In the oligotrophic North Atlantic and North Pacific, ultrafiltration studies show that concentrations of soluble iron and soluble iron-binding organic ligands are much lower than previously presumed "dissolved" concentrations, which were operationally defined as that passing through a 0.4-micrometer pore filter. Our studies indicate that substantial portions of the previously presumed "dissolved" iron (and probably also iron-binding ligands) are present in colloidal size range. The soluble iron and iron-binding organic ligands are depleted at the surface and enriched at depth, similar to distributions of major nutrients. By contrast, colloidal iron shows a maximum at the surface and a minimum in the upper nutricline. Our results suggest that "dissolved" iron may be less bioavailable to phytoplankton than previously thought and that iron removal through colloid aggregation and settling should be considered in models of the oceanic iron cycle.

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Tropospheric ozone (O 3 ) is a trace gas playing important roles in atmospheric chemistry, air quality and climate change. In contrast to North America and Europe, longterm measurements of surface O 3 are very limited in China. We compile available O 3 observations at Mt. Tai -the highest mountain over the North China Plain -during 2003-2015 and analyze the decadal change of O 3 and its sources. A linear regression analysis shows that summertime O 3 measured at Mt. Tai has increased significantly by 1.7 ppbv yr −1 for June and 2.

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Surface-water samples were collected from Galveston Bay, Corpus Christi Bay, Sabine Lake, and Laguna Madre and analyzed for Hg in the <0.45-pm, 0.45~pm-l-kDa, and < 1-kDa fractions. Colloidal material was isolated with an Amicon cross-flow ultrafiltration system which was rigorously tested. Filter-passing Hg concentrations (< 0.45 pm) ranged from 0.12 to 13.6 pM, with the: highest values in Corpus Christi Bay. Within Galveston and Corpus Christi Bays, filter-passing Hg exhibited nonconservative estuarine mixing behavior. The colloidal Hg fraction ranged from 12 to 93% of the filter-passing pool and averaged 57 +20%, indicating that a major portion of the operationally defined "dissolved" Hg is associated with submicron particles and macromolecules > 1 kDa. Colloidal Hg covaries with colloidal organic C concentrations in Galveston Bay waters, suggesting that most of the filter-passing Hg pool is associated with large organic macromolecules. The log of the particle-water partition coefficient (&) ranged from 4.6 to 5.2, and covaried with suspended particulate matter concentrations, likely due to the presence of colloidal Hg.