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Photochemical Alteration of Dissolved Organic Nitrogen in the Surface Ocean of the North Atlantic

Photochemical Alteration of Dissolved Organic Nitrogen in the Surface Ocean of the North Atlantic. Max Jacobson Daniel Sigman Katye Altieri Sarah Fawcett. Background: A Discrepancy. Sargasso sea water has <30nM ammonium Barely measurable

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Photochemical Alteration of Dissolved Organic Nitrogen in the Surface Ocean of the North Atlantic

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  1. Photochemical Alteration of Dissolved Organic Nitrogen in the Surface Ocean of the North Atlantic Max Jacobson Daniel Sigman Katye Altieri Sarah Fawcett

  2. Background: A Discrepancy Sargasso sea water has <30nM ammonium Barely measurable Ammonia effluxes from the surface of the oligotrophic oceanic gyre The measured efflux is significantly higher than the efflux calculated by Henry’s law (Johnson et al., 2008, Lipschultz 2001)

  3. NH4+ in the Nitrogen cycle NO3- NH4+ Particles NH4HSO4 (NH4)2SO4 NH4NO3 NaNO3 NH3(g) Rain [NH4+ ] = 3.5 μM DON NH4+ Oceanic concentrations from the Bermuda Atlantic Time-series Study (BATS) Lipschultz 2001, Knapp et al., 2005 4 μM <30 nM

  4. Project goal • To test the hypothesis that the photo-oxidation of the high concentration surface DON pool contributes to ammonia efflux from the subtropical surface ocean

  5. Testing the hypothesis • Design an apparatus and/or system to demonstrate the conversion of dissolved organic nitrogen (DON) into ammonium. • Test the closed system at Princeton and test the open system at the Bermuda Institute of Ocean Sciences.

  6. Methods • Acidified filters to capture NH3 efflux: Filter impregnation (Quinn 1990) • Extracted captured NH4+ from filters – sonication • Collected and filtered Sargasso seawater using 8L Niskin bottles • Orthophthalaldehde (OPA) fluorescence techniques for measuring nanomolar concentrations of NH4+(Holmes et al 1999) • Irradiated systems in ambient sunlight with dark control

  7. OPA Fluorescence Measures nM concentrations of ammonium Very accurate Example: Standard curve with concentrations 0, 10, 25, 100, 250, and 1000 nM

  8. The Closed System

  9. Testing the Closed System • Filters were exposed to air on a lab bench for 10, 20, 30, and 40 minutes to investigate contamination • Filters exposed to air for 10 minutes were stored in Ziploc bags for 10, 20, 30, and 40 minutes, 24 and 48 hours to assess contamination associated with storage • One filter was left in the closed system and another filter was left in a fume hood overnight to confirm that the system was air tight • The experiment was run with two bottles: one filled with 10mM NH4+ and the other with HPW

  10. Results Air exposure contaminated filters Storing in ziploc did not contaminate filters The closed system is definitely airtight: The filter left in the fume hood had several orders of magnitude more NH4+ than that stored in the closed system Exposure [NH4+] Time(minutes) Ziploc [NH4+] Time(minutes)

  11. The Open System Vacuum pump Filters (capture ammonium) Reflux Condenser Ring Stand and clamps Filter packs (removes ammonia from air)

  12. Testing the Open System • Physically construct the system • Develop and test the standard operating procedure in order to optimize the performance of the open system • Incubated filtered Sargasso Sea water collected during scientific cruise under sunlight and dark conditions

  13. Seawater collection Seawater was collected aboard the R/V Stommel using Niskin bottles and transferred to 4L polycarbonate bottles. The water was filtered through a 0.2 μm polycarbonate filter using vacuum filtration. Seawater was collected aboard the R/V Atlantic Explorer by inline filtration directly from the ship’s underway system.

  14. Lessons Learned • The temperature controlled experiments were supposed to be run on the Atlantic Explorer 2 day cruise. However, it rained continuously and irradiation experiments were not possible. • Ammonium samples could not be analyzed at BIOS. The samples were frozen and brought back to Princeton. • The fluorometer was stolen, and the samples could not be analyzed until it was replaced. • Results from the open system were inconclusive: contamination and/or detection limit problems (see above)

  15. Summary • Closed system: airtight limiting contamination from ambient air. Could be useful for concept testing if modified to increase air/water surface interaction. • Open system: more representative , can control temperature, enhance air/water surface interaction Future work • Continue experimenting with open system • Amend with higher ammonium concentration • Analyze samples same day to minimize storage and/or contamination issues

  16. Acknowledgements Princeton Environmental Institute Bermuda Institute of Ocean Sciences Andrew Peters Karen Ellis Amy Gobel Rosie Zhang

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