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Sensitivity Evaluation of Gas-phase ReductionMechanisms of Divalent Mercury
Using CMAQ-Hg in a Contiguous US Domain
Pruek Pongprueksaa, Che-Jen Lina, and Thomas C. Hob
a Department of Civil Engineering, Lamar University, Beaumont, TX, USA
b Department of Chemical Engineering, Lamar University, Beaumont, TX, USA
5th Annual CMAS Conference
October 16, 2006
Friday Center, UNC-Chapel Hill
Reduction of Divalent Mercury
• Occurs in surface water and atmospheric droplets
• Photolytically assisted in the aqueous phase
• Gaseous-phase reduction of RGM in plume was suggested from
measurement and modeling studies
• No deterministic mechanism with reliable kinetic parameters was
reported
Objectives
• To evaluate possible gaseous phase reduction mechanisms of
divalent Hg using CMAQ-Hg
• To project the likely kinetic parameters of alternative mercury
reduction pathways in addition to the sulfite and the controversial
HO2˙ reduction pathways
• To demonstrate model performance with implementation of other
reduction mechanisms
CategoryCMAQ-Hg by
Bullock and Brehme (2002)
CMAQ-Hg V4.5.1 Updates
(March, 2006)
Gas Chemistry O3, Cl2, H2O2, and OH˙, PHg as the
GEM oxidation product by OH˙,O3,
and H2O2
Product by H2O2 changed to RGM,
Product by OH˙ and O3˙ changed to 50% RGM and 50% PHg,
Kinetics of GEM oxidation by OH scaled down to 7.7×10-14 from 8.7×10-14
cm3/molec/s.
Aqueous Chemistry Ox: O3, OH, HOCl, and OCl-
Red: HgSO3, Hg(OH)2+hv, HO2˙
Unchanged
Aqueous Speciation SO32-, Cl-, OH- Unchanged
Aqueous Sorption Sorption of Hg(II) to ECA, bi-directional non-eq. kinetics w/ linear sorption isotherm
Unchanged
Cloud Mixing Scheme RADM Cloud Scheme Asymmetrical Convective Model
(ACM) Mixing Scheme
Dry Deposition Vdep of HNO3 for RGM deposition,
no GEM deposition
Vdep of I,J modes for PHg
deposition
Both GEM & RGM deposition treated explicitly using resistance models in M3DRY
Wet Deposition Scavenged PHg, dissolved and sorbed Hg(II)aq
Unchanged
Summary of Major Updates in CMAQ-Hg v. 4.5.1
Kinetic Uncertainties in Hg Models
• Widely varied kinetic data reported for same mechanisms (e.g. GEM
oxidation by OH˙ & O3 and aqueous Hg(II) reduction by sulfite)
• Extrapolation of laboratory results may not be appropriate [e.g.
aqueous Hg(II) reduction by HO2˙ (Gårdfeldt and Jonsson, 2003),
GEM oxidation by OH˙ and O3 (Calvert and Lindberg, 2005)]
• Unidentified chemical transformation maybe present [e.g. photo-
induced decomposition of RGM and reduction of RGM (Fay and
Seeker, 1903)]
• Uncertain GEM oxidation products (Lin et al., 2006)
Model Configuration
• Hg oxidation products – 100% RGM (this study)
• No Hg(II) reduction mechanism by HO2˙/O2˙-
• Hg reduction mechanism by CO
HgO(s,g) + CO(g) → Hg(g) + CO2(g) (1)
– Exothermic -130.7 kJ mol-1
– Sensitivity simulation for k = 10-20 to 10-14 cm3 molecule-1 s-1
• Hg photoreduction mechanism
HgO(s,g) + hv → Hg(g) + ½ O2(g) (2)
J(HgO) = f * J(NO2) (3)
– Varying photolysis rate by proportion of J(NO2)
