Global impacts of tropospheric halogens (Cl, Br, I) on oxidants and composition in GEOS-Chem

Abstract

We present a simulation of the global present-day composition of the troposphere which includes the chemistry of halogens (Cl, Br, I). Building on previous work within the GEOS-Chem model we include emissions of inorganic iodine from the oceans, anthropogenic and biogenic sources of halogenated gases, gas phase chemistry, and a parameterised approach to heterogeneous halogen chemistry. Consistent with Schmidt et al. (2016) we do not include sea-salt debromination. Observations of halogen radicals (BrO, IO) are sparse but the model has some skill in reproducing these. Modelled IO shows both high and low biases when compared to different datasets, but BrO concentrations appear to be modelled low. Comparisons to the very sparse observations dataset of reactive Cl species suggest the model represents a lower limit of the impacts of these species, likely due to underestimates in emissions and therefore burdens. Inclusion of Cl, Br, and I results in a general improvement in simulation of ozone (O₃) concentrations, except in polar regions where the model now underestimates O₃ concentrations. Halogen chemistry reduces the global tropospheric O₃ burden by 18.6%, with the O₃ lifetime reducing from 26 to 22 days. Global mean OH concentrations of 1.28 × 10⁶moleculescm^-3 are 8.2% lower than in a simulation without halogens, leading to an increase in the CH₄ lifetime (10.8%) due to OH oxidation from 7.47 to 8.28 years. Oxidation of CH₄ by Cl is small (∼ 2%) but Cl oxidation of other VOCs (ethane, acetone, and propane) can be significant (∼ 15-27%). Oxidation of VOCs by Br is smaller, representing 3.9% of the loss of acetaldehyde and 0.9% of the loss of formaldehyde.

Atmos. Chem. Phys., 16, 1223912271, 2016 www.atmos-chem-phys.net/16/12239/2016/ doi:10.5194/acp-16-12239-2016 Author(s) 2016. CC Attribution 3.0 License.

Global impacts of tropospheric halogens (Cl, Br, I) on oxidants and composition in GEOS-Chem

Toms Sherwen1, Johan A. Schmidt2, Mat J. Evans1,3, Lucy J. Carpenter1, Katja Gromann4,a,Sebastian D. Eastham5, Daniel J. Jacob5, Barbara Dix6, Theodore K. Koenig6,7, Roman Sinreich6, Ivan Ortega6,7, Rainer Volkamer6,7, Alfonso Saiz-Lopez8, Cristina Prados-Roman8,b, Anoop S. Mahajan9, and Carlos Ordez10

1Wolfson Atmospheric Chemistry Laboratories (WACL), Department of Chemistry, University of York, York, YO10 5DD, UK

2Department of Chemistry, University of Copenhagen, Universitetsparken, 2100 Copenhagen O, Denmark

3National Centre for Atmospheric Science (NCAS), University of York, York, YO10 5DD, UK

4Institute of Environmental Physics, University of Heidelberg, Heidelberg, Germany

5School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA

6Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215, USA

7Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309-021, USA

8Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano, CSIC, Madrid, 28006, Spain

9Indian Institute of Tropical Meteorology, Maharashtra, 411008, India

10Dpto. Fsica de la Tierra II, Facultad de Ciencias Fsicas, Universidad Complutense de Madrid, 28040 Madrid, Spain

anow at: Joint Institute For Regional Earth System Science and Engineering (JIFRESSE), University of California Los Angeles, Los Angeles, CA, 90095, USA

bnow at: Atmospheric Research and Instrumentation Branch, National Institute for Aerospace Technology (INTA), Madrid, Spain

Correspondence to: Toms Sherwen ([email protected])

Received: 18 May 2016 Published in Atmos. Chem. Phys. Discuss.: 20 May 2016Revised: 12 September 2016 Accepted: 16 September 2016 Published: 29 September 2016

Abstract. We present a simulation of the global present-day composition of the troposphere which includes the chemistry of halogens (Cl, Br, I). Building on previous work within the GEOS-Chem model we include emissions of inorganic iodine from the oceans, anthropogenic and biogenic sources of halogenated gases, gas phase chemistry, and a parameterised approach to heterogeneous halogen chemistry. Consistent with Schmidt et al. (2016) we do not include sea-salt debromination. Observations of halogen radicals (BrO, IO) are sparse but the model has some skill in reproducing these.Modelled IO shows both high and low biases when compared to different datasets, but BrO concentrations appear to be modelled low. Comparisons to the very sparse observations dataset of reactive Cl species suggest the model represents a lower limit of the impacts of these species, likely due to underestimates in emissions and therefore burdens.

Inclusion of Cl, Br, and I results in a general improvement in simulation of ozone (O3) concentrations, except in polar regions where the model now underestimates O3 concentrations. Halogen chemistry reduces the global tropospheric O3 burden by 18.6 %, with the O3 lifetime reducing from 26 to 22 days. Global mean OH concentrations of1.28 [notdef] 106 molecules cm3 are 8.2 % lower than in a simula

tion without halogens, leading to an increase in the CH4 lifetime (10.8 %) due to OH oxidation from 7.47 to 8.28 years. Oxidation of CH4 by Cl is small ( 2 %) but Cl oxidation of

other VOCs (ethane, acetone, and propane) can be signicant ( 1527 %). Oxidation of VOCs by Br is smaller, represent

ing 3.9 % of the loss of acetaldehyde and 0.9 % of the loss of formaldehyde.

Published by Copernicus Publications on behalf of the European Geosciences Union.

sions budgets and evaluate these on a global scale (Bell et al., 2002; Ziska et al., 2013; Hossaini et al., 2013; Ordez et al., 2012). Global studies have considered impacts of halogens in the troposphere (Parrella et al., 2012; Saiz-Lopez et al., 2012a, 2014; Schmidt et al., 2016; Sherwen et al., 2016a) and reported reductions in the tropospheric O3 burden by up to 15 %. However, this eld of research is quickly evolv

ing, with new halogen sources such as inorganic ocean iodine (Carpenter et al., 2013; MacDonald et al., 2014) and ClNO2 produced from N2O5 hydrolysis on sea salt (Roberts et al., 2009; Bertram and Thornton, 2009; Sarwar et al., 2014) now appearing to be globally important.

Previous studies of halogen chemistry within the GEOSChem (http://www.geos-chem.org

Web End =http://www.geos-chem.org ) model have focussed on either bromine or iodine chemistry. Parrella et al. (2012) presented a bromine scheme and its effects on oxidants in the past and present atmosphere. Eastham et al. (2014) presented the Unied troposphericstratospheric Chemistry eXtension (UCX), which added a stratospheric bromine and chlorine scheme. This chlorine scheme was then employed in the troposphere with an updated heterogeneous bromine and chlorine scheme by Schmidt et al. (2016). An iodine scheme was employed in the troposphere to consider present-day impacts of iodine on oxidants (Sherwen et al., 2016a), which used the representation of bromine chemistry from Parrella et al. (2012). Up to this point, the coupling of chlorine, bromine, and iodine in the GEOS-Chem model and its subsequent impact on the simulated present-day composition of the atmosphere have not been described.

Here we present such a coupled halogen model built within the GEOS-Chem framework and consider the present-day tropospheric impacts of halogens. The model presented here includes recent updates to chlorine (Eastham et al., 2014; Schmidt et al., 2016), bromine (Parrella et al., 2012;Schmidt et al., 2016), and iodine (Sherwen et al., 2016a) chemistry with further updates and additions described in Sect. 2. In Sect. 3 we describe the modelled distribution of inorganic halogens (Sects. 3.13.3) and compare with observations (Sect. 3.4). We then outline the impact on oxidants (Sects. 4.14.2), organic compounds (Sect. 4.3), and other species (Sect. 4.4).

2 Model description

This work uses the GEOS-Chem chemical transport model (http://www.geos-chem.org

Web End =http://www.geos-chem.org , version 10) run at 4 [notdef] 5 spa

tial resolution. The model is forced by assimilated meteorological and surface elds from NASAs Global Modelling and Assimilation Ofce (GEOS-5). The model chemistry scheme includes Ox, HOx, NOx, and VOC chemistry as described in Mao et al. (2013). Dynamic and chemical time steps are 30 and 60 min, respectively. Stratospheric chemistry is modelled using a linearised mechanism as described by Murray et al. (2012).

