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- The distribution and speciation of mercury in the California Current: Implications for mercury transport via fog to land
- Unfiltered seawater samples from vertical profiles collected at 60 stations within the California Current during four summer cruises spanning a two-year period from 2014 to 2015, were analyzed for elemental mercury (Hg°), monomethyl mercury (MMHg), dimethyl mercury (DMHg) and total mercury (THg). Fog water samples, taken at sea and throughout a network of land based stations were also analyzed for MMHg and THg. Vertical profiles indicate that midwater regions around 300 m are associated with concentration maxima in methylated species. Cyclonic mesoscale eddies were shown to be strong sources of the gaseous mercury species to the lower atmosphere and a likely source of these species to fog. Calculated evasive flux of Hg0 and DMHg were greatest in these regions (34 and 11 pmol m−2 d−1, respectively), whereas anticyclonic eddies support little or no sea-air evasion. Incubation experiments showed that DMHg is stable over short time scales at natural seawater pH (7.8 to 8.2) but degrades rapidly to MMHg at low pH. Demethylation of only a small percent of the evading DMHg, on acidic marine aerosols associated with fog condensation nuclei, can account for over 100% of the MMHg observed in fog. The surface microlayer, enriched in MMHg (by 30×), may also contribute to sea-air flux through aerosol production. Neither shelf sediments nor oxygen minimum zones appear to be a major source of methylated mercury in the California Current.
- Coale, Heim, Negrey, Weiss-Penzias, Fernandez, Olson, Chiswell, Byington, Bonnema, Martenuk, Newman, Beebe, Till
- An evaluation of ISFET sensors for coastal pH monitoring applications
- Abstract The accuracy and precision of ion sensitive field effect transistor (ISFET) pH sensors have been well documented, but primarily by ocean chemistry specialists employing the technology at single locations. Here we examine their performance in a network context through comparison to discrete measurements of pH, using different configurations of the Honeywell DuraFET pH sensor deployed in six coastal settings by operators with a range of experience. Experience of the operator had the largest effect on performance. The average difference between discrete and ISFET pH was 0.005 pH units, but ranged from −0.030 to 0.083 among operators, with more experienced operators within ± 0.02 pH units of the discrete measurement. In addition, experienced operators achieved a narrower range of variance in difference between discrete bottle measurements and ISFET sensor readings compared to novice operators and novice operators had a higher proportion of data failing quality control screening. There were no statistically significant differences in data uncertainty associated with sensor manufacturer or deployment environment (pier-mounted, flowthrough system, and buoy-mounted). The variation we observed among operators highlights the necessity of best practices and training when instruments are to be used in a network where comparison across data streams is desired. However, while opportunities remain for improving the performance of the ISFET sensors when deployed by less experienced operators, the uncertainty associated with their deployment and validation was several-fold less than the observed natural temporal variability in pH, demonstrating the utility of these sensors in tracking local changes in acidification.
- McLaughlin, Dickson, Weisberg, Coale, Elrod, Hunter, Johnson, Kram, Kudela, Martz, Negrey, Passow, Shaughnessy, Smith, Tadesse, Washburn, Weis
- Factors influencing methylmercury contamination of black bass from California reservoirs
- Melwani, Negrey, Heim, Coale, Stephenson, Davis