Atmospheric Mercury Simulation with CMAQ Version 4'5'1

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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
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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
  210-13 mol/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.

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Pre-Industrial Mercury Cycling
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Current Mercury Cycling
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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

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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.

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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 EPAs 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

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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
  Henrys 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.

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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.

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Gaseous Chemistry The Hg Reactions
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Aqueous Chemistry The Hg Reactions
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CMAQ Cloud Chemistry Mechanism for Mercury
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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 APHGI,J Vd for AECI,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.

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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.

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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.

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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.