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12240 T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem

1 Introduction

To address problems such as air-quality degradation and climate change, we need to understand the composition of the troposphere and its oxidative capacity. A complicated relationship exists between key chemical families and species such as ozone (O3), HOx (HO2 + OH), NOx

(NO2 + NO), and organic compounds which include carbon

monoxide (CO), methane (CH4), hydrocarbons, and oxygenated volatile organic compounds (VOCs) (for example, see Monks et al., 2015). The most important tropospheric oxidant is OH, which is itself produced indirectly through photolysis of O3. Oxidants control the concentrations of key climate and air-quality gases and aerosols (including O3, methane, sulfate aerosol, and secondary organic aerosols) (Monks et al., 2009; Prather et al., 2012; Unger et al., 2006).O3 itself is not directly emitted, and its tropospheric burden is controlled by its sources through chemical production from NOx and organic compounds, transport from the stratosphere, and loss via deposition and chemical reactions (Monks et al., 2015).

Halogens (Cl, Br, I) are known to destroy O3 through catalytic cycles, such as that shown in Reactions (R1)(R3) (Chameides and Davis, 1980). Tropospheric halogens have also been shown to change OH concentrations (Bloss et al., 2005) and perturb OH to HO2 ratios towards OH (Chameides and Davis, 1980). Halogens perturb the NO to NO2 ratio and reduce NOx concentrations by hydrolysis of XNO3.These perturbations also indirectly decrease O3 formation (von Glasow et al., 2004). Halogens directly oxidise organics species, with Cl radical reactions proceeding the fastest (Atkinson et al., 2006; Sander et al., 2011). This can cause signicant O3 formation through increased RO2 concentrations (Knipping and Dabdub, 2003), notably in regions with elevated ClNO2 (Sarwar et al., 2014). Halogens also play an important role in determining the chemistry of mercury (Holmes et al., 2009; Parrella et al., 2012; Wang et al., 2015;Coburn et al., 2016). The literature on tropospheric halogens has been the topic of several recent reviews, which cover the background in more detail (Simpson et al., 2015; Saiz-Lopez et al., 2012b). However, many uncertainties still exist, notably with heterogeneous halogen chemistry (Abbatt et al., 2012; Simpson et al., 2015) and gas phase iodine chemistry (Saiz-Lopez et al., 2014; Sommariva and von Glasow, 2012).

O3 + X ! XO + O2 (R1)

HO2 + XO ! HOX + O2 (R2)

HOX + h ! OH + X (R3)

Net: HO2 + O3 ! 2O2 + OH (R4)

Tropospheric halogen chemistry has been studied in box model studies (see Simpson et al., 2015, and citations within) and more recently in global models (e.g. Parrella et al., 2012;Saiz-Lopez et al., 2012a, 2014; Schmidt et al., 2016; Sherwen et al., 2016a). Modelling has sought to quantify emis-

T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem 12241

Table 1. Additional halogen reactions included in this simulation that are not described in previous work (Eastham et al., 2014; Schmidt et al., 2016; Sherwen et al., 2016a). The full reaction scheme is given in Appendix B (Tables B2B5). The rate constant is calculated using a standard Arrhenius expression A [notdef] exp(

Ea RT ).

Rxn ID Reaction A Ea/R Citation

cm3 molecules1 s1 K

M29 IO + ClO ! I + OClO 2.59 [notdef] 10

12 280 Atkinson et al. (2007)

M30 IO + ClO ! I + Cl + O2 1.18 [notdef] 10

12 280 Atkinson et al. (2007)

M31 IO + ClO ! ICl + O2 9.40 [notdef] 10

13 280 Atkinson et al. (2007)

M32 Cl + HCOOH ! HCl + CO2 + H2O 2.00 [notdef] 10

13 Sander et al. (2011)

M33 Cl + CH3O2 ! ClO + CH2O + HO2

a 1.60 [notdef] 10

10 Sander et al. (2011)

M34 Cl + CH3OOH ! HCl + CH3O2 5.70 [notdef] 10

11 Sander et al. (2011)

M35 Cl + C2H6 ! HCl + C2H5O2 7.20 [notdef] 10

11

70 Sander et al. (2011)

M36 Cl + C2H5O2 ! ClO + HO2+ ALD2

b 7.40 [notdef] 10

11 Sander et al. (2011)

M37 Cl + EOH ! HCl + ALD2

c 9.60 [notdef] 10

11 Sander et al. (2011)

M38 Cl + CH3C(O)OH ! HCl + CH3O2, + CO2 2.80 [notdef] 10

14 Sander et al. (2011)

M39 Cl + C3H8 ! HCl + A3O2 7.85 [notdef] 10

11

80 Sander et al. (2011)

M40 Cl + C3H8 ! HCl + B3O2 6.54 [notdef] 10

11 Sander et al. (2011)

M41 Cl + ACET ! HCl + ATO2 7.70 [notdef] 10

11 1000 Sander et al. (2011)

M42 Cl + ISOP ! HCl + RIO2 7.70 [notdef] 10

11 500 Sander et al. (2011)

M43 Cl + MOH ! HCl + CH2O + HO2 5.50 [notdef] 10

11 Sander et al. (2011)

M61 Cl + ALK4 ! HCl + R4O2 2.05 [notdef] 10

10 Atkinson et al. (2006)

M62 Br + PRPE ! HBr + PO2 3.60 [notdef] 10

12 Atkinson et al. (2006)

M63 Cl + PRPE

M

!HCl + PO2

d 2.80 [notdef] 10

10 Atkinson et al. (2006)

H1 N2O5

!HNO3 + ClNO2

e (see table footnote)

H2 HOI

!0.85ICl + 0.15IBr

f (see table footnote)

H3 INO2

!0.85ICl +.015IBrf (see table footnote)

H4 INO3

!0.85ICl + 0.15IBr

f (see table footnote)

P1 ICl h

!I + Cl Sander et al. (2011) P2 IBr h

!I + Br Sander et al. (2011) P3 BrCl h

!Cl + Br Sander et al. (2011)

a Reaction from JPL, only considering the major channel (Daele and Poulet, 1996); product of CH3O reacts to form CH2O + HO2

(CH3O + O2 ! CH2O + HO2).

b Only the rst channel from JPL was considered. The second channel forms a criegee (HCl + C2H4O2) and therefore

cannot be represented by reduced GEOS-Chem chemistry scheme. c Reaction dened by JPL and interpreted as proceeding via hydrogen abstraction; therefore the acetaldehyde product is assumed. d K(innity) rate given in table, K(0) rate = 4.00 [notdef] 10

28 with Fc = 0.6 as shown in Table B3.

e Reaction

only proceeds on sea-salt aerosol, with value as described in Evans and Jacob (2005). f Reactions which were included in previous work (Sherwen et al., 2016a), but dihalogen products have been updated, split between ICl and IBr (see Sect 2), and these reactions only proceed on acidic sea-salt aerosol following McFiggans et al. (2000). Acidity of aerosol is calculated as described in Alexander (2005). values for uptake of halogen species are given in Table B4. Abbreviations for tracers are expanded in Appendix C.

tion for I2O4. A quantum yield of unity was assumed for all I2Ox species. It is noted that recent work has used an unpublished spectrum for I2O4 that is much lower than that of I2O3 (Saiz-Lopez et al., 2014), but this is not expected to have a large effect on conclusions presented here.

The parameterisation for oceanic iodide concentration was changed from Chance et al. (2014), as used in Sherwen et al. (2016a), to MacDonald et al. (2014) because the latter resulted in an improved comparison with observations (see Sect. 7.5 of Sherwen et al., 2016a).

The product of acid-catalysed dihalogen release following I+ (HOI, INO2, INO3) uptake was updated from I2 as in

Sherwen et al. (2016a) to yield IBr and ICl following McFiggans et al. (2002). Acidity is calculated online through titra-

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We update the standard model chemistry to give a representation of chlorine, bromine, and iodine chemistry. We describe this version of the model as Cl+Br+I in this paper.

It is based on the iodine chemistry described in Sherwen et al. (2016a) with updates to the bromine and chlorine scheme described by Schmidt et al. (2016) and Eastham et al. (2014). We have made a range of updates beyond these. Updated or new reactions not included in Sherwen et al. (2016a), Schmidt et al. (2016), or Eastham et al. (2014) are given in Table 1 with a full description of the halogen chemistry scheme used given in Appendix B Tables B2B5.

