2.3 Visibility Characterizations

[section requires major rewrites and coordination with staff paper, addition of tables,

figures]

2.3.1 Overview of Current Visibility Conditions

Annual average visibility conditions vary regionally across the U.S. The rural East generally has higher levels of impairment than remote sites in the West, with the exception of the San Gorgonio Wilderness, Point Reyes National Seashore, and Mount Rainier, which have annual average levels comparable to rural sites in the Northeast. Higher averages in the East are due to generally higher concentrations of anthropogenic fine particles and precursors, higher background levels of fine particles, and higher average relative humidity levels. Visibility conditions also vary significantly by season of the year. With the exception of remote sites in the northwestern U.S., visibility is typically worse in the summer months. This is particularly true in the Appalachian region, where average extinction in the summer exceeds the annual average by 40% (Sisler, 1996).

Figure 1 presents 1990 quarterly averages (75th percentile) of monitored visibility levels based on a fusion of IMPROVE, NESCAUM and Federal Aviaition Authority (FAA) data (Husar, 1996) nationally. Clear regional and seasonal patterns are evident. The eastern U.S. is strongly affected by summertime visibility degradation. Western (southern California) visibility degradation is apparent in summer and winter seasons.

[staff paper visibility characterization to be added with delivery of Figures]

2.3.2	Estimated Background Levels of Fine Particles and Associated Light Extinction

Total light extinction is determined by the combined effects of fine particles from background (i.e., geogenic and biogenic) sources, Rayleigh scattering (i.e., the degree of light extinction that would be found even in a particle-free atmosphere; see further discussion in section 5.C. below), and fine particles from anthropogenic sources. Chapter 6 of the CD identifies several alternative definitions of "background" levels of fine particles. For the purposes of this document, background PM is defined as the distribution of PM concentrations that would be observed in the U.S. in the absence of anthropogenic emissions of PM and precursor emissions of VOC, NOx, and SOx in North America. Table IV-4 describes the range for annual average regional background fine PM mass in the East as 2-5 g/m3, and in the West 1-4 g/m3. The lower bounds of these ranges, taken from estimates in the 1990 report of the National Acid Precipitation Assessment Program, are based on compilations of natural versus human-made emission levels, ambient measurements in remote areas, and regression studies using human-made and/or natural tracers (NAPAP, 1991; Trijonis, 1982). The upper bounds are derived from the multi-year annual averages of remote monitoring sites in the IMPROVE network (Malm et al., 1994). It is important to note, however, that IMPROVE data used here reflect the effects of background and anthropogenic emissions from within North America and therefore provide conservative estimates of the upper bounds. Table VIII- 2 from the NAPAP report includes estimates of annual average background levels of fine particles by aerosol constituent, as well as their related contributions to light extinction, expressed in inverse megameters (Mm-1) (NAPAP, 1991). On an hourly or daily basis, however, background concentrations will vary considerably depending on seasonal, meteorological, and geographic factors.

2.3.4	Role of Humidity in Light Extinction

As mentioned previously, humidity plays a significant role in the impairment of visibility by fine particles, particularly in the East, where annual average relative humidity levels are 70-80% as compared to 50-60% in the West (Sisler et al., 1993). Table VIII-2 accounts for relative humidity effects by assigning an extinction efficiency for water associated with aerosols, while extinction efficiencies found in table VIII-3 are modified by a relative humidity adjustment factor in calculating total extinction. The adjustment factor represents 1) the hygroscopic nature of the aerosol constituent, and 2) the average annual humidity for the relevant location (Sisler et al., 1993).

Because annual average relative humidity is higher in the East, the same ambient concentration of sulfate, for example, will on average lead to greater light extinction in an eastern location rather than a western one. The top map in figure VIII-9 illustrates the regional variability of annual mean relative humidity nationwide. The bottom map depicts the variability of the relative humidity correction factor used for sulfates in the IMPROVE analysis (Sisler et al., 1993). For example, when corrected for humidity, the overall extinction efficiency for sulfates in the East may exceed 11-12 m2/g, whereas the extinction efficiency for sulfate in the West may be one-third to one-half of that.

2.3.5	Rayleigh Scattering

Rayleigh scattering represents the degree of light extinction found in a particle-free atmosphere, caused by the gas molecules that make up "blue sky" (e.g., N2, 02, CO2 )(U.S. EPA, 1979). The concept of Rayleigh scattering can be used to establish a theoretical maximum horizontal visual range in the earth's atmosphere, absent visibility-impairing particles. At sea level, this maximum visual range would be approximately 330 kilometers (equivalent to light extinction of 10-12 Mm-1). Table VIII-2 shows the contribution to total light extinction from "Rayleigh scattering." While Rayleigh scattering can be used in calculating this maximum visual range, it is analagous to a baseline or boundary condition and should not be considered to contribute to visibility impairment. Rather, only fine and coarse particles from natural and anthropogenic sources should be treated as factors in determining visibility impairment.

Table VIII-2 illustrates that estimated extinction contributions from Rayleigh scattering plus background levels of fine and coarse particles, in the absence of anthropogenic emissions of visibility-impairing particles, are 26 plus or minus 7 Mm-1 in the East, and 17 plus or minus 2.5 Mm-1 in the West. These equate to a visual range in the East of 150 plus or minus 45 kilometers and 230 plus or minus 40 kilometers in the West. Excluding light extinction due to Rayleigh scatter, annual average background levels of fine and coarse particles are estimated to account for 14 Mm-1 in the East and about 6 Mm-1 in the West. Major contributors that reduce visibility from the Rayleigh maximum (approximately 330 km visual range) to the ranges noted above are naturally-occurring organics, water, and suspended dust (including coarse particles) in the East, and suspended dust, organics, and water in the West. In these ranges of fine particle concentrations, small changes have a large effect on total extinction. Thus, one can see from Table VIII-2 that higher levels of background fine particles and associated humidity in the East result in a fairly significant difference between estimates of naturally- occurring visual range in the East and West.