ATMOS ENVIRON 35 (30): 5205-5210 OCT 2001
Haze Trends over the United States, 1980-1995
Bret A. Schichtela, Rudolf B. Husara, Stefan R. Falkea, 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
The patterns and trends of haze over the U.S., for the period 1980-1995, are presented.
Haze measurements are based on human visual range observations at 298 synoptic
meteorological stations operated by the U.S. Weather Service. There was a significant (~10%)
decline in haziness over the 15 year period. The reductions were evident throughout the eastern
U.S. as well as over the hazy air basins of California. During the same period, the eastern U.S.
sulfur emissions also declined by about 10%. However, a causality for the reductions has not
been established. This report is an update of an earlier survey of haze patterns and trends from
1950-1980.
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 as part of scientific and
regulatory studies [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, 1982a; Sloane, 1982b; 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, formation and transport [NAPAP, 1990; Sloane et al., 1991; Malm, 1992; Malm et al.,
1994; White et al., 1994; Malm and Kreidenweis, 1997]. This report updates 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 (km), 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 Koschmieder constant,
K, is determined by the threshold sensitivity of the human eye, by the contrast of the visible
objects against the horizon sky and by the availability of visual targets. In this report, we use
1
K=1.9 in accordance with the data by Griffing [1980]. The extinction coefficient is roughly
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 aggregates of noontime light extinction coefficient. Visibility is
influenced by both haze aerosol and natural obstructions to vision, such as rain, fog, and snow.
The role of these natural obstructions was eliminated by discarding observations during rain, fog
and snow. The effect of relative humidity (RH) was compensated for by applying a RH
correction factor to yield a "dry extinction coefficient" [Husar and Holloway, 1984]. Data were
quality assured as described by Husar and Wilson [1993].
The 75th percentile of the seasonal bext distribution function is the specific parameter
chosen for use in this haze analysis. While unconventional, this 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 and thereby skews other statistical measures such as the mean [Husar
and Wilson, 1993]. The mean can be estimated from the 75th percentiles. Previous research,
Husar et al., [1979], has shown that the extinction coefficient is roughly lognormal with a typical
logarithmic standard deviation of 2.5 and 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 as contour maps. The contours were derived from the
station-point observations using a inverse distance squared spatial interpolation scheme,
described previously [Husar et al., 1994; Falke, 1999]. The nearest 6 sites within a 250 km
radius were used in calculating the interpolated estimates. Elevation data [NOAA, 1995] were
incorporated as horizontal and vertical barriers, that prevented the use of observations in valleys
for estimating bext at high elevations, and prevented observations from crossing mountain ranges
during interpolation [Falke, 1999].
3. National seasonal trend maps
The U.S. haze patterns and trends since 1980 are shown in 12 seasonal maps covering 5year periods, centered at 1983, 1988, and 1993. (Figure 1). The seasons are defined by calendar
quarter. The national view shows two large contiguous haze regions, one over the eastern U.S.
and the other along the Pacific coast. Between the two haze regions lies a low-haze territory that
spans from the Rocky Mountains to the Sierra-Cascade mountain ranges. This general pattern
has been preserved over the last 30-years [Husar and Wilson, 1993]. However, notable trends
have occurred over both the western and the eastern haze regions.
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The maps in Figure 1 (1980-1995) show that over the eastern U.S., the dry extinction
coefficient is highest during the summer season (Quarter 3). The highest extinction coefficient
(bext>0.2 km-1, equivalent to a visibility distance of 6 miles) is observed adjacent to the
Appalachian Mountains in Tennessee and the Carolinas. In comparison 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 (Figure 1) over the eastern U.S. during the cold season,
Quarters 1 and 4, depict elevated haze values (bext>0.2 km-1) between the Great Lakes and the
Ohio River Valley. Another region of cold-season haze is found over the Gulf states between
Texas and Florida, and along the mid-Atlantic coast from North Carolina to New Jersey.
The summertime bext trends over the eastern U.S. for the fifteen year period 1980-95 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. The decreases over the Northeast for the 90th and 75th percentiles where 16% and
8%, respectively. As a result of these declines, the 75th percentile extinction coefficient was
below 0.2 km-1 by 1991-95, throughout most of the eastern U.S., as shown in the right columns
in Figure 1.
The haze pattern and trends over the visually pristine inter-mountain western U.S. can not
be evaluated due to the poor spatial resolution of the visual range database. Very few monitoring
sites report visibility above 30-50 km (0.038<bext<0.063). To compensate for this deficiency, the
topographical data were incorporated into the mapping. Locations above the scale height were
defined to have 75th bext<0.05 km-1.
