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

Sensitivity Evaluation of Gas-phase Reduction Mechanisms of Divalent Mercury Using CMAQ-Hg in a Contiguous US Domain. Pruek Pongprueksa a , Che-Jen Lin a , and Thomas C. Ho b a Department of Civil Engineering, Lamar University, Beaumont, TX, USA

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

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  1. 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

  2. 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

  3. 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

  4. Summary of Major Updates in CMAQ-Hg v. 4.5.1

  5. Kinetic Uncertainties in Hg Models • Widely varied kinetic data reported for same mechanisms (e.g. GEM oxidation by OH˙ & O3and 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)

  6. Model Configuration k • 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 J(NO2)

  7. 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

  8. MDN vs. MCIP precipitation, July 2001 2.0 * MDN 0.5 * MDN

  9. Hg wet deposition MDN vs. CMAQby photoreduction, July 2001

  10. Hg wet deposition MDN vs. CMAQby CO reduction, July 2001

  11. Hg wet deposition influenced byphotoreduction (blue) and CO reduction (red) Minimum Maximum Optimum

  12. July Hg Wet Deposition, 2001 (a) CMAQ-Hg 4.5.1 (b) 100%RGM & 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

  13. 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)

  14. 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

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