The visual range patterns and trends, observed and discussed in this paper, are caused by changes in the concentration of fine particles in the lower atmosphere. These changes in fine particles can in turn be attributed either to (1) changes in emissions of primary particles or precursors of secondary particles, (2) changes in photochemical smog which influences the rate of formation of secondary particles, or (3) changes in meteorological conditions which influence the mixing depth or ventilation rate. Thus, the haze trends provide an indication of trends in fine particle pollution.
The change from a winter maximum in haze in the 1960s to be summer maximum in the 1980s can be attributed in part to increased sulfate from increased SO2 emissions due to increased combustion of coal to produce electrical power for air conditioning or to increased photochemical smog which lead to more complete conversion of precursors (NOX, SO2, and organics) to particulate matter during the summer. Other changes in trends and patterns are due to the complex interplay between emissions and meteorology.
The two areas of low visibility observed in the third quarter in the 1990 period, one in North Carolina and one in Kentucky and Tennessee, must be due to a combination of the factors listed above. Both areas lie in a region of high meteorological potential for air pollution due to low-wind speeds and low-mixing heights (Holzworth, 1972) and both are in the region where summer SO2 emissions have increased. The North Carolina area, especially, has experienced rapid growth in population and automobile and truck traffic which would be expected to lead to higher photochemical smog and a more complete conversion of precursors to particles.
It should be possible to express the extinction coefficient as a function of the concentration of fine particles in the atmosphere. It has been known for many years that there is a good correlation between light scattering and fine particle mass and that this relationship changes with relative c and to a lesser extent with composition. A recent analysis (Schichtel et al., 1992) demonstrated that by using relationships that vary by season and geographical region a much better correlation can be obtained. These relationships between extinction and fine particle mass take into account the regional and seasonal variations in composition and the variations in the mass/extinction ratios of the different components.
It is possible to use these relationships to quantitatively transform trends and patterns of haze to trends and patterns of fine particle mass. Thus, the human observer visual range database of daily measurements at 280 sites in the U.S. for over 30 years provides a surrogate measurement of fine particle concentrations. This surrogate fine particle database may be used to (1) evaluate the effectiveness of regional and urban particle models; (2) study the relationships between emissions and fine particle concentrations and how these relationships are perturbed by variable meteorological processes; (3) interpolate particle concentrations when daily measurements are needed for epidemiological studies but only every sixth day measurements are available; and (4) investigate the relationships between particle pollution and weather parameters on an urban or regional scale.