Elevated haze (bext>0.1 km-1) can be observed throughout the Pacific coast of the U.S.,
particularly in central and southern California (San Joaquin and the Los Angeles basins). 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 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 U.S., 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
3
1985 were from the National Emissions Trend (NET) emission inventory [US EPA, 1998]. The
National Acid Precipitation Assessment Program and US EPA report regularly on progress and
trends in the effects of SO2 and NOx emissions, including the effects on visibility [NAPAP, 1998;
US EPA, 1999]. 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
declined by about 10% during the 15-year period. Declines are also evident for the northeastern
and the southeastern U.S. However, the relationship varies significantly from year to year.
4. Discussion
This haze trend update indicates that during the 1980-1995 period, haziness declined
significantly (~10%) throughout the country. The frequency of eastern U.S. haze episodes (90th
percentile of bext) declined at an even faster rate (17%).
Since the visibility-derived haziness is a surrogate for PM2.5 concentrations, these results
imply that there was also a substantial (~10%) decline in the national PM2.5 levels. 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 reductions in sulfur emissions over the eastern U.S. In order to fully
establish 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 work further supports the notion that the haze trend data provide means of
monitoring the effectiveness of the 1990 Clean Air Act Amendment emission reductions.
Previous work [Husar and Wilson, 1993] has also linked the regional and seasonal shifts in
eastern U.S. haziness to haze precursor emission patterns.
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. It is regrettable that this may be the last U.S.
haze trend update based on the surface visibility observations. Beginning in the early 1990's, an
increasing number of sites started collecting visibility data with automatic light scattering sensors
that are not fully compatible with the 50-year long human observations.
Acknowledgments
The information in this document has been funded in part by the United States
Environmental Protection Agency under Order No. 9C-R498-NASA. It has been subjected to
Agency review and approved for publication. Mention of trade names or commercial products
does not constitute endorsement or recommendation for use.
4
References
Elridge, R. G., Climatic visibilities of the United States. J. Appl. Meteorol. 5, 227-282, 1986.
Falke, S. R., Mapping air quality: spatial estimation of pollutant concentrations from point monitoring data, D.Sc.
Thesis, Washington University, St. Louis, Missouri, 1999.
Ferman, M. A., G. T. Wolff, and N. A. Kelly, The nature and sources of haze in the Shenandoah Valley/Blue Ridge
Mountains area, J. Air Poll. Contr. Assoc. 31, 1074, 1981.
Griffing G. W., Relationships between the prevailing visibility, nephelometer scattering coefficient, and
sunphotometer turbidity coefficient, Atmos. Environ. 14, 577-584, 1980.
Holzworth, G. C., 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, 1972.
Husar, R. B., N. V. Gillani, J. D. Husar, C. C. Paley, and P. N. Turcu, 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,
1976.
Husar, R. B., D. E. Poll, J. M. Holloway, W. E. Wilson, and T. G. Ellestad, 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, 1979.
Husar, R. B., J. M. Holloway, D. E. Poll, and W. E. Wilson, Spatial and temporal pattern of eastern U.S. haziness: a
summary, Atmos. Environ. 15, 1919-1928, 1981.
Husar, R. B., J. M. Holloway, The properties and climate of atmospheric haze, in Hygroscopic Aerosols, edited by L.
H. Ruhnke, A. Deepak, pp. 129-170, A. Deepak Publishing, 1984
Husar, R. B., Emissions of sulfur dioxide and nitrogen oxides and trends for eastern north America, in Acid
Deposition: Long-Term Trends, pp 48-92, National Academy Press , Washington, D.C., 1986.
Husar R. B. and D. E. Patterson, Haze Climate of the United States, U.S. Environmental Protection Agency, EPA600/3-86-071, Research Triangle Park, NC, 1986.
Husar R. B. and W. E. Wilson, Haze and sulfur emission trends in the eastern United States, Environ Sci. Technol.,
27, 12-16, 1993.
Husar R. B., J. B. Elkins, and W. E. Wilson, U.S. visibility trends, 1960-1992, regional and national, paper presented
at 87th Annual A&WMA Meeting, 19-24 June, Cincinnati, Ohio, 1994.
Knudson, D. A., 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, 1985.
Koschmieder H., Theorie der horizontalen Sichtweite, Beit. Phys. Atmos., 12, 33-55, 1926.
Leaderer, B. P. and J. A. Stolwijk, Optical properties of urban aerosol and their relation to chemical composition,
paper presented at the New York Academy of Science Symposium on Aerosols: Anthropogenic and Natural
Sources and Transport, Jan. 9-12, 1979.
Malm, W. C., Characteristics and origins of haze in the continental United States, Earth-Sci. Rev. 33, 1-36, 1992.
