The overall national view shows two large contiguous haze regions, one over the eastern U.S. and another over the western Pacific states. The two haze regions are divided by a low-haze territory between the Rocky Mountains and the Sierra-Cascade mountain ranges. This general pattern is preserved over the past 30-year period. However, notable trends have occurred over both the western and the eastern haze regions. The haze trends over the visually pristine mountainous western U.S. can not be evaluated here due to the poor distance resolution of the visual range database, particularly at high visibilities. We recognize that this is unfortunate since much of the recent interest in the optical environment arises from the possible significant deterioration in the pristine southwestern states. The haze pattern and trends in this region can only be assessed by using higher resolution dedicated aerosol/haze networks.
The haziness in the western Pacific states covers all of the coastal states, California exhibiting the highest values. In the 1960s a large fraction of western California had low visibility particularly during Quarters 1 and 4. By the 1990s the visibility along the Pacific Coast has improved markedly for all seasons.
The eastern haze region extends from the East Coast to the Rocky Mountains. The western boundary of the eastern haze region is remarkably constant over both the seasons and the years. In fact, the mid-section of the U.S. extending from the Rocky Mountains to the Mississippi River changed little over the 30-year history.
The most dynamic pattern can be observed over the eastern U.S. extending from the Mississippi River to the East Coast. There is both a significant seasonal variation over the region and also a significant secular trend over the past 30 years. Furthermore, these seasonal and secular (long-term) trends are different for sub-regions within the eastern U.S., such as the Northeast, Mid-Atlantic and Gulf States regions.
In the 1960s the highest extinction values were recorded for the cold season, Quarters 1 and 4, with significantly lower values for the warm quarters (Q2, Q3). The remarkable reduction in the cold season haziness and the strong increase during the warm season has shifted the haze peak from winter to summer. Consequently, there was also a regional shift in the highest haze pattern. In the 1960s the worst haziness occurred during the cold season around Lake Erie and the New York-Washington megalopolis. By the 1990s the worst haziness has drifted southward toward Tennessee and Carolinas and it now occurs in the summer season.
The decade of the 1980s shows less change than the earlier decades. However, there was a continued haze reduction in the Northeast, north of the Ohio and east of the Mississippi Rivers. The southeastern U.S. as well as the Pacific states have remained virtually unchanged in the 1980s. It is evident that the haze trend database 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 linked the regional and seasonal shifts in eastern U.S. haziness to haze precursor emission patterns, which include sulfur and organics. However, it is beyond the scope of this report to examine such haze-emission relationships.
Haze Trend Background
The spatial and temporal distribution as well as the anthropogenic causes
of atmospheric haze have received considerable attention from researchers
on this continent since the late 1970s.
(e.g. Ferman, 1981;
Leaderer et al., 1979;
Miller et al., 1972;
Munn, 1973;
Robinson and Valente, 1982;
Sloane, 1982;
Sloane, 1982;
Sloane, 1983;
Sloane, 1984;
Trijonis, 1982;
Weiss et al. 1977;
Wolff et al., 1982).
Much of the recent literature deals with physico-chemical properties of haze,
with the aim of understanding its sources and formation mechanisms. Intermittently,
various investigators have reported haze maps covering parts of North America.
The first and most valuable set of such haze maps for the U.S. were published by
Elridge (1966).
In a sense, this report is the continuation of the haze trend research at Washington University's Center for Air Pollution Impact and Trend Analysis that was initiated in 1976 (Husar et al., 1976; Husar et al. 1979; Husar et al., 1981; Husar and Patterson, 1986; Husar and Wilson, 1993). The availability of such a large scale database on a timely basis permits the monitoring of the regional shifts in atmospheric haziness.