Air Quality Program Manager WESTAR/WRAP Fort Collins, CO
There are 156 Federally-mandated Class I Areas (CIAs) that are offered special visibility protection with most of them being in the western U.S. Section 7491 of the Clean Air Act (CAA) has a national goal to remedy any existing visibility impairment at CIAs that is caused by manmade air pollution. The regional haze rule addresses this national goal by requiring states to show they are making progress toward natural conditions by 2064 for the 20% Most Impaired Days (MID) and have no worsening in visibility for the 20% clearest days. EPA has developed the IMPROVE visibility impairment metric (focusing on the 20% MID) that assumes visibility extinction due to ammonium sulfate and ammonium nitrate is anthropogenic in origin and screens out days with high fire and windblown dust impacts using, respectively, extinction due to carbon and crustal material as proxies. The second round of RH state implementation plans are to show progress by 2028 toward natural conditions that are estimated based on IMPROVE 2000-2014 observations. EPA defines the Uniform Rate of Progress (URP) Glidepath as the straight line (in deciview) from a 2000-2004 IMPROVE MID Baseline to natural conditions in 2064. Photochemical modeling is used to project visibility in 2028 that is compared to the URP Glidepath at 2028 to see whether the CIA is on a path to natural visibility conditions in 2064 (i.e., on or below the URP Glidepath). This paper discusses the western states regional haze modeling and an alternative approach for showing progress toward no manmade impairment at CIAs 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 (RepBase2 representing 2014-2018), the 2028 future year (2028OTBa2) and a 2002 past year (2002 Hindcast). The CAMx Particulate Source Apportionment (PSAT) tool was used to isolate the contributions of U.S. anthropogenic, international anthropogenic, fires and natural sources to PM concentrations and visibility extinction at CIAs in the three modeling years. International anthropogenic, fire, and natural emissions were held at RepBase2 levels for 2002 Hindcast and 2028 simulations. Only U.S. anthropogenic emissions differ between the 2002, RepBase2, and 2028 model runs. A RoP trend line is drawn from the 2002 Hindcast simulation total visibility (in deciview) to visibility in 2064 with no visibility contributions from U.S. anthropogenic emissions, where the CAMx 2028 PSAT results are used for the no U.S. anthropogenic visibility contributions 2064 end-point. The CAMx total visibility results for RepBase2 and 2028 are then compared with the RoP trend line to determine whether visibility at CIAs are on a path toward no U.S. anthropogenic emission contributions by 2064. The U.S. anthropogenic RoP approach has several advantages over the URP Glidepath because it does not rely on uncertain and unknown parameters (e.g., IMPROVE MID, natural conditions, and emissions from fires and international sources) and focuses on what we know best and is available to control (i.e., U.S. anthropogenic emissions).