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