For the photolysis of I2Ox (x = 2, 3, 4) we have adopted

the absorption cross sections reported by Gmez Martn et al. (2005) and Spietz et al. (2005) and used the I2O2 cross sec-

12242 T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem

Table 2. Global sources of reactive tropospheric inorganic halogens. Sources with xed concentration in the model for Cly (CH3Cl, CH3Cl2, CHCl3) and Bry (CHBr3) are shown in terms of chemical release (e.g. +Cl, +OH, +h ) and are in bold. Inclusion of chlorine and bromine

organic species has been reported before in GEOS-Chem (Eastham et al., 2014; Parrella et al., 2012; Schmidt et al., 2016). X2 (I2) and HOX (HOI) are the inorganic ocean source from Carpenter et al. (2013); XNO2 is the source from the uptake of N2O5 on sea salt (ClNO2).

Sources Iy (Tg I year1) Bry (Tg Br year1) Cly (Tg Cl year1)

CH3X 0.26 0.06 2.10 CH2X2 0.33 0.09 0.57

CHX3 0.41 0.25 HOX 1.97 X2 0.14

IX 0.30 0.73

XNO2 0.65

Stratosphere 0.00 0.06 0.43

Total source 2.70 0.91 4.72

Acid-catalysed sea-salt dihalogen IX (X = Cl, Br) ux is only stated for Cly and Bry as it does not

represent a net Iy source.

tion of sea-salt aerosol by uptake of sulfur dioxide (SO2), nitric acid (HNO3), and sulfuric acid (H2SO4) as described by Alexander (2005). Re-release of IX (X = Cl, Br) is only per

mitted to proceed if the sea salt is acidic (Alexander, 2005). Thus aerosol cycling of IX in the model is not a net source of Iy (and may be a net sink on non-acid aerosol) but alters the speciation (Sherwen et al., 2016a). The ratio between IBr and ICl was set to be 0.15 : 0.85 (IBr : ICl), instead of the0.5 : 0.5 used previously (Saiz-Lopez et al., 2014; McFiggans et al., 2000). A ratio of 0.5 : 0.5 gives a large overestimate of bromine monoxide (BrO) with respect to the observations used in Sect. 3.4.2 (Read et al., 2008; Volkamer et al., 2015). We attributed this reduction to the debromination of sea salt, which we do not consider here, and the potential for the model to overestimate the BrOx lifetime. This is discussed further in the next section but future laboratory and eld studies of these heterogenous process are needed to help constrain these parameters.

Iodine on aerosol is represented in the model with separate tracers based on the aerosol on which irreversible uptake oc-curs (see Table B4). We include three iodine aerosol tracers to represent iodine on accumulation and coarse-mode sea salt and on sulfate aerosol. The physical properties of the iodine aerosol tracers are assumed to be the same as their parent aerosol, as previously described for sulfate (Alexander et al., 2012) and sea-salt aerosol (Jaegl et al., 2011). As in Sherwen et al. (2016a), no nucleation of iodine species is considered in this work, with only photolytic and heterogeneous loss being treated.

We have added to the chlorine chemistry scheme described by Eastham et al. (2014) to include more tropospheric 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).

Deposition and photolysis of dihalogen species (ICl, BrCl, IBr) and the reaction between ClO and iodine monoxide (IO) were also included (Sander et al., 2011).

3 Model results

We run the model for 2 years (1 January 2004 to 1 January 2006), discarding the rst year as a spin-up period and using the second year (2005) for analysis. Non-halogen emissions are described in Sherwen et al. (2016a). A reference simulation without any halogens (NOHAL) was also performed. Where comparisons with observations are shown, the model is run for the appropriate year with a 3-month spin-up before the observational dates, unless explicitly stated otherwise. The appropriate month from the 2005 simulation is used as the initialisation for these observational comparisons to account for interannual variations. The model is sampled at the nearest timestamp and grid box. The model only calculates chemistry in the troposphere. To avoid confusion we do not show results above the tropopause (lapse rate of temperature falls below 2 K km1).

3.1 Emissions

The emissions uxes of chlorine, bromine, and iodine species are shown in Fig. 1 with global totals in Table 2. We do not consider the Cl and Br contained within sea salt as emitted in our simulation, following Schmidt et al. (2016), until a chemical process liberates them into the gas phase. These liberation processes are the uptake of N2O5 on sea salt and uptake of I+ species on sea salt. We do not include explicit sea-salt debromination for reasons described in Schmidt et al. (2016).

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T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem 12243

Figure 1.

The organic iodine (CH3I, CH2I2, CH2ICl, CH2IBr) emissions are from Ordez et al. (2012) as described in Sherwen et al. (2016a). Inorganic iodine emissions (HOI, I2) (Carpenter et al., 2013; MacDonald et al., 2014) are 30 % lower here than reported by Sherwen et al. (2016a) due to use of the MacDonald et al. (2014) parameterisation for ocean surface iodide rather than that of Chance et al. (2014). Heterogeneous iodine aerosol chemistry (Sects. 2 and B1 in Appendix B4) does not lead to a net release of iodine, instead just recycling it from less active forms (INO2, INO3, HOI)

into more active forms (ICl / IBr).

The organic bromine (CH3Br, CHBr3, CH2Br2) emissions have been reported previously (Parrella et al., 2012; Schmidt et al., 2016) and our simulation is consistent with this work. A further source of 0.031 Tg Br year1 (3.5 % of total) is included here from CH2IBr photolysis. The heterogeneous cycling for Bry (family dened in Appendix C) has been updated here from Schmidt et al. (2016), as described in Sect. 2/Appendix B1. An additional Bry source not considered by Schmidt et al. (2016) is iodine-activated IBr release from sea salt, which amounts to 0.30 Tg Br year1 and the majority (67 %) of this is tropical (22 N22 S).

The organic chlorine emission (CH3Cl, CHCl3, CH2Cl2) for this simulation (Table 2) has been described previously

Schmidt et al. (2016) and set using xed surface concentrations. An additional source of 0.046 Tg Cl year1(0.96 %

of total) is present from CH2ICl photolysis (Sherwen et al., 2016a). ClNO2 production from the heterogeneous uptake of N2O5 provides a source of 0.66 Tg Cl year1 (14 % of total) with the vast majority (95 %) being in the Northern Hemisphere, with strongest sources in coastal regions north of 20 N. For June we calculate a global source of 21 Gg Cl month1, which is substantially less than the62 Gg Cl month1 (Sarwar Golam, personal communication, 2016) calculated in a previous study (Sarwar et al., 2014).The difference in N2O5 concentrations due to differences in model resolution may contributes to this. Uptake of HOI, INO2 and INO3 to sea-salt aerosol leads to the emission of

ICl, giving an additional source of 0.76 Tg Cl year1 (15.7 % of total) mostly (67 %) in tropical (22 N22 S) locations.

Most of the emissions of Br and I species in our simulation occur in the tropics. It is notable that the chlorine emissions are more widely distributed (Fig. 1). This is a result of longer lifetimes of chlorine precursor gases, which moves their destruction further from their emissions, and the ClNO2 source being primarily in the northern extratropics.

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12244 T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem

Figure 1. Average annual halogen surface emission of species and column-integrated uxes for species that have xed surface concentrations in the model (CH3Cl, CH3Cl2, CHCl3, CHBr3) or those with vertically variable sources (ClNO2 from N2O5 uptake on sea-salt and IX (X = Cl, Br) production from HOI, lNO2, and lNO3 uptake). Values are given in kg X m

2 s1 (X = Cl, Br, I).

Figure 2. Annual global Xy (X = Cl, Br, I) deposition (Xy dened in Appendix C). Values are given in terms of mass of halogen deposited

(kg X m2 s1, X = Cl, Br, I).

3.2 Deposition of halogens

Figure 2 shows the global annual integrated wet and dry deposition of inorganic Xy (X = Cl, Br, I). Much of the deposi

tion of the halogens occurs over the oceans (70, 73, and 90 % for Cly, Bry, and Iy respectively). It is high over regions of signicant tropical precipitation (Intertropical Convergence

Zone, Maritime continents, Indian Ocean) and much lower at the poles, reecting lower precipitation and emissions.

