# Sensitivity of tropospheric ozone to halogen chemistry in the chemistry–climate model LMDZ-INCA vNMHC

Cyril Caram, Sophie Laval-Szopa, Anne Cozic, Slimane Bekki, Carlos A.

Cuevas, Alfonso Saiz-Lopez

► To cite this version:

Cyril Caram, Sophie Laval-Szopa, Anne Cozic, Slimane Bekki, Carlos A. Cuevas, et al.. Sensitivity of tropospheric ozone to halogen chemistry in the chemistry–climate model LMDZ-INCA vNMHC. Geoscientific Model Development, 2023, 16, pp.4041-4062. 10.5194/gmd-16-4041-2023 . insu-03466412v2

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# Sensitivity of tropospheric ozone to halogen chemistry in the chemistry–climate model LMDZ-INCA vNMHC

Cyril Caram¹, Sophie Szopa¹, Anne Cozic¹, Slimane Bekki², Carlos A. Cuevas³, and Alfonso Saiz-Lopez³

¹Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ,
Université Paris-Saclay, Gif-sur-Yvette, France

²Laboratoire Atmosphère, Milieux, Observations Spatiales, Institut Pierre Simon Laplace, LATMOS/IPSL,
CNRS-UVSQ-Sorbonne Université, Guyancourt and Paris, France

³Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano, CSIC, Madrid, Spain

**Correspondence:** Sophie Szopa (sophie.szopa@lsce.ipsl.fr)

Received: 21 November 2021 – Discussion started: 30 November 2021

Revised: 5 May 2023 – Accepted: 10 May 2023 – Published: 18 July 2023

**Abstract.** The atmospheric chemistry of halogenated species (Cl, Br, I) participates in the global chemical sink of tropospheric ozone and perturbs the oxidising capacity of the troposphere, notably by influencing the atmospheric lifetime of methane. Global chemistry–climate models are commonly used to assess the global budget of ozone and its sensitivity to emissions of its precursors, as well as to project its long-term evolution. Here, we report on the implementation of tropospheric sources and chemistry of halogens in the chemistry–climate model LMDZ-INCA (Laboratoire de Météorologie Dynamique general circulation model, LMDZ, and Interactions with Chemistry and Aerosols, INCA, version with Non-Methane HydroCarbon chemistry, vNMHC) and evaluate halogen effects on the tropospheric ozone budget. Overall, the results show that the model simulates satisfactorily the impact of halogens on the photo-oxidising system in the troposphere, in particular in the marine boundary layer. To quantify the effects of halogen chemistry in LMDZ-INCA, standard metrics representative of the behaviour of the tropospheric chemical system ($O_x$, $HO_x$, $NO_x$, $CH_4$ and non-methane volatile organic compounds – NMVOCs) are computed with and without halogens. The addition of tropospheric halogens in the LMDZ-INCA model leads to a decrease of 22 % in the ozone burden, 8 % in OH and 33 % in $NO_x$. Sensitivity simulations show for the first time that the inclusion of halogen chemistry makes ozone more sensitive to perturbations in $CH_4$, $NO_x$ and NMVOCs. Consistent with other global model studies, the sensitivity of the tropospheric ozone burden to changes from pre-industrial to present-day

emissions is found to be ~20% lower when tropospheric halogens are taken into account.

## 1 Introduction

The atmospheric chemistry of halogenated species was first studied for its importance for the composition of the stratosphere and in particular the ozone layer (Molina and Rowland, 1974; Stolarski and Cicerone, 1974), but its importance for tropospheric chemistry was also suggested as soon as the early 1980s (Chameides and Davis, 1980). With the progress of measurement techniques, marine aerosols were thought to harbour and release reactive halogens into the gas-phase (Cicerone, 1981). Later on, halogenated species were found to be responsible for ozone destruction in the Arctic troposphere (Bottenheim et al., 1990; Oltmans and Komhyr, 1986). Meanwhile, natural and anthropogenic sources of halogens were starting to be identified and characterised (Graedel and Keene, 1995; Lobert et al., 1999; McCulloch et al., 1999; Carpenter, 2003; Platt and Hönninger, 2003; Saiz-Lopez and Plane, 2004). In recent years, the addition of halogenated chemistry into global chemistry transport models has revealed its significant influence on the photo-oxidising chemistry of the troposphere on a global scale (Saiz-Lopez et al., 2012b; Simpson et al., 2015). This influence arises from the high reactivity of atomic halogens (e.g. Cl, Br, I) and halogen oxide radicals (e.g. $ClO$, $BrO$, $IO$ and higher oxides), which are produced in the atmosphere by the decomposition
of very short-lived halogenated substances (VSL) and by het-
erogeneous reactions on sea salt aerosols and which, in the
case of iodine, are also released via ozone deposition to the
ocean surface. They can also be generated by polar-specific
halogen activation mechanisms on sea ice surfaces, blowing
snow and the snowpack (as reviewed by Abbatt et al., 2012).

Details on the tropospheric chemistry of chlorine, bromine and iodine can be found in review articles (e.g. Saiz-Lopez and von Glasow, 2012; Simpson et al., 2015). Reactive halogen species mainly affect tropospheric chemistry by (1) modifying the partitioning of HOₓ (HO₂+OH) and nitrogen oxides (NOₓ = NO₂+NO) and reacting with NO₂ to form species that remove NOₓ from the system via deposition or other loss processes, (2) being involved in the oxidation of non-methane volatile organic compounds (NMVOCs) and mercury, (3) participating in the formation of new particles (Giorgi and Chameides, 1985; O'Dowd et al., 2002; von Glasow et al., 2004; Gómez Martín et al., 2020; Saiz-Lopez and Von Glasow, 2012), and (4) constituting a sink for tropospheric ozone at the global scale. Halogenated species play a role in the ozone and hydroxyl radical (OH) budgets and therefore influence the oxidative capacity of the troposphere (Iglesias-Suarez et al., 2020; Badia et al., 2021). In addition, by influencing the abundances of tropospheric ozone, which is an important greenhouse gas, halogens can indirectly affect the climate (Saiz-Lopez et al., 2012a; Hossaini et al., 2015; Sherwen et al., 2017).

The loss of tropospheric ozone by halogens is mainly in-
duced by iodine and bromine species with a lower contribu-
tion of chlorine species (Sherwen et al., 2016b). It can be
caused by direct catalytic ozone destruction and reduction
in ozone production through the removal of NO₂ by halogen
radicals. Halogen chemistry is thought to be especially active
in pristine atmospheres that host 60% to 70% of the global
burden of tropospheric ozone (Holmes et al., 2013). Based
on model calculations, the halogen-driven ozone loss is esti-
mated to be around 30% in the marine boundary layer (von
Glasow et al., 2002, 2004; Saiz-Lopez et al., 2014; Sherwen
et al., 2016b) and 5% to 20% globally (Yang et al., 2005;
Saiz-Lopez et al., 2012a, 2014; Sherwen et al., 2016b), go-
ing up to 25% to 30% in Badia et al. (2019). Consequently,
it might be important to consider halogens in chemistry-
climate modelling. It could also help to reconstruct more reli-
ably past atmospheric compositions (Saiz-Lopez et al., 2014;
Young et al., 2013).

Despite its potential importance, only three global chem-
istry models have so far implemented detailed representa-
tions of halogenated (Cl, Br and I) tropospheric chemistry:
CAM-Chem (Saiz-Lopez et al., 2012), GEOS-Chem (Sher-
wen et al., 2016b) and TOMCAT (Hossaini et al., 2015).
Halogen chemistry is complex with a high computational
cost (Fernandez et al., 2021), Earth system models used to
assess how the evolution of atmospheric composition influ-
ences climate need to find the right compromise between
the level of details (e.g. number of species and processes)

here for halogen chemistry representations and the com-
puting efficiency (box 6.1 in Szopa et al., 2021). Indeed,
the computational cost has to be sufficiently low for mul-
tiple, long chemistry–climate simulations. So far, none of
the chemistry–climate models involved in recent large inter-
comparison projects included tropospheric chemistry of very
short-lived halogens and hence the impacts of their chem-
istry on key short-lived climate forcers (such as ozone and
methane). Such multi-model projects are important. They are
used to assess the budget of key chemical species, which
can impact directly or indirectly climate or global air qual-
ity, and their sensitivity to changes in anthropogenic and
natural emissions. The results are also used to evaluate the
effects of international policies (e.g. the Gothenburg Proto-
col to Abate Acidification, Eutrophication and Ground-level
Ozone) by quantifying source–receptor relations at continen-
tal scales (in the case of the Hemispheric Transport of Air
Pollution project, HTAP; Fiore et al., 2009), to explore the
sensitivity of short-lived climate forcers to changes in emis-
sions of their precursors since the pre-industrial period, or
to project their possible future evolutions under a range of
scenario assumptions: in the case of the Atmospheric Chem-
istry and Climate Model Intercomparison Project (ACCMIP;
Young et al., 2013) or, more recently, the Aerosol Chemistry
Model Intercomparison Project (AerChemMIP; Stevenson et
al., 2020).

The primary purpose of this work is to assess whether io-
dine, bromine and chlorine chemistry can, beyond their al-
ready established effect on global tropospheric ozone bud-
get, affect the sensitivity of ozone to the changes in its pre-
cursors. In order to do so, we implemented a simplified rep-
resentation of tropospheric halogen chemistry in a global
chemistry–climate model, the LMDZ-INCA model (Labo-
ratoire de Météorologie Dynamique (LMD) general circu-
lation model, LMDZ, and Interactions with Chemistry and
Aerosols, INCA) with a focus on reproducing the overall ef-
fect of halogen chemistry on global tropospheric ozone. Re-
sults of the three global tropospheric chemistry models that
already include detailed representations of this chemistry are
used to guide the choices of the chemical reactions and emis-
sions in our simple halogen scheme and to assess the LMDZ-
INCA simulations. First, we evaluate the ability of the model
to simulate the global tropospheric budget of halogens and
their effect on ozone by using the results of the CAM-Chem
and GEOS-Chem models as references. The limitations of
our model are also discussed, notably how some simplifica-
tions in our model affect the simulation of the spatial dis-
tribution of some specific halogenated species. Second, we
investigate how the sensitivities of ozone, methane and hy-
droxyl radicals to perturbations in anthropogenic emissions
of photo-oxidant precursors change when halogen chem-
istry is accounted for. For instance, we explore changes in
ozone burden and net destruction in simulations consider-
ing present-day (PD) and pre-industrial (PI) emissions with
and without halogen chemistry. Finally, the last section re-
calls key findings, and we conclude with implications for chemistry-climate simulations of tropospheric ozone evolution.

## 2 Methodology

### 2.1 Model description

#### 2.1.1 LMDZ-INCA

LMDZ is an atmospheric general circulation model (GCM) developed by Sadourny and Laval (1984). The primitive equations of the GCM are solved with a time step of 3 min, a large-scale transport of tracers is carried out every 15 min, and the physical processes are calculated at a time interval of 30 min. The LMDZ model provides at each time step the physical quantities (temperature, pressure, water vapour, turbulence, boundary layer, etc.) needed to compute the transport and chemistry in INCA.

INCA is an atmospheric chemistry model developed at LSCE (Laboratoire des Sciences du Climat et de l'Environnement). The model takes into account the primary emissions from natural sources or by anthropogenic activities. It computes physical processes such as dry and wet deposition and photochemical reactions (Hauglustaine et al., 2004). The atmospheric concentration fields are integrated in time by solving differential equations when called by LMDZ, with a time step of 30 min. For this development, we used a version of INCA (version v_5.2.1) simulating interactively the gaseous photochemistry, whereas aerosol distributions obtained with a more complex version of INCA are prescribed through climatologies. This version represents in particular the chemistry of the inorganic species $O_x$, $NO_x$ and $HO_x$, as well as $CH_4$, CO, non-methane volatile organic compounds (NMVOCs) and their photo-oxidised products (Folberth et al., 2006).

