GLOBAL MODELING OF MERCURY WITH Br AS ATMOSPHERIC OXIDANT

Document Sample
GLOBAL MODELING OF MERCURY WITH Br AS ATMOSPHERIC OXIDANT Powered By Docstoc
					 GLOBAL MODELING OF MERCURY
WITH Br AS ATMOSPHERIC OXIDANT


 Chris D. Holmes and Daniel J. Jacob




     and funding from EPRI and NSF
    RISING MERCURY IN THE ENVIRONMENT
                              Mercury in polar bear fur

Wyoming ice core
                                            Dietz et al., 2006




                           US fish consumption advisories (EPA)




   Schuster et al., 2002


                                    EPA, 2007
ANTHROPOGENIC         THE MERCURY CYCLE: MAJOR PROCESSES
  PERTURBATION:
   fuel combustion
 waste incineration
            mining                      oxidation (~1 y)
                                Hg(0)                      Hg(II)
                                          reduction        highly water-soluble

  volcanoes
    erosion            volatilization   deposition                ATMOSPHERE

                                                                  SOIL/OCEAN
                                         oxidation
                                                                             particulate
                                Hg(0)                 Hg(II)
                                         reduction              biological      Hg
                                                                 uptake

       uplift
                                                                          burial



                                 SEDIMENTS
   ATMOSPHERIC REDOX CHEMISTRY OF MERCURY

Standard models            OH, X3 Br, Cl
                           X O
          Hg(0)                                  Hg(II)
                              ?      X
                                    HO2 (aq)
                                                 Calvert and Lindberg, AE 2005
                                                 Hynes et al., UNEP 2008
  • Oxidation of Hg(0) by OH or O3 is endothermic

  • Oxidation by Br and Cl may be important:

            Hg  Br  M   HgBr  M
            HgBr  X  M  HgBrX  M                  X  OH , Br , Cl
  • Oxidation by NO3, BrO, O3 (aq) is probably negligible

  • Atmospheric reduction of Hg(II) is hypothetical
MERCURY DEPLETION EVENTS (MDEs) IN ARCTIC SPRING
ARCTAS-A aircraft campaign (April 2008) showed ubiquitous MDEs over sea ice



                               • MDEs are confined to below 0.5 km altitude,
                               occur concurrently with ODEs and in presence
                               of soluble bromide

                               • Mercury depletion is consistent with Hg + Br




                                     Hg(0) vs. O3
                                     in near-surface data


                                                            Mao et al.,
                                                            Kim et al.,
                                                            submitted
 DIURNAL CYCLE OF REACTIVE GASEOUS MERCURY (RGM)
            IN MARINE BOUNDARY LAYER
Early a.m. rise, midday peak suggests Br chemistry, deposition via sea salt uptake
                                             MBL budget
        Subtropical Pacific cruise data




        Observed [Laurier et al., 2003]
        Model Hg(0)+Br
        Model Hg(0)+OH


 Model predicts that ~80% of Hg(II) in MBL should be in sea salt:

                                                        sea-salt
                 Br             Br, OH                  aerosol
       Hg(0)             HgBr             HgBrX
                                                     HgCl32-, HgCl42-
                   T
   kinetics from Goodsite et al. [2004]

Holmes et al. [2009]                                          deposition
            WHAT DO ATMOSPHERIC DATA TELL US
             ABOUT GLOBAL Hg(0) OXIDATION?

•    Atmospheric Hg lifetime against deposition must be ~ 1 year
      – Observed variability of Hg(0)
•    Oxidant must be photochemical
      – Observed late summer minimum at northern mid-latitudes
•    Oxidant must be in gas phase and present in stratosphere
      – Hg(II) increase with altitude, Hg(0) depletion in stratosphere

    Oxidation by Br atoms can satisfy these constraints [Holmes et al., 2006]


          …WHAT DO ATMOSPHERIC DATA TELL US
           ABOUT GLOBAL Hg(II) REDUCTION?
•    If it happens at all it’s mostly in lower troposphere (clouds?)
       – RGM increase with altitude, Hg(0) depletion in stratosphere
         TROPOSPHERIC BROMINE CHEMISTRY
 simulated in GEOS-Chem global chemical transport model
                                        GEOS-Chem
                                        Observed
Northern mid-latitudes
profiles of short-lived                   CHBr3
bromocarbons                                                          CH2Br2
                                          440 Gg a-1                   62 Gg a-1



