&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
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
                     PS    0000329

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