We nd that the major Cly depositional sink is HCl (94 %), with HOCl contributing 5.1 % and ClNO3 1.1 %. The Bry sink is split between HBr, HOBr, and BrNO3 with fractional contributions of 33, 30, and 28 % respectively. The major Iy sink is HOI deposition, which represents 59 % of the deposi-

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Figure 3. Tropospheric distribution of Cly, Bry, and Iy (dened in Appendix C) concentrations. Upper plots show surface and lower plots show zonal values. Only boxes that are entirely tropospheric are included in this plot. The Cly colour bar is capped at 20 pmol mol1, with a maximum plotted value of 116 pmol mol1 at the surface over the North Sea. The Iy colour bar is capped at 10 pmol mol1, with a maximum plotted value of 16.4 pmol mol1 at the surface over the Red Sea.

Figure 4. Tropospheric distribution of IO, BrO, and Cl concentrations. Upper plots show surface and lower plots show zonal values. Only boxes that are entirely tropospheric are included in this plot.

tional ux. The two next largest sinks are deposition of INO3 and iodine aerosol (22 and 15 %).

3.3 Halogen species concentrations

Figure 3 shows the surface and zonal concentration of annual mean Iy, Bry, and Cly, with Fig. 4 showing the same for

IO, BrO, and Cl, key halogen compounds in the atmosphere.

Figure 5 shows the global molecule weighted mean vertical prole of the halogen speciation.

Inorganic iodine concentrations are highest in the tropical marine boundary layer, consistent with their dominant emission regions. The highest concentrations are calculated in the coastal tropical regions, where enhanced O3 concentrations

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12246 T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem

Figure 5. Modelled global average vertical Xy (X = Cl, Br, I) (Xy dened in Appendix C). Units are pmol mol

1 of X (where X = Cl, Br,

I). For Cly the y axis is capped at 20 pmol mol1 to show speciation. A Cly maximum of 1062 pmol mol1 is found within the altitudes shown due to additional HCl contributions increasing with altitude.

Figure 6. Annual mean integrated model tropospheric column for BrO and IO in molecules cm2.

from industrial areas ow over high predicted oceanic iodide concentrations and lead to increased oceanic inorganic iodine emissions. Within the vertical there is an average of 0.5

1 pmol mol1 of Iy, consistent with previous model studies (Saiz-Lopez et al., 2014; Sherwen et al., 2016a). The lowest concentrations of Iy are seen just above the marine boundary layer, where Iy loss via wet deposition is most favourable due to partitioning towards water-soluble HOI. At higher altitudes, lower temperature and high photolysis rates push the Iy speciation to less-water-soluble compounds (IO, INO3)

and hence the Iy lifetime is longer. IO concentrations (Fig. 4) follow those of Iy, with high values in the tropical marine boundary layer. IO increases into the upper troposphere, reecting a partitioning of Iy in this region towards IO (and INO3) and away from HOI. The global mean tropospheric lifetimes of Iy and IOx (IO + I) are 2.2 days and 1.3 min,

respectively. IOx loss proceeds predominately via reaction of IO with HO2 (78 %), with smaller losses via IO + BrO

(7.9 %) and IO + NO2 (7.4 %).

Total reactive bromine is more equally spread through the atmosphere than iodine. This reects the longer life-

time of source species with respect to photolysis, which gives a more signicant source higher in the atmosphere. The highest concentrations are still found in the tropics. Unlike Iy, Bry increases signicantly with altitude, with BrNO3 and HOBr being the two most dominant species. BrO concentrations (Fig. 4) follow those of inorganic bromine. In the boundary layer the highest concentrations are found in the tropics. BrO and IO do not strongly correlate in the tropical marine boundary layer reecting their differing sources. BrO concentrations increase towards the upper troposphere associated with the increase in total Bry.

The global annual-average (molecule weighted) tropospheric

BrO mixing ratio in our simulation is 0.49 pmol mol1 (Bry = 3.25 pmol mol1). When previous implementations

(Parrella et al., 2012; Schmidt et al., 2016) are run for the same year and model version as this work (GEOS-Chem v10), the modelled BrO concentrations are found to be 11 % higher than Schmidt et al. (2016) and 33 % higher than Parrella et al. (2012). We calculate tropospheric lifetimes of 18 days for Bry and 8.1 min for BrOx (BrO + Br). Similarly

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Figure 7. Iodine oxide (IO) surface observations (black) by campaign compared against the simulation with halogen chemistry (Cl+Br+I,

red). Cape Verde measurements are shown against hour of day and others are shown as a function of latitude. Values are considered in 20 bins, with observations and modelled values at the same location and time (as described in Sect. 2) shown side-by-side around the midpoint of each bin. The extent of the bins is highlighted with grey dashed lines. Observations are from Cape Verde (tropical Atlantic; Mahajan et al., 2010; Read et al., 2008), TransBrom (western Pacic; Gromann et al., 2013), the Malaspina circumnavigation (Prados-Roman et al., 2015b), HaloCAST-P (eastern Pacic; Mahajan et al., 2012), and TORERO ship (eastern Pacic; Volkamer et al., 2015). The number of data points within latitudinal bin is shown as n. The box plot extents give the interquartile range, with the median shown within the box. The whiskers give the most extreme point within 1.5 times the interquartile range. Locations of observations are shown in Fig. 20.

to IOx, BrOx loss proceeds predominately via reaction of

BrO with HO2 (71 %) and NO2 (18 %).

Total inorganic chlorine has a highly non-uniform distribution at the surface, reecting the ClNO2 source from

N2O5 uptake on sea salt. At the surface ClNO2, HCl, BrCl, and HOCl represent around 25 % of the total Cly each. Away from the surface the ClNO2 concentrations drop off rapidly due to the short lifetime of sea salt. HCl concentrations increase signicantly into the middle and upper troposphere and dominates the Cly distribution. This suggests that stratospheric chlorine freed from CFCs and organic chlorine strongly contributes to free tropospheric concentrations of Cly. Cl mixing ratios are very low ( 0.075 fmol mol1

or 2000 cm3) in the marine boundary layer. Reactive Cl

(i.e. Cly excluding HCl) drops from the surface to around 10 km, where it then increases again towards the stratosphere. Cl shows a wider distribution than IO and BrO, reecting the source wider distribution of Cly. We calculate

tropospheric lifetimes of 5 days for Cly and 3.8 h for ClOx (Cl + ClO + ClOO + 2Cl2O2). A global tropospheric mean

inorganic chlorine (Cly) concentration of 71 pmol mol1 in seen in our simulation. ClOx loss proceeds through reaction of Cl with CH4 (27 %), ClO reaction with HO2 (21 %), and

ClO reaction with NO2 (10 %). The longer XOx lifetime of

ClOx, compared to BrOx and IOx, can be explained through the importance of the relatively slow dominant loss route through reaction with CH4.

The chemistry of halogens and sea salt is highly uncertain (Simpson et al., 2015; Saiz-Lopez et al., 2012b; Abbatt et al., 2012). Estimates for sea-salt debromination range from0.51 Tg year1 (Parrella et al., 2012, implemented in GEOS

Chem v10 and v9-2) to 2.9 Tg year1 (Fernandez et al., 2014). Other studies have not included sea-salt debromination (von Glasow et al., 2004; Schmidt et al., 2016) as we do not in this work. Schmidt et al. (2016) found that including debromination of sea-salt aerosol improved the simula-

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12248 T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem

Figure 8. Vertical comparison of the model (Cl+Br+I) and measured iodine oxide (IO) during TORERO aircraft campaign (Volkamer

et al., 2015; Wang et al., 2015). Model and observations are in red and black respectively. Values are considered in 0.5 km bins, with observations and modelled values at the same location and time (as described in Sect. 2) shown side-by-side around the midpoint of each bin. Measurements were taken aboard the NSF/NCAR GV research aircraft by the University of Colorado airborne multi-axis DOAS instrument (CU AMAX-DOAS) in the eastern Pacic in January and February 2012 (Volkamer et al., 2015; Wang et al., 2015). The box plot extents give the interquartile range, with the median shown within the box. The whiskers give the most extreme point within 1.5 times the interquartile range. Locations of observations are shown in Fig. 20.

tion of the BrO and HOBr observations reported during the Combined Airborne Studies in the Tropics (CAST; Harris et al., 2016) campaign but resulted in overprediction of the Tropical Ocean tRoposphere Exchange of Reactive halogen and Oxygenated VOC campaign (TORERO; Volkamer et al., 2015; Wang et al., 2015) BrO observations. Arguably this work provides a lower estimate of bromine and chlorine sources in the troposphere, with further work needed to understand the Bry budget.