Biogenic emissions like those of isoprene are provided by the global vegetation model ORCHIDEE (Organising Carbon and Hydrology In Dynamic Ecosystems) (Folberth et al., 2006). Anthropogenic emissions are based on the Community Emissions Data System (CEDS) v2016-07-2 emission inventory for the year 2010 (Hoesly et al., 2018).

The LMDZ-INCA model has been used and assessed in numerous previous model intercomparison exercises aimed at simulating the pre-industrial, present and future tropospheric composition (e.g. Szopa et al., 2013); the long range transport of air pollution in the HTAP exercise (Fiore et al., 2009); or the chemistry-climate interactions in ACCMIP (Naik et al., 2013).

#### 2.1.2 Halogen chemistry

The chemistry of halogenated compounds implemented in LMDZ-INCA is essentially based on the scheme developed for CAM-Chem (Ordóñez et al., 2012; Prados-Roman et al.,

2015a; Saiz-Lopez et al., 2012a, 2014, 2015) and GEOS-Chem (Sherwen et al., 2016a), with a priority given to the reactions of importance for the ozone sinks (Fig. 1).

A total of 42 halogenated species have been added to the INCA chemical scheme (Table 1). The list of the 160 chemical reactions added to the INCA chemical scheme can be found in Tables 2, 3, 4 and 5. Photolysis reactions of halogens as well as relative quantum yields and absorption cross-sections are listed in Table 2. Uptake coefficients for heterogeneous reactions are based on experimental values and data selected for other models and are reported in Table 5. Recycling through heterogeneous chemistry on sea salt and sulfate aerosols is included, while the heterogeneous bromine reactions in water droplets and on ice crystals in clouds are not. Dry deposition is calculated with the Wesely scheme (Wesely, 1989), while washout of gases by precipitation is simulated by using Henry's law constants detailed in Table S1 in the Supplement. Some VSL halocarbons, $CH_2BrCl$, $CHBr_2Cl$ and $CHBrCl_2$, represent a few percent of the emissions of halocarbons (Ordóñez et al., 2012). Considering the high uncertainty in their emissions and their limited abundances, we chose not to consider their chemistry in this version.

### 2.2 Simulations

For these developments, LMDZ-INCA is used with a 3.75° longitude × 1.9° latitude resolution over 39 vertical layers (up to 70 km with 15 levels in the stratosphere). This resolution, coarser than the resolution now commonly used in the CMIP6 model exercise, was the standard LMDZ resolution for CMIP5 (Hourdin et al., 2013) and allows faster simulations necessary in model development. The LMDZ wind fields and sea surface temperatures are nudged on ECMWF reanalysis for the year 2010 in all simulations. The aerosol distribution and monthly variations used for heterogeneous reaction calculations are specified from climatologies pre-computed with another version of the same model but with fully interactive calculations of aerosols. In this version, the stratospheric composition is prescribed. All simulations cover 1 year (2010) and follow a spin-up period of 1 year. Three types of simulations have been performed to simulate (i) present-day conditions, (ii) pre-industrial conditions and (iii) sensitivity to ozone precursors. The surface emissions of $NO_x$, CO and NMVOCs and $CH_4$ concentrations considered in the present-day and pre-industrial simulations are presented in Table S2. For sensitivity tests, simulations with individual emission perturbations of photo-oxidant precursors (−20%) of $NO_x$, CO and NMVOCs and concentration reduction (−20%) of $CH_4$ were performed. For each type of simulation, two versions of the model are applied: one accounting for the halogen chemistry (called Halo simulations hereafter) and a reference simulation without halogen chemistry called "NoHalo".
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Schematic of the most important tropospheric chemical processes involving halogenated compounds (containing Cl, Br, I) as represented in LMDZ-INCA.</figcaption></figure>

## 2.2.1 Present-day set-up

Primary halogenated compounds can originate from both inorganic and organic sources. Inorganic species are reactive species emitted by different sources (e.g. oceans, sea salts) but also result from atmospheric reactions (Simpson et al., 2015). Organic species are only of primary origin and are photo-oxidised in the atmosphere to form halogen radicals (Simpson et al., 2015). Table 6 shows the emissions of halogenated compounds as considered in INCA. Organobromines (e.g. CHBr<sub>3</sub>, CH<sub>2</sub>Br<sub>2</sub>), organo-iodines (e.g. CH<sub>3</sub>I, CH<sub>2</sub>I<sub>2</sub>) and inter-halogen species (e.g. CH<sub>2</sub>IBr, CH<sub>2</sub>ICl) are those used by the CAM-Chem model (Ordóñez et al., 2012).

Sources of organic chlorine are diverse, mainly originating from biological activity on the surface of the oceans and from forest fires. The emissions of these chlorocarbons are detailed in Table S2. In INCA, inorganic chlorine, HCl – the major reservoir of chlorine in the atmosphere – is considered to be emitted by sea salts (90 000 Gg Cl yr⁻¹) and by forest fires (6400 Gg Cl yr⁻¹) with total based on Lobert et al. (1999) and Hossaini et al. (2016). Note that more comprehensive but hard to implement approaches relying on pH-dependent parameterisation of the dechlorination of sea salts

have been recently implemented in GEOS-Chem. HCl emissions from industrial sources (McCulloch et al., 1999) are neglected as recommended by Wang et al. (2019) to avoid overestimating HCl in urban areas. ClO₂ is emitted by the heterogeneous reaction of N₂O₅ on sea salts and produces 2217 Gg Cl yr⁻¹.

CH₃Br emissions are from oceanic origin (40%) and from agriculture (60%) (Barker et al., 2016) and represent 91 Gg of Br per year (Schmidt et al., 2016). IBr and ICl originate from the heterogeneous reaction of HOI, ClO₂ and ClO₃ on sea salt aerosols (Table 5) as considered in other models (Saiz-Lopez et al., 2014; Sherwen et al., 2016a) but without accounting for pH dependence (McFiggans et al., 2000).

The inorganic iodine distribution of emissions (HOI and I₂) are prescribed from a climatology computed by the CAM-Chem model considering sea-air fluxes as a function of the oxidation of aqueous iodide by atmospheric ozone on the ocean surface parameterised by Carpenter et al. (2013) and MacDonald et al. (2014) and which depends on the ozone deposition to the ocean surface, the sea surface temperature and the wind speed (Prados-Roman et al., 2015b).
<table><caption>**Table 1.** List of halogen tracers in INCA.</caption><thead><tr><th rowspan="2">Tracer</th><th rowspan="2">Name</th><th colspan="5">Processes affecting the tracer</th></tr><tr><th>Chemistry</th><th>Advection/convection</th><th>Emissions*</th><th>Dry deposition</th><th>Wet deposition</th></tr></thead><tbody><tr><td>Br<sub>2</sub></td><td>Dibromine</td><td>•</td><td>•</td><td>–</td><td>•</td><td>•</td></tr><tr><td>Br</td><td>Bromine</td><td>•</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>BrO</td><td>Bromine monoxide</td><td>•</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>HOBr</td><td>Hypobromous acid</td><td>•</td><td>•</td><td>–</td><td>•</td><td>•</td></tr><tr><td>HBr</td><td>Hydrogen bromide</td><td>•</td><td>•</td><td>–</td><td>•</td><td>•</td></tr><tr><td>BrNO<sub>2</sub></td><td>Bromine nitrite</td><td>•</td><td>•</td><td>–</td><td>•</td><td>•</td></tr><tr><td>BrNO<sub>3</sub></td><td>Bromine nitrate</td><td>•</td><td>•</td><td>–</td><td>•</td><td>•</td></tr><tr><td>CH<sub>3</sub>Br</td><td>Bromomethane</td><td>•</td><td>•</td><td>•</td><td>–</td><td>–</td></tr><tr><td>CH<sub>2</sub>Br<sub>2</sub></td><td>Dibromomethane</td><td>•</td><td>•</td><td>•</td><td>–</td><td>–</td></tr><tr><td>CHBr<sub>3</sub></td><td></td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr><tr><td>I<sub>2</sub></td><td>Diiodine</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr><tr><td>I</td><td>Iodine</td><td>•</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>IO</td><td>Iodine monoxide</td><td>•</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>OIO</td><td>Iodine dioxide</td><td>•</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>HOI</td><td>Hypoiiodous acid</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr><tr><td>HI</td><td>Hydrogen iodide</td><td>•</td><td>•</td><td>–</td><td>•</td><td>•</td></tr><tr><td>INO</td><td>Nitrosyl iodide</td><td>•</td><td>•</td><td>–</td><td>–</td><td>–</td></tr><tr><td>INO<sub>2</sub></td><td>Iodine nitrite</td><td>•</td><td>•</td><td>–</td><td>•</td><td>•</td></tr><tr><td>INO<sub>3</sub></td><td>Iodine nitrate</td><td>•</td><td>•</td><td>–</td><td>•</td><td>•</td></tr><tr><td>CH<sub>3</sub>I</td><td>Iodomethane</td><td>•</td><td>•</td><td>•</td><td>–</td><td>–</td></tr><tr><td>CH<sub>2</sub>I<sub>2</sub></td><td>Diiodomethane</td><td>•</td><td>•</td><td>•</td><td>–</td><td>–</td></tr><tr><td>I<sub>2</sub>O<sub>2</sub></td><td>Iodooxy hypoiodite</td><td>•</td><td>•</td><td>–</td><td>•</td><td>•</td></tr><tr><td>I<sub>2</sub>O<sub>3</sub></td><td>Iodo iodate</td><td>•</td><td>•</td><td>–</td><td>•</td><td>•</td></tr><tr><td>I<sub>2</sub>O<sub>4</sub></td><td>Iodosyl iodate</td><td>•</td><td>•</td><td>–</td><td>•</td><td>•</td></tr><tr><td>I<sub>aer</sub></td><td>Aerosol iodine</td><td>–</td><td>•</td><td>–</td><td>•</td><td>•</td></tr><tr><td>Cl<sub>2</sub></td><td>Dichlorine</td><td>•</td><td>•</td><td>–</td><td>•</td><td>•</td></tr><tr><td>Cl</td><td>Chlorine</td><td>•</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>HCl</td><td>Hydrogen chloride</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr><tr><td>ClO</td><td>Chlorine monoxide</td><td>•</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>ClNO<sub>2</sub></td><td>Chlorine nitrite</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr><tr><td>ClNO<sub>3</sub></td><td>Chlorine nitrate</td><td>•</td><td>–</td><td>–</td><td>•</td><td>•</td></tr><tr><td>ClOO</td><td>Chlorine superoxide</td><td>•</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>OCIO</td><td>Chlorine dioxide</td><td>•</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>HOCl</td><td>Hypochlorous acid</td><td>•</td><td>•</td><td>–</td><td>•</td><td>•</td></tr><tr><td>Cl<sub>2</sub>O<sub>2</sub></td><td>Dichlorine dioxide</td><td>•</td><td>•</td><td>–</td><td>–</td><td>–</td></tr><tr><td>CH<sub>3</sub>Cl</td><td>Chloromethane</td><td>•</td><td>•</td><td>•</td><td>–</td><td>–</td></tr><tr><td>CH<sub>2</sub>Cl<sub>2</sub></td><td>Dichloromethane</td><td>•</td><td>•</td><td>•</td><td>–</td><td>–</td></tr><tr><td>CHCl<sub>3</sub></td><td>Trichloromethane</td><td>•</td><td>•</td><td>•</td><td>–</td><td>–</td></tr><tr><td>CH<sub>2</sub>IBr</td><td>Bromoiodomethane</td><td>•</td><td>•</td><td>•</td><td>–</td><td>–</td></tr><tr><td>CH<sub>2</sub>ICl</td><td>Chloroiodomethane</td><td>•</td><td>•</td><td>•</td><td>–</td><td>–</td></tr><tr><td>IBr</td><td>Iodine monobromide</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr><tr><td>ICl</td><td>Iodine monochloride</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr><tr><td>BrCl</td><td>Bromine monochloride</td><td>•</td><td>•</td><td>–</td><td>•</td><td>•</td></tr></tbody></table>

