Acid rain and mercury NATURAL pH OF RAIN Equilibrium with natural CO2 (280 ppmv) results in a rain pH of 5.7: H2O CO2 H 2O CO2 ( g )
K H 3 10 2 M atm -1 HCO3 H K1 9 10 7 M CO2 H 2O CO32 H HCO3
K 2 7 10 10 M [ H ] ( K1 K H PCO2 )1/ 2 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 PRECIPITATION PH OVER THE UNITED STATES
CHEMICAL COMPOSITION OF PRECIPITATION Neutralization by NH3 is illusory because NH4+ g NH3 + H+ in ecosystem LONG-TERM TREND IN US SO2 EMISSIONS Sulfate wet deposition and aerosol concentrations, 1980-2010 Circles = observations; background = model
Leibensperger et al.  Nitrate wet deposition and aerosol concentrations, 1980-2010 Leibensperger et al.  Ammonium wet deposition and aerosol concentrations, 1980-2010 Leibensperger et al. 
Trends in precipitation pH Nitrogen deposition in the US GEOS-Chem simulation for 2006-2008 Critical loads for ecosystems Nitrogen deposition exceeds critical loads in much of the country Most of that deposition is as nitric acid originating from NO x emissions Zhang et al. , Ellis et al. 
Critical load exceedances for N deposition at US national parks More deposition is expected to originate from ammonia in future Ellis et al.  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 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 ANTHROPOGENIC
biological uptake Hg burial SEDIMENTS RISING MERCURY IN THE ENVIRONMENT Global mercury deposition has roughly tripled since preindustrial times
Dietz et al.  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 g kgg kg-1 d-1 (about 2 fish meals per week)
Atmospheric transport distributes Hg on a global scale Implies global-scale transport of anthropogenic emissions Anthropogenic Hg emission (2006) Mean Hg(0) concentration in surface air: circles = observed, background = GEOS-Chem model Hg(0) lifetime = 0.5-1 year
Streets et al. ; Soerensen et al.  UNEP Minimata Convention on Mercury Opened for signatures in October 2013; already signed by 91 countries 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 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 Reservoir fraction Time scale for dissipation of an atmospheric emission pulse 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.  Global source contributions to Hg in present-day surface ocean emissions Human activity has increased 7x the Hg content of the surface ocean
Half of this human influence is from pre-1950 emissions natural pre-1850 ROW former USSR N America
N America, Europe and Asia share similar responsibilities for anthropogenic Hg in present-day surface ocean S America Europe Asia
Amos et al.  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.
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