Recommended metric for tracking visibility progress in the regional haze rule
For many Class I areas (national parks and wilderness areas with special air quality protections) in the western U.S., wildfire smoke and dust events can have a large impact on visibility. Currently, the Environmental Protection Agency’s (EPA) 1999 Regional Haze Rule uses the 20% haziest days to track visibility changes over time even if they are dominated by smoke or dust. Visibility on the 20% haziest days has remained constant or degraded over the last 16 years at some Class I areas despite widespread emission reductions from anthropogenic sources. To better track visibility changes specifically associated with anthropogenic pollution sources rather than natural sources, the EPA as finalized a requirement to track visibility on the 20% most anthropogenically-impaired days rather than the haziest days. To support the implementation of this requirement, the EPA has also proposed (but not finalized) a specific recommended metric for characterizing the anthropogenic and natural portions of the daily extinction budget at each site. This metric selects the 20% most impaired days based on these portions using a “delta deciview” approach to quantify the deciview scale impact of anthropogenic light extinction. Using this metric, sulfate and nitrate make up the majority of the anthropogenic extinction in 2015 on these days, with natural extinction largely made up of organic carbon mass in the eastern U.S. and a combination of organic carbon mass, dust components, and sea-salt in the western U.S. For sites in the western U.S., the seasonality of days selected as the 20% most impaired is different than the seasonality of the 20% haziest days, with many more winter and spring days selected. Applying this new metric to the 2000–2015 period across sites representing Class I areas results in substantial changes in the calculated visibility trend for the Northern Rockies and southwest U.S.
Changing the approach for tracking visibility in Regional Haze Rule would allow the EPA, States, and the public to track visibility on days when reductions in anthropogenic emissions have to greatest potential to improve the view. The calculations involved with the recommended metric can be incorporated into the routine IMPROVE data processing, enabling rapid analysis of current and future visibility trends. Natural visibility conditions are important in the calculations for the recommended metric, necessitating the need for additional analysis and potential refinement of their values.