### 2.2.2 Pre-industrial set-up

The aim of this simulation is to explore how the sensitivity of ozone to anthropogenic emissions perturbations have changed since the pre-industrial period (a present-day climate is thus considered). Only emissions from natural sources are included for the simulation of the pre-industrial

troposphere, following the approach used by other modelling teams to quantify this perturbation. Emissions of halogenated compounds from biomass burning are reduced to 10% of their current values (Wang and Jacob, 1998). The methane concentration is set at 700 ppbv (IPCC, 2013). Emissions of biogenic volatile organic compounds are kept at their present-day level due to uncertainties in the effect of their

<footnote>* Surface source • or net chemical source from sea salts (dynamically represented in INCA) ■.</footnote>
<table><caption>**Table 2.** Photolysis reactions of halogens included in the scheme.</caption><thead><tr><th>Reaction</th><th>Quantum yield φ</th><th>Reference for absorption cross-section</th></tr></thead><tbody><tr><td>Br<sub>2</sub> + hv → 2Br</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>BrNO<sub>2</sub> + hv → Br + NO<sub>2</sub></td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>BrONO<sub>2</sub> + hv → BrO + NO<sub>2</sub></td><td>0.15</td><td>Sander et al. (2011)</td></tr><tr><td>BrONO<sub>2</sub> + hv → Br + NO<sub>2</sub></td><td>0.85</td><td>Sander et al. (2011)</td></tr><tr><td>BrO + hv → Br</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>CH<sub>3</sub>Br + hv → Br</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>CH<sub>2</sub>Br<sub>2</sub> + hv → 2Br</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>CHBr<sub>3</sub> + hv → 3Br</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>HOBr + hv → Br + OH</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>BrCl + hv → Br + Cl</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>CH<sub>2</sub>IBr + hv → I + Br</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>IBr + hv → I + Br</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>CH<sub>2</sub>I<sub>2</sub> + hv → 2I</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>CH<sub>3</sub>I + hv → I</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>HOI + hv → I + OH</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>INO<sub>3</sub> + hv → I + NO<sub>3</sub></td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>I<sub>2</sub> + hv → 2I</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>INO<sub>2</sub> + hv → I + NO<sub>2</sub></td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>INO + hv → I + NO</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>IO + hv → I</td><td>0.91</td><td>Sander et al. (2011)</td></tr><tr><td>OIO + hv → I + O<sub>2</sub></td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>ICl + hv → I + Cl</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>CH<sub>2</sub>ICl + hv → I + Cl</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>CH<sub>2</sub>Cl<sub>2</sub> + hv → 2Cl</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>CH<sub>3</sub>Cl + hv → Cl + CH<sub>3</sub>O<sub>2</sub></td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>Cl<sub>2</sub>O<sub>2</sub> + hv → Cl + ClOO</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>Cl<sub>2</sub> + hv → 2Cl</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>ClNO<sub>2</sub> + hv → Cl + NO<sub>2</sub></td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>ClONO<sub>2</sub> + hv → Cl + NO<sub>2</sub></td><td>a</td><td>Sander et al. (2011)</td></tr><tr><td>ClONO<sub>2</sub> + hv → ClO + NO<sub>2</sub></td><td>b</td><td>Sander et al. (2011)</td></tr><tr><td>ClOO + hv → ClO + O<sub>2</sub></td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>ClO + hv → Cl</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>HOCl + hv → Cl + OH</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>OClO + hv → ClO + O</td><td>1</td><td>Sander et al. (2011)</td></tr><tr><td>I<sub>2</sub>O<sub>2</sub> + hv → I + OIO</td><td>0.21</td><td>Gomez Martin et al. (2005); Spietz et al. (2005)</td></tr><tr><td>I<sub>2</sub>O<sub>3</sub> + hv → IO + OIO</td><td>0.21</td><td>Gomez Martin et al. (2005); Spietz et al. (2005)</td></tr><tr><td>I<sub>2</sub>O<sub>4</sub> + hv → 2OIO</td><td>0.21</td><td>Gomez Martin et al. (2005); Spietz et al. (2005)</td></tr></tbody></table>

drivers over the past century and the resulting high uncertainty regarding the quantification of change in their emissions (Szopa et al., 2021).

Organo-iodine emissions are almost unchanged between the PI and PD. For organo-bromines, pre-industrial emissions of CHBr<sub>3</sub> and CH<sub>2</sub>Br<sub>2</sub> are equivalent to the PD ones, following the methodology of Parrella et al. (2012). The concentration of CH<sub>3</sub>Br is set to ~5 ppbv and is considered to be in agreement with the measurements from ice cores (Saltzman et al., 2004). Pre-industrial emissions of CH<sub>3</sub>Cl, CHCl<sub>3</sub> and CH<sub>2</sub>Cl<sub>2</sub> are from Carpenter et al. (2014), Hu (2012), Montzka et al. (2010), and Worton et al. (2006). HOI and I<sub>2</sub> emissions, pre-computed by CAM-Chem, are ~40% lower

than today because they are sensitive to atmospheric ozone content which was lower in the pre-industrial era (Prados-Roman et al., 2015a; Sherwen et al., 2017). Global emissions for each primary halogenated compound are reported in Table 6.

### 3 Evaluation of the representation of the halogen-driven ozone sinks

We first assess the ozone sensitivity to halogen chemistry in LMDZ-INCA, and then we evaluate the performance of the model in simulating the climatology of halogen species which are important for the global ozone loss. In order to do

<footnote>a $\varphi_1(\lambda < 308 \text{ nm}) = 0.6$, $\varphi_1(\lambda = 308-364 \text{ nm}) = 7.143 \times 10^{-3} \lambda(\text{nm}) - 1.60$, $\varphi_1(\lambda > 364 \text{ nm}) = 1.0$</footnote>

