Halogen chemistry mechanism

Overview

The halogen simulation present in GEOS-Chem v11-2d was build atop of GEOS-Chem v10-01 and brought into the main code branch at v11-01g. This code brings together published halogen developments in GEOS-Chem as described in detail by Sherwen et al [2016b], including the methyl iodide simulation [Bell et al 2002], the iodine simulation from Sherwen et al [2016a], the original bromine simulation in GEOS-Chem [Parrella et al 2012], the UCX simulation including stratospheric halogens [Eastham et al 2014], updates to bromine mechanism [Schmidt et al 2016]. Further updates were made use NASA JPL15-10 halogen cross-sections and rates by Sherwen et al [2017].

The gas-phase chemistry follows Sherwen et al [2016b], with updates made to the heterogenous code to work with structural changes from v10-01 to v11-01.

Key additions vs. v10-01:

• Addition of 21 new tracers (for further details on these tracers, please see the GEOS-Chem species wiki page). All halogen tracers are shown below, with new ones in bold. This includes five aerosol tracers for bromine (BrSALA, BrSALC) and iodine (ISALA, ISALC, AERI).

CHCl3	IONO	BrNO2	HCFC142b	Cl2O2
CH2Cl2	IONO2	BrNO3	CFC11	INO
CH3I	I2O2	CHBr3	CFC12	HBr
CH2I2	I2O3	CH2Br2	HCFC22	HCFC141b
CH2ICl	I2O4	CH3Br	H1211	Cl2
CH2IBr	ISALA	BrCl	H1301	OIO
HOI	ISALC	HCl	H2402	HOBr
I2	AERI	CCl4	Cl	HCFC123
IBr	BrSALA	CH3Cl	ClO	OClO
ICl	BrSALC	CH3CCl3	HOCl
I	Br2	CFC113	ClNO3
IO	Br	CFC114	ClNO2
HI	BrO	CFC115	ClOO

• Emissions of iodocarbons (CH3I, CH2I2, CH2ICl, CH2IBr) have been included following Ordonez et al [2012]. Emissions of inorganic iodine (HOI, I2) has been using the parameterisation from Carpenter et al [2013] and Macdonald et al [2014].
• Wet deposition of X species (HOCl, ClNO3, HI, HOI, INOy, I2OX, AERI) and dry deposition of Y species (HI, HOI, INOy, I2OX, AERI).

Chlorine

Stratospheric chlorine chemistry is from UCX [Eastham et al 2014]. Tropospheric chlorine chemistry follows that described in Eastham et al [2014] and Schmidt et al [2016], with additions described in Sherwen et al [2016b]. These additions include tropospherically relevant reactions based on the JPL 10-6 compilation [Sander et al., 2011] and IUPAC [Atkinson et al., 2006]. The heterogenous reaction of N2O5 on aerosols was updated to yield products of ClNO2 and HNO3 [Bertram and Thornton, 2009; Roberts et al., 2009] on sea salt and 2HNO3 on other aerosol types. Reaction probabilities are unchanged [Evans and Jacob, 2005].

Bromine

The bromine simulation incorporates the work of Parrella et al [2012] (See wiki article here on incorporation in earlier versions), with updates (especially to heterogenous process) as described in detail by Schmidt et al [2016]. Minor updates to Schmidt et al [2016] are described in Sherwen et al [2016b].

Iodine

The iodine simulation is described in Sherwen et al [2016a], with updates to coupling with bromine and chlorine described in Sherwen et al [2016b]. Notably, the cycling of iodine on aerosol now results in production of ICl and IBr, instead of I2 as in Sherwen et al [2016a].

Key differences from publication

Certain changes were made during implementation of the halogens code in v10-01 into v11-02 that are not described in Sherwen et al (2016b). These include:

• In combination with UCX [Eastham et al 2014], halogen chemistry is calculated online for both troposphere and stratosphere. In Sherwen et al [2016a, 2016b, 2107] the troposphere was calculated offline.
• Updates to heterogeneous chemistry (see separate section with detail below).
• Minor deposition routes included in previous publications were removed for simplicity (Br2, ICl, IBr, I2).
• Rates were updated following the JPL15-10 complication [Sherwen et al 2017]. Rates were also updated to more consistently use JPL over alternative options (e.g. vs IUPAC).
• Updates to VOCs (inc. oceanic acetaldehyde source) in the simulation (see “PAN updates”) will decrease Bry mixing ratios and therefore halogen impacts.
• Uptake of HOBr by Br-/Cl-/S(IV) in aerosols and cloud droplets now treated as a "one-bulk" process following Chen et al [2017], rather than "multiple" uptake of HOBr by Br-/Cl-/S(IV) in aerosol and cloud droplets.

