Atmospheric Mercury Simulation with CMAQ Version 4.5.1

1. Atmospheric Mercury Simulation with CMAQ Version 4.5.1 Russ Bullock - NOAA Air Resources Laboratory* Kathy Brehme - Computer Sciences Corp. 5th Annual CMAS Conference Chapel Hill, NC 16 October 2006 *in partnership with the U.S. Environmental Protection Agency

2. Atmospheric Mercury 101 – The Basics • Mercury (Hg) is a naturally-occuring component of the atmosphere mostly present as atomic elemental mercury (Hg0) gas. • The average concentration of Hg0 gas is currently ≈2×10-13mol/mol (≈ 0.2 ppt), but it can be much higher near emission sources. • This average concentration is about 3x that of pre-industrial times. • Compounds of divalent mercury (Hg2+) can occur as gases and aerosols, but generally at much lower concentrations than Hg0. • Hg cycles through air, water and soil with an atmospheric residence time in air of about one year. Residence times in soil and water can be much longer, possibly on the order of hundreds of years. • Hg cycles between the deep earth and the biosphere on geologic time scales. Volcanic activity and geothermal vents are important sources of truly natural Hg in the biosphere.

3. Pre-Industrial Mercury Cycling

4. Current Mercury Cycling

5. Currently Measurable Forms (Species)of Atmospheric Mercury • Elemental Mercury (Hg0): mildly reactive gas (in most cases); sparingly soluble in water; subject to very long range transport throughout the entire atmosphere • Reactive Gaseous Mercury (RGM): operational term for gaseous Hg compounds that are water soluble and/or chemically reactive; readily deposited to water, soils and vegetation by wet and dry processes • Particulate Mercury (HgP): various condensed Hg compounds and semi-volatile Hg bound to receptive aerosols; two size modes simulated in CMAQ

6. Consequences of Mercury Exposure • Inhalation of atmospheric Hg species is not a serious health hazard, even at the highest ambient levels. • Ingestion of Hg0 is not a serious problem either. • Highly toxic methylmercury compounds and dimethylmercury can be formed in aquatic systems from inorganic Hg deposited from the atmosphere. • Exposure to these methylated forms of Hg is the primary health hazard. • Hg(CH3)2 is one of the most potent neurotoxins known.

7. History of Atmospheric Mercury Modeling with CMAQ • 1999: Mercury added to CMAQ cloud chemistry module AQCHEM • 2000: European mercury model inter-comparison study begins • 2001: First full-scale version of CMAQ mercury model is operational • 2002: Article in Atmospheric Environment describes adaptations for mercury and compares wet deposition results to observations. • 2003: European study shows “large differences” between models and observations of oxidized mercury air concentrations • 2004: New computational efficiencies applied to CMAQ allowing full calendar year simulations, even for mercury model • 2005: EPA’s Clean Air Mercury Rule developed using CMAQ mercury model with minor modifications from 2002 version • 2006: Mercury simulation capabilities included in CMAQ version 4.5.1 and version 4.6 code releases after Hg0 dry deposition is added

8. Additions Associated with the CMAQ Mercury Modeling Option • Emissions: Special point and non-point industrial emission inventories for Hg and molecular chlorine (Cl2) are processed by the Sparse Matrix Operator Kernel Emissions (SMOKE) module. • Gaseous Chemistry: Hg0, RGM and Cl2 are added to the CB-IV gas-phase chemical mechanisms where oxidation of Hg0 can form RGM and/or HgP. (CMAQ v4.6 will add to CB-05 mechanism) • Aqueous Chemistry: Special version of AQCHEM is used to add the simulation of a Hg redox system with compound-specific reactions and Hg2+ sorption to particles. Total dissolved Hg2+ in water and RGM in air are partitioned using the Henry’s Law constant for HgCl2. • Deposition: Wet deposition of Hg treated just like other species. Dry deposition of HgP is based on that of elemental carbon aerosol. Dry deposition velocity (Vd) of Hg0 and RGM are computed in MCIP using the same type of parameterization as for other gases.

9. Emissions: Industrial and “Natural” • Emissions of mercury species (Hg0, RGM and HgP) and Cl2 gas are needed. • Mercury emissions inventories rarely specify the chemical or physical form, just total mercury mass. • Mercury emissions inventories do not include emissions from natural processes which are mostly emissions of previously deposited anthropogenic mercury. • These natural processes are not yet well understood. Nonetheless, they must be accounted for to prevent unrealistic depletion of Hg0 in model simulations. • Chlorine was added as an option to CMAQ v4.5 and Cl2 emissions data are prepared as specified for that option.

10. Gaseous Chemistry: The Hg Reactions

11. Aqueous Chemistry: The Hg Reactions

12. CMAQ Cloud Chemistry Mechanism for Mercury

13. Wet and Dry Deposition of Mercury • Wet deposition of mercury species is treated the same as for all pollutant species. The species concentration in cloud water multiplied by the precipitation rate of cloud water gives the wet deposition flux. • Dry deposition of HgP is based on the assumption that it is bound to elemental carbon aerosol. Vd for APHG[I,J] = Vd for AEC[I,J]. • Dry deposition of RGM is based on Vd estimates for HgCl2 that are calculated in MCIP using the same type of parameterization as for all other gaseous species. • Dry deposition of Hg0 to vegetation is based on Vd estimates calculated in MCIP that include an additional factor for mesophyll resistance to account for mercury already in leaf tissue. Evasion of Hg0 from vegetation also occurs and is treated separately as an emissions input. • Dry deposition of Hg0 to water bodies is set to zero based on the observation that most are already supersaturated with Hg0. Evasion of Hg0 from water bodies is treated separately as an emissions input.

14. Mercury Model Application Requirements • CMAQ model code for mercury simulation is part of the v4.5.1 public release available from the CMAS web site. • The mercury option requires the same J-value files and meteorology as used to simulate criteria air pollutants. • Emissions files must include the CB-IV criteria species plus Hg0, RGM, HgP and Cl2. Hg0 emissions from soils, vegetation and water bodies should be included, but no standard method yet exists. • IC/BC files should also include Hg0, RGM, HgP and Cl2. Boundary conditions can be static (temporally constant) or time-variable based on previous modeling at a larger scale. • Shell scripts for mercury are similar to those from other applications. We have typically set these up to run the CCTM for one simulation day at a time.

15. Further CMAQ-Hg Developments • The mercury option for CMAQ v4.6 has been modified to use the CB-05 gaseous chemistry mechanism and the AERO4 aerosol module. • Hg emissions from natural processes will be better characterized with separate treatments for first-time emissions to the mercury cycle and re-emission of previously deposited Hg using an explicit multi-media modeling treatment of soil and water-body reservoirs. • Chemical and physical reactions of Hg in both air and cloud water are still being identified and described. Reactions will be added, kinetic rate constants will be modified and heterogeneous mercury chemistry may be added based on the outcomes of basic scientific research.

16. Disclaimer The research presented here was performed under the Memorandum of Understanding between the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and under agreement number DW13921548. This work constitutes a contribution to the NOAA Air Quality Program. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views.