<footnote>b $\varphi_2(\lambda) = 1 - \varphi_1(\lambda)$</footnote>
<table><caption>**Table 3.** Bimolecular halogen reactions included in the INCA scheme. The rate constant is calculated using a standard Arrhenius expression $Ax \exp(-E_a/RT)$.</caption><thead><tr><th>Reaction</th><th>A<sub>0</sub> (molecules<sup>-2</sup> cm<sup>6</sup> s<sup>-1</sup>)</th><th>-E<sub>a</sub>/R(K)</th><th>Reference</th></tr></thead><tbody><tr><td>Cl + CH<sub>3</sub>O<sub>2</sub> → ClO + CH<sub>2</sub>O + HO<sub>2</sub></td><td>1.60 × 10<sup>-10</sup></td><td></td><td>Sander et al. (2011)</td></tr><tr><td>Cl + CH<sub>3</sub>OOH → HCl + CH<sub>3</sub>O<sub>2</sub></td><td>5.70 × 10<sup>-11</sup></td><td></td><td>Sander et al. (2011)</td></tr><tr><td>Cl + C<sub>2</sub>H<sub>6</sub> → HCl + C<sub>2</sub>H<sub>5</sub>O<sub>2</sub></td><td>7.20 × 10<sup>-11</sup></td><td>-70</td><td>Sander et al. (2011)</td></tr><tr><td>Cl + C<sub>2</sub>H<sub>5</sub>O<sub>2</sub> → ClO + HO<sub>2</sub> + CH<sub>3</sub>CHO</td><td>7.40 × 10<sup>-11</sup></td><td></td><td>Sander et al. (2011)</td></tr><tr><td>Cl + C<sub>2</sub>H<sub>5</sub>OH → HCl + CH<sub>3</sub>CHO</td><td>9.60 × 10<sup>-11</sup></td><td></td><td>Sander et al. (2011)</td></tr><tr><td>Cl + CH<sub>3</sub>COOH → HCl + CH<sub>3</sub>O<sub>2</sub> + CO<sub>2</sub></td><td>2.80 × 10<sup>-14</sup></td><td></td><td>Sander et al. (2011)</td></tr><tr><td>Cl + C<sub>3</sub>H<sub>8</sub> → HCl + C<sub>3</sub>H<sub>7</sub>O<sub>2</sub></td><td>7.85 × 10<sup>-11</sup></td><td>-80</td><td>Sander et al. (2011)</td></tr><tr><td>Cl + C<sub>3</sub>H<sub>8</sub> → HCl + PROPAO<sub>2</sub></td><td>6.54 × 10<sup>-11</sup></td><td></td><td>Sander et al. (2011)</td></tr><tr><td>Cl + CH<sub>3</sub>COCH<sub>3</sub> → HCl + PROPAO<sub>2</sub></td><td>7.70 × 10<sup>-11</sup></td><td></td><td>Sander et al. (2011)</td></tr><tr><td>Cl + ISOP → HCl + ISOPO<sub>2</sub></td><td>7.70 × 10<sup>-11</sup></td><td>500</td><td>Sander et al. (2011)</td></tr><tr><td>Cl + CH<sub>3</sub>OH → HCl + CH<sub>2</sub>O + HO<sub>2</sub></td><td>5.50 × 10<sup>-11</sup></td><td></td><td>Sander et al. (2011)</td></tr><tr><td>Cl + ALKAN → HCl + ALKANO<sub>2</sub></td><td>2.05 × 10<sup>-10</sup></td><td></td><td>Atkinson et al. (2006)</td></tr><tr><td>Cl + C<sub>3</sub>H<sub>6</sub> → HCl + PROPEO<sub>2</sub></td><td>3.60 × 10<sup>-12</sup></td><td></td><td>Atkinson et al. (2006)</td></tr><tr><td>Cl + CH<sub>3</sub>Cl → CO + 2HCl + HO<sub>2</sub></td><td>2.17 × 10<sup>-11</sup></td><td>-1130</td><td>Sander et al. (2011)</td></tr><tr><td>Cl + H<sub>2</sub>O<sub>2</sub> → HCl + HO<sub>2</sub></td><td>1.10 × 10<sup>-11</sup></td><td>-980</td><td>Sander et al. (2011)</td></tr><tr><td>Cl + HO<sub>2</sub> → HCl + O<sub>2</sub></td><td>1.40 × 10<sup>-11</sup></td><td>270</td><td>Sander et al. (2011)</td></tr><tr><td>Cl + HO<sub>2</sub> → ClO + OH</td><td>3.60 × 10<sup>-11</sup></td><td>-375</td><td>Sander et al. (2011)</td></tr><tr><td>Cl + O<sub>3</sub> → ClO + O<sub>2</sub></td><td>2.30 × 10<sup>-11</sup></td><td>-200</td><td>Sander et al. (2011)</td></tr><tr><td>Cl + ClNO<sub>3</sub> → Cl<sub>2</sub> + NO<sub>3</sub></td><td>6.50 × 10<sup>-12</sup></td><td>135</td><td>Sander et al. (2011)</td></tr><tr><td>ClO + ClO → Cl<sub>2</sub> + O<sub>2</sub></td><td>1.00 × 10<sup>-12</sup></td><td>-1590</td><td>Sander et al. (2011)</td></tr><tr><td>ClO + ClO → ClOCl + Cl</td><td>3.50 × 10<sup>-13</sup></td><td>-1370</td><td>Sander et al. (2011)</td></tr><tr><td>ClO + ClO → Cl + ClOO</td><td>3.00 × 10<sup>-11</sup></td><td>-2450</td><td>Sander et al. (2011)</td></tr><tr><td>ClO + HO<sub>2</sub> → O<sub>2</sub> + HOCl</td><td>2.60 × 10<sup>-12</sup></td><td>290</td><td>Sander et al. (2011)</td></tr><tr><td>ClO + NO → Cl + NO<sub>2</sub></td><td>6.40 × 10<sup>-12</sup></td><td>290</td><td>Sander et al. (2011)</td></tr><tr><td>ClOO + Cl → 2ClO</td><td>1.20 × 10<sup>-11</sup></td><td></td><td>Sander et al. (2011)</td></tr><tr><td>ClOO + Cl → Cl<sub>2</sub> + O<sub>2</sub></td><td>2.30 × 10<sup>-10</sup></td><td></td><td>Sander et al. (2011)</td></tr><tr><td>ClO + CH<sub>3</sub>O<sub>2</sub> → ClOO + HO<sub>2</sub> + CH<sub>2</sub>O</td><td>3.30 × 10<sup>-12</sup></td><td>-115</td><td>Sander et al. (2011)</td></tr><tr><td>OH + CH<sub>3</sub>Cl → Cl + HO<sub>2</sub> + H<sub>2</sub>O</td><td>3.90 × 10<sup>-12</sup></td><td>-1411</td><td>Sander et al. (2011)</td></tr><tr><td>OH + CH<sub>2</sub>Cl<sub>2</sub> → 2Cl + HO<sub>2</sub> + H<sub>2</sub>O</td><td>1.90 × 10<sup>-12</sup></td><td>-870</td><td>Sander et al. (2011)</td></tr><tr><td>OH + CHCl<sub>3</sub> → 3Cl + HO<sub>2</sub> + H<sub>2</sub>O</td><td>2.20 × 10<sup>-12</sup></td><td>-920</td><td>Sander et al. (2011)</td></tr><tr><td>OH + Cl<sub>2</sub> → HOCl + Cl</td><td>2.60 × 10<sup>-12</sup></td><td>-1100</td><td>Sander et al. (2011)</td></tr><tr><td>OH + Cl<sub>2</sub>O<sub>2</sub> → HOCl + ClOO</td><td>6.00 × 10<sup>-13</sup></td><td>670</td><td>Sander et al. (2011)</td></tr><tr><td>OH + ClNO<sub>2</sub> → HOCl + NO<sub>2</sub></td><td>2.40 × 10<sup>-12</sup></td><td>-1250</td><td>Sander et al. (2011)</td></tr><tr><td>OH + ClNO<sub>3</sub> → HOCl + NO<sub>3</sub></td><td>1.20 × 10<sup>-12</sup></td><td>-330</td><td>Sander et al. (2011)</td></tr><tr><td>OH + ClO → HCl + O<sub>2</sub></td><td>6.00 × 10<sup>-13</sup></td><td>230</td><td>Sander et al. (2011)</td></tr><tr><td>OH + ClO → HO<sub>2</sub> + Cl</td><td>7.40 × 10<sup>-12</sup></td><td>270</td><td>Sander et al. (2011)</td></tr><tr><td>OH + HCl → H<sub>2</sub>O + Cl</td><td>1.80 × 10<sup>-12</sup></td><td>-250</td><td>Sander et al. (2011)</td></tr><tr><td>OH + HOCl → H<sub>2</sub>O + ClO</td><td>3.00 × 10<sup>-12</sup></td><td>-500</td><td>Sander et al. (2011)</td></tr><tr><td>OH + ClOCl → HOCl + O<sub>2</sub></td><td>1.50 × 10<sup>-12</sup></td><td>600</td><td>Sander et al. (2011)</td></tr><tr><td>Cl + CH<sub>4</sub> → HCl + CH<sub>3</sub>O<sub>2</sub></td><td>9.60 × 10<sup>-12</sup></td><td>-1360</td><td>Atkinson et al. (2004)</td></tr><tr><td>Cl + C<sub>2</sub>H<sub>4</sub> → HCl + C<sub>2</sub>H<sub>5</sub>O<sub>2</sub></td><td>1.00 × 10<sup>-10</sup></td><td></td><td>Lurmann et al. (1986)</td></tr><tr><td>Cl + CH<sub>2</sub>O → HCl + HO<sub>2</sub> + CO</td><td>8.10 × 10<sup>-11</sup></td><td>-30</td><td>Sander et al. (2003)</td></tr><tr><td>Cl + PAN → HCl + CH<sub>2</sub>O + NO<sub>3</sub></td><td>1.00 × 10<sup>-14</sup></td><td></td><td>Sander et al. (2003)</td></tr><tr><td>Cl + HNO<sub>3</sub> → HCl + NO<sub>2</sub></td><td>1.00 × 10<sup>-16</sup></td><td></td><td>Sander et al. (2003)</td></tr><tr><td>Br + O<sub>3</sub> → BrO + O<sub>2</sub></td><td>1.60 × 10<sup>-11</sup></td><td>-780</td><td>Sander et al. (2011)</td></tr><tr><td>Br + HO<sub>2</sub> → HBr + O<sub>2</sub></td><td>4.80 × 10<sup>-12</sup></td><td>-310</td><td>Sander et al. (2011)</td></tr><tr><td>Br + CH<sub>2</sub>O → HO<sub>2</sub> + CO + HBr</td><td>1.70 × 10<sup>-11</sup></td><td>-800</td><td>Sander et al. (2011)</td></tr><tr><td>Br + C<sub>2</sub>H<sub>6</sub> → C<sub>2</sub>H<sub>5</sub>O<sub>2</sub> + HBr</td><td>2.36 × 10<sup>-10</sup></td><td>-6411</td><td>Seakins et al. (1992)</td></tr><tr><td>Br + C<sub>3</sub>H<sub>8</sub> → C<sub>3</sub>H<sub>7</sub>O<sub>2</sub> + HBr</td><td>8.77 × 10<sup>-11</sup></td><td>-4330</td><td>Seakins et al. (1992)</td></tr><tr><td>Br + CH<sub>3</sub>CHO → CH<sub>3</sub>CO<sub>3</sub> + HBr</td><td>1.30 × 10<sup>-11</sup></td><td>-360</td><td>Atkinson et al. (2007)</td></tr><tr><td>Br + CH<sub>3</sub>COCH<sub>3</sub> → PROPAO<sub>2</sub> + HBr</td><td>1.66 × 10<sup>-10</sup></td><td>-7000</td><td>King et al. (1970)</td></tr><tr><td>Br + C<sub>3</sub>H<sub>6</sub> → PROPEO<sub>2</sub> + HBr</td><td>3.60 × 10<sup>-12</sup></td><td></td><td>Atkinson et al. (2006)</td></tr></tbody></table>
<table><caption>**Table 3. Continued.**</caption><thead><tr><th>Reaction</th><th>A<sub>0</sub> (molecules<sup>-2</sup> cm<sup>6</sup> s<sup>-1</sup>)</th><th>-E<sub>a</sub>/R(K)</th><th>Reference</th></tr></thead><tbody><tr><td>Br + ALKEN → ALKENO<sub>2</sub> + HBr</td><td>3.60 × 10<sup>-12</sup></td><td></td><td>Atkinson et al. (2006)</td></tr><tr><td>Br + BrNO<sub>3</sub> → Br<sub>2</sub> + NO<sub>3</sub></td><td>4.90 × 10<sup>-11</sup></td><td></td><td>Orlando and Tyndall (1996)</td></tr><tr><td>Br + NO<sub>3</sub> → BrO + NO<sub>2</sub></td><td>1.60 × 10<sup>-11</sup></td><td></td><td>Sander et al. (2011)</td></tr><tr><td>HBr + OH → Br + H<sub>2</sub>O</td><td>5.50 × 10<sup>-12</sup></td><td>200</td><td>Sander et al. (2011)</td></tr><tr><td>BrO + OH → Br + HO<sub>2</sub></td><td>1.70 × 10<sup>-11</sup></td><td>250</td><td>Sander et al. (2011)</td></tr><tr><td>BrO + HO<sub>2</sub> → HOBr + O<sub>2</sub></td><td>4.50 × 10<sup>-12</sup></td><td>460</td><td>Sander et al. (2011)</td></tr><tr><td>BrO + NO → Br + NO<sub>2</sub></td><td>8.80 × 10<sup>-12</sup></td><td>260</td><td>Sander et al. (2011)</td></tr><tr><td>BrO + BrO → 2Br + O<sub>2</sub></td><td>2.40 × 10<sup>-12</sup></td><td>40</td><td>Sander et al. (2011)</td></tr><tr><td>BrO + BrO → Br<sub>2</sub> + O<sub>2</sub></td><td>2.80 × 10<sup>-14</sup></td><td>860</td><td>Sander et al. (2011)</td></tr><tr><td>Br<sub>2</sub> + OH → HOBr + Br</td><td>2.10 × 10<sup>-11</sup></td><td>240</td><td>Sander et al. (2011)</td></tr><tr><td>CHBr<sub>3</sub> + OH → 3Br + CO</td><td>1.35 × 10<sup>-12</sup></td><td>-600</td><td>Sander et al. (2011)</td></tr><tr><td>CH<sub>2</sub>Br<sub>2</sub> + OH → 2Br + CO</td><td>2.00 × 10<sup>-12</sup></td><td>-840</td><td>Sander et al. (2011)</td></tr><tr><td>CH<sub>3</sub>Br + OH → Br + CO</td><td>2.35 × 10<sup>-12</sup></td><td>-1300</td><td>Sander et al. (2011)</td></tr><tr><td>I + O<sub>3</sub> → IO + O<sub>2</sub></td><td>2.10 × 10<sup>-10</sup></td><td>-830</td><td>Atkinson et al. (2007)</td></tr><tr><td>I + HO<sub>2</sub> → HI + O<sub>2</sub></td><td>1.50 × 10<sup>-11</sup></td><td>-1090</td><td>Sander et al. (2011)</td></tr><tr><td>I<sub>2</sub> + OH → HOI + I</td><td>2.10 × 10<sup>-10</sup></td><td></td><td>Atkinson et al. (2007)</td></tr><tr><td>HI + OH → I + H<sub>2</sub>O</td><td>1.60 × 10<sup>-11</sup></td><td>440</td><td>Atkinson et al. (2007)</td></tr><tr><td>HOI + OH → IO + H<sub>2</sub>O</td><td>5.00 × 10<sup>-12</sup></td><td></td><td>Riffault et al. (2005)</td></tr><tr><td>IO + HO<sub>2</sub> → HOI + O<sub>2</sub></td><td>1.40 × 10<sup>-11</sup></td><td>540</td><td>Atkinson et al. (2007)</td></tr><tr><td>IO + NO → I + NO<sub>2</sub></td><td>7.15 × 10<sup>-12</sup></td><td>300</td><td>Atkinson et al. (2007)</td></tr><tr><td>CH<sub>3</sub>I + OH → H<sub>2</sub>O + I</td><td>4.30 × 10<sup>-12</sup></td><td>-1120</td><td>Atkinson et al. (2008)</td></tr><tr><td>INO + INO → I<sub>2</sub> + 2NO</td><td>8.40 × 10<sup>-11</sup></td><td>-2620</td><td>Atkinson et al. (2007)</td></tr><tr><td>INO<sub>2</sub> + INO<sub>2</sub> → I<sub>2</sub> + 2NO<sub>2</sub></td><td>4.70 × 10<sup>-12</sup></td><td>-1670</td><td>Atkinson et al. (2007)</td></tr><tr><td>I<sub>2</sub> + NO<sub>3</sub> → I + INO<sub>3</sub></td><td>1.50 × 10<sup>-12</sup></td><td></td><td>Atkinson et al. (2007)</td></tr><tr><td>INO<sub>3</sub> + I → I<sub>2</sub> + NO<sub>3</sub></td><td>9.10 × 10<sup>-11</sup></td><td>-146</td><td>Kaltsoyannis and Plane (2008)</td></tr><tr><td>OIO + OIO → I<sub>2</sub>O<sub>4</sub></td><td>1.50 × 10<sup>-10</sup></td><td></td><td>Gómez Martín et al. (2007)</td></tr><tr><td>OIO + NO → NO<sub>2</sub> + IO</td><td>1.10 × 10<sup>-12</sup></td><td>542</td><td>Atkinson et al. (2007)</td></tr><tr><td>IO + IO → I + OIO</td><td>2.16 × 10<sup>-11</sup></td><td>180</td><td>Atkinson et al. (2007)</td></tr><tr><td>IO + IO → I<sub>2</sub>O<sub>2</sub></td><td>3.24 × 10<sup>-11</sup></td><td>180</td><td>Atkinson et al. (2007)</td></tr><tr><td>IO + OIO → I<sub>2</sub>O<sub>3</sub></td><td>1.50 × 10<sup>-10</sup></td><td></td><td>Gómez Martín et al. (2007)</td></tr><tr><td>I<sub>2</sub>O<sub>2</sub> → IO + IO</td><td>1.00 × 10<sup>+12</sup></td><td>-9770</td><td>Ordóñez et al. (2012)</td></tr><tr><td>I<sub>2</sub>O<sub>2</sub> → OIO + I</td><td>2.50 × 10<sup>+14</sup></td><td>-9770</td><td>Ordóñez et al. (2012)</td></tr><tr><td>I<sub>2</sub>O<sub>4</sub> → 2OIO</td><td>3.80 × 10<sup>-02</sup></td><td></td><td>Kaltsoyannis and Plane (2008)</td></tr><tr><td>INO<sub>2</sub> → I + NO<sub>2</sub></td><td>9.94 × 10<sup>+17</sup></td><td>-11859</td><td>McFiggans et al. (2000)</td></tr><tr><td>INO<sub>3</sub> → IO + NO<sub>2</sub></td><td>2.10 × 10<sup>+15</sup></td><td>-13670</td><td>Kaltsoyannis and Plane (2008)</td></tr><tr><td>IO + ClO → I + OClO</td><td>2.59 × 10<sup>-11</sup></td><td>280</td><td>Atkinson et al. (2007)</td></tr><tr><td>IO + ClO → I + Cl + O<sub>2</sub></td><td>1.18 × 10<sup>-12</sup></td><td>280</td><td>Atkinson et al. (2007)</td></tr><tr><td>IO + ClO → ICl + O<sub>2</sub></td><td>9.40 × 10<sup>-13</sup></td><td>280</td><td>Atkinson et al. (2007)</td></tr><tr><td>I + BrO → IO + Br</td><td>1.20 × 10<sup>-11</sup></td><td></td><td>Sander et al. (2011)</td></tr><tr><td>IO + Br → I + BrO</td><td>2.70 × 10<sup>-11</sup></td><td></td><td>Bedjanian et al. (1997)</td></tr><tr><td>IO + BrO → Br + I + O<sub>2</sub></td><td>3.00 × 10<sup>-12</sup></td><td>510</td><td>Atkinson et al. (2007)</td></tr><tr><td>IO + BrO → Br + OIO</td><td>1.20 × 10<sup>-11</sup></td><td>510</td><td>Atkinson et al. (2007)</td></tr><tr><td>ClO + BrO → OClO + Br</td><td>1.60 × 10<sup>-12</sup></td><td>430</td><td>Atkinson et al. (2004)</td></tr><tr><td>ClO + BrO → Br + Cl + O<sub>2</sub></td><td>2.90 × 10<sup>-12</sup></td><td>220</td><td>Atkinson et al. (2004)</td></tr><tr><td>ClO + BrO → BrCl + O<sub>2</sub></td><td>5.80 × 10<sup>-13</sup></td><td>170</td><td>Atkinson et al. (2004)</td></tr></tbody></table>

