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Abstract Mercury (Hg) is a bioaccumulative neurotoxin that can concentrate to potentially harmful levels in higher levels of marine food webs following conversion to methylmercury (MeHg). This is of public health concern as seafood is a main protein source for many in the Pacific region. To better understand Hg partitioning and transformations in the Pacific Ocean, Hg species and phases were measured along a meridional section from Alaska to Tahiti in 2018. This allowed the description of Hg concentrations and speciation under a variety of biogeochemical conditions such as the Alaskan shelf, the oligotrophic North Pacific gyre, and near the hydrothermally active Loihi seamount. Filtered HgT concentrations were elevated below 1,000 m near the Loihi Seamount with an average concentration of 1.45 pM, possibly indicating enrichment from hydrothermal venting. Filtered MeHg concentrations were notably higher at depth at the equator and generally lower south of the equator. Total Hg in suspended particles was greatest in the upper 1,000 m near the Alaskan Shelf and decreased in concentration southward. Suspended particle MeHg was greatest in the surface ocean in the upper 300 m near the Intertropical Convergence Zone (ITCZ). For both HgT and MeHg, particle‐associated concentrations appear to be related to organic fraction, and concentrations decreased southward. In general, all measured Hg species had greater concentrations in the northern than southern Pacific Ocean consistent with prior measurements. 

Mercury (Hg) is a global pollutant with substantial human health impacts. While most studies focus on atmospheric total Hg (THg) deposition, contributions of methylated Hg (MeHg), including monomethylmercury (MMHg) and dimethylmercury (DMHg), remain poorly understood. To examine this, we use rain and aerosol Hg speciation data and high-resolution surface DMHg measurements, collected on a transect from Alaskan coastal waters to the Bering and Chukchi Seas. We observed a significant fivefold increase in the MeHg:THg fraction in rain and a 10-fold increase for aerosols, closely linked to elevated surface DMHg and the highest DMHg evasion (~9.4 picomoles per square meter per hour) found in upwelling waters near the Aleutian Islands. These data highlight a previously underexplored aspect of MeHg air-sea exchange and its importance to Hg cycling and human health concerns. Our findings emphasize the importance of DMHg evasion by demonstrating that atmospheric MeHg can be transported long distances (~1700 kilometers) in the Arctic, posing risks to human health and ecosystems. 

Abstract Hygroscopic growth and cloud condensation nuclei activation are key processes for accurately modeling the climate impacts of organic particulate matter. Nevertheless, the microphysical mechanisms of these processes remain unresolved. Here we report complex thermodynamic behaviors, including humidity-dependent hygroscopicity, diameter-dependent cloud condensation nuclei activity, and liquid–liquid phase separation in the laboratory for biogenically derived secondary organic material representative of similar atmospheric organic particulate matter. These behaviors can be explained by the non-ideal mixing of water with hydrophobic and hydrophilic organic components. The non-ideality-driven liquid–liquid phase separation further enhances water uptake and induces lowered surface tension at high relative humidity, which result in a lower barrier to cloud condensation nuclei activation. By comparison, secondary organic material representing anthropogenic sources does not exhibit complex thermodynamic behavior. The combined results highlight the importance of detailed thermodynamic representations of the hygroscopicity and cloud condensation nuclei activity in models of the Earth’s climate system.