The chemistry of mercury in the atmosphere

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The chemistry of mercury in the atmosphere

• John Munthe
• IVL Swedish Environmental Research Institute

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Presentation outline

• Overview of known chemical reactions of
  atmospheric mercury
• Overview of chemistry descriptions in atmospheric
  models
• Results from model intercomparison Stage 1
• Missing information and future challenges

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HYSPLIT-Hg Atmospheric Mercury Fate Processes
Cloud droplet or deliquesced aerosol particle
Wet deposition
Dry deposition
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Hg0 X room temperature reaction rates in the
gas phase and mid-latitude lifetimes
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Mercury room temperature reaction rates in the
aqueous phase
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Chemistry in atmospheric models

• Varying degree of detail in different models.
• Purpose of model development and application
  determines the degree of detail in chemistry (and
  other processes) i.e. process studies, episodes,
  monthly-annual fluxes.
• Highly detailed process descriptions increases
  requirements on computer resources and time.

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Model intercomparison Stage 1 Chemistry

• Organised by MSC East, results published in
  Ryaboshapko et al., Atmospheric Environment 36,
  3881-3898, 2002.
• Participating models ADOM TCM (GKSS), AER (USA),
  CAM (Sweden), CMAQ (USA), MSC-E.
• Models run with purpose of comparing chemistry.
  Simulations of concentration changes over limited
  time period, in a situation of stable
  meteorology, no wet deposition etc.
• Different chemical descriptions in models

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ADOM-Main Features Atmospheric Mercury Chemistry
Scheme (G. Petersen, GKSS)
21 reactions among 14 species based on the
Chemistry of Atmospheric Mercury (CAM) model of
the Swedish Environmental Research Institute
(IVL) G(1) - G(4) mercury species in
ambient air AQ(1) - AQ(10) mercury species in
the aqueous phase R1 - R5 mass
transfer rate expressions R6 - R
17 aqueous phase reaction rate expressions
R18 - R19 equilibrium rate expressions for
adsorption R20 - R21 gas phase rection rate
expressions chemical solver using the Young
and Boris predictor-corrector scheme
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(No Transcript)
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Chemistry in CMAQ (R. Bullock, US EPA)
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Chemistry in CAM model

• Full chemistry according to current knowledge
• EMEP MSC West simpified scheme for oxidants
• Descriptions of SO2 oxidations, radical chemistry
  in gas and aqueous phases, Hg complexes in
  aqueous phase etc.
• Only for process studies (very simple
  meteorology).

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Chemistry in MSC-East model (A. Ryaboshapko)
Chemical scheme
Temperature dependences
EMEP/MSC-E
EMEP/MSC-E
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Chemistry in MSC-East model
MSCE-Hg chemical scheme
50 of particulate Hg is soluble
EMEP/MSC-E
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Chemistry in MSC-East model
MSCE-Hg chemical scheme
Evaporation of a drop results in an aerosol
particle formation
EMEP/MSC-E
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Chemistry in MSC-East model
Temperature dependences
MSCE-Hg chemical scheme
EMEP/MSC-E
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Chemistry in MSC-East model
Three groups identified
First order differential equations
Analytical solution possible and used for
calculations of concentration changes of A, B, C
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Current HYSPLIT-Hg Atmospheric Chemistry for
Mercury (M Cohen, NOAA)
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Simulated concentrations in cloud droplets during
48 hr
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Variation of Hg concentrations (ng/L)
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Lessons learned from Intercomparison stage 1 (and
new research since then...)

• Different models apply different chemistry and
  get different results.
• Differences are moderatly large.
• Chemistry has been updated in most models since
  the intercomparison and is now more
  homogeneous(?)
• OH oxidation in gas phase, halogen reactions (Br,
  Cl), new results for SO2 induced reduction of
  Hg(II), no reaction Hg(II) HO2 ....
• Remaining issues - Solubility of particulate Hg
  (what is particulate Hg?- Partitioning between
  dissolved and particultate Hg in aqueous phase

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Lessons learned from Intercomparison stage 1 (and
new research since then...)

• Arctic mercury depletion events Largest
  research effort ever to understand atmospheric Hg
  dynamics. Numerous field measurement campaigns,
  laboratory reaction studies and modelling
  exercises.

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Conclusions and discussion

• MSC-East model represents the current
  understanding of mercury chemistry in the
  atmosphere.
• Remaining uncertainties are moderate to large,
  but the resulting errors caused by uncertainites
  in chemistry are limited, and probably smaller
  than errors caused by uncertainties in e.g.
  emissions.
• Modelling can be used to describe variability in
  Hg concentrations and deposition with respect
  to- Location (variability with distance from
  sources)- Emission changes (future scenarios or
  historical changes)
• "All models are wrong but some are useful"

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Conclusions and discussion

• Flexibility of models to introduce new chemistry?
• Uncertainty in currently used rate constants
• Reduction of RGM in emission plumes
• Formation and characteristics of particulate
  mercury
• Boundary conditions for oxidised Hg (RGM)
• "Rapid oxidation events" Arctic Mercury Depletion
  Events, oxidation in free troposphere (Manua Loa
  data)
• .....

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TGM from Mace Head 1991 to 1997
Slide courtesy of Dr Ralf Ebinghaus, GKSS
Research Centre (ralf.ebinghaus_at_gkss.de)
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Mercury Depletion Events - example from Ny
Aalesund
Slide courtesy of Torunn Berg, NILU
(torunn.berg_at_nilu.no)
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Mauna Loa, Hawaii Monitoring Site
Slide courtesy of Dr Matthew Landis, US EPA
(landis.matthew_at_epa.gov)
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Mauna Loa Hg Time Series 2001 Downslope
Slide courtesy of Dr Matthew Landis, US EPA
(landis.matthew_at_epa.gov)