Text with suggested deletions in strikeout and additions in red, notes in green.
Also change hae to haze in reference, Malm 1992.
With these revisions and a reference for RH correction I'll start internal review. No
comments on lack of EPA support for this type of work. It won't make it through review! I'll
have OAQPS folks review it. Maybe that will create some interest in further support now that
the new intrumental data are
available at full resolution. Oaqps is ordering 1999 data.
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
bNational Center for Environmental Assessment, US EPA, Research Triangle Park,
NC, USA
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 at298 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 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. This report is an update of an earlier survey of haze patterns and trends from
1950-1980 (Husar and Wilson, 1993).
1. Introduction
The spatial and temporal trends as well as the man-made causes ofatmospheric
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, 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, formation and transport (strikethrough: and formation mechanisms,
transport and relationship to sources )(NAPAP, 1990, Sloan 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,
b(subscript: ext )(km(superscript: -1)) via the Koschmieder relationship b(subscript: ext)=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 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(strikethrough: . The 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 (strikethrough: were) was eliminated by discarding the values
(strikethrough: that occur when) caused by these meteorological phenomena (strikethrough:
occur). The effect of relative humidity was compensated for by applying a RH correction
factor to yield a "dry extinction coefficient". (Need a reference for the RH correction factor)
Data were quality assured as described in Husar and Wilson (1993).
(strikethrough: The specific parameter that is plotted for the haze maps is the 75th
percentile of the seasonal bext distribution function. While this is unconventional, it is) The
75th percentile of the seasonal bext distribution function is the specific parameter chosen for
use in the 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 (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 lognormal with a 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 (i.e., summer is June, July and August). The overall national view shows two
large contiguous haze regions, one over the eastern U.S. and the other along the Pacific coast.
In between the two haze regions lies a low-haze (strikethrough: free) territory that spans from
the Rocky Mountains to the Sierra-Cascade mountain ranges. This general pattern was preserved
over the past 30-year period (Husar and Wilson, 1993). However, notable trends have
occurred over both the western and the eastern haze regions.
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 (strikethrough: (June, July, August)) 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 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 distance resolution of the visual range database. Very few
monitoring sites report visibility above 30-50 km (0.038<bext<0.063).
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 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 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; 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 have 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,
(strikethrough: the haziness has ) haziness declined significantly (~10%) throughout the
country. The frequency of eastern U.S. haze episodes (90th percentile of bext) has declined
even faster (17%).
Since the visibility-derived haziness is a surrogate for PM2.5 concentration,
these results imply that there was also a substantial (~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 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 work further supports the notion that the haze trend data provide 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 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 have begun collecting visibility data with automatic light
scattering sensors that are not fully compatible with the 50-year long
uman observations. (strikethrough: In addition, the U.S EPA Trends Reports no
longer report this type of visibility trend analyses. )(strikethrough: [Secondly, the U.S.
EPA has discontinued support for this type of visibility trend analyses])(strikethrough: )