Paper to be submitted to Environmental Science & Technology
May, 2000
Haze Trends over the United States, 1980-1995
Rudolf B. Husara, Bret A. Schichtela and William E. Wilsonb
aCenter
for Air Pollution Impact and Trend Analysis, Washington University, St. Louis, MO, USA
Center for Environmental Assessment, US EPA, Research Triangle Park, NC, USA
bNational
Abstract
This is an update of the haze pattern over the U.S. for the period 1980-95, based
on human visual range observations at 260 synoptic meteorological stations. There has
been a significant (~10%) decline in haziness over the 15 year period. The reductions
were evident throughout the eastern US domain as well as over the hazy air basins of
California. During the same period, the eastern U.S. sulfur emissions have also declined
by about 10%. However, a causality has not been established.
1. Introduction
The spatial and temporal trends as well as the man-made causes of atmospheric
haze have received considerable attention from North American researchers since the late
1960s. (e.g. Elridge, 1966, Miller et al., 1972; Munn, 1973; Husar et al., 1976; Weiss et
al. 1977; Husar et al. 1979; Leaderer et al., 1979; Ferman, 1981; Husar et al., 1981;
Robinson and Valente, 1982; Sloane, 1982; Sloane, 1982; Trijonis, 1982; Wolff et al.,
1982; Sloane, 1983; Sloane, 1984; Husar and Patterson, 1986; Husar and Wilson, 1993).
Much of the recent literature deals with physico-chemical properties of haze, with the aim
of understanding its sources and formation mechanisms (NAPAP, 1990). This report is an
update of the haze trend research at Washington University's Center for Air Pollution
Impact and Trend Analysis that was initiated in 1976.
2. Haze trend data sets
The data set used for the visibility trend analysis consist of hourly prevailing
daytime (noon) visibility, V, recorded at synoptic weather stations by human observers.
The observed visual range (km) is used to calculate the extinction coefficient, bext (km-1)
via the Koschmieder relationship bext=K/V (Koschmieder, 1926). The value of K is
1
determined by the threshold sensitivity of the human eye, by the contrast of the visible
objects against the horizon sky and by the available visual targets. In this report, we use
K=1.9 in accordance with the data by Griffing (1980). The extinction coefficient is in
units of km-1 and is proportional to the concentration of light scattering and absorbing
aerosols and gases (NAPAP, 1990). The terms extinction coefficient and haze are used
here synonymously.
For purposes of spatial-temporal trend analysis, the raw visibility observations
were summarized as quarterly averages of noontime light extinction coefficient. For each
quarter and station, three different extinction coefficients were calculated. The first set
included all visibility data regardless of weather and pollutant conditions (BX). The
second group (FX) is composed of extinction coefficients excluding precipitation and fog.
For the third group (RH) a relative humidity correction was performed to compensate for
water vapor effects. This latter parameter is closely related to the dry fine particle aerosol
mass concentration. The extinction coefficient calculated from the visual range is
influenced by both haze aerosol and natural obstructions to vision, such as rain, fog, and
snow. The role of these natural obstructions can be eliminated by discarding the values
that occur when these meteorological phenomena occur.
The specific parameter that is plotted for the haze maps is the 75th percentile.
While this is unconventional, it constitutes the safest approach in that it does not require
any extrapolation or other adjustments to the data. A significant problem with airport
visual range observations is that there is a furthest marker beyond which the visual range
is not resolved (Husar and Wilson, 1993). More conventional statistical measures, e.g. the
mean can be estimated as follows: from previous research, e.g. Husar et al. (1979), the
extinction coefficient is roughly lognormally distributed 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.
The spatial patterns are presented on contour maps. The contours were derived
from the station-point observations using a spatial extrapolation scheme, described
previously (Husar et al., 1994)
3. National seasonal trend maps
The U.S. haze patterns and trends since 1980 are shown in 12 seasonal maps
covering 5-year periods, centered at 1983, 1988, and 1993. (Figure 1). The seasons are
defined by calendar quarter. The overall national view shows two large contiguous haze
regions, one over the eastern U.S. and another one along the Pacific coast. In between the
2
two haze regions lies a haze free territory that spans from the Rocky Mountains to the
Sierra-Cascade mountain ranges. This general pattern was preserved over the past 30year period (Husar and Wilson, 1993). However, notable trends have occurred over both
the western and the eastern haze regions.