– Sensitivity simulation for f = 10-5 to 10
k
J(NO2)
Model Input
• Meteorological data - 2001 MM5 and MCIP v. 3.1 with M3Dry option
• Emission inventory - U.S. and Canada 1999 NEI + vegetative Hg EI
(Lin et al. 2005)
• Initial and boundary conditions – default profile files [1.4 - 1.5 ng m-3
for Hg(0), 16.4 – 57.4 pg m-3 for Hg(II)gas, and 1.6 - 10.8 pg m-3 for
Hg(P)]
• Model verification with MDN archived wet deposition in July 2001 (at
least 80% continuous monitoring)
• Normalized CMAQ-Hg wet deposition according to MDN
precipitation field use for scattered plots
y = 1.07x
R2 = 0.55
0
50
100
150
200
250
300
350
400
0 50 100 150 200 250 300 350 400
MDN Precipitation, mm month-1
MC
IP P
reci
pita
tion
, mm
mon
th-1
Good
Bad
MDN vs. MCIP precipitation, July 2001
0.5 * MDN
2.0 * MDN
10
100
1000
10000
100 1000 10000 100000MDN Total Hg Wet Deposition Flux, ng m-2 month-1
CM
AQ
Tot
al H
g W
et D
epos
itio
n F
lux,
ng
m-2
mon
th-1
J = 10-7 s-1, y = 1.45x, R2 = 0.77J = 10-6 s-1, y = 1.04x, R2 = 0.77
J = 10-5 s-1, y = 0.41x, R2 = 0.72
J = 10-4 s-1, y = 0.15x, R2 = 0.48
J = 10-3 s-1, y = 0.10x, R2 = 0.33
Hg wet deposition MDN vs. CMAQby photoreduction, July 2001
10
100
1000
10000
100 1000 10000 100000
MDN Total Hg Wet Deposition Flux, ng m-2 month-1
CM
AQ
Tot
al H
g W
et D
epos
ition
Flu
x, n
g m
-2
mon
th-1
kCO=1x10-19, y=1.52x, R2 = 0.77
kCO=1x10-18, y=1.36x, R2 = 0.77kCO=5x10-18, y=0.97x, R2 = 0.78
kCO=1x10-17, y=0.74x, R2 = 0.78
kCO=1x10-16, y=0.26x, R2 = 0.77
kCO=1x10-15, y=0.11x, R2 = 0.61
Hg wet deposition MDN vs. CMAQby CO reduction, July 2001
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.0E-011.0E-021.0E-031.0E-041.0E-051.0E-061.0E-07Photoreduction Rate, s-1
CM
AQ
:MD
N f
rom
Red
uct
ion
by C
O
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.0E-141.0E-151.0E-161.0E-171.0E-181.0E-191.0E-20
Reduction Rate by CO, cm3 s-1 molecule-1
CM
AQ
:MD
N f
rom
Ph
otor
educ
tion
Minimum
Optimum Maximum
Hg wet deposition influenced byphotoreduction (blue) and CO reduction (red)
July Hg Wet Deposition, 2001
(a) CMAQ-Hg 4.5.1 (b) 100%RGM & no HO2˙ reduction
(c) kCO = 5 x 10-18 cm3 molecule-1 s-1 (d) JHg(II) = 10-3 JNO2 ≈ 8.82 x 10-6 s-1
Summary
• Sensitivity simulations of Hg(II) reduction constants by photoreduction and by CO reduction are demonstrated
• CMAQ-Hg is very sensitive to reduction rates• The minimum rates
– CO reduction = 1 x 10-20 cm3 molecule-1 s-1
– Photoreduction = 1 x 10-7 s-1
• The optimum rates– CO reduction = 5 x 10-18 cm3 molecule-1 s-1
– Photoreduction = 1 x 10-5 s-1
• More studies are needed for the combination of these reduction mechanisms
• These mechanisms provide a preliminary estimate for further verification by more kinetic laboratory studies (i.e. temperature-dependent reaction)
Acknowledgements
• US Environmental Protection Agency (USEPA, RTI subcontract No.
3-93U-9606)
• Texas Commission on Environmental Quality (TCEQ work order No.
64582-06-15)
• Robert Yuan, Lamar University
• Pattaraporn Singhasuk, University of Warwick