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NATURAL pH OF RAIN

NATURAL pH OF RAIN. Equilibrium with natural CO 2 (280 ppmv) results in a rain pH of 5.7:. This pH can be modified by natural acids (H 2 SO 4 , HNO 3 , RCOOH…) and bases (NH 3 , CaCO 3 ) e natural rain has a pH in range 5-7. “Acid rain” refers to rain with pH < 5 e damage to ecosystems.

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NATURAL pH OF RAIN

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  1. NATURAL pH OF RAIN • Equilibrium with natural CO2 (280 ppmv) results in a rain pH of 5.7: • This pH can be modified by natural acids (H2SO4, HNO3, RCOOH…) and bases (NH3, CaCO3) e natural rain has a pH in range 5-7 “Acid rain” refers to rain with pH < 5 e damage to ecosystems

  2. PRECIPITATION PH OVER THE UNITED STATES

  3. CHEMICAL COMPOSITION OF PRECIPITATION Neutralization by NH3 is illusory because NH4+g NH3 + H+ in ecosystem

  4. LONG-TERM TREND IN US SO2 EMISSIONS

  5. Power plant emission trend: “NOx SIP Call” Long-term trend in US NOx emissions

  6. Sulfate wet deposition and aerosol concentrations, 1980-2010 Leibensperger et al. [2011]

  7. Ammonium wet deposition and aerosol concentrations, 1980-2010 Leibensperger et al. [2011]

  8. Nitrate wet deposition and aerosol concentrations, 1980-2010 Leibensperger et al. [2011]

  9. TREND IN FREQUENCY OF ACID RAIN (pH < 5) 1994-1996 2002-2004 Lehmann et al. [2007]

  10. BUT ECOSYSTEM ACIDIFICATION IS PARTLY A TITRATION PROBLEM FROM ACID INPUT OVER MANY YEARS Acid flux FH+ Acid-neutralizing capacity (ANC) from CaCO3 and other bases

  11. BIOGEOCHEMICAL CYCLING OF MERCURY ATMOSPHERE Hg (gas) combustion industry mining volcanoes erosion deposition re-emission SOIL OCEAN burial SEDIMENTS DEEP EARTH

  12. Electronic structure of mercury Mass number = 80: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 • Complete filling of subshells gives elemental Hg(0) stability, high volatility • Two stable oxidation states: Hg(0) and Hg(II)

  13. Orbital energies vs. atomic number Energetic arrangement of orbitals is such that mercury (Z=80) has all its subshells filled

  14. BIOGEOCHEMICAL CYCLING OF MERCURY ATMOSPHERE Hg (gas) combustion industry mining volcanoes erosion deposition re-emission SOIL OCEAN burial SEDIMENTS DEEP EARTH

  15. Global mercury deposition has roughly tripled since preindustrial times RISING MERCURY IN THE ENVIRONMENT Dietz et al. [2009[

  16. Tuna is the #1 contributor HUMAN EXPOSURE TO MERCURY IS MAINLY FROM FISH CONSUMPTION Mercury biomagnification factor State fish consumption advisories EPA reference dose (RfD) is 0.1 μg kg-1 d-1 (about 2 fish meals per week)

  17. MERCURY CYCLING INVOLVES CHEMICAL TRANSFORMATION elemental mercury VOLATILE mercuric compounds WATER-SOLUBLE ATMOSPHERE Hg(0) Hg(II) oxidation TRANSPORT deposition re-emission Hg(0) Hg(II) reduction SURFACE RESERVOIRS (Ocean, Land) microbes MeHg Methylmercury TOXIC

  18. ELEMENTAL MERCURY IS GLOBALLY DISTRIBUTED IN ATMOSPHERE Human emission (2006) Mean Hg(0) concentration in surface air: circles = observed, background = model Transport around northern mid-latitudes: 1 month Transport to southern hemisphere: 1 year Streets et al. [2009]; Soerensen et al. [2010]

  19. High-temperature combustion emits both Hg(0) and Hg(II) LOCAL POLLUTION INFLUENCE FROM EMISSION OF Hg(II) 60% Hg(0) GLOBAL MERCURY POOL Photoreduction? 40% Hg(II) NEAR-FIELD WET DEPOSITION Hg(II) concentrations in surface air: circles = observed, background=model MERCURY DEPOSITION “HOT SPOT” Large variability of Hg(II) implies atmospheric lifetime of only days against deposition Thus mercury is BOTH a global and a local pollutant! Selin et al. [2007]

  20. MERCURY WET DEPOSITION FLUXES,2004-2005 Circles: observations Background: GEOS-Chem model Model contribution from North American anthropogenic sources Model contribution from external sources Selin and Jacob [2008]

  21. SOURCE ATTRIBUTION FOR U.S. MERCURY DEPOSITION % contribution of North American sources to annual total mercury deposition Legacy anthropogenic re-emitted from soil and ocean on centurial time scale (17%) Natural (32%) North American anthropogenic (20%) Rest of world anthropogenic (31%) Selin and Jacob [2008]

  22. Historical inventory of global anthropogenic Hg emissions • Large legacy contribution from N. American and European emissions; Asian dominance is a recent phenomenon • Time integrals of global emissions imply that legacy reservoirs are not globally enriched relative to the surface Streets et al. , submitted

  23. Observed decrease of total gaseous Hg (TGM) since 1996 20-38% worldwide decrease Slemr et al. [2011] • Explanation by decline of legacy emissions would imply much higher past emissions than in Streets et al. historical inventory • Faster atmospheric oxidation of Hg(0) does not seem likely

  24. Disposal of Hg in commercial products: a missing source of legacy anthropogenic Hg? David Streets, unpublished • Hg use peaked in the 1970s: fungicide, batteries, thermometers,… • discarded Hg would enter the environment through incineration, wastewater, emission/leakage from landfills • The 1996-present decline in atmospheric concentrations could reflect the transfer of this Hg from surface reservoirs to more stable geochemical reservoirs

  25. Anthropogenic perturbation to the global Hg cycle GEOS-Chem model natural atmosphere + present-day human enhancement Primary emissions x7 Atmospheric deposition x3 Surface ocean x3 Soil +15% Deep ocean + 15% Selin et al. [2008]; Selin [2009]

  26. February 2009: Governing Council of UNEP agrees on need for global legally binding instrument on mercury • Goal is to complete negotiations by 2013 • In US; Clean Air Mercury Rule (CAMR) to reduce power plant emissions was struck down by courts in 2008; new effort is underway TOWARDS A GLOBAL MERCURY TREATY:Focus activity of United Nations Environmental Program (UNEP) • CHALLENGES: • How to regulate in the face of considerable uncertainty? • How to account for legacy mercury from past US and European emissions? • How to account for possible major effects of climate change?

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