particles and gases that impair visibility and, to the extent possible, to apportion the impairment among contributing sources. The source apportionment methods (see Chapter 5 and Appendix C) should be appropriate to the temporal and geographic scale of the visibility problem. A visibility strategy could include controls on several different pollutants and source types. Although, sulfur dioxide, the precursor to sulfates, is often the most important single cause of impairment, control of fine and coarse particulate matter, oxides of nitrogen, ammonia (which is possibly a limiting factor in nitrate formation), and volatile organic compounds (VOCs) also must be considered.

In this chapter the committee provides an example, using a speciated rollback model, to apportion anthropogenic light extinction among source types in the eastern, southwestern, and northwestern United States. This exercise is not intended as an operational evaluation of regional haze in those regions. Instead, it is presented to illustrate some of the issues that arise in any apportionment of visibility impairment.

The next step in designing a strategy is to determine whether control measures exist or can be developed to reduce the emissions that impair visibility. Appendix D describes control options for the principal source categories identified in the example. (There could, of course, be situations where other sources are also of concern.) Many of these control techniques are effective and commercially available; others are still under development.

Next, it is necessary to assess the effectiveness of alternative control measures. The committee used the speciated rollback modeling exercise to estimate the visibility improvements that would result from application of commercially available pollution control methods and technologies to major contributing sources in the three regions modeled. The committee's analysis shows that the application of these controls could noticeably improve visibility in the areas modeled.

This exercise is not intended to signify the committee's recommendation for adopting any specific control strategy. Such a decision involves issues outside the bounds of science and the committee's expertise. Rather, the exercise allows an estimate to be made of the extent to which visibility could be improved with commercially available technology. Others must decide whether the public welfare requires the improvement and whether a program to do so would best use a technology-based approach or some alternative, such as an air-quality management

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