Future year (2028) source apportionment modeling to support Regional Haze Rule planning in the western U.S.

ABSTRACT

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 (NO_x), 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, NO_x 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). NO_x 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.

Acknowledgement

The results of this paper were made possible by the air quality and emissions modeling team at Ramboll of Tejas Shah, Marco Rodriguez, Chao-Jung Chien and Pradeepa Vennam. 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 author(s).

Data availability statement

The data that support the findings of this study are openly available at the Western Regional Air Partnership’s Technical Support System (WRAP TSS):

http://views.cira.colostate.edu/tssv2/Express/ModelingTools.aspx

http://views.cira.colostate.edu/tssv2/Express/EmissionsTools.aspx

Additional information

Notes on contributors

Michael Barna

Michael Barna is a physical scientist with the Air Resources Division of the National Park Service. His work focuses on identifying emission sources that impact air quality at national parks and other sensitive ecosystems through the application of regional air quality models.

Ralph Morris

Ralph Morris is a Managing Principal at Ramboll Environment and Health. He has over 40 years of experience in the development and application of air quality models including as one of the leads in the development of the Comprehensive Air-quality Model with extensions (CAMx).

Patricia Brewer

Patricia Brewer worked for the National Park Service Air Resources Division in regional haze and acid deposition science and policy until retirement in 2019. From 2019-2021 she supported the Western Regional Air Partnership in delivery of regional haze modeling analyses for use in western states’ regional haze implementation plans.

Tom Moore

Tom Moore is the Technical Services Program manager for the State of Colorado’s Air Pollution Control Division in the Dept. of Public Health and the Environment. He previously worked for WESTAR/WRAP for more than 20 years, coordinating and managing the regional analyses and planning support for visibility, ozone, and other western U.S. regional programs and issues.

Gail Tonnesen

Gail Tonnesen is an air quality modeler at the EPA Region 8 office in Denver, CO.

Kevin Briggs

Kevin Briggs is the Regional Modeling and Emission Inventory Supervisor at the Colorado Department of Public Health and Environment-Air Pollution Control Division. He has been at the Colorado APCD for 33 years and leads the regional modeling and emission inventory development efforts for such projects as the State Implementation Plans.