Stable Isotopes Reveal Photoreduction of Particle-Bound Mercury Driven by Water-Soluble Organic Carbon during Severe Haze

Watershed runoff as a dominant pathway for mercury loading into a karst lake as unraveled by a novel mercury isotope and flux balance model

Identification of sources, transport pathways, and transformation mechanisms of mercury (Hg) is fundamental to our understanding of the biogeochemical cycling of Hg in lake ecosystems. In this study, we conducted a comprehensive investigation on the isotopic compositions of Hg in water columns, sediments, atmospheric deposition endmembers, watershed river, and runoff in Hongfeng Lake (HFL) ecosystem in southwestern China. We observed significant difference in δ202Hg between dissolved Hg (DHg) and particulate Hg (PHg) in lake, river, and runoff samples, with lighter isotopes preferentially adsorbed onto particulate matter. ∆199Hg was also significantly higher for DHg than PHg in lake and river waters, primarily due to the higher susceptibility of DHg to photochemical reduction processes. ∆199Hg and ∆200Hg of Hg in sediments were similar to those of PHg in water column, but significantly different from those of DHg, reflecting the dominant role of sedimentation of PHg from water column in the Hg accumulation in sediments. The isotopic compositions of Hg in lake water and sediments lay between those of atmospheric deposition and watershed runoff end-members. Using a Hg isotope mass balance model, we estimated that runoff input accounted for 62.4 % of total Hg input into the HFL ecosystem, followed by riverine input (14.5 %), particulate-bound Hg (PBM) dry deposition (11.0 %), Hg(II) wet deposition (5.5 %), Hg(0) dry deposition (4.2 %), and gaseous oxidized Hg (GOM) dry deposition (2.4 %). These results highlight the key role of watershed runoff as a source of Hg in lake ecosystems and improve our understanding of the biogeochemical cycling of Hg such ecosystems.

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

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 δ202Hg (-1.86 % to -0.32 %; mean = -1.17 ± 0.65 %, 2 SD, n = 34) and small variations in Δ199Hg (-0.27 % to 0.04 %; mean = -0.13 ± 0.14 %, 2 SD, n = 34) and Δ200Hg (-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 δ202Hg isotopic compositions of TGM at MLO were different from those of reported values of high-altitude mountains; the δ202Hg 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 Δ199Hg, 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.

Atmospheric Mercury Concentrations and Isotopic Compositions Impacted by Typical Anthropogenic Mercury Emissions Sources

Coal-fired power plants (CFPPs) and cement plants (CPs) are important anthropogenic mercury (Hg) emission sources. Mercury speciation profiles in flue gas are different among these sources, leading to significant variations in local atmospheric Hg deposition. To quantify the impacts of Hg emissions from CFPPs and CPs on local-scale atmospheric Hg deposition, this study determined concentrations and isotopes of ambient gaseous elemental mercury (GEM), particulate-bound mercury (PBM), and precipitation total Hg (THg) at multiple locations with different distances away from a CFPP and a CP. Higher concentrations of GEM and precipitation THg in the CFPP area in summer were caused by higher Hg emission from the CFPP, resulting from higher electricity demand. Higher concentrations of GEM, PBM, and precipitation THg in the CP area in winter compared to those in summer were related to the higher output of cement. Atmospheric Hg concentration peaked near the CFPP and CP and decreased with distance from the plants. Elevated GEM concentration in the CFPP area was due to flue gas Hg0 emissions, and high PBM and precipitation Hg concentrations in the CP area were attributed to divalent Hg emissions. It was estimated that Hg emissions from the CFPP contributed 58.3 ± 20.9 and 52.3 ± 25.9% to local GEM and PBM, respectively, and those from the CP contributed 47.0 ± 16.7 and 60.0 ± 25.9% to local GEM and PBM, respectively. This study demonstrates that speciated Hg from anthropogenic emissions posed distinct impacts on the local atmospheric Hg cycle, indicating that Hg speciation profiles from these sources should be considered for evaluating the effectiveness of emission reduction policies. This study also highlights the Hg isotope as a useful tool for monitoring environmental Hg emissions.

Photoreduction and origin of dissolved and particulate mercury in cloud water: Insights from stable mercury isotopes

The photoreduction of mercury (Hg) in clouds is crucial for determining global Hg cycling. The recently-developed isotope approach provides new insight into the fate of atmospheric Hg, however, limited data have been reported on the dynamics of Hg isotopes in clouds. This study presented the isotopic compositions of dissolved mercury (DHg) and particulate mercury (PHg) in cloud water collected at Mt. Tai (1545 m a.s.l.) in eastern China during summer 2021. Both DHg and PHg exhibited positive mass-independent fractionation of odd isotopes (odd-MIF, denoted as Δ199Hg), with averaged Δ199Hg values of 0.83 ± 0.34‰ and 0.20 ± 0.11‰, respectively. This high odd-MIF likely resulted from aqueous photoreduction in clouds, with DHg being more susceptible to photolysis than PHg. Our findings indicated that the photoreduction was promoted by sunlight and influenced by the chemical compositions of cloud water that controlled the Hg(II) speciation. The isotope mixing model estimation revealed that particulate-bound Hg and reactive gaseous Hg constituted the principal sources of Hg in cloud water, accounting for 55% to 99% of the total, while gaseous element Hg also made a notable contribution. Additionally, cloud water samples with faster reduction rates of Hg(II) were located outside of the isotope mixing models, which indicated an enhanced photoreduction process in cloud water.

Atmospheric Mercury Isotope Shifts in Response to Mercury Emissions from Underground Coal Fires

Pollutant emissions from coal fires have caused serious concerns in major coal-producing countries. Great efforts have been devoted to suppressing them in China, notably at the notorious Wuda Coalfield in Inner Mongolia. Recent surveys revealed that while fires in this coalfield have been nearly extinguished near the surface, they persist underground. However, the impacts of Hg volatilized from underground coal fires remain unclear. Here, we measured concentrations and isotope compositions of atmospheric Hg in both gaseous and particulate phases at an urban site near the Wuda Coalfield. The atmospheric Hg displayed strong seasonality in terms of both Hg concentrations (5-7-fold higher in fall than in winter) and isotope compositions. Combining characteristic isotope compositions of potential Hg sources and air mass trajectories, we conclude that underground coal fires were still emitting large amounts of Hg into the atmosphere that have been transported to the adjacent urban area in the prevailing downwind direction. The other local anthropogenic Hg emissions were only evident in the urban atmosphere when the arriving air masses did not pass directly through the coalfield. Our study demonstrates that atmospheric Hg isotope measurement is a useful tool for detecting concealed underground coal fires.