The natural geochemistry of tetrafluoromethane and sulfur hexafluoride : : studies of ancient Mojave Desert groundwaters, North Pacific seawaters and the summit emissions of Kilauea Volcano

Tetrafluoromethane (CF₄) and sulfur hexafluoride (SF₆) are potent, long-lived greenhouse gases whose natural atmospheric sources and sinks are poorly understood. CF₄ and SF₆ concentrations were measured in groundwater, deep and surface seawater, and volcanic gas samples to provide a better constraint on their lithospheric sources to the atmosphere. Groundwaters collected from the Mojave Desert and nearby Big Bear Lake Watershed contain CF₄ and SF₆ concentrations well in excess of air-saturated water concentrations for the conditions of recharge, providing in situ evidence for a crustal degassing of CF₄ and SF₆. Excess CF₄ and SF₆ concentrations can be attributed to release during weathering of the surrounding granitic alluvium and to a deeper crustal flux of CF₄ and SF₆ entering the study aquifers through the crystalline basement. The crustal flux of CF₄, but not SF₆, is enhanced in the vicinity of local active fault systems due to release of crustal fluids during episodic crustal fracturing driven by tectonic activity. When the crustal degassing rate of CF₄ and SF₆ into studied groundwaters is extrapolated to a global scale, it is consistent with the lithospheric flux required to sustain their preindustrial atmospheric abundances using best-estimate atmospheric lifetimes. CF₄ and SF₆ in volcanic emissions from Kilauea summit originate from air entrained into rising volcanic gases and from gases exsolved from Kilauea's hydrothermal system. An upper limit to a hypothetical volcanic flux of CF₄ and SF₆ is negligible when compared to the continental flux, indicating that the upper mantle is not a significant source of either gas to the atmosphere. Surface seawaters collected off of Scripps Pier during calm weather are in equilibrium with expected air- saturated seawater CF₄ concentrations. Deep Pacific seawater samples are oversaturated by roughly 4%, consistent with a predicted 5% oversaturation for these waters. The oceanic crust is therefore not a significant source of lithospheric CF₄. This suggests that CF₄ is conservative in seawater, and, combined with its rapid accumulation in the atmosphere, indicates that dissolved anthropogenic CF₄ concentrations are an effective time- dependent tracer of ocean circulation and mixing processe

Assessing California Groundwater Susceptibility Using Trace Concentrations of Halogenated Volatile Organic Compounds

Twenty-four halogenated volatile organic compounds (hVOCs) and SF₆ were measured in groundwater samples collected from 312 wells across California at concentrations as low as 10^–12 grams per kilogram groundwater. The hVOCs detected are predominately anthropogenic (i.e., “ahVOCs”) and as such their distribution delineates where groundwaters are impacted and susceptible to human activity. ahVOC detections were broadly consistent with air-saturated water concentrations in equilibrium with a combination of industrial-era global and regional hVOC atmospheric abundances. However, detection of ahVOCs in nearly all of the samples collected, including ancient groundwaters, suggests the presence of a sampling or analytical artifact that confounds interpretation of the very-low concentration ahVOC data. To increase our confidence in ahVOC detections we establish screening levels based on ahVOC concentrations in deep wells drawing ancient groundwater in Owens Valley. Concentrations of ahVOCs below the Owens Valley screening levels account for a large number of the detections in prenuclear groundwater across California without significant loss of ahVOC detections in shallow, recently recharged groundwaters. Over 80% of the groundwaters in this study contain at least one ahVOC after screening, indicating that the footprint of human industry is nearly ubiquitous and that most California groundwaters are vulnerable to contamination from land-surface activities

Food industries

Measurement of oxidized mercury, Hg­(II), in the atmosphere poses a significant analytical challenge as Hg­(II) is present at ultra-trace concentrations (picograms per cubic meter air). Current technologies are sufficiently sensitive to measure the total Hg present as Hg­(II) but cannot determine the chemical speciation of Hg­(II). We detail here the development of a soft ionization mass spectrometric technique coupled with preconcentration onto nano- or microparticle-based traps prior to analysis for the measurement of mercury halides in air. The current methodology has comparable detection limits (4–11 pg m^–3) to previously developed techniques for the measurement of total inorganic mercury in air while allowing for the identification of HgX₂ in collected samples. Both mercury chloride and mercury bromide have been sporadically detected in Montreal urban and indoor air using atmospheric pressure chemical ionization-mass spectrometry (APCI-MS). We discuss limitations and advantages of the current technique and discuss potential avenues for future research including quantitative trace measurements of a larger range of mercury compounds

Mercury Physicochemical and Biogeochemical Transformation in the Atmosphere and at Atmospheric Interfaces: A Review and Future Directions

Mercury Physicochemical and Biogeochemical Transformation in the Atmosphere and at Atmospheric Interfaces: A Review and Future Direction