The difference in lifetimes of inorganic halogen families (Xy) can be understood from the change in loss routes, which shifts HX to HOX following the order of group 17 in the periodic table (Cl ! Br ! I).

Figure 6 shows column-integrated BrO and IO, which are the major halogen species for which we have observations (see Sect. 3.4). Tropospheric ClO concentrations are small (see Fig. 5) and are therefore not shown in Fig. 6. Tropical maxima are seen for both BrO and IO, with BrO concentrations decreasing towards the equator. For IO a localised maximum is seen in the Arabian Sea. The IO maximum in Antarctica reported from satellite retrievals (Schnhardt et al., 2008) is not reproduced in our model, potentially reecting the lack of polar-specic processes in the model.

3.4 Comparison with halogen observations

The observational dataset of tropospheric halogen compounds is sparse. Previous studies that this work is based on have shown comparisons for the oceanic precursors for chlorine (Eastham et al., 2014; Schmidt et al., 2016), bromine (Parrella et al., 2012; Schmidt et al., 2016), and iodine (Bell et al., 2002; Sherwen et al., 2016a; Ordez et al., 2012). The model performance in simulating these compounds has not changed since these previous publications so we focus here on the available observations of concentrations of IO, BrO, and some inorganic chlorine species (ClNO3, HCl, and

Cl2).

3.4.1 Iodine monoxide

A comparison of IO to a suite of recent remote surface observations is shown in Fig. 7. The model shows an overall negative bias of 23 %. This compares with the 90 % positive bias previously reported in Sherwen et al. (2016a). This reduction in bias to IO observations is due to the use of the MacDonald et al. (2014) iodide parameterisation over that of Chance et al. (2014) which has reduced the inorganic emission of iodine, along with the restriction of iodine recycling to acidic aerosol.

Figure 8 shows a comparison between modelled IO with altitude against observations in the eastern Pacic

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Tropospheric BrO Column: Seasonal Variation

90N-60N

60N-30N

4

2

Tropospheric BrO column / 1013 cm2

0

4

GOME-2 Cl+Br+I

30N-Eq.

Eq.-30S

2

0

30S-60S

60S-90S

4

2

0 J F M A M J J A S O N

F M A M J J A S O N D

Month

Figure 9. Seasonal variation of zonal mean tropospheric BrO columns in different latitudinal bands. Observations from the GOME-2 satellite instrument in 2007 (Theys et al., 2011) are compared to GEOS-Chem values at the GOME- 2 local overpass time (09:0011:00).

(Volkamer et al., 2015; Wang et al., 2015). In general, the model agreement with observations is good. There is an average bias of +37 % in the free troposphere (350 hPa < p < 900 hPa), which increases to +54 % in the

upper troposphere (350 hPa > p > tropopause). As with the surface measurements, the model bias when comparing to IO observations (Volkamer et al., 2015; Wang et al., 2015) in the free and upper troposphere is decreased from previously reported positive biases of 73 and 96 %, respectively (Sherwen et al., 2016a).

3.4.2 Bromine monoxide

Comparisons of BrO against seasonal satellite tropospheric BrO observations from GOME-2 (Theys et al., 2011) are shown in Fig. 9. As shown previously (Parrella et al., 2012; Schmidt et al., 2016) the model has some skill in capturing both the latitudinal and monthly variations in tropospheric BrO columns. However, it underestimates the column BrO in the lower southern latitudes (6090 S) and to a smaller degree also in lower northern latitudes (6090 N), which may reect the lack of bromine from polar (blown snow, frost owers, etc.) sources and sea-salt debromination processes.

As shown in Fig. 10, comparisons between the model and observations of BrO made at Cape Verde (Read et al., 2008; Mahajan et al., 2010) show a negative bias of 22 %. We attribute this to the high local sea-salt loadings at this site

Figure 10. Bromine oxide (BrO) surface observations (black) at Cape Verde (Read et al., 2008; Mahajan et al., 2010) compared against the simulation with halogen chemistry (Cl+Br+I, red).

Values are binned by hour of day. Locations of observations are shown in Fig. 20.

TORERO Mean BrO Vertical profiles

Observation Cl+Br+I

1 2 3

Subtropics

Tropics

12

Altitude / km

8

4

0 0 1 2 3

BrO / ppt

Figure 11. Vertical comparison of the model (Cl+Br+I) and

measured bromine oxide (BrO) during TORERO aircraft campaign (Volkamer et al., 2015; Wang et al., 2015) in the subtropics (left) and tropics (right). Model and observations are in red and black, respectively. Observations and modelled values at the same location and time (as described in Sect. 2) are shown side-by-side around the midpoint of each bin. Measurements were taken aboard the NSF/NCAR GV research aircraft by the University of Colorado airborne multi-axis DOAS instrument (CU AMAX-DOAS) in the eastern Pacic in January and February 2012 (Volkamer et al., 2015;Wang et al., 2015). Locations of observations are shown in Fig. 20.

(Carpenter et al., 2010), which is situated in the surf zone.

This may locally increase the BrO concentrations. The model concentrations of 1 pmol mol1 are, however, consistent

with other ship-borne observations made in the region (Leser et al., 2003).

Figure 11 shows modelled vertical BrO concentrations against observations in the eastern Pacic (Volkamer et al.,

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Table 3. Comparison between modelled and observed ClNO2. Concentrations are shown as the maximum and average of the daily maximum value for the observational and equivalent model time period. The model values are taken for the nearest time step and location within the analysis year (2005).

Obs. Cl+Br+I

Location Lat. Long. Max Mean Max Mean Reference

Coastal

Pasadena, CA, US (2010) 34.2 118.2 3.46 1.48 0.43 0.20 Mielke et al. (2013)

Southern China, CN (2012) 22.2 114.3 2.00 0.31 0.60 0.18 Tham et al. (2014)

Los Angeles, CA, US (2010) 34.1 118.2 1.83 0.50 0.43 0.20 Riedel et al. (2012)

Houston, TX, US (2006) 30.4 95.4 1.15 0.80 0.19 0.04 Osthoff et al. (2008)

London, GB (2012) 51.5 0.2 0.73 0.23 0.50 0.17 Bannan et al. (2015)

TX, US (2013) 30.4 95.4 0.14 0.08 0.19 0.04 Faxon et al. (2015)

Continental

Hessen, DE (2011) 50.2 8.5 0.85 0.20 0.16 0.02 Phillips et al. (2012)

Boulder, CO, US (2009) 40.0 105.3 0.44 0.14 0.00 0.00 Thornton et al. (2010); Riedel et al. (2013)

Calgary, CA, US (2010) 51.1 114.1 0.24 0.22 0.02 0.01 Mielke et al. (2011)

2015; Wang et al., 2015). We nd a reasonable agreement within the free troposphere (350 hPa < p < 900 hPa) in both the tropics and subtropics, with an average bias of 3.5 and +4.2 %, respectively. A similar comparison is seen in the upper troposphere (350 hPa > p > tropopause) with negative biases for the tropics and subtropics, of 6.3 and 9.7 %, respectively. The decrease in agreement seen in the TORERO comparison (Fig. 11) relative to that previously presented in Schmidt et al. (2016) is due to reduced BrCl and BrO production from slower cloud multiphase chemistry (see Sects. B1

B3 in Appendix B). We model higher BrO concentrations in the tropical marine boundary layer which are above those ob-served (Volkamer et al., 2015). Our modelled concentrations are lower than those reported previously (Miyazaki et al., 2016; Long et al., 2014; Pszenny et al., 2004; Keene et al., 2009).

