Atmospheric Environment 42 (2008) 1828–1845 Scientific uncertainties in atmospheric mercury models III: Boundary and initial conditions, model grid resolution, and Hg(II) reduction mechanism Pruek Pongprueksa a , Che-Jen Lin a,b, , Steve E. Lindberg c,d,1 , Carey Jang e , Thomas Braverman e , O. Russell Bullock Jr. f , Thomas C. Ho g , Hsing-Wei Chu h a Department of Civil Engineering, Lamar University, Beaumont, TX 77710, USA b School of Environmental Science and Engineering, South China University of Technology, Guangzhou City, China c Environmental Sciences Division, Oak Ridge National Laboratory, USA d Department of Natural Resources and Environmental Science, University of Nevada in Reno, USA e Office of Air Quality Planning and Standards, USEPA, Research Triangle Park, NC 27711, USA f NOAA Air Resources Laboratory 2 , Research Triangle Park, NC 27711, USA g Department of Chemical Engineering, Lamar University, TX 77710, USA h Department of Mechanical Engineering, Lamar University, TX 77710, USA Received 22 June 2007; received in revised form 11 November 2007; accepted 13 November 2007 Abstract In this study, the model response in terms of simulated mercury concentration and deposition to boundary condition (BC), initial condition (IC), model grid resolution (12 km versus 36 km), and two alternative Hg(II) reduction mechanisms, was investigated. The model response to the change of gaseous elemental mercury (GEM) concentration from 0 to 2 ng m 3 in IC/BC is found to be very linear (r 2 40.99) based on the results of sensitivity simulations in July 2001. An increase of 1 ng m 3 of GEM in BC resulted in an increase of 0.81 ng m 3 in the monthly average of total mercury concentration, and 1270 ng m 2 in the monthly total deposition. IC has similar but weaker effects compared to those of BC. An increase of 1 ng m 3 of GEM in IC resulted in an increase of 0.14 ng m 3 in the monthly average of total mercury concentration, and 250 ng m 2 in the monthly total deposition. Varying reactive gaseous mercury (RGM) or particulate mercury (PHg) in BC/IC has much less significant impact. Simulation results at different grid resolutions show good agreement (slope ¼ 0.950–1.026, r ¼ 0.816–0.973) in mercury concentration, dry deposition, and total deposition. The agreement in wet deposition is somewhat weaker (slope ¼ 0.770–0.794, r ¼ 0.685–0.892) due to the difference in emission dilution and simulated precipitation that subsequently change reaction rates in the aqueous phase. Replacing the aqueous Hg(II)-HO 2 reduction by either RGM reduction by CO (5  10 18 cm 3 molecule 1 s 1 ) or photoreduction of RGM (1  10 5 s 1 ) gives significantly better model agreement with the wet deposition measured by Mercury Deposition Network (MDN). Possible ranges of the reduction rates are estimated based on model sensitivity results. The kinetic estimate requires further verification by laboratory studies. r 2007 Elsevier Ltd. All rights reserved. Keywords: Atmospheric mercury; Boundary and initial conditions; CMAQ-Hg; Grid resolution; Mercury reduction mechanism ARTICLE IN PRESS www.elsevier.com/locate/atmosenv 1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.11.020 Corresponding author at: Department of Civil Engineering, Lamar University, 211 Redbird Lane, ML 10024, Beaumont, TX 77710, USA. Tel.: +1 409 880 8761; fax: +1 409 880 8121. E-mail address: Jerry.Lin@lamar.edu (C.J. Lin). 1 Now in Graeagle, CA 96103, USA. 2 In partnership with the US Environmental Protection Agency.