&EFA
United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-82-076 Sept. 1982
Project Summary
Atmospheric Turbidity
Over the United States
from 1967 to 1976
Elmer Robinson and Ralph J. Valente
The purpose of this study was to
analyze the observational data from
the U.S. Environmental Protection
Agency-National Oceanic and Atmos-
pheric Administration turbidity net-
work in the United States for the
1967-1976 decade. The research also
compared patterns and trends of
background turbidity with a previous
report, which covered the six-year
period 1961 to 1966.
The results of the turbidity clima-
tological analysis for the 1967 to
1976 time period assessed the geo-
graphical, seasonal, and temporal
variations in mean background (i.e.,
nonurban) turbidity. Maximum annual
average background turbidity occurs
over the Southeast and the Smoky
Mountain region and minimum annual
average background turbidity occurs
over the Rocky Mountains and the
interior Southwest. This geographical
variation occurred in all four seasons.
The annual turbidity cycle was also
analyzed; maximum seasonal average
turbidity occurred in the summer and
minimum seasonal average turbidity
occurred in the winter in all regions of
the United States. The amplitude of
this seasonal change was greatest
over the Southeast and smallest over
the Rocky Mountain region.
Results of trend analyses indicated
increases in turbidity during the 1967
to 1976 decade, especially in the
summer season, in the Southeast and
the Smoky Mountain region. Increasing
urbanization and industrialization in
the South are suggested as possible
causes for this trend. No increases in
background turbidity could be docu-
mented in the western states.
To develop a simple, regionally
stratified model relating turbidity to
urbanization, climatological average
turbidity was treated as the sum of
two terms, background and excess
average turbidity. The relationship
between urban population and excess
average turbidity was shown to be
linear and well correlated (r = 0.76).
The application of this relationship to
predict increases in annual average
turbidity based on population growth
projections for urban areas was also
described.
The technique of separating long-
term average values into background
and local effects may be useful in
0redictive investigations of other
properties of the atmosphere which
are influenced by man's activity.
Regional case studies of turbidity
during air pollution episodes produced
only marginal results. These poor
results are attributed to the fragmented
nature of the turbidity record at most
stations. Some air mass transport
could be hypothesized from the
turbidity data as long as visibility
records were available for guidance.
The results were in contrast to some
single station records that have been
published showing day-to-day turbidity
changes with weather patterns at a
given location.
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This Project Summary was developed
by EPA's Environmental Sciences
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
Air pollutants, when dispersed in the
atmosphere, can change the optical
properties of the atmosphere. These
changes may be due to increased
absorption and scattering effects result-
ing from both pollutant gases and
particles. Common atmospheric optical
properties are the visibility and the
turbidity most commonly related to a
change in solar intensity and thus to a
more or less vertical sight path. By
definition, turbidity in the meteorologi-
cal context is "any condition of the
atmosphere which reduces its trans-
parency to radiation, especially to visible
radiation." In a quantitative manner, a
turbidity coefficient can be calculated
from solar intensity measurements.
On hazy days, reductions in total
irradiance at the surface can be of
considerable magnitude. Some investi-
gators have demonstrated a 20%
reduction in total spectral irradiance at
the surface (direct + diffuse) on a hazy
day as compared to a clear day in Texas.
Episodes of high turbidity in the eastern
United States have produced conditions
in which as much as 85% of the
incoming solar radiation appeared as
diffuse skylight. In a "clean" atmosphere,
approximately 10% to 15% of the
incoming radiation appears as skylight.
The data for the present study were
obtained from the turbidity observation
network set up in the United States in
1960 to 1961 and operated since then
as a cooperative U.S. Environmental
Protection Agency-National Oceanic
and Atmospheric Administration (EPA/
NOAA) program. Since 1971, the United
States turbidity network has operated
as part of a global program guided by the
World Meteorological Organization
(WMO). The basic instrument used in
this turbidity program is the Volz
sunphotometer.
The results of the initial six years of
operation of the United States network,
i.e., 1961 to 1966, were described in an
earlier report The current study contin-
ues the analysis of turbidity data and
covers the period 1967 to 1976. Since
1966, the size Of the network has
ranged from 25 to 40 stations. A
tabulation of yearly and seasonal
average turbidity data for all network
stations is given in an appendix to the
project report.
Research Techniques and
Results
The data for this study was developed
from magnetic tape and hard copy
versions of the raw data. The nature of
the turbidity measurement and the
seasonal cycle in turbidity present
obstacles to climatological averaging.