The eastern U.S. shows significant seasonal variation and there is also a
significant trend over the past 30 years. The maps in Figure 1 (1980-1995) show that
over the eastern U.S., the RH corrected dry extinction coefficient is highest during the
summer season (Quarter 3). The highest extinction coefficient (bext>0.2 km-1) is observed
adjacent to the Appalachian Mountains in Tennessee and the Carolinas. It is observed,
that on the periphery of the eastern U.S. (Maine, Florida, Texas, North Dakota) the
summer-time extinction coefficient is less than half (bext<0.1 km-1) of the values near the
center of the eastern U.S.
The dry extinction coefficient over the eastern U.S. during the cold season,
Quarters 1 and 4, is shown on the top and bottom row of maps. In the 1981-1985 period,
elevated Quarter 1 haze values (bext>0.2 km-1) are observed between the Great Lakes and
the Ohio River Valley. Another region of cold-season haze is observed over the Gulf
states between Texas and Florida. Elevated winter-time haze is also evident along the
mid-Atlantic coast from North Carolina to New Jersey.
The summer time (June, July, August) bext trends for the fifteen year period 198095 are presented in Figure 2. The trends were computed for the 75th and 90th percentile
using data from all stations east of the Mississippi River (eastern U.S.), north of Virginia
and east of Ohio (northeastern U.S.) and south of Tennessee and east of Mississippi
(southeastern U.S.). As shown, over the eastern U.S. there was a 17% decrease in the 90th
percentile bext over the fifteen year period, and a 9% decrease of the 75th percentile.
Larger decreases in bext were observed in the southeastern U.S., where the 90th and 75th
percentiles decreased by 20% and 12 % respectively. Both trends are significant at the
95th percentile using student's t-distribution. The decreases over the Northeast for the 90th
and 75th percentiles where 16% and 8% respectively. As a result of the decline over the
eastern U.S., by 1991-95, the extinction coefficient was below 0.2 km-1 throughout most
including the summer season (Figure 1).
The haze pattern and trends over the visually pristine inter-mountain western U.S.
can not be evaluated due to the poor distance resolution of the visual range database.
Very few monitoring sites report visibility above 30-50 km (0.038<bext<0.063).
3
Elevated haze (bext>0.1 km-1) can be observed throughout the Pacific coast of the
U.S. The worst haze can be found in California, throughout the San Joaquin and the Los
Angeles basin. The haze is worst during the cold season (Quarters 4 and 1) when bext
exceeds 0.2 km-1). During the period 1981-1995, the level of haze has significantly (10%)
declined throughout the Pacific coast, including the San Joaquin and Los Angeles basins.
Since sulfate constitutes 40-70% (NAPAP 1990??) of the light scattering aerosol
over the eastern US, the trend of bext is compared to the sulfur emission trends in Figure
3. The emission data prior to 1985 were taken from Knudson (1985) and Husar (1986)
while the values after 1985 were from the NEI (19??). An unusual feature of the sulfur
emissions data is the sharp drop in 1995. The comparison in Figure 3. indicates that both
the eastern U.S. average bext and sulfur emissions have declined about 10% during the 15year period. Declines are also evident for the Northeastern and the Southeastern U.S.
However, the relationship varies significantly from year to year.
4. Discussion
Since the late 1980s, visibility trend maps, similar to the ones presented here,
were used in the National Air Quality and Emissions Trends report issued by EPA. The
Trend Report is the official yearly report card on the nations air quality. This haze trend
update indicates that during the 1990-1995 period, the haziness has declined significantly
(~10%) throughout the country.
The frequency of eastern U.S. haze episodes (90th percentile of bext) has declined
even faster. Since the visibility-derived haziness is a surrogate for PM2.5 concentration,
this report implies that there was also a significant (~10%) decline in the national PM2.5
levels as well. The causes of the haze and PM2.5 decline were not investigated here in
detail. However, it is noted that the haze decline coincided with comparable reductions in
the sulfur emissions in the eastern US. In order to establish fully the causality of the
observed reductions, additional work needs to be focused on the trends of specific aerosol
chemical species, possible changes in the oxidation capacity of the atmosphere and trends
of relevant meteorological variables.
