Understanding High Gaseous Elemental Mercury at Carlsbad Caverns National Park During CarCavAQS-21: Insights from Observations and CMAQ-newHg-Br v2 Simulations 
Lin Wu1, Huiting Mao1, Winston Luke2, and Barkley C. Sive3
1 Department of Chemistry, State University of New York, College of Environmental Science and Forestry, Syracuse, NY, 2 Air Resources Laboratory, National Ocean and Atmospheric Administration, College Park, MD, 3 Air Resources Division, National Park Service, Denver, CO
While mercury (Hg) pollution has been studied extensively in urban and highly polluted areas, work in Clean Air Act defined Class I areas, including National Parks in the United States (U.S.), remains limited. Measurement from the 2021 Carlsbad Caverns Air Quality Study (CarCavAQS-21) showed that Carlsbad Caverns National Park (CAVE), located in the southwestern U.S., experienced higher concentrations of gaseous elemental mercury (GEM) than most northeastern monitoring sites typically influenced by anthropogenic sources. One-month simulations (19 July - 19 August 2021), designed for the field intensive period, were conducted using CMAQ-newHg-Br v2 (Wu et al., 2024), which includes updated Hg redox chemistry and a theoretical gas-particle partitioning scheme. Our model simulations revealed significant contributions of transboundary transport from Mexico to ambient Hg concentrations at CAVE, reflected in episodic increases of up to 14% (27 ppqv) for GEM, 58% (1.2 ppqv) for gaseous oxidized mercury (GOM) and 88% (4.0 pg/m3) for particulate-bound mercury (PBM). In contrast, the direct impact of oil and natural gas (O&G) emissions from the Permian Basin - one of the most productive O&G regions in the U.S., on whose edge CAVE is located - was limited, accounting for <1% of total Hg emissions. However, their indirect effects via O3-driven oxidation of GEM could be non-negligible as a result of O3 increases of 31% (~14 ppbv) on average. During three episodes of peak O&G influence, GOM and PBM increased by ~5% on average while GEM decreased slightly (0.2%). Gas-particle partitioning was found to play an important role in shaping the vertical distribution of PBM in the planetary boundary layer. These findings suggested that the southwestern U.S. may represent an underrecognized region of elevated GEM background concentrations and provided quantitative evidence that O&G emissions and transboundary transport from Mexico could together shape atmospheric Hg cycling in the region.