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 b Department of Chemical Engineering, Lamar University, Beaumont, TX, USA 5 th Annual CMAS Conference October 16, 2006 Friday Center, UNC-Chapel Hill

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

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 HO 2 ˙ reduction pathways To demonstrate model performance with implementation of other reduction mechanisms

Category CMAQ-Hg by Bullock and Brehme (2002) CMAQ-Hg V4.5.1 Updates (March, 2006) Gas ChemistryO 3, Cl 2, H 2 O 2, and OH˙, PHg as the GEM oxidation product by OH˙,O 3, and H 2 O 2 Product by H 2 O 2 changed to RGM, Product by OH˙ and O 3 ˙ changed to 50% RGM and 50% PHg, Kinetics of GEM oxidation by OH scaled down to 7.7× from 8.7× cm 3 /molec/s. Aqueous ChemistryOx: O 3, OH, HOCl, and OCl - Red: HgSO 3, Hg(OH) 2 +hv, HO 2 ˙ Unchanged Aqueous SpeciationSO 3 2-, Cl -, OH - Unchanged Aqueous SorptionSorption of Hg(II) to ECA, bi- directional non-eq. kinetics w/ linear sorption isotherm Unchanged Cloud Mixing SchemeRADM Cloud SchemeAsymmetrical Convective Model (ACM) Mixing Scheme Dry DepositionV dep of HNO 3 for RGM deposition, no GEM deposition V dep of I,J modes for PHg deposition Both GEM & RGM deposition treated explicitly using resistance models in M3DRY Wet DepositionScavenged PHg, dissolved and sorbed Hg(II) aq Unchanged Summary of Major Updates in CMAQ-Hg v

Kinetic Uncertainties in Hg Models Widely varied kinetic data reported for same mechanisms (e.g. GEM oxidation by OH˙ & O 3 and aqueous Hg(II) reduction by sulfite) Extrapolation of laboratory results may not be appropriate [e.g. aqueous Hg(II) reduction by HO 2 ˙ (Gårdfeldt and Jonsson, 2003), GEM oxidation by OH˙ and O 3 (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)

Model Configuration Hg oxidation products – 100% RGM (this study) No Hg(II) reduction mechanism by HO 2 ˙/O 2 ˙ - Hg reduction mechanism by CO HgO (s,g) + CO (g) → Hg (g) + CO 2(g) (1) –Exothermic kJ mol -1 –Sensitivity simulation for k = to cm 3 molecule -1 s -1 Hg photoreduction mechanism HgO (s,g) + hv → Hg (g) + ½ O 2(g) (2) J(HgO) = f * J(NO 2 )(3) –Varying photolysis rate by proportion of J(NO 2 ) –Sensitivity simulation for f = to 10 k J(NO 2 )

Model Input Meteorological data 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 [ ng m -3 for Hg(0), 16.4 – 57.4 pg m -3 for Hg(II) gas, and 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

MDN vs. MCIP precipitation, July * MDN 2.0 * MDN

Hg wet deposition MDN vs. CMAQ by photoreduction, July 2001

Hg wet deposition MDN vs. CMAQ by CO reduction, July 2001

Minimum Optimum Maximum Hg wet deposition influenced by photoreduction (blue) and CO reduction (red)

July Hg Wet Deposition, 2001 (a) CMAQ-Hg 4.5.1(b) 100%RGM & no HO 2 ˙ reduction (c) kCO = 5 x cm 3 molecule -1 s -1 (d) JHg(II) = JNO 2 ≈ 8.82 x s -1

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 cm 3 molecule -1 s -1 –Photoreduction = 1 x s -1 The optimum rates –CO reduction = 5 x cm 3 molecule -1 s -1 –Photoreduction = 1 x 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)

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