This suggests that the haze trend data provides a way to monitor the effectiveness
of the emission reductions from the 1990 Clean Air Act Amendment. Previous work
(Husar and Wilson, 1993) has also linked the regional and seasonal shifts in eastern U.S.
haziness to haze precursor emission pattern.
4
It is regrettable that this may be the last U.S. haze trend update based on the
surface visibility observations. After 1995, increasing number of visibility data are
collected by automatic light scattering sensors that are not fully compatible with the 50year long human observations. [Secondly, the U.S. EPA has discontinued support for this
type of visibility trend analyses]
References
Elridge, R. G., 1986. Climatic visibilities of the United States. J. Appl. Meteorol. 5, 227-282.
Ferman, M.A., Wolff, G.T., and Kelly, N.A., 1981. The nature and sources of haze in the Shenandoah
Valley/Blue Ridge Mountains area, J. Air Poll. Contr. Assoc. 31, 1074.
Griffing G.W., 1980. Relationships between the prevailing visibility, nephelometer scattering coefficient,
and sunphotometer turbidity coefficient. Atmos. Environ. 14, 577-584.
Holzworth, G.C., 1972. Mixing heights, wind speeds, and potential for urban air pollution throughout the
contiguous United States, Environmental Protection Agency, Office of Air Programs, RTP, NC,
January.
Husar, R.B., Gillani, N.V., Husar, J.D., Paley, C.C., and Turcu, P.N., 1976. Long-range transport of
pollutants observed through visibility contour maps, weather maps and trajectory analysis. Preprint
volume: Third Symposium on Turbulence, Diffusion and Air Pollution, American Meteorological
Society, Reno, NV pp. 344-347.
Husar, R.B., Poll, D.E., Holloway, J.M., Wilson, W.E. and Ellestad, T.G., 1979. Trends of eastern U.S.
haziness since 1948. Preprint Volume: Fourth Symposium on Turbulence, Diffusion and Air
Pollution, American Meteorological Society, Boston, MA pp. 249-256.
Husar, R.B., Holloway, J.M., Poll, D.E. and Wilson, W.E., 1981. Spatial and temporal pattern of eastern
U.S. haziness: a summary. Atmos. Environ. 15, 1919-1928.
Husar, R.B., 1986. Emissions of sulfur dioxide and nitrogen oxides and trends for eastern north America.
In Acid Deposition: Long-Term Trends. National Academy Press , Washington, D.C. 1986; pp
48-92.
Husar R.B. and Patterson D.E., 1986. Haze Climate of the United States. U.S. Environmental Protection
Agency, EPA-600/3-86-071, Research Triangle Park, NC.
Husar R.B. and Wilson W.E., 1993. Haze and Sulfur Emission Trends in the Eastern United States, Environ
Sci. Technol., 27, 12-16.
Husar R.B., Elkins, J.B. and Wilson, W.E., 1994. U.S. Visibility Trends, 1960-1992, Regional and
National. Presented at 87th Annual A&WMA Meeting, 19-24 June, Cincinnati, Ohio.
Knudson, D.A., 1985. Estimated monthly emissions of sulfur dioxide and oxides of nitrogen for the 48
contiguous states, 1975-1984. U.S. Department of Energy, ANL/EES-TM-318, Vol 1.
Koschmieder H., 1926. Theorie der horizontalen Sichtweite, Beit. Phys. Atmos., 12, 33-55.
Leaderer, B.P. and Stolwijk, J.A., 1979. Optical properties of urban aerosol and their relation to chemical
composition. Presented at the New York Academy of Science Symposium on Aerosols:
Anthropogenic and Natural Sources and Transport, Jan. 9-12.
Miller, M.E., Canfield, N.L., Ritter, T.A. and Weaver, C.R., 1972. Visibility changes in Ohio, Kentucky,
and Tennessee from 1962 to 1969. Monthly Weather Review 100, 67-71.
Munn, R.E., 1973. Secular increases in summer haziness in the Atlantic provinces, Atmosphere, 11, 156161.
NAPAP, 1990. Acid Deposition: State of Science and Technology. Visibility: Existing and Historical
Conditions-Causes and Effects. Report 24, National Acid Precipitation Assessment Program,
Washington D.C.;
Robinson, E. and Valente, R.J., 1982. Atmospheric turbidity over the United States, 1948-1978. Research
Publication GMR-3474, Env #92, General Motors Corp.
