The Western Regional Air Partnership (WRAP) has developed a modeling platform to simulate the formation of haze-causing particles that impact federally-protected lands in the western United States. To assist state air quality planners in determining which emission sources are likely candidates for future mitigation, several source apportionment scenarios were evaluated, and two sets of results for the year 2028 are presented here: 1) a "high-level important regional sources" version, with broad emission categories (i.e. U.S. anthropogenic, international anthropogenic, natural, and fires), and 2) a "low-level anthropogenic emission sources within individual states" version, which refines the U.S. anthropogenic contribution to specific emission sectors within individual WRAP region states. Eight examples are discussed, which reflect the variation in source apportionment results at national parks, wilderness areas, and wildlife refuges in the western U.S. and suggest which emission sectors are candidates for mitigation to improve future visibility. In 2028, the contribution of domestic anthropogenic emissions at the eight sites ranges from 17% to 58%, with significant impacts from oil and gas production, fossil fuel electric generation, and federally-regulated mobile sources. The contribution from international anthropogenic sources can also be considerable, and ranges from 17% to 43%. Most sectors that are emitting sulfur dioxide (SO2) and nitrogen oxides (NOx), which are the two most likely particle precursors to be curtailed in the states' Regional Haze plans, are declining. For example, in the 13 contiguous WRAP region states, NOx emissions from on-road mobile sources and electric generating units (EGUs) declined by 738 kton/yr (29% decrease) and 65 kton/yr (31% decrease), respectively, in 2028 as compared to current emission estimates, and SO2 emissions from EGUs declined by 42 kton/yr (29% decrease). NOx emissions from oil and gas development also declined by 25 kton/yr (9% decrease) but rose for SO2 emissions by 12 kton/yr (20% increase).Implications: The goal of the Regional Haze Rule (RHR) is to improve visibility at federally-protected areas, and to eventually arrive at natural conditions by the year 2064. Source apportionment tools within regional air quality models are useful for identifying which emission regions and sectors are contributing to haze-causing particles and can indicate to air quality planners where additional emission controls may be warranted.