Changes in heterogeneous chemistry

Due to updates structuctural changes in GEOS-Chem (aka Flexchem making the smvgear mechanism obsolete), the heterogenous chemistry routines have been updated to allow for use of more complex heterogenous chemistry previously calculated in Calcrate.F [Eastham et al 2014, Schmidt et al. 2016, Sherwen et al 2016b].

This work required a revamp of the heterogeneous chemistry. As a result, all heterogeneous reactions except for direct uptake (e.g. HO2) and those involving sea salt Cl- (which is not explicitly tracked) are now calculated using a second-order rate constant. This should help to make the het-chem easier to extend, as new functions have been implemented which will automatically convert a pseudo-first-order rate constant into a second order constant while performing safety checks and imposing (if requested) limits on depletion rates to avoid instability.

Inclusion of sulfur-halogen chemistry from Chen et al [2017]

Sulfur oxidation by reactive halogens, as described by Chen et al [2017], was included in the same development branch (v11-2d) of the GEOS-Chem code as the updates to the halogen simulation detailed here.

From Chen et al [2017]:

Sulfur and reactive bromine (Bry) play important roles in tropospheric chemistry and the global radiation budget. The oxidation of dissolved SO2 (S(IV)) by HOBr increases sulfate aerosol abundance and may also impact the Bry budget, but is generally not included in global climate and chemistry models. In this study, we implement HOBr + S(IV) reactions into the GEOS-Chem global chemical transport model and evaluate the global impacts on both sulfur and Bry budgets. Modeled HOBr mixing ratios on the order of 0.1–1.0 parts per trillion (ppt) lead to HOBr + S(IV) contributing to 8% of global sulfate production and up to 45% over some tropical ocean regions with high HOBr mixing ratios (0.6–0.9 ppt). Inclusion of HOBr + S(IV) in the model leads to a global Bry decrease of 50%, initiated by the decrease in bromide recycling in cloud droplets. Observations of HOBr are necessary to better understand the role of HOBr + S(IV) in tropospheric sulfur and Bry cycles.

Text S2 in the supporting document for Chen et al [2017] describes the parameterisation of HOBr + S(IV) reactions now included in GEOS-Chem.

Key differences in Simulation

In addition to changes seen on implementation of halogen chemistry in v10-01 detailed in publications [Eastham et al 2014, Schmidt et al. 2016, Sherwen et al 2016b]. Please find some details on changes to the standard simulation seen (full 1yr benchmark plots are found inked here).

Differences in oxidants

< insert benchmark comparisons of O3 code here - using the gridded O3 dataset from Sofen et al >

Source code and data files

A monthly inventory of Iodocarbon emissions are included from Ordonez et al. [2012]. These files were the result of a project lead by Alfonso Saiz-Lopez and he has kindly allowed for them to be distributed with the main GEOS-Chem code.

These new data files are contained in the HEMCO data directory tree. For detailed instructions on how to download these data files to your disk server, please see our Downloading the HEMCO data directories wiki post.

Known issues

• The reimplementation into GEOS-Chem v11-2 code is currently only compatible with the standard simulation (e.g. cannot be compiled with a setting of “NO_REDUCED=no”).
• CH3I is not currently calculated on-line. Instead the emissions of Bell et al [2002] are currently read in from an offline NetCDF.
• For consistency with published work. Certain reactions that are included in the stratosphere (e.g. ClNO3+HCl, BrNO3+HCl, HOCl+HCl and HOCl+HBr) remain unconsidered in the troposphere.
• ClNO2 production on sulfate following condensation of HCl is not considered (HCl+N2O5=(sulfate aerosol)>ClNO2) for consistency with Sherwen et al 2016b.
• The “uptake” reaction of HCl results in destruction of Cl, as there is no aerosol-phase Cl tracer.
• There are only aerosol-phase bromine tracers for sea-salt, so Bry uptake cannot occur on sulfate.
• Release of IX by heterogenous iodine cycling (HOI, INOy) leads to net production of Cly and Bry as no aqueous phase Cl/Br is considered in the reaction (e.g. excess Cl-/Br- is assumed).

On-going development

The halogen simulation will developed further in the future. Please see the GEOS-Chem model development priorities for more details.

Previous issues that are now resolved

Accounting for seasalt bromide in SSA heterogeneous chemistry

Lei Zhu Wrote:

This bug was found by Xuan Wang. I tested it, and found it has a great impact on Bry budget.

About the bug: GEOS-Chem v11-02d doesn’t allow sea salt bromide (BrSALA and BrSALC) to be emitted as part of sea salt aerosol. Uptake of gas-phase HBr becomes the only source of bromide in sea salt aerosol. This bug has a great impact on total Bry burden and speciation through heterogeneous reactions.

To fix the bug: Turn on sea salt Br- in HEMCO_Config.rc (line 145) as default.

--> Model sea salt Br-  :  true

--Melissa Sulprizio (talk) 20:18, 8 February 2018 (UTC)

References

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