United States
Environ
Agency
itection
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-86/071 Apr. 1987
Project Summary
Haze Climate of the United States
Rudolf B. Husar and D. E. Patterson
The historical data base of visual range
at 137 surface synoptic meteorological
stations is examined. The original ob-
servations of visual range each noon
are converted to light extinction coef-
ficient. BEXT= 24 / VISIBILITY, a mea-
sure of haziness. The b.xt is summarized
for each calendar quarter from 1948 to
1983 as percentiles of the distribution
function, namely the 25th, 50th
(median), 75th and 90th percentile.
Detailed examination of station by
station behaviour indicate that the
median is often obscured by an upper
threshold of visual range reported; thus,
75th percentile results are used to
illustrate the secular trend of haziness
at each site. The results consist of trend
graphs, as well as seasonal contour
maps for each decade of the time
period. The most pronounced changes
over the 35 year period have been in
the summer season over the Eastern
U.S. The past decade has seen strong
increases in haziness in the southeastern
U.S. and, more recently, in the deep
South.
This Project Summary was developed
by EPA's Atmospheric Sciences Re-
search Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that Is fully docu-
mented In a separate report of the same
title (see Project Report ordering In-
formation at back).
Introduction
This is a climatic atlas of the haze over
the contiguous United States for the time
period 1948 to 1983. It covers the spatial
and temporal pattern of atmospheric
haziness in a climatic sense: in terms of
synoptic distribution and secular trends.
It also contains a rough attribution of
haze between man-induced and natural
causes. The attribution of the man-made
haze to specific sources is beyond the
scope of this atlas.
The spatial and temporal distribution
as well as the man-made causes of
atmospheric haze have received consider-
able attention from researchers on this
continent since the late 1970's. Much of
the recent literature deals with physical-
chemical properties of haze and has the
aim of understanding its sources and
formation mechanisms. Over the years
various investigators have reported haze
maps covering parts of North America.
This report is one of a series on the
continuing haze research at Center for
Air Pollution Impact and Trend Analysis
(CAPITA) that was initiated in 1976. Since
1982, EPA has supported through Co-
operative Agreements the maintenance
and updating of a visibility data base. This
report is a product of that data acquisition,
maintenance and analysis. The availability
of such large scale data base on a timely
basis permits the monitoring of the
regional shifts in atmospheric haziness.
Undoubtedly, such data can be of benefit
for the monitoring of the nation's atmo-
spheric environment, effectiveness of
existing control strategies, and the devel-
opment of future ones.
Visibility Data Base
The sources and the gross features of
visibility data have been described in the
past by almost all investigators dealing
with the subject. The following discussion
will be limited to those items that are
directly relevant to the climatic maps and
trend graphs in this atlas.
The trend data base consists of 137
stations for which computerized data
exists since the 1948-1952 period. The
spatial coverage of stations is particularly
dense in the Northeast. The temporal
coverage for most stations started in
1948, although some stations only have
computerized data since 1952. The main
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data base of 3-hourly weather observa-
tions resides on over 50 magnetic tapes.
For purposes of spatial-temporal trend
analysis, raw visibility observations were
summarized as monthly averages of
noontime light extinction coefficient (24
/ visual range, miles). For each month
and station, three different extinction co-
efficients were calculated: the first set
includes all visibility data regardless of
weather and pollutant conditions (BX);
the second group (FX) is composed of
extinction coefficients excluding precipi-
tation (rain and snow) and fog events; the
third group (RX) excludes precipitation,
fog and includes an RH correction that
was performed to compensate for water
and vapor effects. This latter parameter is
closely related to the dry fine particle
aerosol mass concentration. The correc-
tion, RX = BX/f(RH), serves primarily to
reduce the apparent extinction coefficient
at relative humidities above 80%.
A problem with visual range measure-
ments is that there is always a furthest
marker beyond which the visual range is
not resolved. This translates to a lower
threshold value for the computed extinc-
tion coefficient. For this reason, the mean
is inappropriately biased upward, and
more reliable, nonparametric statistical
indices are more useful. In our analysis,
the 25th, 50th, 75th and 90th percentiles
of each of the three extinction coefficients
are calculated monthly for each station.
Other statistical quantities such as means
and standard deviations can be computed
from the percentiles once it is established
which percentile is valid, i.e. above the
threshold.
The utility of the computer percentile
values is demonstrated in Figure 1. For
Sioux City, IA, the trend graph shows
that from 1948 to 1968, the threshold
extinction coefficient (FX) was at 16, and
then dropped to 8 x 10"6m~1. Since the
50th percentile was near the high initial
threshold, this median would indicate a
drop in the later periods; the mean would
show a strong decrease in the late 1970's.
The 90th percentile, however, appears to
be a robust reliable measure which is
above the threshold influence; it indicates
a clear increase in extinction over the
entire period. Thus, depending on whether
one follows the 50th or 90th percentiles,
one would arrive at opposite conclusions
about the trend.
The excellent spatial and temporal
coverage of the visibility data base can be
utilized only after careful site by site
scrutiny for anomolous behaviour such
as this. The following results were
compiled after extensive examination at
16
14
12
10
afrfta-B a a a en ar» frfla a a a a an i&^ \ /
..* '*' '^
\
^^ / .
