Assessment of progress toward regional haze rule visibility goals using United States anthropogenic emissions rate of progress

ABSTRACT

The U.S. EPA developed the Regional Haze Rule to address Section 7491 of the 1977 Clean Air Act Amendments to prevent any future and remedy any existing visibility impairment due to manmade air pollution at Federal Class I areas (CIAs). The rule addresses this national goal by requiring states to show they are making progress toward estimated natural conditions by 2064 for the 20% anthropogenically Most Impaired Days (MID). For the MID, days that have high haze contributions from wildfires and windblown dust tend to be excluded using haze contributions from Carbon and crustal material as surrogates. To show progress toward natural conditions in 2064, a Uniform Rate of Progress Glidepath is defined as a straight line from measured 2000–2004 IMPROVE MID Baseline to natural conditions in 2064. Photochemical modeling is used to project the observed IMPROVE 2014–2018 MID visibility to 2028 that is compared to the Glidepath at 2028 to determine whether the MID visibility at a CIA is on a path toward natural visibility conditions in 2064. This paper discusses an alternative approach for showing progress toward no manmade impairment by using modeling results to generate a U.S. Anthropogenic Emissions Rate of Progress (RoP). The CAMx photochemical grid model was run for a current year (representing 2014–2018), 2028 future year and a 2002 past year and source apportionment was used to isolate the contributions of U.S. anthropogenic emissions to PM concentrations and visibility extinction. A RoP slope line is drawn from the 2002 visibility extinction due to U.S. anthropogenic emissions to zero in 2064 and the CAMx 2028 visibility for U.S. anthropogenic emissions is compared with the RoP slope line at 2028 to determine whether visibility due to U.S. anthropogenic emissions is on a path toward no U.S. manmade impairment in 2064.

Implications: The U.S. EPA Regional Haze Rule guidance to show progress toward no U.S. manmade visibility impairment at Class I Areas by 2064 backs into the U.S. manmade impairment contribution by using total atmospheric haze based on measured PM concentrations and subtracting uncertain estimates of routine natural and episodic (i.e. wildfires and windblown dust) natural conditions. The guidance also recommends accounting for visibility contributions due to international anthropogenic and prescribed fire emissions that are also uncertain. This paper presents an alternative approach that models the contributions of U.S. anthropogenic emissions to visibility for past, current and future years using source apportionment to show that U.S. anthropogenic emissions visibility impairment at Class I areas are on a path toward no contribution in 2064. Many U.S. anthropogenic emissions (e.g. power plants with continuous emissions monitoring systems) are better known and characterized than international, fire and natural emissions so the alternative approach should provide a better assessment of whether U.S. anthropogenic emissions are on a path toward no manmade impairment in 2064 than using trends in the measured visibility most impaired days that rely on uncertain estimates of haze due to wildfire, windblown dust, and international emissions and uncertain estimates of natural conditions in 2064.

Acknowledgment

The results of this paper were made possible by the air quality and emissions modeling team at Ramboll of Tejas Shah, Chao-Jung Chien, Yesica Alvarez and Pradeepa Vennam under the direction of the co-authors and the WRAP Regional Technical Operations Work Group with co-chairs Mike Barna of the National Park Service and Kevin Briggs of the Colorado Department of Public Health and Environment. The authors would also like to acknowledge Shawn McClure of Colorado State University who implemented the visualization of the modeling results on the WRAP Technical Support System. This paper does not reflect EPA policy, guidance or recommendations.

Disclosure statement

No potential conflict of interest was reported by the authors.

Data availability statement

The data that support the findings of this study are available on the Intermountain West Data Warehouse (IWDW) and Western Regional Air Partnership (WRAP) Technical Support System (TSS) websites. Specifically, the CAMx 2014 modeling platform files for the 2014v2, RepBase2, 2029OTBa2 and 2002 Hindcast emissions scenarios can be downloaded from the IWDW website (https://views.cira.colostate.edu/iwdw/RequestData/Default.aspx) with the definitions of the scenarios also available on the IWDW website (https://views.cira.colostate.edu/iwdw/Modeling/Platforms.aspx). The graphics and data used in the URP Glidepath displays are available from Chart 4 and the data used in the U.S. Anthropogenic Emissions Rate of Progress (RoP) displays are available in Chart 6 of the WRAP TSS Modeling Express Tools website (https://views.cira.colostate.edu/tssv2/Express/ModelingTools.aspx).