Revisiting atmospheric Hg oxidation
mechanisms in GEOS-Chem:
constraints from observations
Hannah M. Horowitz
Daniel J. Jacob, Helen M. Amos, Theodore S. Dibble, Franz Slemr, Johan A. Schmidt,
Daniel A. Jaffe, Elizabeth S. Corbitt, and Elsie M. Sunderland
IGC7 – 6 May 2015
Contact: hmhorow@fas.harvard.edu
Reconciling advances in our scientific understanding
with observational constraints
Robust observational constraints for atmospheric Hg redox cycling:
• Atmospheric burden: 4600 – 5600 Mg of total Hg
• Hg0 lifetime against oxidation + deposition: 0.4 – 1.0 years
Example of prior GEOS-Chem Hg model results, within constraints:
Burden: 5000 Mg
Hg0 lifetime: 10.4 months
• Recent research suggests these
fluxes are incorrect or incomplete
• How should we “improve” the
model, and what will happen?
Holmes et al., 2010
Proposing new standard atmospheric chemical mechanism for Hg in GEOS-Chem
Standard v9-02
Proposed changes
• GC Br fields from Parrella et al. (2012)
• GC Br fields from Schmidt et al. in
prep.
Change in Br concentrations
v9-02 annual zonal mean [Br]
New annual zonal mean [Br]
Altitude
20
Schmidt et al.
tropospheric Br is on
average 70% higher
10
0
-60
0
-30
0
30
Latitude
60
-60
1.16E6 2.33E6 3.5E6 molec cm-3
New [Br] – standard v9-02
0
-30
0
30
Latitude
1.16E6
60
2.33E6 3.5E6 molec cm-3
% difference New – standard v9-02
Altitude
20
10
0
-2.5E6 -1.25E6 0 1.25E6 2.5E6 molec cm-3
-200 -100
0
100
200 %
Burden?
Lifetime?
Proposing new standard atmospheric chemical mechanism for Hg in GEOS-Chem
Standard v9-02
Proposed changes
• GC Br fields from Parrella et al. (2012)
• GC Br fields from Schmidt et al. in
prep.
• No in-plume reduction
• Strong in-plume reduction (75% of HgII)
Fossil fuel emissions speciation
HgII
Hg0
Strong IPR
More wet deposition
No IPR
Burden?
Lifetime?
Proposing new standard atmospheric chemical mechanism for Hg in GEOS-Chem
Standard v9-02
Proposed changes
• GC Br fields from Parrella et al. (2012)
• GC Br fields from Schmidt et al. in
prep.
• No in-plume reduction
• Strong in-plume reduction (75% of
II)
•HgIn-cloud
Hg(II) photoreduction
• No in-cloud Hg(II) reduction
If in-cloud, why not also in sulfate aerosol in UT/LS?
Reduction here is not compatible with understood vertical profile of Hg
Geopotential height (km)
O3
THg
HgII
Hg0
Lyman and Jaffe, 2011
Based on aircraft observations
Proposing new standard atmospheric chemical mechanism for Hg in GEOS-Chem
Standard v9-02
Proposed changes
• GC Br fields from Parrella et al. (2012)
• GC Br fields from Schmidt et al. in
prep.
• No in-plume reduction
• Strong in-plume reduction (75% of
II)
•HgIn-cloud
Hg(II) photoreduction
• Oxidation initiated by Br atom,
completed by Br & OH
• No in-cloud Hg(II) reduction
• Additional oxidation reactions and updated
rate coefficients based on literature review
-32
-1.86
6
-2 -1
1) Hg 0 + X + M ® HgX + M k1 = 1.5´10 (T / 298) cm molec s 7.3×10-33 (T / 298)-1.86
-10
0.9
-1 -3.35
-9 (T -1
/ s298)
exp(-7600/T)
cm 3molec
2) HgX + M ® Hg 0 + X + M k2 = 1.6 ´10 (T / 298) exp(-7292 / T )2.0×10
2a) HgX + X ® Hg 0 + X 2
k2a = 3.9 ´10-11 cm 3molec-1s-1
3) HgX + Y ® HgXY
k3 = 2.5´10-10 (T / 298)-0.57 cm 3molec-1s-14.1×10-11
2.8×10-11
X = Br,
Br Cl
Y = Br, OH,
OH HO2, NO2, BrO, ClO
+ in-cloud aqueous oxidation by O3, OH, HOCl/OClsmaller rate coefficients + more oxidants: effects on burden and lifetime not immediately clear
New reactions and rates from Dibble et al., 2012, 2014; Wilcox, 2009; Balabanov et al., 2005; Donohoue et al., 2005; Wang and Pehkonen,
2004; Lin and Pehkonen, 1997, 1998; Munthe 1992; Buxton et al., 1988
Importance of new oxidation pathways
Fraction of total annual oxidation of Hg0:
Zonal total HgII production per year
HgBr+HO2
HgBr+NO2
Altitude (km)
20
10
HgBr+OH
HgBr+Br
aqueous (O3, HOCl)
Hg+Cl
HgBr+ClO
0
HgBr+BrO
-60
-30
0
• Br atom (all blue segments) is still dominant
initiator of oxidation
• Initial second-step oxidants (Br, OH)
outcompeted by more abundant radicals
0
Latitude
1.5
30
3.0
60
kg m-3 a-1
• Oxidation now at lower altitudes, due to
NO2, HO2
• New Br fields affect polar oxidation
Observational comparison: reasonable lifetimes, seasonality, spatial distribution,
but low bias in concentrations
New model // Std v9-02
3200 Mg // 4500 Mg
Burden:
Hg0 lifetime against ox. + dep.: 6.4 months // 11.2 months
ng m-3
Combined effect of differences in
oxidation and reduction
ng m-3
Obs
Std v9-02
New model
Improved Br?