^ ^ "-^-
40
50
G 25 Percentile
Figure 1. Trends of percentiles at Sioux City. IA
60 70
/ear
<- 50% * 75%
80
90%
each site's percentile trends. The ad-
vantageous feature of systematic thres-
hold offset is that the properly presented
data can identify its limitations and pro-
vide remedies for them.
Climatic Maps of the U.S. Haze
The essence and the summary of the
present haze atlas is given in Figure 2.
The specific parameter that is plotted is
the 75th percentile of the extinction ex-
cluding precipitation and fog and corrected
to RH = 60%. While this is unconventional,
it constitutes the safest approach in that
it did not require any extrapolations or
other adjustments to the data. More con-
ventional statistical measures can be
estimated as follows: from previous re-
search it is known that the extinction
coefficient is roughly lognormally distri-
buted with typical logarithmic standard
deviation of 2.5. For such a distribution,
the 50th percentile is 0.5 times the 75th
percentile, and the mean is 0.76 times
the 75th percentile. Thus, if one is to
convert the maps, the scales of the
intervals must be multiplied by the ap-
propriate fractions. We recognize that
even if the haze is lognormally distributed
everywhere, its log standard deviation
will tend to vary geographically and
seasonally. The available data suggest,
however, that its range is confined to 1.6
to 3.4.
Figure 2 is a composite of 16 maps,
each representing four time periods and
four seasons. The time periods are
selected to center around 1950, 1960,
1970 and 1980, while the four quarters
are Jan.-March, Apr.-June, etc. These
maps are indicative of the dry fine particle
concentration over the nation. Hence,
they represent a "pollution index" for
visuaT air quairty. The wiKte'r s'da§on^ary"
haziness is most pronounced over the
Great Lakes states, California and the
Gulf states. Pennsylvania and New York
show declines of dry haze from the 1950's
to the 1980's. Ohio has not changed
significantly. The California stations,
particularly in the south coast basin, show
increased winter dry haze, particularly
from 1940 to 1950. The most significant
wintertime increase is noted for the Gulf
states, LA, AL, MS, and GA. Second and
third quarter haziness shows an increase
over all states east of the Rocky Moun-
tains. The increase is most pronounced
for the Gulf states, and least over the
northeast and California. Quarter four
closely resembles the spatial and temporal
trends over the first quarter. Again, not-
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1948-1954
1955-1964
1966-1974
1975-1983
Figure 2. Trend maps for U.S. haziness for winter IQ-\), spring (Qi), summer Qs), and fall (Q4). The units are mean extinction coefficient
able exceptions are the improved late fall
visibility in the Ohio region.
Trends of Haze at Selected Sites
The secular midday summertime trends
of RH BEXT percentiles for all stations
are presented in the Appendix of the full
Report. A few sites representative of
regional behavior will be discussed briefly.
The haziness at Long Beach, CA, was
among the worst in the nation in the
1950's and 1960's, with median visibility
of about 6-8 miles. Beginning in the early
1970's, the visibility has shown a clear
and consistent trend to improved visibility.
Generally low levels of extinction have
been present at Phoenix, AZ, throughout
the period since 1948, with clear indica-
tions of reduced haziness since about
1974. There is no evident trend in the
median haziness at Des Moines, IA,
particularly since much of the trend is
obscured by the threshold problem up to
1970. The higher percentiles, however.
appear to exhibit an upward trend over
the entire period. Madison, Wl trends
also indicate a consistent secular increase
in haziness, although the 25th and 50th
percentile information is lost until 1980.
A sharp increase in haziness is evident
about 1978, likely due to local influences.
The trends indicate that New Orleans,
LA, has experienced increased haziness
since the late 1950's. A rather sharp
jump in the lower percentiles occurred
about 1968. The trend at Jackson, MS,
indicate a rapid rise in haze at the lower
percentiles, including the median, be-
ginning about 1976. The median extinc-
tion coefficient rose by 50% in the past
decade. At Columbus, OH, the gradual
increase in haziness from about 1960
reversed itself in the mid-70's, indicating
little change over the past decade in
summertime haziness there.
Our earlier studies, found evidence of
remarkably rapid increase in average
haziness at Charlotte, NC from the mid
1960's to 1974. The current analysis of
percentiles shows that the strong in-
creases have halted about 1974, with no
evidence of further trend over the past
decade. In New York City, visual range
observations at LaGuardia airport show a
flat secular .trend, with some improve-
ment in visibility in the upper percentiles
since the mid 1960's. Nearby sites, e.g.
Newark, NJ, are similar. Evidently the
haze in the industrialized northeast has
not worsened.
Finally, the Burlington, VT, site may be
regarded as a remote receptor of regional
haze. The median extinction coefficient
there has changed very little over the last
thirty years, while substantial swings have
occurred at upper percentiles. Where such
a receptor is periodically hit with heavily
polluted air masses, the highest per-
centiles serve as an index of the aerosol
pollution contributed by upwind sources.
Here it appears that the higher percentiles
increased during the 1960's decade, but
have declined since about 1970.
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/?. B. Husar and D. E, Patterson are with CAPITA, Washington University, St.
Louis, MO 63130.
William E. Wilson is the EPA Project Officer (see below).
The complete report, entitled "Haze Climate of the United States," {Order No.
PB 87-141 057/AS; Cost: $18.95, 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:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-86/071
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