The reactions come from GEOS-Chem (Parrella et al., 2012; Eastham et al., 2014; Schmidt et al., 2016; Sherwen et al., 2016a, b) and TOMCAT (Hossaini et al., 2016) and from Thomas et al. (2011). Lumped species are detailed in Folberth et al. (2006).
<table><caption>**Table 4.** Termolecular halogen reactions included in the INCA scheme.</caption><thead><tr><th>Termolecular reaction</th><th>A<sub>0</sub> (molecules<sup>-2</sup> cm<sup>6</sup> s<sup>-1</sup>)</th><th>x</th><th>k<sub>∞</sub></th><th>m</th><th>Fc</th><th>Reference</th></tr></thead><tbody><tr><td>Cl + O<sub>2</sub> + M → ClOO</td><td>2.20 × 10<sup>-33</sup></td><td>0</td><td>1.80 × 10<sup>-10</sup></td><td>3.1</td><td>0.6</td><td>Sander et al. (2011)</td></tr><tr><td>ClO + ClO + M → Cl<sub>2</sub>O<sub>2</sub></td><td>1.60 × 10<sup>-21</sup></td><td>2</td><td>3.0 × 10<sup>-12</sup></td><td>4.5</td><td>0.6</td><td>Sander et al. (2011)</td></tr><tr><td>ClO + NO<sub>2</sub> + M → ClNO<sub>3</sub></td><td>1.80 × 10<sup>-31</sup></td><td>3.4</td><td>1.50 × 10<sup>-11</sup></td><td>1.9</td><td>0.6</td><td>Sander et al. (2011)</td></tr><tr><td>ClOO + M → Cl + O<sub>2</sub></td><td>3.30 × 10<sup>-9</sup></td><td>0</td><td>2.73 × 10<sup>14</sup></td><td>3.1</td><td>0.6</td><td>Sander et al. (2011)</td></tr><tr><td>Cl<sub>2</sub>O<sub>2</sub> + M → Cl + O<sub>2</sub></td><td>9.30 × 10<sup>-6</sup></td><td>2</td><td>1.74 × 10<sup>15</sup></td><td>4.5</td><td>0.6</td><td>Sander et al. (2011)</td></tr><tr><td>Cl + C<sub>3</sub>H<sub>6</sub> + M → ALKANO<sub>2</sub></td><td>4.0 × 10<sup>-28</sup></td><td>0</td><td>2.80 × 10<sup>-10</sup></td><td>–</td><td>0.6</td><td>Atkinson et al. (2006)</td></tr><tr><td>Br + NO<sub>2</sub> + M → BrNO<sub>2</sub></td><td>4.20 × 10<sup>-31</sup></td><td>2.4</td><td>2.70 × 10<sup>-11</sup></td><td>–</td><td>0.6</td><td>Sander et al. (2011)</td></tr><tr><td>BrO + NO<sub>2</sub> + M → BrNO<sub>3</sub></td><td>5.20 × 10<sup>-31</sup></td><td>3.2</td><td>6.90 × 10<sup>-12</sup></td><td>–</td><td>0.6</td><td>Sander et al. (2011)</td></tr><tr><td>I + NO + M → INO</td><td>1.80 × 10<sup>-32</sup></td><td>1</td><td>1.70 × 10<sup>-11</sup></td><td>–</td><td>0.6</td><td>Atkinson et al. (2007)</td></tr><tr><td>I + NO<sub>2</sub> + M → INO<sub>2</sub></td><td>3.0 × 10<sup>-31</sup></td><td>1</td><td>6.60 × 10<sup>-11</sup></td><td>–</td><td>0.63</td><td>Atkinson et al. (2007)</td></tr><tr><td>IO + NO<sub>2</sub> + M → INO<sub>3</sub></td><td>7.70 × 10<sup>-31</sup></td><td>5</td><td>1.60 × 10<sup>-11</sup></td><td>–</td><td>0.4</td><td>Atkinson et al. (2007)</td></tr></tbody></table>

The reactions come from previous updates to halogen chemistry in GEOS-Chem (Parrella et al., 2012; Eastham et al., 2014; Schmidt et al., 2016; Sherwen et al., 2016a, b). The lower pressure limit rate ($k_0$) is given by $A_0(\frac{300}{T})^x$. The high pressure limit rate is given by $k_\infty$. Fc characterises the fall-off curve of the reaction as described by Atkinson et al. (2007).

<table><caption>**Table 5.** Halogen multiphase reactions and reactive uptake coefficients ($\gamma$).</caption><thead><tr><th>Reaction</th><th>Reactive uptake coefficient on sea salt (γ)</th><th>Reactive uptake coefficient on sulfate aerosols (γ)</th><th>Reference</th></tr></thead><tbody><tr><td>HOBr + HBr → Br<sub>2</sub> + H<sub>2</sub>O<sup>c</sup></td><td>0.2</td><td>0.2</td><td>Parrella et al. (2012)</td></tr><tr><td>HOBr + HCl → BrCl + H<sub>2</sub>O<sup>c</sup></td><td>0.2</td><td>0.2</td><td>Sander et al. (2011)</td></tr><tr><td>CINO<sub>3</sub> + HBr → BrCl + HNO<sub>3</sub><sup>c</sup></td><td>0.2</td><td>0.2</td><td>Compromise between Sander et al. (2011) and Badia et al. (2019)</td></tr><tr><td rowspan="2">CINO<sub>3</sub> → HOCl + HNO<sub>3</sub></td><td>0.001<sup>a</sup></td><td>0.001<sup>a</sup></td><td rowspan="2">Badia et al. (2019)</td></tr><tr><td>0.01<sup>b</sup></td><td>0.01<sup>b</sup></td></tr><tr><td rowspan="2">BrNO<sub>3</sub> → HOBr + HNO<sub>3</sub></td><td>0.03<sup>a</sup></td><td>0.03<sup>a</sup></td><td rowspan="2">Badia et al. (2019)</td></tr><tr><td>0.8<sup>b</sup></td><td>0.8<sup>b</sup></td></tr><tr><td>I<sub>2</sub>O<sub>x</sub> → 2 I<sup>d</sup><sub>aer</sub></td><td>0.02</td><td>2.10<sup>-2</sup></td><td>Sherwen et al. (2016a)</td></tr><tr><td>HI → I<sub>aer</sub></td><td>0.1</td><td>–</td><td>Sherwen et al. (2016a)</td></tr><tr><td>HOI → 0.85 ICl + 0.15 IBr + HNO<sub>3</sub></td><td>0.01</td><td>–</td><td>Sherwen et al. (2016a)</td></tr><tr><td>INO<sub>3</sub> → 0.85 ICl + 0.15 IBr + HNO<sub>3</sub></td><td>0.01</td><td>–</td><td>Sherwen et al. (2016a)</td></tr><tr><td>INO<sub>2</sub> → 0.85 ICl + 0.15 IBr + HNO<sub>3</sub></td><td>0.02</td><td>–</td><td>Sherwen et al. (2016a)</td></tr><tr><td>N<sub>2</sub>O<sub>5</sub> → 1.5 HNO<sub>3</sub> + 0.5 ClNO<sub>2</sub></td><td>3.10<sup>-2</sup></td><td>–</td><td>Hossaini et al. (2016)</td></tr></tbody></table>

so, model outputs for compounds of interest in this chemistry are compared to a range of halogen observations and simulations from other models with halogen chemistry. An analysis of the model tropospheric budgets of key halogen species is also performed.

