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<p>Mercury (<span class="inline-formula">Hg</span>) is a global atmospheric pollutant. In its oxidized form (<span class="inline-formula">Hg^II</span>), it can readily deposit to ecosystems, where it may bioaccumulate and cause severe health effects. High <span class="inline-formula">Hg^II</span> concentrations are reported in the free troposphere, but spatiotemporal data coverage is limited. Underestimation of <span class="inline-formula">Hg^II</span> by commercially available measurement systems hinders quantification of <span class="inline-formula">Hg</span> cycling and fate. During spring–summer 2021 and 2022, we measured elemental (<span class="inline-formula">Hg⁰</span>) and oxidized <span class="inline-formula">Hg</span> 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 <span class="inline-formula">Hg</span> concentrations displayed diel and episodic behavior similar to previous work at SPL but were approximately 3 times higher in magnitude due to improved measurement accuracy. We identified 18 multi-day events of elevated <span class="inline-formula">Hg^II</span> (mean enhancement of 36 <span class="inline-formula">pg m⁻³</span>) that occurred in dry air (mean <span class="inline-formula">±</span> SD of relative humidity <span class="inline-formula">=</span> 32 <span class="inline-formula">±</span> 16 %). Lagrangian particle dispersion model (HYSPLIT–STILT, Hybrid Single-Particle Lagrangian Integrated Trajectory–Stochastic Time-Inverted Lagrangian Transport) 10 <span class="inline-formula">d</span> back trajectories showed that the majority of transport prior to events occurred in the low to middle free troposphere. Oxidized <span class="inline-formula">Hg</span> was anticorrelated with <span class="inline-formula">Hg⁰</span> during events, with an average (<span class="inline-formula">±</span> SD) slope of <span class="inline-formula">−</span>0.39 <span class="inline-formula">±</span> 0.14. We posit that event <span class="inline-formula">Hg^II</span> resulted from upwind oxidation followed by deposition or cloud uptake during transport. Meanwhile, sulfur dioxide measurements verified that three upwind coal-fired power plants did not influence ambient <span class="inline-formula">Hg</span> at SPL. Principal component analysis showed <span class="inline-formula">Hg^II</span> consistently inversely related to <span class="inline-formula">Hg⁰</span> and generally not associated with combustion tracers, confirming oxidation in the clean, dry free troposphere as its primary origin.</p>