Sensitivity Evaluation of Gas-phase Reduction Mechanisms of Divalent Mercury Using CMAQ-Hg in a Contiguous US Domain

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Sensitivity Evaluation of Gas-phase
ReductionMechanisms of Divalent MercuryUsing
CMAQ-Hg in a Contiguous US Domain

• Pruek Pongprueksaa, Che-Jen Lina, and Thomas C.
  Hob
• a Department of Civil Engineering, Lamar
  University, Beaumont, TX, USA
• b Department of Chemical Engineering, Lamar
  University, Beaumont, TX, USA
• 5th Annual CMAS Conference
• October 16, 2006
• Friday Center, UNC-Chapel Hill

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Reduction of Divalent Mercury

• Occurs in surface water and atmospheric droplets
• Photolytically assisted in the aqueous phase
• Gaseous-phase reduction of RGM in plume was
  suggested from measurement and modeling studies
• No deterministic mechanism with reliable kinetic
  parameters was reported

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Objectives

• To evaluate possible gaseous phase reduction
  mechanisms of divalent Hg using CMAQ-Hg
• To project the likely kinetic parameters of
  alternative mercury reduction pathways in
  addition to the sulfite and the controversial
  HO2? reduction pathways
• To demonstrate model performance with
  implementation of other reduction mechanisms

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Summary of Major Updates in CMAQ-Hg v. 4.5.1
Category CMAQ-Hg by Bullock and Brehme (2002) CMAQ-Hg V4.5.1 Updates (March, 2006)
Gas Chemistry O3, Cl2, H2O2, and OH?, PHg as the GEM oxidation product by OH?,O3, and H2O2 Product by H2O2 changed to RGM, Product by OH? and O3? changed to 50 RGM and 50 PHg, Kinetics of GEM oxidation by OH scaled down to 7.710-14 from 8.710-14 cm3/molec/s.
Aqueous Chemistry Ox O3, OH, HOCl, and OCl- Red HgSO3, Hg(OH)2hv, HO2? Unchanged
Aqueous Speciation SO32-, Cl-, OH- Unchanged
Aqueous Sorption Sorption of Hg(II) to ECA, bi-directional non-eq. kinetics w/ linear sorption isotherm Unchanged
Cloud Mixing Scheme RADM Cloud Scheme Asymmetrical Convective Model (ACM) Mixing Scheme
Dry Deposition Vdep of HNO3 for RGM deposition, no GEM deposition Vdep of I,J modes for PHg deposition Both GEM RGM deposition treated explicitly using resistance models in M3DRY
Wet Deposition Scavenged PHg, dissolved and sorbed Hg(II)aq Unchanged
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Kinetic Uncertainties in Hg Models

• Widely varied kinetic data reported for same
  mechanisms (e.g. GEM oxidation by OH? O3 and
  aqueous Hg(II) reduction by sulfite)
• Extrapolation of laboratory results may not be
  appropriate e.g. aqueous Hg(II) reduction by
  HO2? (Gårdfeldt and Jonsson, 2003), GEM oxidation
  by OH? and O3 (Calvert and Lindberg, 2005)
• Unidentified chemical transformation maybe
  present e.g. photo-induced decomposition of RGM
  and reduction of RGM (Fay and Seeker, 1903)
• Uncertain GEM oxidation products (Lin et al.,
  2006)

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Model Configuration

• Hg oxidation products 100 RGM (this study)
• No Hg(II) reduction mechanism by HO2?/O2?-
• Hg reduction mechanism by CO
• HgO(s,g) CO(g) ? Hg(g) CO2(g) (1)
• Exothermic -130.7 kJ mol-1
• Sensitivity simulation for k 10-20 to 10-14 cm3
  molecule-1 s-1
• Hg photoreduction mechanism
• HgO(s,g) hv ? Hg(g) ½ O2(g) (2)
• J(HgO) f J(NO2) (3)
• Varying photolysis rate by proportion of J(NO2)
• Sensitivity simulation for f 10-5 to 10

k
J(NO2)
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Model Input

• Meteorological data - 2001 MM5 and MCIP v. 3.1
  with M3Dry option
• Emission inventory - U.S. and Canada 1999 NEI
  vegetative Hg EI (Lin et al. 2005)
• Initial and boundary conditions default profile
  files 1.4 - 1.5 ng m-3 for Hg(0), 16.4 57.4 pg
  m-3 for Hg(II)gas, and 1.6 - 10.8 pg m-3 for
  Hg(P)
• Model verification with MDN archived wet
  deposition in July 2001 (at least 80 continuous
  monitoring)
• Normalized CMAQ-Hg wet deposition according to
  MDN precipitation field use for scattered plots

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MDN vs. MCIP precipitation, July 2001
2.0 MDN
0.5 MDN
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Hg wet deposition MDN vs. CMAQby photoreduction,
July 2001
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Hg wet deposition MDN vs. CMAQby CO reduction,
July 2001
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Hg wet deposition influenced byphotoreduction
(blue) and CO reduction (red)
Minimum
Maximum
Optimum
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July Hg Wet Deposition, 2001
(a) CMAQ-Hg 4.5.1
(b) 100RGM no HO2? reduction
(c) kCO 5 x 10-18 cm3 molecule-1 s-1
(d) JHg(II) 10-3 JNO2 8.82 x 10-6 s-1
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Summary

• Sensitivity simulations of Hg(II) reduction
  constants by photoreduction and by CO reduction
  are demonstrated
• CMAQ-Hg is very sensitive to reduction rates
• The minimum rates
• CO reduction 1 x 10-20 cm3 molecule-1 s-1
• Photoreduction 1 x 10-7 s-1
• The optimum rates
• CO reduction 5 x 10-18 cm3 molecule-1 s-1
• Photoreduction 1 x 10-5 s-1
• More studies are needed for the combination of
  these reduction mechanisms
• These mechanisms provide a preliminary estimate
  for further verification by more kinetic
  laboratory studies (i.e. temperature-dependent
  reaction)

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Acknowledgements

• US Environmental Protection Agency (USEPA, RTI
  subcontract No. 3-93U-9606)
• Texas Commission on Environmental Quality (TCEQ
  work order No. 64582-06-15)
• Robert Yuan, Lamar University
• Pattaraporn Singhasuk, University of Warwick