Elevated oxidized mercury in the free troposphere: Analytical advances and application at a remote continental mountaintop site

Abstract. Mercury (Hg) is a global atmospheric pollutant. In its oxidized form (Hg^II), atmospheric Hg can readily deposit to ecosystems, where it may bioaccumulate and cause severe health effects. High Hg^II concentrations are reported in the free troposphere, but spatiotemporal data coverage is limited. Underestimation of Hg^II by commercially available measurement systems hinders quantification of Hg cycling and fate. During spring-summer 2021 and 2022, we measured elemental (Hg⁰) and oxidized Hg using a calibrated dual-channel system alongside trace gases, aerosol properties, and meteorology at the high-elevation Storm Peak Laboratory (SPL) above Steamboat Springs, Colorado. Oxidized Hg concentrations displayed temporal behavior similar to previous work at SPL, but were approximately three times higher in magnitude due to improved measurement accuracy. We identified 18 multi-day events of elevated Hg^II (mean enhancement: 36 pg m^-3) that occurred in dry air (mean ± s.d. RH = 32 ± 16 %). Lagrangian particle dispersion model (HYSPLIT-STILT) 10-day back-trajectories showed that the majority of transport prior to events occurred in the low to mid-free troposphere. Oxidized Hg was anticorrelated with Hg⁰ during events, with an average (± s.d.) slope of -0.39 ± 0.14, suggestive of upwind oxidation followed by deposition during transport. Concurrent sulfur dioxide measurements verified that three upwind coal-fired power plants did not measurably contribute ambient Hg at SPL. Principal Components Analysis revealed Hg^II consistently inversely related with Hg⁰ and was generally not associated with combustion tracers, confirming oxidation in the clean, dry free troposphere as its primary origin.