1. Interaction between sulfur and reactive bromine in clouds
 Qianjie Chen, Johan Schmidt, Viral Shah, Lyatt Jaeglé, Thomas Sherwen and Becky Alexander
HOBr + S(IV)  Sulfate + Br-
I am Qianjie Chen from University of Washington. First I would like to acknowledge all the coauthors for your contribution to this work and NSF for the funding. In this work, we’ve added one reaction into GEOS-Chem, the oxidation of SO2 by reactive bromine HOBr in cloud droplets, and we found this reaction is very important for both the sulfur and reactive bromine budgets in the troposphere, especially the reactive bromine budgets.
My name is Qianjie Chen and I am a graduate student in University of Washington, working with Prof. Becky Alexander. Today I am going to talk about a reaction that is not well-known but potentially very important for tropospheric chemistry. That is the in-cloud oxidation of bisulfite and sulfite by HOBr, one of the reactive bromine species, to produce sulfate and bromide ions. This is the reaction we added into GEOS-Chem. We want to see the impacts on both sulfur and reactive bromine budgets in the troposphere.

2. Our previous work Δ17O (‰)
SO2
SO2· H2O
HSO3-
SO32-
SO42- OH
H2O2 O3
O3, HOBr/HOCl
gas
aerosols
HOBr/HOCl
clouds
This study is motivated by our previous work that measured oxygen isotopes of sulfate in the aerosol samples collected in the remote MBL. We quantified the contribution of each pathway to sulfate formation for those aerosol samples and found that 33-50% of sulfate in the remote MBL is produced from HOBr/HOCl oxidation, which is a large contribution.
a large fraction of sulfate in the MBL is produced by HOBr/HOCl oxidation. In that study, we measured oxygen isotopes of sulfate collected from two ship cruises, and calculated that 30-50% of sulfate is produced from HOBr/HOCl. We further calculated the HOBr/HOCl concentration needed to explain the observations, which is on the order of ppt. Then, in this study, we add the HOBr+S(IV) reactions into the model, by assuming a first-order uptake of HOBr onto the cloud droplets.
Oxygen isotopes observations indicate 33~50% of sulfate in the remote MBL is produced via HOBr/HOCl oxidation.
[Chen et al., ACP, 2016]

3. HOBr + S(IV)  Sulfate + Br-
GEOS-Chem v9-02; 4°x5°; tropospheric sulfur chemistry from Alexander et al. [2012] and bromine chemistry from Schmidt et al. [2016]. 4
90N-60N
60N-30N
In Schmidt et al. [2016], tropospheric BrO column was overestimated when sea salt debromination (HOBr+Br-SS+H+Br2) was switched on.
In our study, sea salt debromination is switched on, since it is the largest source of Bry. 2 4
30N-Eq.
Eq.-30S
Tropospheric BrO column (1013 cm-2) 2
So we added this new sulfate formation mechanism into GEOS-Chem. We used V9-02 version, with 4x5 degree resolution. The tropospheric sulfur chemistry is from Alexander et al. (2012), with updates on cloud pH and some bug fixes. The bromine chemistry is from Schmidt et al. (2016), with updates on the alkalinity of sea salt aerosols and bromide recycling on aerosols. In Schmidt et al., the model overestimated the satellite observations of BrO column when sea salt debromination was switched on. So for their standard simulations, they switched off sea salt debromination source, to better match the observations. However, sea salt debromination is the biggest source of Bry in the troposphere. So in our simulations, we always turn it on. 4
30S-60S
60S-90S 2
J F M A M J J A S O N D
J F M A M J J A S O N D
Month
[Schmidt et al., 2016]

4. Changes in Bry budget after adding HOBr+S(IV)
-20%
-40%
-60%
-80%
Let me show you some results. This is the fractional change of tropospheric Bry burden after adding the HOBr+S(IV) reactions in the model. The tropospheric Bry burden decreases by 50% globally. The decrease is larger over the ocean and at higher latitudes, for example about 80% decrease over Southern Ocean. The high-latitude ocean is the place where we have a lot of clouds for the HOBr+S(IV) reactions to occur. It is also the place where bromocarbon decomposition is weak so sea salt debromination is more important. The reaction we add into the model decreases sea salt debromination rates by competing with HOBr+Br-/Cl- reactions and lowering HOBr abundance.
It is also the place where sunlight is weaker so that more HOBr was used to recycle HBr in aerosols and cloud droplets before adding the reaction.
Tropospheric Bry burden decreases by 50% globally
Decrease of Bry is larger at: high-latitude ocean (MBL)
Reasons: HOBr+S(IV) decreases sea salt debromination rates by competing with HOBr+Br-/Cl- and lowering HOBr abundance.