Since the measurement can be made
only when an unobstructed line of sight
to the sun (i.e., no clouds blocking the
direct beam) exists, the number of
observations varies from station-to-
station and from season-to-season as
well as with prevailing synoptic weather.
Data analysis techniques were developed
to minimize the potential for bias caused
by observing or sampling problems.
Annual Average Background
Turbidity
This study of turbidity began with an
examination of the average background
data from rural and other nonurban
stations. Figure 1 shows the annual
average background turbidity over the
United States for 1967 to 1976. Individ-
ual values range from about 0.04 to
0.17. Values for most cities with
populations of 100,000 or more are
significantly higher than these back-
ground levels. The effect of large cities
on turbidity was examined with the
modeling study.
The annual average turbidity pattern
of Figure 1 shows significant differences
between eastern and western regions.
The highest annual average turbidities
occur in the southern Appalachian and
Smoky Mountain regions and thus are
similar to the results reported for the
1961 to 1966 period. In the West, the
minimum average turbidity values
occur in the interior basin region generally
defined on the east by the crest of the
Rockies and on the west by the High
Sierra and Cascade ranges.
Background turbidity in summertime,
the season with maximum turbidity
values, is shown by Figure 2. In the
western states and the northern Great
Plains, little apparent change occurs
from spring to summer. However, in the
Southeast, turbidity is approximately
two times the spring values, increasing
from a spring maximum of 0.16 in the
Appalachians to a turbidity coefficient of
0.30 in the summer. More detailed
analyses show that this seasonal cycle
with a summer maximum is observed
generally at all network sites, although
the amplitude of the cycle varies from
region-to-region. Important factors that
can contribute to the summer maximum
in turbidity are slower moving stagnant
air masses, greater solar radiation
leading to increased production of
natural and anthropogenic photochemi-
cal aerosols, and higher relative humid-
.70-0.
.06
.06
Figure 1. Annual average background turbidity. 1967 to 1976.
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ities associated with maritime tropical
air masses.
Turbidity Time Trends During
the 1967 to 1976 Period
Details of regional differences were
determined through an examination of
seasonal time trends over the 1967 to
1976 decade at specific sites across the
country. The plot for Oak Ridge, Ten-
nessee, (Figure 3), is typical for sites in
the Smoky Mountain and southeastern
region of the United States. Slight
increases in annual average turbidity
over the period (shown at the bottom of
the figure) are due mainly to strong
increases in the summer average
values (shown by the seasonal trends in
the upper portion). Interestingly, time
trends in the winter, spring, and fall
average values at Oak Ridge are not
pronounced. The strong summer turbidity
trend seems to be relatively limited in
geographical extent to an area generally
east of the Mississippi River.
Background turbidity trends in the
upper Midwest are illustrated in Figure
4, which shows the annual and seasonal
trends for Green Bay, Wisconsin.
Average annual turbidity is about 0.1
compared to about 0.15 at Oak Ridge,
and no consistent trend at Green Bay
exists over the 10-year period. Green
Bay shows, in general, the greatest
average turbidity during the summer,
but no trend over the 1967 to 1976
decade is discernible in either the
summer data or in any of the other
.10
seasons. The differences between
winter and summer turbidity are also
relatively small compared to the stations
in the Southeast.
Model Relating Turbidity to
Urbanization
Long-term average atmospheric tur-
bidity at an urban location can be
considered as the sum of two effects.
The first, background turbidity, is
influenced by regional differences. The
second, the local contribution to turbidity,
is influenced by the extent of the
urbanization and industrialization of the
city where the measurement is made
and the influence of any nearby major
urban areas. Combining these two
factors gives an expression for the long-
term average turbidity at a given
location and could be applied to predict
long-term changes in atmospheric
turbidity based on growth projections
for urbanized areas. One measure of the
relative urbanization of a given city that
is frequently projected for a variety of
purposes is its population. The source of
population data for our study was the
United States Census for 1970.
To examine the relationship between
population and turbidity, excess turbidity
was calculated for the urban network
sites by subtracting the background
levels (obtained by interpolating on
Figure 1) from the observed 1967 to
1976 average turbidity. The results of
the turbidity-population correlation are
presented in Figure 5. This figure shows
Figure 2. Summer average background turbidity, 1967 to 1976.
the correlation between 1970 popula-
tion, P, and average 1967 to 1976
turbidity in excess of background levels,
Be, for 55 available turbidity sites.