## 3.1 Impact on ozone loss in the present day

Table 7 presents the ozone budget in LMDZ-INCA with and without consideration of halogen chemistry. Overall, when halogens are considered, the tropospheric ozone burden is reduced by 22% for present-day conditions, which is in the 5% to 25% range reported from other models which include Cl, Br and I chemistry (Saiz-Lopez et al., 2012a, b, 2015;

Sherwen et al., 2016a; Badia et al., 2019, 2021). Over the global troposphere, it is associated with a decrease in the annual-average ozone mixing ratio (mass-weighted) of ozone of 9.7 ppbv (compared to 9.4 ppbv in GEOS-Chem; Sherwen et al., 2016a). Over the tropospheric tropical column, the annual-average ozone mixing ratio (mass-weighted) decreases by 8 ppbv (21%), which is twice as large as the relative change reported in Saiz-Lopez et al. (2014). Surface ozone averaged over continental areas undergoes a decrease of 6 ppbv (23%), while surface ozone in oceanic areas, where the majority of primary halogenated compounds are emitted, decreases by 7.7 ppbv (32%) (range reported in other studies is 20%–33%; Saiz-Lopez et al., 2014; Long et al., 2014; Prados-Roman et al., 2015a; Sherwen et al., 2016a).

<footnote><sup>a</sup> Uptake coefficient for moderate temperatures. <sup>b</sup> Uptake coefficient for cold temperatures. <sup>c</sup> For second-order reactions, rate constants are calculated by assuming that the first reactant is limiting, and thus the first-order rate constant is divided by the concentrations of the adsorbed species. <sup>d</sup> $I_{2}O_{x}$ represents $I_{2}O_{2}$, $I_{2}O_{3}$ and $I_{2}O_{4}$.</footnote>
<table><caption>**Table 6.** Emission of halogen gases for pre-industrial and present-day simulations.</caption><thead><tr><th rowspan="2">Compounds</th><th colspan="2">I (Gg I yr<sup>-1</sup>)</th><th colspan="2">Br (Gg Br yr<sup>-1</sup>)</th><th colspan="2">Cl (Gg Cl yr<sup>-1</sup>)</th></tr><tr><th>Pre-industrial</th><th>Present</th><th>Pre-industrial</th><th>Present</th><th>Pre-industrial</th><th>Present</th></tr></thead><tbody><tr><td>CH<sub>3</sub>X</td><td>209</td><td>219</td><td>36</td><td>91</td><td>2081</td><td>2166</td></tr><tr><td>CH<sub>2</sub>X<sub>2</sub></td><td>108</td><td>108</td><td>62</td><td>62</td><td>199</td><td>628</td></tr><tr><td>CHX<sub>3</sub></td><td>—</td><td>—</td><td>506</td><td>506</td><td>236</td><td>315</td></tr><tr><td>HOI</td><td>1361</td><td>2448</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>I<sub>2</sub></td><td>34</td><td>59</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>ClNO<sub>2</sub></td><td>—</td><td>—</td><td>—</td><td>—</td><td>169</td><td>2217</td></tr><tr><td>CH<sub>2</sub>IBr</td><td>50</td><td>50</td><td>32</td><td>32</td><td>—</td><td>—</td></tr><tr><td>CH<sub>2</sub>ICl</td><td>168</td><td>168</td><td>—</td><td>—</td><td>47</td><td>47</td></tr><tr><td>IBr, ICl</td><td>—</td><td>—</td><td>495</td><td>799</td><td>1244</td><td>2008</td></tr><tr><td>HCl</td><td>—</td><td>—</td><td>—</td><td>—</td><td>96 429</td><td>96 429</td></tr><tr><td>Total</td><td>1930</td><td>3052</td><td>1131</td><td>1490</td><td>100 406</td><td>103 810</td></tr></tbody></table>

Sources of organic species (CH₃X, CH₂X₂, CHX₃) are shown in terms of emissions. I₂ and HOI are the inorganic oceanic source that depends on surface ozone (Carpenter et al., 2013). IBr and ICl are released following gaseous iodine uptake on sea salt. ClNO₂ is emitted after absorption of N₂O₅ on sea salt.

Figure 2 shows the absolute changes in surface ozone and relative change in zonally averaged ozone in a simulation with halogen chemistry relative to a simulation without it. Significant relative losses are found in the Southern Hemisphere, which are due to the strong emissions of halogenated species from the ocean in this region (Long et al., 2014; Saiz-Lopez et al., 2012a; Schmidt et al., 2016; Sherwen et al., 2016a, b) and the smaller absolute abundance of ozone. Most of the change in total tropospheric ozone concentration occurs in the free troposphere between 350 and 900 hPa (63%). This result is similar in magnitude to the relative change reports in GEOS-Chem (65%; Sherwen et al., 2016a) and CAM-Chem (65%; Iglesias-Suarez et al., 2020).

The annual production of ozone decreases by 4.7% when the chemistry of halogens is considered. This decrease is due to a reduction in NOₓ concentrations of 32.8% (Table 7) due to the hydrolysis of XNO₃ (X = Cl, Br, I) leading to deposition losses, in accordance with previous studies (Saiz-Lopez et al., 2012a; Long et al., 2014; von Glasow et al., 2004; Parrella et al., 2012; Schmidt et al., 2016; Sherwen et al., 2016a).

The vertical profile of the different contributions to chemical ozone loss is shown in Fig. 3. The processes leading to ozone loss differ considerably between simulations with and without halogenated compounds (Table 7). Halogens account for nearly ~22% of the ozone loss in the boundary layer in the halogen simulation, as shown in Fig. 3. Halogens represent 55% of the Oₓ (Oₓ = odd oxygen) loss in the upper troposphere (350 hPa > pressure > tropopause), while this value is about 33% and 40% in GEOS-Chem with the iodine plus bromine and the iodine plus bromine plus chlorine chemistry, respectively. Such differences could be partly due to differences in convection schemes which induce different uplifts of IO in the upper troposphere resulting in different IO concentrations (see Sect. 3.1.3 below) but also to

different representations of the stratospheric composition or chemistry between models.

When chemistry of halogenated compounds is considered, the total annual chemical ozone loss decreases by 2.4% (Table 7). The relative contributions of different halogens to ozone loss are very similar to those of GEOS-Chem in Sherwen et al. (2016b). The loss of ozone by chlorine, bromine and iodine represents 0.5%, 4.7% and 18% of the total losses, respectively; thus, halogens represent 23.2% of the total loss of ozone. The sum of the ozone loss caused by the halogens is 984 Tg yr⁻¹. Iodine is responsible for 78% of that halogen-mediated loss, while bromine accounts for 19%, which is close to the values reported by Iglesias-Suarez et al. (2020) (80%, 16%, respectively). Iodine is responsible for an overall tropospheric ozone loss of 762 Tg yr⁻¹ (comparable to the 748 Tg yr⁻¹ reported by Sherwen et al., 2016b). Much of this loss arises from the photolysis of HOI after its production from the reaction of IO with HO₂ and the photolysis of OIO after its production from reactions implicating IO and IₓOᵧ. Bromine is responsible for an overall tropospheric loss of 187 Tg yr⁻¹ which mostly arises from the photolysis of HOBr after its production from the reaction of BrO with HO₂.

The inclusion of halogens leads to an overall decrease in the net production of tropospheric ozone (PO₃–LO₃) of 20% (126 Tg yr⁻¹). Consequently, the lifetime of ozone (=tropospheric burden / losses) is significantly decreased by 3.2 d and reaches 17.9 d.

## 3.2 Present-day budgets for halogenated species in LMDZ-INCA

In order to reflect the role played by the halogenated species in their respective cycle, inorganic halogens are presented hereafter under the name Xᵧ which can be divided into HX
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Surface ozone changes (ppbv) and relative change in zonal tropospheric ozone (%) between simulations with and without halogen chemistry.</figcaption></figure>

Global vertical ozone loss in LMDZ-INCA without halogen chemistry

<figure><img src="image_5.png" /></figure>

Global vertical ozone loss in LMDZ-INCA with halogen chemistry

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Contribution of the chemical ozone sinks as a function of the altitude without (a) and with (b) consideration of the chemistry of halogens in LMDZ-INCA.</figcaption></figure>

(the acidic forms of halogenated compounds) and X* (composed of the nocturnal reservoirs of reactive halogenated species and the reactive halogenated species). HX in LMDZ-INCA takes part in heterogeneous bimolecular reactions. In CAM-Chem and GEOS-Chem, the representation of this chemistry is more detailed: HX can be incorporated in droplets or particles and dissociate (Ordóñez et al., 2012; Prados-Roman et al., 2015a; Saiz-Lopez et al., 2012a, 2014, 2015; Sherwen et al., 2016a). X* is subdivided into the “reactive halogen” XOₓ; grouping X + XO*; and the reservoir species X₂, HOX, XNO₂ and XNO₃, which have a relatively longer lifetime. These last species are formed mostly at night and serve as reservoirs of reactive halogens far from their primary sources.

In the following subsections, we present, for each halogen, the species considered in each reservoir, their respective tropospheric mass burden and the main fluxes between these reservoirs. The purpose is to evaluate the representation of halogenated species of interest for ozone and to discuss the limitations of the model in the computation of the budget of each halogen family.

### 3.2.1 Iodine

Iodine chemistry is the most impactful with regard to the ozone loss. The simplified iodine cycle is shown schematically in Fig. 4. The burden of inorganic iodine excluding IBr or ICl and iodine in aerosols is ~ 22.0 Gg in the troposphere in LMDZ-INCA, comparable to the value of ~ 27.9 Gg reported by Sherwen et al. (2016a, Fig. 7). HOI and OIO
<table><caption>**Table 7.** Comparison of global tropospheric Oₓ budgets of “Halo” and “NoHalo” simulations in INCA for the PI and PD. STE signifies stratosphere–troposphere exchange.</caption><thead><tr><th></th><th>Pre-industrial<br>Halo</th><th>Pre-industrial<br>NoHalo</th><th>Present day<br>Halo</th><th>Present day<br>NoHalo</th></tr></thead><tbody><tr><td>Ozone burden (Tg)</td><td>170.7</td><td>218.9</td><td>249.9</td><td>317</td></tr><tr><td>Ozone chemical sources (Tg yr<sup>-1</sup>)</td><td></td><td></td><td></td><td></td></tr><tr><td>NO + HO<sub>2</sub></td><td>1459</td><td>1554</td><td>2954</td><td>3190</td></tr><tr><td>NO + CH<sub>3</sub>O<sub>2</sub></td><td>474.9</td><td>477.2</td><td>1134</td><td>1159</td></tr><tr><td>NO + RO<sub>2</sub></td><td>380.7</td><td>355.5</td><td>625.8</td><td>596.7</td></tr><tr><td>Total chemical ozone sources (PO<sub>3</sub>)</td><td>2314</td><td>2387</td><td>4714</td><td>4946</td></tr><tr><td>Ozone chemical sinks (Tg yr<sup>-1</sup>)</td><td></td><td></td><td></td><td></td></tr><tr><td>O<sub>3</sub> + H<sub>2</sub>O → 2OH + O<sub>2</sub></td><td>1099</td><td>1497</td><td>1905</td><td>2460</td></tr><tr><td>O<sub>3</sub> + HO<sub>2</sub> → OH + O<sub>2</sub></td><td>431.5</td><td>648</td><td>917</td><td>1295</td></tr><tr><td>O<sub>3</sub> + OH → HO<sub>2</sub> + O<sub>2</sub></td><td>207.1</td><td>297.8</td><td>349.4</td><td>488.7</td></tr><tr><td>Bromine O<sub>3</sub> sinks</td><td>155.3</td><td>—</td><td>186.9</td><td>—</td></tr><tr><td>Iodine O<sub>3</sub> sinks</td><td>536.9</td><td>—</td><td>762.3</td><td>—</td></tr><tr><td>Chlorine O<sub>3</sub> sinks</td><td>23.51</td><td>—</td><td>22.16</td><td>—</td></tr><tr><td>Other O<sub>3</sub> sinks</td><td>78.06</td><td>85.11</td><td>88.5</td><td>92.87</td></tr><tr><td>Total chemical ozone sink (LO<sub>3</sub>)</td><td>2531</td><td>2528</td><td>4231</td><td>4337</td></tr><tr><td>Ozone PO<sub>3</sub>–LO<sub>3</sub> (Tg yr<sup>-1</sup>)</td><td>-216.6</td><td>-141</td><td>483</td><td>609.1</td></tr><tr><td>Ozone dry deposition (Tg yr<sup>-1</sup>)</td><td>403.4</td><td>593.4</td><td>875.5</td><td>1139</td></tr><tr><td>Ozone lifetime (days)</td><td>21.2</td><td>25.6</td><td>17.9</td><td>21.1</td></tr><tr><td>Ozone STE (LO<sub>3</sub> + dry deposition PO<sub>3</sub>) (Tg yr<sup>-1</sup>)</td><td>620</td><td>734.5</td><td>392.4</td><td>530.2</td></tr></tbody></table>

species, whose photolysis result in atomic iodine production, are the main intermediates for direct ozone loss. HOI is the most abundant iodine species, and its wet deposition controls the total atmospheric iodine burden. Its tropospheric burden of 12.8 Gg I in INCA is very close to the 13 Gg simulated by Sherwen et al. (2016b). The tropospheric burden of OIO in INCA, 0.43 Gg I, is comparable to the 0.55 Gg I also reported in Sherwen et al. (2016b). Overall, the global levels of HOI and OIO simulated by INCA are similar to their simulation by other models described in the literature.