Malm, W. C., J. F. Sisler, D. Huffman, R. A. Eldred, and T. A. Cahill, Spatial and seasonal trends in particle
concentrations and optical extinction in the United-States, J. Geophys. Res. 99, 1347-1370, 1994.
Malm, W. C. and S. M. Kreidenweis, The effects of models of aerosol hygroscopicity on the apportionment of
extinction, Atmospheric Environment 31, 1965-1976, 1997.
Miller, M. E., N. L. Canfield, T. A. Ritter, and C. R. Weaver, Visibility changes in Ohio, Kentucky, and Tennessee
from 1962 to 1969, Monthly Weather Review 100, 67-71, 1972.
Munn, R. E., Secular increases in summer haziness in the Atlantic provinces, Atmosphere, 11, 156-161, 1973.
NAPAP, 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., 1990.
NAPAP, National Acid Precipitation Assessment Program. Biennial Report to Congress: An Integrated
Assessment. National Science and Technology Council, Committee on Environment and Natural
Resources, May, 1998.
NOAA, U.S. National Oceanic and Atmospheric Administration, National Geophysical Data Center, Terrain Base
Global Digital Terrain Model, Version 1.0. 1995.
Parkhurst, W. J., R. L. Tanner, F. P. Weatherford, R. J. Valente, J. F. Meagher, Historic PM2.5/PM10
concentrations in the southeastern United States - Potential implications of the revised particulate matter
standard, J. Air Waste Management Assoc., 49, 1060-1067, 1999.
5
Robinson, E. and R. J. Valente, Atmospheric turbidity over the United States, 1948-1978, Research Publication
GMR-3474, Env #92, General Motors Corp., 1982.
Schichtel, B. A., R. B. Husar, W. E. Wilson, R. Poirot, and W. C. Malm, Reconciliation of visibility and aerosol
composition data over the U.S, psper presented at the 84th Annual A&WMA meeting, June16-21,
Vancouver, 1991.
Sloane, C. S., Visibility trends I: methods of analysis, Atmos. Environ. 16, 41, 1982a.
Sloane, C. S., Visibility trends II: mideastern United States, Atmos. Environ. 16, 2309, 1982b.
Sloane, C. S., Summertime visibility declines: meteorological influences, Atmos. Environ. 17, 763-774, 1983.
Sloane, C. S., Meteorologically adjusted air quality trends: visibility, Atmos. Environ. 18, 1217-1229, 1984.
Sloane, C. S., Watson J., Chow J., Pritchett L. and Richards L. W., Size-segregated fine particle measurements by
chemical species and their impacts on visibility impairment in Denver, Atmos. Environ. 25A, 1013-1024,
1991.
Trijonis, J., Existing and natural background levels of visibility and fine particles in the rural East, Atmos. Environ.
16, 2431, 1982.
US EPA, National air pollutant emission trends, procedures document 1900-1996. Office of Air Quality Planning
and Standards, Research Triangle Park NC 27711 EPA Report# EPA-454/R-98-008, 1998.
US EPA, Progress Report on the EPA Acid Rain Program. EPA430-R-99-011. November 1999. U.S.
Environmental Protection Agency, 1999.
Waggoner, A. P. and R. E. Weiss, Impact of the chemical and physical properties of fine and coarse particles on
visibility, in Aerosols, edited by S. D. Lee, T. Schneider, L. D. Grant and P. J. Verkerk P.J., Lewis
Publishers, Chelsea, MI., 1986.
Weiss, R. E., A. P. Waggonner, R. J. Charlson, and N. C. Ahlquist, Sulfate aerosol: its geographical extent in the
midwestern and southern United States, Science 197, 997-998, 1977.
White, W. H., E. S. Macias, R. C. Nininger and D Schorran, Size-resolved Measurements of light scattering by
ambient particles in the southwestern U.S.A. Atmospheric Environment 28, 909-921, 1994.
Wolff, G. T., N. A. Kelly, and M. A. Ferman, Source regions of summertime ozone and haze episodes in the eastern
United States, Water, Air and Soil Pollution, 18, 65-81, 1982.
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Figure 1. United States trend maps for the 75th percentile RH corrected extinction coefficient, bext 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.
7
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
0.14
80
83
86
89
92
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
12% / 15 Yrs
95% Conf.
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 U.S. 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 75 th percentile light
extinction coefficient for the eastern U.S., northeastern U.S. and southeastern U.S. During 1980-95,
the eastern U.S. haze and sulfur emission have declined at a comparable rate of 10%.
8
(east central states decline in PM2.5 of 3-5 g/m3 out of annual average range of 12.6-21.3 g/m3
[Parkhurst et al., 1999]
9