Linear and polynominal fits were
performed; however, three-constant
polynominal fits did not give a signifi-
cantly better correlation than the linear
least squares fit. The correlation coeffi-
cient is 0.76. The line appears curved in
Figure 5 because of the semi-log plot.
As might be expected, at populations
below 100,000 only a slight excess
turbidity occurs above background
levels. For cities west of the Mississippi
with populations of about 2,000,000,
the total urban turbidity is roughly
double the background levels. Turbidity
doubling occurs with populations of
about 5,000,000 for cities east of the
Mississippi. This difference results from
the geographical differences in back-
ground levels.
Regional Studies
In an attempt to assess regional
turbidity during conditions of relatively
high air pollution, time periods were
sought that had both a high proportion
of turbidity network observations for
several days at a time and a high air
pollution potential weather pattern —
namely a slow moving anti-cyclone over
the eastern part of the United States.
With such data it was hoped that the
movement of the pollutant air mass
could be tracked across the turbidity
station network. No time period was
found with an acceptable combination
of observational data and synoptic
weather. The usual problem was a very
sporadic turbidity observational record
with large breaks in all the station
records. This lack of data continuity
could be caused by cloud interference or
by the problems of a low priority
observation.
As a substitute, an investigation was
made of one anticyclonic haze incident
that had been studied in some detail by
others in the period of June 25 to July 5,
1975 when a large air mass formed and
moved slowly across the Ohio Valley,
recirculated over the area a second
time, then moved across the south-
eastern states and over the Atlantic
Ocean.
During that air mass haze episode, a
number of network turbidity stations
were within the affected area. The
stations were grouped into four general
geographic groups — Appalachians,
upper Midwest, Ohio Valley, and East
Coast — and daily averages for the
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0.4-*
0.3-
I
I 0.2-
0.7-
0.0-
Summer
Fall
Spring
Winter
67 68 69
0.5-1
0.1
70 71 72 73 74 75
year
76
67 68 69 70 71 72 73 74 75 76
Year
Figure 3. Seasonal and annual trends in mean turbidity at Oak Ridge, Tennessee.
geographic groups were studied. The
very sporadic nature of the data set was
a major problem; unfortunately this sort
of broken data record is very character-
istic of the turbidity record.
The upper Midwest, with turbidity
measurements at four stations, showed
some impact of the air mass haze cloud
between June 30 and July 3. During
this time period, easterly and southerly
flow recirculated the air mass through
the upper Midwest.
Because the Ohio Valley turbidity
data were not available until June 29,
the initial days of the haze episode are
not documented. The highest average
concentrations for the two Ohio Valley
stations occurred on June 29, the day
the air mass began its recirculation
motion. The East Coast group of stations
was, for the most part, on the edge of the
haze air mass until the final trajectory
southward toward the coast on July 3
and 4.
The Appalachian or southeast section
contained a group of seven stations
from North Carolina and Tennessee to
Tallahassee, Florida. In general, this
region had the highest turbidity values
during the latter part of the period,
between July 3 and 5. These observa-
tions are in general agreement with the
southward trajectory of the air mass as
noted by weather observations of visi ble
haze.
In the analysis of this summer 1975
episode and other episodes, attempts
were made to draw meaningful isoline
plots of turbidity with little satisfaction
because of the scattered nature of the
record.
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0.3-,
0.2
s
0.0
57 68 69 70
71 72
Year
73 74 75 76
0.2 n
§
0.0
67 68 69 70, 71 72 73 74 75 76
Year
Figure 4. Seasonal and annual trends in mean turbidity at Green Bay, Wisconsin.
.18-
.16-
.14-
.12-
.10-
.08-
06.
.04-
.02-
.00-
Least squares best fit line
fl, = 3.976 x 10-* P + 0.009584; r = 0.76
I
I
T
/ o
J SD_
o° o
U
o
o o
103 10* 10*
Population (1970)
Figure 5. Excess turbidity versus population at network sites.
rrp
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Elmer Robinson and Ralph J. Valente are with Washington State University,
Pullman, WA 99164.
Herbert Vielfrock is the EPA Project Officer (see below).
The complete report, entitled "Atmospheric Turbidity Over the United States,
from 1967 to 1976." (Order No. PB 82-239 369; Cost: $12.00, 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
OUSGPO: 1982 — 559-092/0508
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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Fees Paid
Environmental
Protection
Agency
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Official Business
Penalty for Private Use $300
PS 0000329
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