The tropospheric burdens of HI, INO₂ and I₂ sum up to 1 Gg I, lower than the burdens reported for GEOS-Chem and totalling 2.48 Gg I (Sherwen et al., 2016b). The global burden of I₂ in INCA is lower than that in GEOS-Chem (Sherwen et al., 2016b) because in Sherwen et al. (2016b) heterogeneous reactions transform INO₃, INO₂ and HOI into I₂, whereas in INCA, these reactions produce IBr and ICl, similarly to the heterogeneous reactions adopted by Sherwen et al. (2016a). Other quantifications in the literature are lacking to explain the differences in INO₂ and HI; however, Saiz-Lopez et al. (2014) indicate that minor iodine species (Imin = HI + OIO + INO₂ + INO) represent less than 5 % of the mass in tropospheric inorganic iodine (2.3 % in our case). In INCA, the tropospheric Iₓ mean is 1.3 pptv (parts per trillion volume), whereas previous model studies found a range of ~0.5 to 1 pptv (Saiz-Lopez et al., 2014; Sherwen

et al., 2016a, b). A large set of observations has been assembled by Sherwen et al. (2016b) for IO. Measurements of IO during oceanic campaigns show a strong diurnal variation (IO being present only during daytime) with climatological concentrations (local observations averaged over 20° bins) comprised between 0.3 and 1 pptv. These IO concentrations show a weak latitudinal variability and a rapid decrease with altitude to reach values of about 0.2 pptv from 2 km to the tropopause (observations gathered and reported by Sherwen et al., 2016b). Surface values simulated by INCA agree within a factor of 2 with these observations. Over oceans, the annual mean surface concentrations of IO is approximately 0.2 pptv with concentrations of between 0 and 2 pptv. This is similar to the range simulated by Sherwen et al. (2016b) (0.25–1 pptv) and to the observations reported by Prados-Roman et al. (2015b). In the tropics (20° S–20° N), levels of surface IO in INCA (0.35 pptv) are in agreement with model results reported for CAM-Chem (~0.4 pptv) (Saiz-Lopez et al., 2014). In the free troposphere, IO levels slightly increase with altitude to reach diurnal values of ~0.5 pptv at 10 km in INCA. This overestimation of IO in the upper troposphere is common to all current model-simulated iodine and remains unclear (Sherwen et al., 2016b; Badia et al., 2019).

The IOₓ family influences the oxidative capacity of the troposphere through the catalytic cycles of ozone depletion (Brasseur, and Solomon, 2005), modifies the HOₓ (HO₂/OH)
and NOₓ (NO₂/NO) ratios (Bloss et al., 2010), produces photolabile higher-order iodine oxides (IₓOᵧ) (Gómez Martín et al., 2020; Lewis et al., 2020), and generates different forms of inorganic iodine (Saiz-Lopez et al., 2012b). For that reason, a correct representation of the branching ratios and the generation and loss of IOₓ is crucial. Globally, IOₓ production in INCA is dominated by inorganic photolysis of HOI (80.5 %), OIO (10.2 %), INO₃ (5.4 %) and I₂Oₓ (3.9 %). This partition is comparable to the one reported in Sherwen et al. (2016a), who found a 76 % HOI photolysis contribution and a 11 % OIO photolysis contribution. The main IOₓ loss pathway is the production of HOI by IO reaction with HO₂ (78.5 %), with additional loss pathways by self-reaction and reactions with NOₓ and BrO contributing 11.3 %, 5.7 % and 4.5 %, respectively – numbers that are very close to the branching ratios reported in Sherwen et al. (2016a) (77 %, 10 %, 7.7 % and 4.6 %, respectively).

### 3.2.2 Bromine

The tropospheric bromine budget and simplified recycling cycle are illustrated in Fig. 5. The tropospheric inorganic bromine burden is ~27 Gg in LMDZ-INCA, close to the 28 Gg reported for the most recent version of GEOS-Chem (Wang et al., 2021). On an annual average, INCA simulates daytime BrO values in the free troposphere of ~0.3 pptv within the tropics, close to the results in CAM-Chem (~0.2 pptv) (Fernandez et al., 2014). At the tropical tropopause layer, the modelled tropical BrO concentrations in INCA are ~3 pptv, close to the observed concentrations of 2.0 ± 1.5 pptv and of 3.2 ± 1.6 pptv at two different altitudes performed by Dorf et al. (2008) and reported by Fernandez et al. (2014) for the evaluation of CAM-Chem. Overall, the global tropospheric mean of BrO simulated by INCA is 0.6 pptv (in agreement with model results from Parrella et al., 2012) but is in the lower end of the 0.5 to 2 pptv range inferred from observations (Dorf et al., 2008; Saiz-Lopez and von Glasow, 2012; Stone et al., 2012).

In this first implementation of halogen chemistry in a version of LMDZ-INCA, the focus is on gaseous chemistry. Aqueous chemistry in droplets or on ice crystals in clouds is not accounted for in the first version of the scheme because it is complex and uncertain and, more importantly, might not be a dominant mechanism for global ozone loss. Nonetheless, the scheme includes recycling through heterogeneous chemistry on sea salt and sulfate aerosols. A study by Badia et al. (2019) using WRF-Chem shows that heterogeneous halogen chemistry (on all types of aerosols) allows a better vertical Brₓ partitioning but contributes little (~6%) to ozone loss compared to the gas-phase halogen chemistry (25%) on a global scale. Our choice to include, in this first step, reactions on sea salt and sulfate aerosols only is suited to reproduce bromine-mediated ozone loss in the marine boundary layer but probably tends to slightly underestimate the halogen-mediated ozone loss in the global troposphere.

Due to this lack of in-cloud halogen chemistry in cloud droplets and ice crystals, whose importance increases with altitude, the bromine chemistry in INCA is less dynamic and the recycling of bromine is less efficient in LMDZ-INCA compared to those reported for some versions of GEOS-Chem (Parrella et al., 2012; Schmidt et al., 2016; Sherwen et al., 2016a; Zhu et al., 2019). Note, however, that the recently updated HOBr heterogeneous chemistry implemented in the most recent GEOS-Chem version (Wang et al., 2021) results in much slower recycling of HOBr in clouds and aqueous aerosols than previously calculated. This results in fluxes from heterogeneous reactions (specifically HOBr + HBr → Br₂ + H₂O and HOBr + HCl → BrCl + H₂O) that sum up to 32 500 Gg Br yr⁻¹, which is smaller for example than our estimates of 86 000 Gg Br yr⁻¹ but remains of the same order of magnitude. Finally, our branching ratios are found to be consistent with those in Schmidt et al. (2016) and Wang et al. (2021). This leads us to conclude that, although LMDZ-INCA does not integrate the same degree of details in tropospheric halogen heterogeneous chemistry compared to its peers, global fluxes in the bromine cycle of INCA appear to be more comparable in terms of the bromine cycle with those in the newest version of GEOS-Chem (see the detailed bromine budget and cycle in Fig. S1) and CAM-Chem (Iglesias-Suarez et al., 2020) than fluxes in the older versions of GEOS-Chem (Parrella et al., 2012; Schmidt et al., 2016; Sherwen et al., 2016a; Zhu et al., 2019). All these estimates remain of the same order of magnitude, but the range of fluxes illustrates the level of uncertainties about the strength of the halogen heterogeneous recycling.

### 3.2.3 Chlorine

The total tropospheric burden of Clᵧ (inorganic chlorine) is 290 Gg of Cl, largely dominated by HCl. The total load of HCl is smaller than the one reported for GEOS-Chem (Wang et al., 2019) essentially because of the difference in the representation of the emissions. However, the total burden of reservoir species has a similar magnitude (10.4 Gg Cl). The halogen chemistry in INCA leads to a 25 % higher quantity of available reactive chlorine compounds: Cl and ClO* (ClO* being ClO + ClOO + OCIO + 2Cl₂O₂).

Atomic chlorine has two main chemical sinks. The first and most important in terms of reaction flux is the transformation of atomic Cl into ClO*. This conversion takes place mainly by reaction with ozone (99.5 %), while the conversion of ClOₓ to Cl is dominated by reaction with NO (93.9 %). The second path is represented by the chemical loss of reactive chlorine Cl to inactive chlorine HCl (Fig. 6), which is due to the reactions of Cl with methane and several NMVOCs. The conversion of Cl to ClO* is in strong competition with the second route. By dividing the fluxes of the conversion Cl to ClO* by the flux of conversion of Cl to HCl, we find that the chain length is short and equal to 1.3 (4.8 × 10⁴/3.6 × 10⁴).
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Iodine budget in the present-day reference simulation with simplified cycle of tropospheric $I_y$ species. Heterogeneous reactions are shown in green. Photolysis reactions are in red. The numbers in the boxes represent the mass balance of the species families (in Gg I), and the numbers near the arrows represent the fluxes through chemical reactions (in Gg I yr$^{-1}$).</figcaption></figure>

This is close to the chain length of 1.6 calculated by Wang et al. (2019) and shows that the branching ratios are of the same order of magnitude in both models despite differences in NMVOC and CH$_4$ concentrations between the two models. Overall, we find that the partition between Cl and ClO* is correct in the model as well as the balance between radical and non-radical species, although the latter is a little more shifted towards reactive species in INCA compared to GEOS-Chem and CAM-Chem.

The average of climatological observations (mean over several months and representative of large areas) over six oceanic and coastal remote locations shows a mean surface HCl mixing ratio of 196±114 pptv, whereas INCA simulates an average of 193 ± 98 pptv, compared to the 246 ± 90 pptv in GEOS-Chem (Wang et al., 2019) (see Fig. S2). However, for continental regions, LMDZ-INCA underestimates the HCl concentrations compared to observations mainly because it does not include industrial emissions. CINO$_2$ over continental areas does not exceed a few tens of pptvs in INCA, whereas observations at several continental locations, as gathered by Wang et al. (2019) (their Table 5), show concentrations of a few hundreds to thousands of pptvs over such locations for hourly maxima. This difference can arise from the coarse resolution of the model which does not resolve the spatial heterogeneity of these local observations (Sherwen et al., 2016a) and the absence of CINO$_2$ pro-

duction in sulfate-nitrate-ammonium aerosols (Wang et al., 2019). The chlorine Cl* measurements to which we compare LMDZ-INCA results represent the sum of Cl$_2$, HOCl, CINO$_2$, CINO$_3$ and other minor components as explained in Pszenny et al. (1993). LMDZ-INCA Cl* values are of the same order of magnitude as measurements (see Table S3).