5. Compared with satellite BrO
Now we compare our model results with satellite observations for tropospheric BrO column, before and after adding the reaction. Both simulations include sea salt debromination. We can see a big decrease in BrO column, especially at mid latitudes. The decrease at polar regions is due to the decrease at mid latitudes because there is no snow source of Bry in the model. Overall, modeled BrO column underestimates observations after adding the reaction. We anticipate an increase in BrO if we have a coupling iodine-bromine-chlorine chemistry scheme in the future.
Polar regions, no snow source.
Both include sea salt debromination
Adding the snow source could improve the model.
Modeled BrO columns underestimate observations after adding HOBr+S(IV).
GOME-2 tropospheric BrO from Theys et al. [2011].

6. Changes in sulfur budget after adding HOBr+S(IV)
OH ( ) (32%)
SO2
(Gg S)
SO42-
(Gg S)
H2O2 O3
S(IV):
SO2H2O
HSO3-
SO32-
O2+Mn(II)/Fe(III)
HOBr ( ) (8%)
Blue: before and Red : after adding HOBr+S(IV) reactions
Fraction of sulfate produced from HOBr
50%
HOBr accounts for 8% of sulfate production globally.
The amount of sulfate produced in gas phase remains unchanged.
40%
For the sulfur budget, HOBr accounts for about 8% of sulfate formation globally. This fraction can be very high when HOBr mixing ratio is high. For example, over tropical ocean, the fraction of sulfate produced from HOBr can reach up to 50%. We see the amount of sulfate produced from OH oxidation in the gas phase remains almost unchanged, which is interesting because there should be less SO2 available to be oxidized in the gas phase after adding a new aqueous phase reaction. The reason why it is unchanged is because Bry decreases after adding the reaction so that O3 and OH concentrations increase. An increase in the OH concentration enhances the DMS oxidation so that more SO2 is produced. So, adding the HOBr+S(IV) reactions in the aqueous phase does not necessarily result in a decrease in gas phase SO2 oxidation and new particle formation. We need to consider the impacts of this reaction on Bry and OH budgets, which were not considered before.
Where is this reaction more important? We can see a higher fraction of sulfate produced from HOBr oxidation over low latitude ocean where HOBr mixing ratio is high. The fraction is pretty small over high latitude ocean, which is likely due to the underestimate of HOBr abundance over those regions in the model.
This global map shows the fraction of sulfate produced from HOBr oxidation.
The global sulfate burden increases by 6% from 432 Gg S to 460 Gg S.
----- Meeting Notes (4/27/17 15:54) -----
take home key message
colorbar
remove HOBr mixing ratio
30%
20%
10%

7. Ongoing work in collaboration with Tom Breider
DMS oxidation
cloud droplets
new particles
OHaq
DMSO
MSA
MSA
SO42-
O3,aq
SO2 OH
DMS OH
H2SO4
SO2
DMSO
OH (low T),
BrO
OH (high T), NO3, Cl, O3
DMS
MSA/nssSO42-
(1) Xue-Long 2012
(2) RITS
I want to show another aspect of sulfur-halogen interactions in the troposphere, which is the DMS oxidation. I am adding a couple of DMS oxidation pathways into GEOS-Chem, both gas phase and multiphase reactions, including oxidation of DMS by BrO, Cl radicals and O3. DMSO is added as a new tracer in the model. We will use the observations of MSA/nssSO4 ratio as a constraint for the model.
In this project, we will add a couple of reactions into GEOS-Chem. DMSO will be added as a chemical tracer in GEOS-Chem, which will undergo transport, deposition and chemical reactions in the model. We will add oxidation of DMS by Cl radical, O3 in both gas and aqueous phase and BrO. We will also add the oxidation of DMSO by OH in both gas and aqueous phase. We can see that these chemical reactions we add will affect the MSA/nssSO4 ratio. So we will use the observations of MSA/nssSO4 ratio as a constraint for the model. We have observations from Xue-Long cruise in 2012 and the RITS cruise in 1993 and 1994.
abstraction
addition
phytoplankton

8. Conclusions
Inclusion of HOBr+S(IV) reactions in the model reduces global tropospheric Bry burden by 50%, initiated by decrease in bromide recycling in cloud droplets.
About 8% of sulfate is produced via HOBr oxidation globally. Our model underestimated this HOX+S(IV) pathway due to too low HOBr and the lack of HOCl+S(IV).
In-cloud sulfate production via HOBr oxidation does not necessarily result in less gas-phase SO2 oxidation.
To conclude, adding the reaction reduces Bry burden by 50%, which is initiated by decrease in bromide recycling in cloud droplets. About 8% of sulfate is produced through HOBr oxidation globally. Our model could have underestimated sulfate production from HOX due to too low HOBr and the lack of HOCl+S(IV) reaction. The in-cloud sulfate production from HOBr oxidation does not necessarily result in less gas-phase SO2 oxidation, due to an increase in OH concentrations. This work is now under review in GRL.
[Chen et al., 2017, under review]