United States
Environmental Protection
Agency
Atmospheric Research and
Exposure Assessment Laboratory
Research Triangle Park NC 27711
\\
Research and Development
EPA/600/S3-89/062 Sept. 1989
&EPA Project Summary
Trends of Seasonal
Haziness and Sulfur
Emissions Over the Eastern U.S.
Rudolf B. Husar
The seasonal sulfur emission
trends for individual regions are
compared to the measured trends in
atmospheric haziness. The monthly
sulfur emissions for individual states
going back to the 1800's were recon-
structed using yearly emission trends
contributed by earlier studies. The
seasonal emission rates prior to 1975
were estimated from seasonal fuel
consumption; data monthly sulfur
emissions are available from 1975-
1984. Our results indicate that, before
1970, emissions were greater in
winter than in summer, but since
1970 the summer emissions have
become comparable in magnitude. In
the Southeast there was a clear
crossover from a winter peak before
1970 to a summer peak after 1970.
The patterns of sulfur emissions and
haziness in the Northeast generally
correspond with each other —
showing a decline in the winter
season and an increase during the
summer — although the haziness
generally shows more random fluctu-
ation. In the southeastern states
there is also a close correspondence
between emissions and haze pat-
terns, particularly in the summer
season. The correspondence be-
tween sulfur emissions and extinc-
tion coefficient, when these are
disaggregated on a regional and sea-
sonal basis, suggests that this rela-
tionship can be used as a first
indication of how visibility might
change with changes in emissions of
sulfur dioxide.
This Project Summary was devel-
oped by EPA's Atmospheric Research
and Exposure Assessment Laboratory,
ftesearcft Triangle Park, NC, to an-
nounce 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 reduction of visual range is caused
primarily by particles between 0.1 and 1
micrometers in size. From the point of
view of visibility, the most significant
chemical species of submicron particles
are sulfates. Therefore, it is expected that
the spatial distribution and temporal trend
of man-made haziness will, to some
extent, correspond to the spatial-temporal
patterns of sulfur emissions.
This report examines the sulfur
emission-haze relationship using sea-
sonally disaggregated SO2 emission and
visual range data. This study was
motivated by observations of the regional
and seasonal dependence of haziness.
Across most of the northeastern states,
for example, the wintertime haziness gen-
erally has been increasing in all areas of
the eastern U. S The question examined
in this study is whether the differences in
seasonal haziness trends can be attrib-
uted to seasonal shifting of the emission
trends.
Sulfur Emission Trends
The emissions of sulfur oxides exhibit a
strongly seasonal pattern over most of
the country. The seasonal trend is driven
largely by climatic variables such as
temperature. In the winter season, the low
temperatures in some areas cause higher
emissions due to heating in the
residential and commercial sector. Simi-
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larly, in the summertime, cooling require-
ments cause high electricity consumption
—and therefore high emissions—in many
areas. Because both heating and cooling
demands depend strongly on geography
the resulting seasonal amplitude varies
from state to state. In the eastern part of
the country, the northern states have a
winter peak, while the southern states
exhibit a peak during the summer.
For purposes of summary and compar-
ison with the haze data, the seasonal
emission trends have been aggregated
over two regions: the Northeast (Illinois,
Indiana, Ohio, Pennsylvania, and New
York), and the Southeast (Tennessee,
North Carolina, South Carolina, Georgia,
Alabama, and Louisiana). In the North-
east, winter (January) emissions were
about 50% higher than summer (July)
emissions over most of the century but
by the 1970's the two seasons' emissions
became comparable in magnitude. In the
Southeast, there was a clear shift from a
strong winter peak to a summer peak; the
crossover occurred in the late 1970's.
Regional Haze Trends
In this section, visibility data are
analyzed to determine regional haze
trends over the eastern U.S. Haziness is
the inverse of the visual range. The visual
range is the maximum distance at which
an observer can discern the outline of a
black object against the horizon sky.
Values of ground-level visual range are
recorded every hour at several hundred
National Weather Service meteorological
stations within the U.S. The visibility
trend database consists of 137 stations
for which computerized data are available
for the years since 1948. The quantitative
measure of haziness is the extinction
coefficient, bext which is calculated from
the visual range using the Koschmieder
relationship bext = 24/(visual range in
miles). The constant 24 results from the
assumption that the eye can just detect a
2% contrast difference between a black
object and the horizon sky.
Within geographic regions, individual
stations show coherent trends. However,
the patterns of the trends differ signifi-
cantly between the Northeast and the
Southeast. In the Northeast, the January
haziness shows about a 40% decline
between 1948 and 1983, while in the
Southeast there was about a 10%
increase in the January haze. The July
haziness in the industrialized north-
eastern states shows a general increase
of about 40% from the 1950's to the
1960's, with evidence of a decline after
1978. In the southeastern states, on the
other hand, summer haziness increased
by more than a factor of two, mainly
between the 1950's and the 1960's.
Comparison of Trends
The seasonal sulfur emission trends for
individual regions are compared to the
measured trends in atmospheric haziness
in Figure 1. The patterns of sulfur
emission and haziness in the Northeast
show a rough correspondence: a decline
in the winter and an increase during the
summer; the haziness generally shows
more random fluctuation., In the south-
eastern states, there is also a close
correspondence between emission and
haze patterns, particularly in the summer
Conclusions
The results of this study show an
interesting relationship between sulfur
emission trends and trends in haziness
when examined by region and season,
although such qualitative comparisons do
not provide conclusive evidence of a
cause-effect relationship. Also, the pat-
terns of haze and sulfur emissions for the
Northeast and Southeast tend to deviate
from each other at times. The causes of
such deviations may include variabilities
due to meteorology as well as potential
errors in both emission and haze data.
Such deviations would probably be
reduced if three month averages were
compared rather than single months.
Finally, a one-to-one relationship cannot
be expected, since the haziness in one
region may be influenced by emissions in
neighboring regions. If the emission
trends differ between each region,
atmospheric processes would tend to
average out the regional haze trend. A
more detailed emission-haze trend anal-
ysis could be conducted using a regional
haze model that incorporates both the
changes in the emissions as well as the
meteorological data for individual years.
Both emission and wind data sets are
available for such retrospective model
studies.
The relative significance of chemical
species and source types that influence
visibility, other than sulfur, were not
examined in this analysis. Other com-
pounds may have emission and trend
patterns similar to sulfur's. Future studies
could examine the emission trends of
other potential visibility-reducing species,
such as organics, flyash, and soot, in
order to estimate their contributions.
From other information it is known that
in the eastern U.S. at least in the last
decade sulfate, formed in the atmosphere
from sulfur dioxide, and the ammonium
and condensed water associated with the
sulfate, dominate the light-scattering
particulate matter in the ambient air. The
remarkable correspondence between sul-
fur emissions and extinction coefficient,
when these are disaggregated on a
regional and seasonal basis, suggests
that this relationship can be used as a
first indication of how visibility might
change with changes in emissions ol
sulfur dioxide.
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Northeast U.S
Winter (January)
Southeast U S.
Winter (January)
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Summer (July)
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Figure 1. Comparison of trends of sulfur emissions and 75th percentile extinction coefficient for northeast and southeast U.S. for January
and July. Q Emissions + extinction coefficient.
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Rudolf B. Husar is with Washington University, St. Louis, MO 63130.
William E. Wilson is the EPA Project Officer (see below).
The complete report, entitled "Trends of Seasonal Haziness and Sulfur Emissions
over the Eastern U.S.," (Order No. PB 89-220 51 I/AS; Cost: $13.95, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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