Unraveling atmospheric mercury dynamics at Mauna Loa through the isotopic analysis of total gaseous mercury

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Our investigation seeks to uncover the intricate nature of mercury dynamics in the free troposphere through analysis of the isotopic composition of total gaseous elemental mercury (TGM) at the high altitude Mauna Loa Observatory (MLO, 3397 m) in Hawaii, USA. By focusing on this unique site, we aim to provide essential insights into the behavior and cycling of mercury, contributing valuable data to a deeper understanding of its global distribution and environmental impacts. Forty-eight hours of TGM sampling from January to September 2022 revealed significant variations in δ 202 Hg (−1.86 % to −0.32 %; mean = −1.17 ± 0.65 %, 2 SD, n = 34) and small variations in Δ 199 Hg (−0.27 % to 0.04 %; mean = −0.13 ± 0.14 %, 2 SD, n = 34) and Δ 200 Hg (−0.20 % to 0.06 %; mean = −0.05 ± 0.13 %, 2 SD, n = 34). During the sampling period, GEM was negatively correlated with gaseous oxidized mercury (GOM). However, the GOM/GEM ratio was not −1, suggesting that GEM oxidation and subsequent scavenging occurred previously. The δ 202 Hg isotopic compositions of TGM at MLO were different from those of reported values of high-altitude mountains; the δ 202 Hg of TGM at MLO was lower than the isotopic ratios that were obtained from other mountain regions. The unique atmospheric conditions at Mauna Loa, with (upslope winds during the day and downslope winds at night, likely result in the) possibly mixing of GEMs from terrestrial (and possibly oceanic GEM emission) sources with and tropospheric sources, influencing and affect the isotopic composition. During the late summer to early fall (September 14–28), negative correlations were found between relative humidity and GOM and between particle number concentrations and Δ 199 Hg, indicating the gas-to-particle partitioning of the atmospheric mercury during this period. This study will improve our understanding on mercury dynamics of marine origin and high altitudes and shed light on its complex interactions with environmental factors. • Hg-isotope ratios of GEM collected at MLO were analyzed. • A negative GOM/GEM correlation indicates GEM oxidation and subsequent scavenging. • GEM oxidation and subsequent GOM partitioning into aerosols may have occurred.

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