Our model does not include sea-salt debromination and yet calculated roughly the reported concentrations of BrO.Inclusion of sea-salt debromination leads to excessively high BrO concentration in the model (Schmidt et al., 2016). Sea-salt debromination is well established; thus the success of the model despite the lack of inclusion of this process suggests model failure in other areas. The BrOx lifetime may be too long. The conversion of BrOx to HBr is dominated by the reaction between Br and organics to produce HBr. Oceanic sources of VOCs such as acetaldehyde have been proposed (Millet et al., 2010; Volkamer et al., 2015) and a signicant increase in the concentration of these species would lead to lower BrOx concentrations. Alternatively, a reduction in the efciency of cycling of Bry through aerosol would also have a similar effect. The aerosol phase chemistry is complex and the parameterisations used here may be too simple or fail to capture key processes (e.g. pH, organics). These all require further study in order to help reconcile models with the

rapidly growing body of observation of both gas and aerosol phase bromine in the atmosphere.

3.4.3 Nitryl chloride (ClNO2), hydrochloric acid (HCl), hypochlorous acid (HOCl), and molecular chlorine (Cl2)

Very few constraints on the concentration of tropospheric chlorine species are available, but an increasing number of ClNO2 observations are becoming available. Table 3 shows a comparison between the model an available observations.We nd that the model does reasonably well in coastal regions but does not reproduce observations in continental regions or regions with very high NOx.

Lawler et al. (2011) reports measurements of HOCl and Cl2 at Cape Verde for a week in June 2009. For the rst 4 days of the campaign, HOCl concentrations were higher and peaked at 100 pmol mol1 with Cl2 concentrations

peaking at 30 pmol mol1. For the later days, HOCl con

centrations dropped to around 20 pmol mol1 and Cl2 concentrations to 010 pmol mol1. We calculate much lower

concentrations of Cl2 ( 1 [notdef] 103 pmol mol1) and slightly

lower HOCl ( 10 pmol mol1). This is similar to ndings of

Long et al. (2014), who also found better comparisons with the later period of observations. Similar to the comparison with observed ClNO2, our simulation underestimates HOCl and Cl2.

The model does not include many sources of reactive chlorine. The failure to reproduce continental ClNO2 is likely due to a lack of representation of sources such as salt plains, direct emission from power station and swimming pools, and HCl acid displacement. The inability to reproduce the very high ClNO2 found in some cities (Pasadena) and industrialised regions (Texas) may be due to the coarse resolution of the model compared to the spatial inhomogeneity of these

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Table 4. Comparison between global tropospheric Ox budgets of simulations Cl+Br+I (with halogen chemistry) and NOHAL (without

halogen chemistry). Recent average model values from ACCENT (Young et al., 2013) are also shown for comparison. For the X1O + X2O

halogen crossover reactions where X1O [negationslash]=X2O, we split the O3 loss equally between the two routes. Values are rounded to the nearest

integer value.

Cl+Br+I NOHAL ACCENT

O3 burden (Tg) 339 416 340 [notdef] 40

Ox chemical sources (Tg yr)

NO + HO2 3436 3607

NO + CH3O2 1288 1316

NO + RO2 525 508

Total chemical Ox sources (POx) 5249 5431 5110 [notdef] 606 Ox chemical sinks (Tg year1 )

O3 + H2O

h

!2OH + O2 1997 2489

O3 + HO2 ! OH + O2 1061 1432

O3 + OH ! HO2 + O2 562 737

HOBr h

!Br + OH 285

HOBr + HCl ! BrCl 54

HOBr + HBr ! Br2 + H2O (aq. aerosol) 22

BrO + BrO ! 2Br + O2 13

BrO + BrO ! Br2 + O2 4

BrO + OH ! Br + HO2 12

IO + BrO ! Br + I + O2 11

ClO + BrO ! Br + ClOO/OClO 4

Other bromine Ox sinks 0 Total bromine Ox sinks 405

HOI h

!I + OH 438 OIO h

!I + O2 140

IO + BrO ! Br + I + O2 11

IO + ClO ! I + Cl + O2/ICl + O2 1

Other iodine Ox sinks 2 Total iodine Ox sinks 591

HOCl h

!Cl + OH 27

CH3O2 + ClO ! ClOO 6

ClO + BrO ! Br + ClOO/OClO 4

ClNO3 + HBr ! BrCl 2

IO + ClO ! I + Cl + O2/ICl + O2 1

Other chlorine Ox sinks 1 Total chlorine Ox sinks 40

Other Ox sinks 184 172 Total chem. Ox sinks (LOx) 4841 4829 4668 [notdef] 727

O3 POx LOx (Tg year

1) 408 602 618 [notdef] 251

O3 dry deposition (Tg year1) 799 980 1003 [notdef],200

O3 lifetime (days) 22 26 22 [notdef] 2

O3 STE (POx LOx-Dry dep.) (Tg year

1) 391 378 552 [notdef] 168

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Figure 12. Change in tropospheric O3 on inclusion of halogen chemistry. Column (left), surface (middle), and zonal (right) changes are shown. Upper plots show absolute change and lower plots below give change in % terms ((Cl+Br+I NOHAL)/NOHAL [notdef] 100).

Figure 13. Seasonal cycle of near-surface O3 at a range of Global Atmospheric Watch (GAW) sites. Observational data shown are 6-year monthly averages (20062012). Model data are for 2005. Data are from GAW, compiled and processed as described in Sofen et al. (2016).

Blue and red lines represent simulations without halogens (NOHAL) with halogens (Cl+Br+I), respectively. Grey shaded area gives 5th

and 95th percentiles of the observations. Locations of observations are shown in Fig. 21.

observations. The failure to reproduce the Cape Verde observations may be due to the very simple aerosol phase chlorine chemistry included in the model. Overall we suggest that the model provides a lower limit estimate of the chlorine emissions and therefore burdens within the troposphere, but constraints of surface concentrations are limited and vertical proles are not available. Further laboratory work to better dene aerosol processes and observations will be necessary to investigate the role of chlorine on tropospheric chemistry.

4 Impact of halogens

We now investigate the impact of the halogen chemistry on the composition of the troposphere. We start with O3 and OH and then move onto other components of the troposphere.

4.1 Ozone

Figure 12 shows changes in column, surface, and zonal

O3 both in absolute and fractional terms between simulations with and without halogen emissions (Cl+Br+I

vs. NOHAL). Globally the mass-weighted, annual-average

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Figure 14. Comparison between annual modelled O3 proles and sonde data (2005). Proles shown are the annual mean of available observations from World Ozone and Ultraviolet Radiation Data Centre (WOUDC, 2014) and model data for 2005 at given locations. Blue and red lines represent simulations without halogens (NOHAL) with halogens (Cl+Br+I), respectively. Observations (in black) show

mean concentrations with upper and lower quartiles given by whiskers. Locations of observations are shown in Fig. 21.

mixing ratio is reduced by 9.4 nmol mol1 with the inclusion of halogens and tropospheric burden decreases by18.6 % (Cl+Br+I NOHAL)/ (NOHAL [notdef] 100). A

much larger percentage decrease of 30.0 % (8.5 nmol mol1) is seen over the ocean surface. Large percentage losses are seen in the oceanic Southern Hemisphere as reported previously (Long et al., 2014; Schmidt et al., 2016; Sherwen et al., 2016a), reecting the signicant oceanatmosphere exchange in this regions. The majority (65 %) of the change in O3 mass due to halogens occurs in the free troposphere (350 hPa < p < 900 hPa). The location of O3 concentration decreases is noteworthy as the climate effect of O3 is highly spatial and vertically variable (Myhre et al., 2013). Effects of halogens on tropospheric O3 from preindustrial to present day are explored elsewhere (Sherwen et al., 2016b).

Comparisons of the model and observed surface and sonde

O3 concentrations are given in Figs. 13 and 14. In the tropics the delity of the simulation improves with the inclusion of halogens, as shown previously by Schmidt et al. (2016) and Sherwen et al. (2016a). Sonde and surface comparisons north of 50 N and south of 60 S, however, show that

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Figure 15. Global annual-average tropospheric vertical odd oxygen loss (Ox) through different reaction routes (Photolysis, HOx, IOx, BrOx, and ClOx).

12254 T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem

Figure 16. Global loss routes (+h , +Br, +NO3, +Cl, +O3, +OH) of organic compounds shown as % of total tropospheric losses.

