Acid rain and mercury

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 )  natural rain has a pH in range 5-7 “Acid rain” refers to rain with pH < 5  damage to ecosystems

PRECIPITATION PH OVER THE UNITED STATES

CHEMICAL COMPOSITION OF PRECIPITATION Neutralization by NH 3 is illusory because NH 4 +  NH 3 + H + in ecosystem

LONG-TERM TREND IN US SO 2 EMISSIONS

Sulfate wet deposition and aerosol concentrations, Leibensperger et al. [2012] Circles = observations; background = model

Nitrate wet deposition and aerosol concentrations, Leibensperger et al. [2012]

Ammonium wet deposition and aerosol concentrations, Leibensperger et al. [2012]

Trends in precipitation pH

Nitrogen deposition in the US GEOS-Chem simulation for Zhang et al. [2012], Ellis et al. [2013] Nitrogen deposition exceeds critical loads in much of the country Most of that deposition is as nitric acid originating from NO x emissions Critical loads for ecosystems

Critical load exceedances for N deposition at US national parks Ellis et al. [2013] More deposition is expected to originate from ammonia in future

Electronic structure of mercury Mass number = 80: 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 10 4f 14 5s 2 5p 6 5d 10 6s 2 Complete filling of subshells gives Hg(0) a low melting point, volatility Two stable oxidation states: Hg(0) and Hg(II)

Orbital energies vs. atomic number Mercury (Z=80) has all its subshells filled

Biogeochemical cycle of mercury Hg(0) Hg(II) particulate Hg burial SEDIMENTS uplift volcanoes erosion oxidation Hg(0)Hg(II) reduction biological uptake ANTHROPOGENIC PERTURBATION: fuel combustion mining ATMOSPHERE OCEAN/SOIL VOLATILE WATER-SOLUBLE (months)

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

HUMAN EXPOSURE TO MERCURY IS MAINLY FROM FISH CONSUMPTION Tuna is the #1 contributor 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)

Atmospheric transport distributes Hg on a global scale Anthropogenic Hg emission (2006) Streets et al. [2009]; Soerensen et al. [2010] Mean Hg(0) concentration in surface air: circles = observed, background = GEOS-Chem model Implies global-scale transport of anthropogenic emissions Hg(0) lifetime = year

UNEP Minimata Convention on Mercury Requires best available control technology for coal-fired power plants Mercury mining to be banned in 15 years Many mercury-containing commercial products to be banned by 2020 Opened for signatures in October 2013; already signed by 91 countries Convention requires ratification by 50 countries to go into effect Only ratifying country so far has been the US (November 6)

History of global anthropogenic Hg emissions Large past (legacy) contribution from N. American and European emissions; Asian dominance is a recent phenomenon Streets et al., 2011

Time scale for dissipation of an atmospheric emission pulse Reservoir fraction Pulse gets transferred to subsurface ocean within a few years and stays there ~100 years, maintaining a legacy in the surface ocean Pulses injected in surface ocean or terrestrial reservoirs have similar fates Amos et al. [2013]

Global source contributions to Hg in present-day surface ocean Human activity has increased 7x the Hg content of the surface ocean Half of this human influence is from pre-1950 emissions N America, Europe and Asia share similar responsibilities for anthropogenic Hg in present-day surface ocean Amos et al. [2013] Europe Asia N America S America former USSR ROW pre-1850 natural emissions

What can we hope from the Minimata Convention? Effect of zeroing global anthropogenic emissions by 2015 Zeroing anthropogenic emissions would decrease ocean Hg by 30% by 2100, while keeping emissions constant would increase it by 40% Elevated Hg in surface ocean will take centuries to fix; the only thing we can do in short term is prevent it from getting worse. Amos et al. [2013]