# 4 Perturbation of the ozone sensitivity to its precursors by halogen chemistry

In this section, the evolution of the model-calculated ozone sensitivity from the pre-industrial period to the present day is assessed, with and without halogen chemistry, and compared with results from other models. Then the impact of halogen chemistry on the sensitivity of ozone, OH radical and CH$_4$ concentrations to the emissions of anthropogenic ozone precursors is discussed.

## 4.1 Pre-industrial to present-day changes

With and without halogen chemistry, the model simulates significantly lower ozone concentrations in the pre-industrial compared to present due to the decrease in NO$_x$ concentrations. Taking into account halogen chemistry reduces the average surface ozone concentrations for pre-industrial condi-
Troposphere

<figure><img src="image_8.png" /><figcaption>**Figure 5.** Bromine budget in the present-day reference simulation with a simplified representation of the cycle of tropospheric Br<sub>y</sub> species. Fluxes through the reactions, indicated next to the arrows, are in gigagrams Br per year (Gg Br yr<sup>-1</sup>). Heterogeneous reactions are represented in green. Percentages in brackets represent the branching ratios for the Br chemical sink (in violet), for the BrO chemical sinks (in grey) and for HOBr chemical sinks (in light maroon). The numbers in the boxes represent the burden of the species or families in gigagrams Br (Gg Br), and the numbers near the arrows represent the fluxes through chemical reactions in gigagrams Br per year.</figcaption></figure>

tions by 5.0 ppbv (38 %), with the largest reductions over the oceans (5.5 ppbv or 42.4 %).

Globally, halogens reduce the tropospheric ozone burden by 67.1 Tg in the present day and 48.2 Tg in pre-industrial conditions (Table 7). Between PI and PD, ozone increases by 98.1 Tg (113 Tg in Sherwen et al., 2017) without halogen chemistry and by 79.2 Tg (90 Tg in Sherwen et al., 2017) with halogen chemistry. Both in PD and PI, halogens are responsible for about 23%–28% of the ozone destruction. Tropospheric ozone lifetimes drop from 21.1 to 17.9 d in the present with the inclusion of halogens and from 25.6 to 21.2 d in the pre-industrial period. The sensitivity of the burden of ground-level ozone to pre-industrial–present changes is ~20% lower when the chemistry of the halogens is considered. The changes observed in LMDZ-INCA and schematised in Fig. 7 are in agreement with the changes in Sherwen et al. (2017).

## 4.2 Sensitivity runs

In order to study the effect of halogen chemistry on the sensitivity of the tropospheric oxidative system, we explore, in Fig. 8, the relative changes (%) in ozone and OH when present-day emissions of photo-oxidant precursors (NO<sub>x</sub>,

CO and NMVOCs and of CH<sub>4</sub> concentrations) are perturbed by a 20% reduction, as done in the HTAP protocol. Our results show that the importance of sensitivities of ozone and OH to individual photo-oxidant precursor emissions increases as follows in terms of importance: NMVOCs, CO, NO<sub>x</sub> and CH<sub>4</sub> (Fig. 8). It is also clear that the OH radical is more sensitive to perturbations in CH<sub>4</sub>, NO<sub>x</sub> and CO than ozone. Notably, when accounting for halogens, ozone becomes more sensitive to changes in CH<sub>4</sub> and NO<sub>x</sub> and to a lesser extent to changes in NMVOCs. On the other hand, in the presence of halogen chemistry, the OH radical becomes more resilient in the presence of halogens to CH<sub>4</sub>, NO<sub>x</sub> and NMVOCs perturbations. Usually, solely the CH<sub>4</sub> consumption by OH is considered when the chemical lifetime of CH<sub>4</sub> in the troposphere is reported in the literature since it represents the main CH<sub>4</sub> sink. It thus provides a useful proxy for global tropospheric oxidising capacity (Wild et al., 2020). The simulations examined here show that the CH<sub>4</sub> lifetime with respect to OH remains a useful proxy for global tropospheric oxidising capacity for all perturbations. However, in the presence of halogen chemistry, when NO<sub>x</sub> emissions are reduced, atomic chlorine, a secondary oxidant of methane, becomes more efficient and increases the sensitivity of CH<sub>4</sub> to NO<sub>x</sub> emission change. This exercise shows for the first
<figure><img src="image_9.png" /><figcaption>**Figure 6.** Simplified representation of the cycle of tropospheric Cl<sub>y</sub> species in the reference simulation. Fluxes through the reactions are in gigagrams Cl per year (Gg Cl yr<sup>-1</sup>). Heterogeneous reactions are represented in green. The main loss pathways for atomic chlorine are in orange. The numbers in the boxes represent the mass balances of the species families in Gg Cl.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 7.** Schematic representation of the evolution of ozone mass budget changes between PI and PD, following the inclusion of halogen chemistry in LMDZ-INCA.</figcaption></figure>

time the influence of halogens on the sensitivity of the tro-
pospheric oxidative system to changes in the emissions of
photo-oxidant precursor (NOₓ, CO and NMVOCs and CH₄).

**4.3 Impact on methane lifetime**

The inclusion of tropospheric halogens affects the atmo-
spheric concentrations of a large number of compounds in
the model. A marked decrease in oxidants (O3, OH, HO2,
H2O2) is then computed. This leads to an increase in the con-

centrations of CO (+8.5 %) and some NMVOCs, including
isoprene (+7.1 %). The lifetime of methane with respect to
chlorine and the OH radical, as well as the share of chlorine
in the losses of methane in the literature, is shown in Table 8.
In the reference simulation, the tropospheric chemical life-
time of methane due to its consumption by OH is 7.9 years.
With the inclusion of halogen chemistry, the OH concentra-
tion decreases, increasing this chemical lifetime of methane
to 9.3 years. The chemical lifetime of methane linked to ox-
idation by Cl is 333 years, less than the 384 years suggested
by Hossaini et al. (2016) but closer to IPCC Fifth Assess-
ment Report (AR5) estimates. This oxidation by Cl repre-
sents 2.7 % of the total loss (close to the values reported in
the literature and in Table 8). Thus, in LMDZ-INCA, ac-
counting for both OH and the chlorinated radical consump-
tion, methane oxidation leads to a total chemical lifetime of
CH₄ of 9.0 years.

**5 Conclusions**

Global chemistry climate models allow us to simulate the
evolution of the atmospheric composition and are used for fu-
ture projections in the framework of international model in-
tercomparisons such as AerChemMIP. Most of these models
do not account for halogen chemistry. We have implemented
a simplified representation of the halogenated chemistry
in the three-dimensional climate–chemistry model LMDZ-
<figure><img src="image_11.png" /><figcaption>**Figure 8.** Tropospheric relative changes (%) in ozone mass budget, OH concentration and methane lifetime for perturbation of NOₓ, CO and NMVOC emissions and CH₄ concentrations with and without halogen chemistry.</figcaption></figure>

<table><caption>**Table 8.** Methane lifetime with respect to chlorine and OH radicals as well as the share of chlorine in methane losses in the literature.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Lifetime (years) relative to</th><th rowspan="2">Total chemical<br>lifetime (years)</th><th rowspan="2">Method</th><th rowspan="2">Share of chlorine in<br>methane loss (%)</th></tr><tr><th>chlorine</th><th>OH radical</th></tr></thead><tbody><tr><td>IPCC (2013) AR5</td><td>200 ± 100</td><td>11.2 ± 1.3</td><td>9.25 ± 0.6</td><td>Multi-evidence assessment</td><td>—</td></tr><tr><td>Hossaini et al. (2016)</td><td>384.4</td><td>10</td><td></td><td>TOMCAT model</td><td>2.5</td></tr><tr><td>Sherwen et al. (2016a)</td><td>—</td><td>8.28</td><td>8.16</td><td>GEOS-Chem model</td><td>2.0</td></tr><tr><td>Li et al. (2022)</td><td></td><td>10.6–10.8</td><td></td><td>CAM-Chem model</td><td></td></tr><tr><td>Szopa et al. (2021)</td><td></td><td>9.7 ± 1.1</td><td>9.1 ± 0.9</td><td>Multi-evidence assessment</td><td></td></tr><tr><td>This work</td><td>—</td><td>7.9</td><td>7.9</td><td>LMDZ-INCA model (NoHalo)</td><td>—</td></tr><tr><td></td><td>332.5</td><td>9.28</td><td>9.0</td><td>LMDZ-INCA model (Halo)</td><td>2.7</td></tr></tbody></table>

INCA to account for its effects on the tropospheric chemistry, notably ozone.

First, the halogenated species, their sources and sinks, their reactions in the gas and heterogeneous phases, and their physicochemical characteristics have been integrated into INCA. In order to simplify the integration of this chemistry, a chemical scheme already tested and evaluated in the CAM-Chem model has been used as a basis for our developments.

The representation of this chemistry has been shown to be sufficient in a first approach to simulate properly the impact of halogens on the photo-oxidising system in the troposphere and particularly in the atmospheric boundary layer. The response of the tropospheric chemical system Oₓ, HOₓ, NOₓ,

CH₄ and NMVOCs to the incorporation of halogen chemistry has been quantified. On a global tropospheric scale, this chemistry induces decreases in the ozone burden by 22%, in OH by 8% and in NOₓ by 33%. An increase in CO and NMVOC concentrations is simulated, and the lifetime of methane increases by 1 year. These changes are consistent with those reported in the literature. Comparisons between simulations considering pre-industrial and present-day emissions show that the sensitivity of the ozone burden to changes in emissions over the pre-industrial era is ~ 20% lower in the presence of halogen chemistry (mainly because of the chemistry of iodine).

Sensitivity tests, consisting of reducing in turn the emissions of ozone precursors and methane concentrations, show
for the first time that ozone becomes more sensitive to NOₓ, CO and NMVOC perturbations in the presence of halogen chemistry.

Although still rarely represented in chemistry climate models, our model results confirm that the chemistry of tropospheric halogens (Cl, Br, I) plays a significant role in the global loss of tropospheric ozone. Since halogen chemistry affects substantially the ozone burden both in pre-industrial and present-day chemical conditions and the ozone sensitivity to changes in anthropogenic emissions, international exercises examining the evolution of ozone between pre-industrial, present and future periods should consider taking into account halogen chemistry.

*Code and data availability.* The main model codes and the chemical scheme with halogenated compounds as well as the data outputs are publicly accessible through https://doi.org/10.14768/56b1b070-deae-4357-a2dd-d75d27659f63 (Szopa et al., 2023).

*Supplement.* The supplement related to this article is available online at: https://doi.org/10.5194/gmd-16-4041-2023-supplement.

*Author contributions.* The implementation of the halogens chemical scheme has been done by CC, SS and AC. The design of the numerical simulations was conducted by CC, SS, ASL and CAC. CC and SS prepared the figures. CC, SS, SB, ASL and CAC analysed the results and drafted the paper.

*Competing interests.* At least one of the (co-)authors is a member of the editorial board of *Geoscientific Model Development*. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.

*Disclaimer.* Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

*Acknowledgements.* The authors are thankful to the NCAR Atmospheric Chemistry Division (ACD) for the distribution of the NCAR/ACD TUV: Tropospheric Ultraviolet and Visible Radiation Model (https://www.acom.ucar.edu/Models/TUV/Interactive_TUV/, last access: 5 July 2023) and the availability of their quick tool.

*Financial support.* This research has been supported by the Agence Nationale de la Recherche (grant no. ANR-16-CE31-0010) and the HPC resources of TGCC under the allocation A0090102212 made by GENCI (Grand Equipement National de Calcul Intensif).

*Review statement.* This paper was edited by Samuel Remy and reviewed by six anonymous referees.

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