                                          Mean tropospheric concentrations (ppt)
                             OH                      In GEOS-Chem
               CH3Br
                            1.1 years
                                              0.09            0.8      0.2
                              OH
               CH2 Br2                        Br          BrO        BrNO3
                             91 days
                             hv, OH
               CHBr3                         HBr       HOBr
                            14 days
                          debromination       5.0       1.5             Bry
industry       Sea salt
                                                    deposition
           plankton
                                                          Justin Parrella, in prep.
       GEOS-Chem MODEL OF ATMOSPHERIC MERCURY
                                                (2006)




• Global 3-D atmospheric
                              Streets et al. [2009]
simulation driven by GEOS
meteorological data and
coupled to 2-D dynamic
surface ocean and land
reservoirs

• Hg(0) oxidation by Br
[Donohoue et al., 2005;
Goodsite et al., 2004;
Balabanov et al. [2005]

• Compare to previous model
with Hg(0) oxidation by OH
and ozone
Holmes et al., in prep.
            SPECIFICATION OF Br CONCENTRATIONS
                   IN GEOS-Chem Hg MODEL
Zonal mean concentrations (ppt) from bromocarbons + hv, OH simulated by
TOMCAT (troposphere) and GMI (stratosphere) with standard gas-phase chemistry




Add 1 ppt BrO in MBL

5 ppt in Arctic spring BL
        PREFERENTIAL REGIONS FOR Hg(0) OXIDATION

                           Annual zonal mean oxidation rates

Hg(0) lifetime
      against      0.45 years                   0.30 years
   oxidation




                 Add aqueous-phase photoreduction of Hg(II) in cloud
                 tuned to yield Hg lifetime against deposition of 0.9 years



                                                        Holmes et al., in prep.
 MODEL EVALUATION AGAINST SURFACE TGM DATA
             Total gaseous mercury (TGM); model is 2006-2008 annual mean


Hg+Br
simulation




    model:                           • Unbiased at land sites (r2 =0.88 for
      Hg + Br
                                     Hg+Br, r2 = 0.87 forHg+OH/O3)
      Hg + OH/O3
                                     • Underestimate over N Atlantic is
                                     corrected in most recent GEOS-Chem
                                     version by using observed subsurface
                                     ocean concentrations (Soerensen et
                                     al., in prep.)
                                     • Hg+Br model has steeper latitudinal
                                     gradient
                                                     Holmes et al., in prep.
                 SEASONAL VARIATION OF TGM

                  15 sites                    3 sites




• Both models reproduce late summer minimum at northern mid-latitudes
• Summer maximum at Cape Point is due to ocean emission
• Only Hg+Br model can simulate polar spring depletion, summer rebound
• Only Hg+Br model can simulate high-RGM subsidence events over Antarctica
                                                        Holmes et al., in prep.
                  VERTICAL PROFILES OF TGM




• Uniform in troposphere, dropping in stratosphere
• Arctic spring observations show much faster drop in stratosphere than
elsewhere – underestimate of halogen oxidants?
                                                        Holmes et al., in prep.
                 WET DEPOSITION FLUX PATTERNS
MDN and EMEP annual means (2006-2008)
Observations as symbols, model as background   Seasonal variation




                               Hg+Br model


 • Hg +Br simulation is too low over Gulf of
 Mexico in summer – missing Br source in
 subtropics?
 • Model is too high at northerly sites in
 winter – insufficient scavenging by snow?


Holmes et al., in prep.
    MODEL DEPOSITION PATTERNS DEPEND ON OXIDANT
            Oxidation by Br causes greater deposition to SH oceans
                               Annual total Hg(II) deposition flux




      Hg+Br




  Hg+OH/O3




Environmental implications depend on cycling through land and ocean reservoirs;
Development of a fully coupled atmosphere-ocean-land model is underway
                                                        Holmes et al., in prep.