Figure 17. Changes in tropospheric burden of species and families on inclusion of halogens (Cl+Br+I) compared to no halogens (NO

HAL). Burdens are considered in elemental terms (e.g Tg S/N/C) and species masses for OH, HO2, H2O2, and O3. The family denoted by VOCs in this plot is dened as the sum of carbon masses of CO, formaldehyde, acetaldehyde, ethane, acetone, isoprene, propane, C4

alkanes, C3 alkenes, and C3 ketones. Abbreviations for tracers are expanded in Appendix C.

the model now underestimates O3. This is clearly the case for Neumayer and the South Pole (Fig. 13).

The global odd oxygen budget Ox in the troposphere with (Cl+Br+I) and without (NOHAL) halogens is shown in

Table 4. The Ox loss through chlorine, bromine, and iodine represents 0.8, 8.4, and 12.2 % of the total Ox loss respectively; thus halogens constitute 21.4 % of the overall O3 loss.

The sum of halogen-driven Ox loss is 1036 Tg Ox year1, which is similar to the magnitude of loss via reaction of O3 with HO2 of 1100 Tg Ox year1 (21.9 % of total). Halo

gen cross-over reactions (BrO + IO, BrO + ClO, IO + ClO)

contribute little to the overall O3 loss. This number compares with 930 Tg Ox year1 reported in GEOS-Chem pre

viously by Sherwen et al. (2016a). Saiz-Lopez et al. (2014) found that, between 50 S and 50 N and over the ocean only, halogens are responsible for the loss of 640 Tg Ox year1. We nd a higher value of 827 Ox year1 with our model.

Halogens represent 39.6 and 33.0 % of Ox loss in the upper troposphere (350 hPa > p > tropopause) and marine boundary layer (900 hPa < p), respectively, as shown in Fig. 15. The marine boundary layer Ox loss attributable to halogens is comparable to the 31 % reported by Prados-Roman et al. (2015a) previously, and it is higher than the 26 % reported

solely for iodine (Sherwen et al., 2016a). The inter-reaction of halogen monoxide species is found to less important here than previous studies (e.g. Read et al., 2008), which has been basis in locations of higher halogen monoxide concentrations. Inclusion of sea salt, which would increase BrO in the marine boundary layer, would increase the magnitude of contribution of theses routes to total halogen-driven Ox loss.

Although the partitioning of the Ox loss processes is signicantly different between the simulations with and without halogens (Table 4), the overall annual Ox loss only increases by 0.25 % (4841 vs. 4829 Tg year1).

The Ox production term decreases by 3.4 %. This decrease is due to a reduction in NOx concentrations via hydrolysis of XNO3 (X = Cl, Br, I). Our tropospheric

NOx burden decreases by 3.1 % to 167 Gg N (see Table A1) on inclusion of halogens consistent with previous model studies (Long et al., 2014; von Glasow et al., 2004; Parrella et al., 2012; Schmidt et al., 2016). Globally NOx losses through ClNO3 and BrNO3 hydrolysis are approximately equal (1 : 0.88) and overall proceed at a rate of 10 %

of the NOx loss through the NO2 + OH pathway. Iodine ni

trite and nitrate (INO2, INO3) hydrolysis is much less signi-cant ( 0.2 % of rate of NO2 + OH). Net Ox is the difference

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Figure 18. Global annual-average surface and zonal change (%) in HOx, NOx, and SOx families (as dened in Appendix C) on inclusion of halogens.

Figure 19. Global annual-average surface and zonal change (%) in ethane (C2H6), propane (C3H8), C4 alkanes, and acetone

(CH3C(O)CH3) on inclusion of halogens. For species where all average changes are negative a continuous colour bar is used (C3H8 and C2H6) and for species where both negative and positive changes are present a divergent colour bar is used ( C4 alkanes and CH3C(O)CH3).

between the production and loss terms and the change here is much greater, leading to an overall decrease in net production of tropospheric O3 (POx LOx) of 32 % (194 Tg year1)

and a resultant decrease in O3 lifetime of 16 %.

4.2 HOx (OH + HO2)

We nd that global molecule weighted average HOx (OH + HO2) concentrations are reduced by 10.2 % with the

inclusion of halogens, with OH decreasing by 8.2 % from1.40 [notdef] 106 to 1.28 [notdef] 106 molecules cm3. Lower O3 concen-

trations decreases the primary OH source (O3 h ! 2OH) by

17.4 % and the secondary OH source (HO2 + NO) by 4.7 %.

The reduction in the sources of OH is buffered by an additional OH source from the photolysis of HOX (X = Cl, Br,

I) which acts to increase the conversion of HO2 to OH. Previously, Sherwen et al. (2016a) showed an increase of 1.8 % in global OH concentrations on inclusion of iodine. However, increased Bry and reduced Iy concentrations in the simulations described here mean that the increased OH source from HOX photolysis does not compensate fully for the re-

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Figure 20. Locations of halogen observations against which the model is compared. IO observations are shown in different colours. ClNO2 observations are shown in gold. BrO observations presented here were made at the same locations as IO observations. 1 indicates Cape

Verde, CV (Read et al., 2008; Mahajan et al., 2010); 2 is TORERO (aircraft-based; Volkamer et al., 2015; Wang et al., 2015); 3 is Malaspina (Prados-Roman et al., 2015b); 4 is TransBrom (Prados-Roman et al., 2015b); 5 = HaloCAST-P (Mahajan et al., 2012); 6 is TORERO (ship-

based; Volkamer et al., 2015; Wang et al., 2015); 7 is Texas, US (Faxon et al., 2015; Osthoff et al., 2008); 8 is California, US (Riedel et al., 2012; Mielke et al., 2013); 9 is Southern China, CN (Tham et al., 2014); 10 is London, GB (Bannan et al., 2015); 11 is Hessen, Germany (Phillips et al., 2012); 12 = Colorado, USA (Thornton et al., 2010; Riedel et al., 2013); 13 is Calgary, CA (Mielke et al., 2011).

duced primary source, resulting in an overall 8.2 % reduction in global mean OH. This buffering contributes to a change in OH smaller than the 11 % reported previously (Schmidt et al., 2016). As reported previously (Long et al., 2014; Schmidt et al., 2016), we also nd the net effect of halogens on the OH : HO2 ratio is a small increase (2.3 %).

4.3 Organic compounds

The oxidation of VOCs by halogens is included in this simulation (see Table B2 for reactions). The global fractional loss due to OH, Cl, Br, O3, NO3, and photolysis (h ) for a range of organics is shown in Fig. 16.

Globally, Br oxidation is small in our simulation and contributes 3.9 % to the loss of acetaldehyde (CH3CHO), 0.8 %

of the loss of formaldehyde (CH2O), 0.63 % of the loss of [greaterorequalslant] C4 alkenes, and <0.001 % of the loss of other compounds. Recent work has suggested a signicant source of oceanic oxygenated VOCs (oVOCs) (Coburn et al., 2014; Lawson et al., 2015; Mahajan et al., 2014; Millet et al., 2010; Myriokefalitakis et al., 2008; Sinreich et al., 2010; Volkamer et al., 2015), which we do not include in this simulation. Furthermore, although our modelled Bry is broadly comparable to some previous work (Parrella et al., 2012; Schmidt et al., 2016), it is lower in the marine boundary layer than in other recent work (Long et al., 2014). The combination of these two factors suggests that our model provides a lower bounds of impacts of bromine on VOCs. Signicantly higher concentrations of oVOC would decrease the BrOx concentrations in the model and might then allow an increased sea-salt source of reactive bromine.

The oxidation of VOCs by chlorine is more signicant. In our simulation chlorine accounts for 27, 15, and 14 % of the global loss of ethane (C2H6), propane (C3H8), and acetone (CH3C(O)CH3), respectively. Loss of other VOCs is globally small. This increased loss due to Cl is to some extent compensated for by the reduction in the OH concentrations that we calculate. Thus the overall lifetime of ethane, propane, and acetone changes from 131, 38, and 85 days in the simulation without halogens to 113, 37, and 80 in the simulation with halogens. Notably the ethane lifetime without halogens is 16 % longer. Given that we consider the chlorine in the model to be a lower limit, ethane oxidation by chlorine may in reality be more signicant than found here.