5
Schichtel, B.A., Husar, R.B. Wilson, W.E., Poirot, R. and Malm, W.C., 1991. Reconciliation of visibility
and aerosol composition data over the U.S, 84th Annual A&WMA meeting, June16-21,
Vancouver.
Sloane, C.S., 1982. Visibility Trends II: Methods of Analysis, Atmos. Environ. 16, 41.
Sloane, C.S., 1982. Visibility Trends II: Mideastern United States, Atmos. Environ. 16, 2309.
Sloane, C.S., 1982. Summertime visibility declines: meteorological influences. Atmos. Environ. 17, 763774.
Sloane, C.S., 1984. Meteorologically adjusted air quality trends: visibility, Atmos. Environ. 18, 12171229.
Trijonis, J., 1982. Existing and natural background levels of visibility and fine particles in the rural East.
Atmos. Environ. 16, 2431.
Waggoner, A.P. and Weiss, R.E., 1986. Impact of the chemical and physical properties of fine and coarse
particles on visibility. In Aerosols Lee S. D., Schneider T., Grant L.D., and Verkerk P.J. (Eds.).
Lewis Publishers, Chelsea, MI.
Weiss, R.E., Waggonner, A.P., Charlson, R.J. and Ahlquist, N.C., 1977. Sulfate aerosol: its geographical
extent in the midwestern and southern United States. Science 197, 997-998.
Wolff, G.T., Kelly, N.A. and Ferman, M.A., 1982. Source regions of summertime ozone and haze episodes
in the eastern United States. Water Air Pollution.
6
7
Figure 1. United States trend maps for the 75th percentile RH corrected extinction coefficient, b ext for winter
(Q1), spring (Q2), summer (Q3), and fall (Q4). b ext [km-1] is derived from visual range, VR, data by bext
=1.9/VR.
Data during natural obstructions to vision (rain, snow, fog) were eliminated.
8
9
Eastern US
0.32
0.3
0.3
16% / 15 Yrs
90% Conf.
0.28
0.26
0.26
0.2
bext (1/km)
bext (1/km)
0.22
0.24
0.22
0.2
9% / 15 Yrs
85% Conf.
0.24
0.22
0.2
8% / 15 Yrs
65% Conf.
0.18
0.18
0.18
0.16
0.16
0.16
0.14
0.14
80
83
86
89
92
12% / 15 Yrs
95% Conf.
0.14
80
95
20% / 15 Yrs
95% Conf.
0.28
0.26
0.24
Southeastern US
0.32
0.3
17% / 15 Yrs
95% Conf.
0.28
bext (1/km)
Northeastern US
0.32
83
86
89
92
95
80
83
86
Year
Year
89
92
95
Year
90th Percentile - Top Trends
75th Percentile - Bottom Trends
Figure 2. Trends of the summertime 90th and 75th percentile light extinction for the eastern,
Northeastern and Southeastern US from 1980-95. The confidence level for each trend is based on the
two sided Student's t-distribution.
Eastern US
Southeastern US
Northeastern US
0.2
0.2
0.2
10.5
75th %-ile bext
Sulfur Emissions
0.14
7.5
6.5
5.5
80
83
86
89
Year
92
95
0.16
75th %-ile bext
3.3
0.12
2.8
0.1
2.3
0.14
0.18
2.3
Sulfur Emissions
0.16
0.14
1.8
0.12
0.12
0.1
80
83
86
89
Year
92
95
bext (1/km)
8.5
3.8
bext (1/km)
0.16
0.18
Yearly Sulfur Emissions
(Million Tons Sulfur)
Sulfur Emissions
75th %-ile bext
Yearly Sulfur Emissions
(Million Tons Sulfur)
9.5
bext (1/km)
Yearly Sulfur Emissions
(Million Tons Sulfur)
4.3
0.18
1.3
0.1
80
83
86
89
92
95
Year
Figure 3. Comparison of the annual sulfur emission trends and summertime 75th percentile light
extinction coefficient for the eastern US, Northeastern US and Southeastern US. During 1980-95, the
eastern US haze and sulfur emission have declined at a comparable rate of 10%.
10