The U.S. Environmental Protection Agency (EPA) issued initial visibility regulations in 19801 that address visibility impacts that could be “reasonably attributed” to individual emissions sources. In 1999, EPA issued further regulations, known as the Regional Haze Rule (RHR),2 that address visibility impairment over a broad geographic region that is attributable to emissions from multiple sources. The RHR calls for each state to adopt emissions control strategies that would reduce haze at every Class I area impacted by its emissions (even if the area is in another state) on the 20% haziest days at that area, without degrading visibility on the 20% clearest days.
This concept is illustrated in Figure 2. The degree of haziness is represented by a haze index, in units of deciviews, which increases with the degree of visibility impairment and in which equal increments are intended to represent approximately equally perceptible changes in visibility. The straight solid diagonal line represents the “uniform rate of progress” (in deciviews) from five-year (2000–2004) baseline visibility conditions to the natural visibility conditions goal in 2064. States must establish a “reasonable rate of progress” toward the national visibility goal for each Class I area by considering the uniform rate of progress and then factoring in the cost, time, and other consequences associated with the requisite emissions controls needed to attain that rate of progress.
The first RHR-required State Implementation Plans (SIPs), which address the emissions strategies for the first phase of haze mitigation (up to 2018), are due at the end of 2007. During the course of the 60-year program, the performance of the emissions controls in yielding visibility improvements at the state-selected reasonable rate of progress is to be evaluated every five years and emissions reduction strategies are to be revised (and new SIPs submitted) every 10 years.
Quantifying the Haze
Instead of characterizing the haze by means of an optical measurement, the RHR calls for measuring aerosol species concentrations to estimate average daily haze levels. This estimate is possible because particle sizes, composition, and concentrations affect the amount of light that is scattered and absorbed by particles. The combined effect constitutes particle light extinction, which is defined as the fraction of light that is attenuated over a unit distance (typically expressed as per million meters [Mm-1]). The amount of particle light extinction is estimated by multiplying the concentration of each of the major components of PM (i.e., fine sulfates, nitrates, organics, black carbon, fine soil, coarse matter, and sometimes fine sea salt) by an extinction efficiency factor that accounts for the typical particle size and composition of each component. The extinction efficiency represents the amount of light extinction per unit mass concentration of the component under the assumed conditions. The light extinction contributions by each component can then be summed to get a measure of the total extinction by the haze.