Impairment-Based Regional Haze Metric - Overview Scott Copeland - 02/19/2019
Visibility-reducing haze is caused by natural and anthropogenic sources. The Clean Air Act sets a goal of eliminating human-caused visibility impairment from Class I areas, which include certain National Parks, National Wildlife Refuges, and Wilderness Areas, by 2064. The United States Environmental Protection Agency’s (U.S. EPA) original 2003 Guidance for Tracking Progress Under the Regional Haze Rule1 was based on the removal of anthropogenic impairment on the 20% of days each year with the highest total haze (natural and anthropogenic). In the western United States, these days regularly include large amounts of haze from wildfire smoke and windblown dust. Meeting the Clean Air Act goals of removing human-caused haze by focusing on days dominated by sources that are not practically controllable is problematic.
U.S. EPA rule revisions and guidance published in 2016 and 2018 propose a new approach to track progress under the Regional Haze Rule2, which is to select 20% of the sample days from each year that have the highest anthropogenic impairment. These represent the days each year that have the largest apparent change in visibility from what would have existed with no anthropogenic haze and that are expected to be most sensitive to emissions control programs.
The Impairment Framework
Total extinction estimates from each daily IMPROVE sample are split into natural and anthropogenic contributions (see Impairment Framework Split and Sort). Anthropogenic impairment is defined as the difference between the total haze and natural haze, expressed in deciviews. Sorting by anthropogenic impairment produces the same result as sorting by the ratio of anthropogenic to total extinction. A deciview value based on total extinction is calculated for each of the 20% most anthropogenically impaired days. The mean of these values becomes the tracking progress metric for that year.
2064 Endpoints (also see Impairment Framework 2064 Endpoints)
Changing the metric for tracking progress requires new estimates of the 2064 goals in order to calculate the Uniform Rate of Progress. Natural haze levels change from year to year, but the expectation is that the average amount of natural haze on the most impaired days will remain roughly constant over time. Fifteen-year averages of the natural haze levels on the 20% most impaired days are used to derive the endpoint estimates.
The impairment framework minimizes issues caused by large natural haze events obscuring progress toward natural conditions. However, it introduces the issue of estimating natural haze levels for each IMPROVE sample day, which is challenging. The U.S. EPA draft guidance impairment algorithm relies heavily on National Acid Precipitation Assessment Program estimates of natural visibility conditions (Trijonis, 1990) as well empirical estimates of natural wildfire and dust events. This introduces problems as evidenced by the fact that measurements of organic carbon extinction across much of the eastern United States are already below the NAPAP estimates on an annual average basis. Therefore, the algorithm that splits daily total extinction into natural and anthropogenic contributions assigns all of the organic carbon extinction on every sample day at many eastern sites to the “natural” category.
The splitting of total extinction into natural and anthropogenic contributions to estimate anthropogenic impairment is only performed to sort the data and select the 20% most impaired days. The resultant daily estimates of natural and anthropogenic contributions to haze from each species should only be interpreted within the context of this sorting process and for estimating 2064 endpoints.
The impairment framework provides a metric that is relatively simple to implement and results that are not overly sensitive to the assumptions used to perform the natural/anthropogenic split (e.g., the precise definition of an episodic contribution). In the eastern United States, there is generally little difference between the haziest and most impaired days, with similar temporal trends for both metrics. In the West, sites with insignificant or increased trends in the haziest days metric show modest but steady reductions in haze with the impairment metric.
1 U.S. Environmental Protection Agency. Tracking Progress Under the Regional Haze Rule. EPA-454/B-03–004, Washington, DC, September 2003.
2 U.S. Environmental Protection Agency. Technical Guidance on Tracking Visibility Progress for the Second Implementation Period of the Regional Haze Program. EPA-454/R-18-010, Washington, DC, December 2018.