Improved atmospheric constraints on Southern Ocean CO2 exchange.

We present improved estimates of air-sea CO2 exchange over three latitude bands of the Southern Ocean using atmospheric CO2 measurements from global airborne campaigns and an atmospheric 4-box inverse model based on a mass-indexed isentropic coordinate (Mθe). These flux estimates show two features not clearly resolved in previous estimates based on inverting surface CO2 measurements: a weak winter-time outgassing in the polar region and a sharp phase transition of the seasonal flux cycles between polar/subpolar and subtropical regions. The estimates suggest much stronger summer-time uptake in the polar/subpolar regions than estimates derived through neural-network interpolation of pCO2 data obtained with profiling floats but somewhat weaker uptake than a recent study by Long et al. [Science 374, 1275-1280 (2021)], who used the same airborne data and multiple atmospheric transport models (ATMs) to constrain surface fluxes. Our study also uses moist static energy (MSE) budgets from reanalyses to show that most ATMs tend to have excessive diabatic mixing (transport across moist isentrope, θe, or Mθe surfaces) at high southern latitudes in the austral summer, which leads to biases in estimates of air-sea CO2 exchange. Furthermore, we show that the MSE-based constraint is consistent with an independent constraint on atmospheric mixing based on combining airborne and surface CO2 observations.

Strong Southern Ocean carbon uptake evident in airborne observations.

The Southern Ocean plays an important role in determining atmospheric carbon dioxide (CO2), yet estimates of air-sea CO2 flux for the region diverge widely. In this study, we constrained Southern Ocean air-sea CO2 exchange by relating fluxes to horizontal and vertical CO2 gradients in atmospheric transport models and applying atmospheric observations of these gradients to estimate fluxes. Aircraft-based measurements of the vertical atmospheric CO2 gradient provide robust flux constraints. We found an annual mean flux of ­0.53 ± 0.23 petagrams of carbon per year (net uptake) south of 45°S during the period 2009­2018. This is consistent with the mean of atmospheric inversion estimates and surface-ocean partial pressure of CO2 (Pco2)­based products, but our data indicate stronger annual mean uptake than suggested by recent interpretations of profiling float observations.

Determining air-water exchange, spatial and temporal trends of freely dissolved PAHs in an urban estuary using passive polyethylene samplers.

Passive polyethylene (PE) samplers were deployed at six locations within Narragansett Bay (RI, USA) to determine sources and trends of freely dissolved and gas-phase polycyclic aromatic hydrocarbons (PAHs) from May to November 2006. Freely dissolved aqueous concentrations of PAHs were dominated by fluoranthene, pyrene, and phenanthrene, at concentrations ranging from tens to thousands of pg/L. These were also the dominant PAHs in the gas phase, at hundreds to thousands of pg/m3. All stations mostly followed the same temporal trends, with highest concentrations (up to 7300 pg/L for sum PAHs) during the second of 11 deployments, coinciding with a major rainstorm. Strong correlations of sum PAHs with river flows and wastewater treatment plant discharges highlighted the importance of rainfall in mobilizing PAHs from a combination of runoff and atmospheric washout. PAH concentrations declined through consecutive deployments III to V, which could be explained by an exponential decay due to flushing with cleaner ocean water during tides. The estimated residence time (tres) of the PAH pulse was 24 days, close to an earlier estimate of tres of 26 days for freshwater in the Bay. Air-water exchange gradients indicated net volatilization of most PAHs closest to Providence. Further south in the Bay, gradients had changed to mostly net uptake of the more volatile PAHs, but net volatilization for the less volatile PAHs. Based on characteristic PAH ratios, freely dissolved PAHs at most sites originated from the combustion of fossil fuels; only two sites were at times affected by fuel spill-derived PAHs.

Dietary uptake from historically contaminated sediments as a source of PCBs to migratory fish and invertebrates in an urban estuary.

Migratory fish and invertebrate samples were analyzed for polychlorinated biphenyls (PCBs) to study bioaccumulation in an urbanized estuary in the northeastern USA. Fish were also analyzed for (13)C, (15)N, and (34)S ratios. Results from several approaches (stable isotopes, total PCB concentrations, congener ratios, and bioaccumulation factors, BAFs) suggested that the fish and invertebrates fell into two distinct dietary groups: the more planktonic butterfish and squid versus a benthic group composed of lobsters, scups, and crabs. Both benthic and pelagic fish obtained the majority of their PCB body burdens from the sediments. Lobsters seemed to have an additional uptake from sediment particles, as observed by an increase in highly chlorinated congeners' bioaccumulation. BAFs were calculated relative to passive sampling-derived dissolved concentrations of PCBs. BAFs exceeded K(ow) values, implying that PCBs were accumulated beyond equilibrium partitioning with the water column. This was supported by comparison of chemical activity gradients, which suggested chemical activities of hexa- and heptachlorobiphenyls in biota exceeded those in water and porewater, but not for tetra- and pentachlorobiphenyls in squids and butterfish.

Detecting air-water and surface-deep water gradients of PCBs using polyethylene passive samplers.

Polyethylene passive samplers (PEs) were deployed in a vertical array (bottom water, surface water, near-surface air) to study the cycling of active polychlorinated biphenyls (PCBs) between reservoirs in an urban estuary (Narragansett Bay, RI), from May to November 2006. Performance reference compounds were used to account for nonequilibrium of PCBs in PEs. Activity gradients were established from direct comparisons of temperature, salt, and nonequilibrium corrected PE concentrations. The uncertainty of determining air-water gradients was < 70%, and < 50% within the water column. Except during the height of summer, PCB activities were up to 30 times higher in the air than in the surface water, but closer to equilibrium in the water column. Surface waters became depleted in PCBs during periods of highest temperature and stratification, leading to the uptake of gaseous PCBs. Our results demonstrate that passive samplers are powerful tools to determine the flux directions of organic contaminants in the environment.