Transcript PPT - Atmospheric Chemistry Modeling Group

GLOBAL MODELING OF MERCURY
WITH Br AS ATMOSPHERIC OXIDANT
Chris D. Holmes and Daniel J. Jacob
and funding from EPRI and NSF
RISING MERCURY IN THE ENVIRONMENT
Mercury in polar bear fur
Wyoming ice core
Dietz et al., 2006
US fish consumption advisories (EPA)
Schuster et al., 2002
EPA, 2007
ANTHROPOGENIC
PERTURBATION:
fuel combustion
waste incineration
mining
THE MERCURY CYCLE: MAJOR PROCESSES
oxidation (~1 y)
Hg(II)
Hg(0)
reduction
volcanoes
erosion
volatilization
highly water-soluble
ATMOSPHERE
deposition
SOIL/OCEAN
oxidation
particulate
Hg(II)
Hg(0)
reduction
uplift
biological
uptake
Hg
burial
SEDIMENTS
ATMOSPHERIC REDOX CHEMISTRY OF MERCURY
Standard models
Hg(0)
OH,
O3 Br, Cl
X X
?
HO
X2 (aq)
Hg(II)
Calvert and Lindberg, AE 2005
Hynes et al., UNEP 2008
• Oxidation of Hg(0) by OH or O3 is endothermic
• Oxidation by Br and Cl may be important:
Hg  Br  M
HgBr  M
HgBr  X  M  HgBrX  M
X  OH , Br , Cl
• Oxidation by NO3, BrO, O3 (aq) is probably negligible
• Atmospheric reduction of Hg(II) is hypothetical
MERCURY DEPLETION EVENTS (MDEs) IN ARCTIC SPRING
ARCTAS-A aircraft campaign (April 2008) showed ubiquitous MDEs over sea ice
• MDEs are confined to below 0.5 km altitude,
occur concurrently with ODEs and in presence
of soluble bromide
• Mercury depletion is consistent with Hg + Br
Hg(0) vs. O3
in near-surface data
Mao et al.,
Kim et al.,
submitted
DIURNAL CYCLE OF REACTIVE GASEOUS MERCURY (RGM)
IN MARINE BOUNDARY LAYER
Early a.m. rise, midday peak suggests Br chemistry, deposition via sea salt uptake
MBL budget
Subtropical Pacific cruise data
Observed [Laurier et al., 2003]
Model Hg(0)+Br
Model Hg(0)+OH
Model predicts that ~80% of Hg(II) in MBL should be in sea salt:
Br, OH
Br
Hg(0)
sea-salt
aerosol
HgBr
HgBrX
T
HgCl32-, HgCl42-
kinetics from Goodsite et al. [2004]
Holmes et al. [2009]
deposition
WHAT DO ATMOSPHERIC DATA TELL US
ABOUT GLOBAL Hg(0) OXIDATION?
•
•
•
Atmospheric Hg lifetime against deposition must be ~ 1 year
– Observed variability of Hg(0)
Oxidant must be photochemical
– Observed late summer minimum at northern mid-latitudes
Oxidant must be in gas phase and present in stratosphere
– Hg(II) increase with altitude, Hg(0) depletion in stratosphere
Oxidation by Br atoms can satisfy these constraints [Holmes et al., 2006]
…WHAT DO ATMOSPHERIC DATA TELL US
ABOUT GLOBAL Hg(II) REDUCTION?
•
If it happens at all it’s mostly in lower troposphere (clouds?)
– RGM increase with altitude, Hg(0) depletion in stratosphere
TROPOSPHERIC BROMINE CHEMISTRY
simulated in GEOS-Chem global chemical transport model
GEOS-Chem
Observed
Northern mid-latitudes
profiles of short-lived
bromocarbons
CH3Br
CH2 Br2
CHBr3
industry
Sea salt
CHBr3
CH2Br2
440 Gg a-1
OH
1.1 years
OH
91 days
62 Gg a-1
Mean tropospheric concentrations (ppt)
In GEOS-Chem
0.09
Br
0.8
BrO
0.2
BrNO3
hv, OH
14 days
debromination
HBr
HOBr
5.0
1.5
Bry
deposition
plankton
Justin Parrella, in prep.
GEOS-Chem MODEL OF ATMOSPHERIC MERCURY
(2006)
• Global 3-D atmospheric
simulation driven by GEOS
meteorological data and
coupled to 2-D dynamic
surface ocean and land
reservoirs
• Hg(0) oxidation by Br
[Donohoue et al., 2005;
Goodsite et al., 2004;
Balabanov et al. [2005]
• Compare to previous model
with Hg(0) oxidation by OH
and ozone
Holmes et al., in prep.
Streets et al. [2009]
SPECIFICATION OF Br CONCENTRATIONS
IN GEOS-Chem Hg MODEL
Zonal mean concentrations (ppt) from bromocarbons + hv, OH simulated by
TOMCAT (troposphere) and GMI (stratosphere) with standard gas-phase chemistry
Add 1 ppt BrO in MBL
5 ppt in Arctic spring BL
PREFERENTIAL REGIONS FOR Hg(0) OXIDATION
Annual zonal mean oxidation rates
Hg(0) lifetime
against
oxidation
0.45 years
0.30 years
Add aqueous-phase photoreduction of Hg(II) in cloud
tuned to yield Hg lifetime against deposition of 0.9 years
Holmes et al., in prep.
MODEL EVALUATION AGAINST SURFACE TGM DATA
Total gaseous mercury (TGM); model is 2006-2008 annual mean
Hg+Br
simulation
model:
Hg + Br
Hg + OH/O3
• Unbiased at land sites (r2 =0.88 for
Hg+Br, r2 = 0.87 forHg+OH/O3)
• Underestimate over N Atlantic is
corrected in most recent GEOS-Chem
version by using observed subsurface
ocean concentrations (Soerensen et
al., in prep.)
• Hg+Br model has steeper latitudinal
gradient
Holmes et al., in prep.
SEASONAL VARIATION OF TGM
15 sites
3 sites
• Both models reproduce late summer minimum at northern mid-latitudes
• Summer maximum at Cape Point is due to ocean emission
• Only Hg+Br model can simulate polar spring depletion, summer rebound
• Only Hg+Br model can simulate high-RGM subsidence events over Antarctica
Holmes et al., in prep.
VERTICAL PROFILES OF TGM
• Uniform in troposphere, dropping in stratosphere
• Arctic spring observations show much faster drop in stratosphere than
elsewhere – underestimate of halogen oxidants?
Holmes et al., in prep.
WET DEPOSITION FLUX PATTERNS
MDN and EMEP annual means (2006-2008)
Observations as symbols, model as background
Hg+Br model
• Hg +Br simulation is too low over Gulf of
Mexico in summer – missing Br source in
subtropics?
• Model is too high at northerly sites in
winter – insufficient scavenging by snow?
Holmes et al., in prep.
Seasonal variation
MODEL DEPOSITION PATTERNS DEPEND ON OXIDANT
Oxidation by Br causes greater deposition to SH oceans
Annual total Hg(II) deposition flux
Hg+Br
Hg+OH/O3
Environmental implications depend on cycling through land and ocean reservoirs;
Development of a fully coupled atmosphere-ocean-land model is underway
Holmes et al., in prep.