown that: (1) only
a small fraction of the haze reported occurs
in one-day-old emissions, less than one
half occurs in two-day-old emissions, but
almost 90% occurs in the three-day-old
emissions, and (2) the majority of the haze
reports are at locations where a superposi-
tion of emissions from different sources or
different times occurs. These results
suggest that much of the haze forms in
blended, aged emissions and at a con-
siderable distance from the sources.
The corresponding diagrams for the
non-haze case revealed one-day-old plumes
that are noticeably larger than those
generated during the haze episodes. And
the two-day-old emissions cover an area
so large that it compares favorably with the
largest of the three-day-old emissions
volume for the haze episodes. It is not
possible to determine how much larger
the three-day-old emissions volume is for
the non-haze case since its boundaries
extend beyond the map boundaries on the
north and in the southeast. It is apparent
that much larger dilution volumes are
generated in this non-haze case than during
haze episodes.
Figure 2. One-day-old emissions from St. Louis. Chicago. Cincinnati, and Pittsburgh. 00 GCT
1 March 1973.
-------
Recommendation
The analysis of the haze episodes sug-
gests that it might be instructive to devise
haze formation models that move the
emissions with the winds at all levels
within the lowest 2 km layer and that
include vertical eddy diffusion but neglect
horizontal eddy diffusion.
Figure 3. Two-day-old emissions from St. Louis, Chicago, Cincinnati, and Pittsburgh, 00 GCT
1 March 1972.
Figure 4. Three-day-old emissions from St. Louis, Chicago, Cincinnati, and Pittsburgh, 00 GCT
1 March 1973.
-------
James G. Edinger and Timothy F. Press are with the University of California, Los
Angeles, CA 90024.
George C. Holzworth is the EPA Project Officer (see below).
The complete report, entitled "Meteorological Factors in the Formation of
Regional Haze," (Order No. PB 83-209 742; Cost: $10.00, 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
-------
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Fees^aid"
Environmental
ProtectTn
Agency
EPA 335
Official Business
Penalty for Private Use $300
------- |