Methane is a signicant climate gas, as it has the second-highest forcing amongst well-mixed greenhouse gases from preindustrial to present day (Myhre et al., 2013). In our simulation without halogens we calculate a tropospheric chemical lifetime due to OH of 7.47 years. With the inclusion of halogen chemistry the OH concentration drops, extending the methane lifetime due to OH to 8.28 years (an increase of10.8 %). However, in our halogen simulations, chlorine radicals also oxidise methane (2.0 % of the total loss), shortening the lifetime to 8.16 years (1.52 %). As noted previously, the models chlorine concentrations appear to be underestimated. Allan et al. (2007) estimate a 25 Tg year1 sink for methane from Cl ( 4 %), signicantly higher than our esti

mate (4 Tg). Overall the models CH4 lifetime still appears to be short compared to the observationally based estimation of 9.1 [notdef] 0.9 from Prather et al. (2012), but halogens decrease

this bias.

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T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem 12257

Figure 21. Locations of O3 observations the model is compared against. Observations made by O3 sonde are shown in brown; surface observations at GAW sites are shown in gold. Where a site is a location of both sonde release and surface O3 observation it is shown in brown (Samoa, Neumayer, Lauder, and Milo).

In our simulations, halogens (essentially chlorine) have a signicant but not overwhelming role in the concentrations of hydrocarbons (from 1 % of methanol loss to 27 % of

ethane loss). However, as discussed earlier, the low biases seen with the very limited observational dataset of chlorine compounds would suggest that the impacts calculated here are probably lower limits.

4.4 Other species

With the inclusion of halogens in the troposphere there are a large number of changes in the composition of the tropo-sphere. Figure 17 illustrates the fractional global change in burden by species (for abbreviation see Appendix C). The spatial and zonal distribution of these changes by species family (HOx, NOx, SOx as dened in Appendix C) are shown in Fig. 18 and for a few VOCs (C3H8, C2H6, acetone, and [greaterorequalslant] C4 alkanes) in Fig. 19. A tabulated form of these changes is given within Appendix A (Table A1)

As discussed in Sects. 4.1 and 4.2, a clear decrease in oxidants (O3, OH, HO2, H2O2) is seen. This drives an increase in the concentrations of some VOCs (4.5 % on a per carbon basis), including CO (6.1 %) and isoprene (6.2 %). However, as discussed, it also adds an additional Cl sink term which leads to an overall decrease in some species (e.g. C2H6, (CH3)2CO, C3H8) particularly in the northern hemispheric oceanic regions. The SOx burden increases slightly (0.5 %), which can be attributed to decreases in oxidants.

5 Summary and conclusions

We have presented a model of tropospheric composition which has attempted to include the major routes of halogen chemistry impacts. Assessment of the model performance

is limited as observations of halogen species are extremely sparse. However, given the available observations we conclude that the model has some useful skill in predicting the concentration of iodine and bromine species and appears to underestimate the concentrations of chlorine species.

Consistent with previous studies, our model shows significant halogen-driven changes in the concentrations of oxidants. The tropospheric O3 burden and global mean OH decreases by 18.6 and 8.2 %, respectively, on inclusion of halogens. The methane lifetime increases by 10.8 %, improving agreement with observations.

There are a range of changes in the concentrations of other species. Direct reaction with Cl atoms leads to enhanced oxidation of hydrocarbons with ethane showing a signicant response. Given that the model appears to provide a lower limit for atomic Cl concentrations, this suggests a major missing oxidation pathway for ethane which is currently not considered. NOx concentrations are reduced by aerosol hydrolysis of the halogen nitrates, which leads to reduced global O3 production. Our simulation of BrO appears to be relatively consistent with observations, but we do not include a sea-salt debromination mechanism. This would suggest that either the cycling of bromine in our model is generally too fast or that we do not have sufciently large BrOx sinks (potentially oVOCs). Both hypotheses warrant further research.

Signicant uncertainties, however, remain in our understanding of halogens in the troposphere. The gas phase chemistry and photolysis parameters of iodine compounds are uncertain, together with the emissions of their organic and inorganic precursors (Sherwen et al., 2016a). For chlorine, bromine, and iodine heterogeneous chemistry, little experimental data exist and suitable parameterisations for the complex aerosols found in the atmosphere are unavailable (Abbatt et al., 2012). The uncertainties of this have been dis-

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12258 T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem

cussed in recent reviews (Saiz-Lopez et al., 2012b; Simpson et al., 2015) and considered in previous model studies (Schmidt et al., 2016; Sherwen et al., 2016a), and they still warrant further exploration.

Understanding fully the impact of halogens on tropospheric composition will require signicant development of new experimental techniques and more eld observations, new laboratory studies, and models which are able to exploit these developments.

6 Data availability

The model code used here will be made available to the community through the standard GEOS-Chem repository (http://www.geos-chem.org

Web End =http: http://www.geos-chem.org

Web End =//www.geos-chem.org http://www.geos-chem.org

Web End = ). Requests for materials should be addressed to Mat Evans ([email protected]).

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T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem 12259

Appendix A: Tabulated burden changes on inclusion of halogens

Table A1 gives the burdens with and without halogens and the fractional change.

Table A1. Tropospheric burden of species and families with (Cl+Br+I) and without halogens (NOHAL), and % change. Burdens are

considered in elemental terms (e.g Gg S/N/C) and species masses for OH, HO2, H2O2, and O3. Families are dened in Appendix C.

NOHAL Cl+Br+I % [Delta1]

NO3 1.49 1.14 23.57

N2O5 9.38 7.48 20.22

C2H6 3258.84 2628.05 19.36

O3 415 843.25 338 708.23 18.55

HNO4 19.84 16.84 15.14

H2O2 3229.09 2764.27 14.39

C3H8 609.76 524.31 14.01 C4 alkanes 488.35 429.02 12.15

PPN 15.82 14.31 9.55

HO2 27.55 24.95 9.44

OH 0.28 0.26 6.31

CH3C(O)CH3 7533.51 7085.23 5.95

PAN 202.89 191.57 5.58

NO2 123.53 118.52 4.06

CH2O 389.55 375.42 3.63

NOx 171.01 165.75 3.07

SO4 on SSA 1.97 1.94 1.74

PMN 0.68 0.67 1.27

NOy 1374.56 1367.26 0.53

NO 47.48 47.24 0.50

NH3 126.61 126.42 0.15

NH4 270.93 270.88 0.02

SOx 398.98 400.80 0.46 SO4 397.01 398.86 0.47

PROPNN 7.46 7.55 1.22 Acetaldehyde 184.93 187.23 1.25 CH3O2NO2 13.80 14.03 1.63

HNO3 463.49 471.53 1.74 > C3 ketones 186.99 190.49 1.87

C3 alkenes 97.93 100.28 2.40 SO2 286.11 298.96 4.49

VOCs 148 193.29 155 234.49 4.75 MMN 3.15 3.32 5.17

C4 alkyl nitrates 64.60 68.00 5.26 HNO2 2.76 2.92 5.84

CO 134 654.88 142 877.06 6.11 Isoprene 788.55 837.40 6.19 ISOPN 0.65 0.71 9.40

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12260 T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem

Appendix B: Gas phase chemistry scheme

Here is described the full halogen chemistry scheme as presented in previous works (Bell et al., 2002; Eastham et al., 2014; Parrella et al., 2012; Schmidt et al., 2016; Sherwen et al., 2016a) and with updates as detailed in Sect. 2 and Table 1. The complete gas phase photolysis, bimolecular, and termolecular reactions are described in Tables B1, B2, and B3.

B1 Heterogenous reactions

The halogen multiphase chemistry mechanism is based on the iodine mechanism (Br + I) described in Sherwen et al.

(2016a) and the coupled (Cl, Br) mechanism of Schmidt et al. (2016). The heterogenous reactions in the scheme are shown in Table B4 and with further detail individual detail on certain reactions below. The loss rate of a molecule X due to multiphase processing on aerosol is calculated following Jacob (2000).

dnXdt = [parenleftbigg]

4HHOBrRT kHOBr+X [X][H+]lrf (r,lr)

c , (B3)

with kHOBr+Cl = 5.9[notdef]109 M2 s1 and kHOBr+Br = 1.6[notdef] 1010 M2 s1. In the equation above c is the average thermal velocity of HOBr, and f (lr,r) is a reacto-diffusive correction factor,

f (lr,r) = coth

[parenleftbigg]

1AnX, (B1)

where r is the aerosol effective radius, Dg is the gas phase diffusion coefcient of X, c is the average thermal velocity of X, is the reactive uptake coefcient, A is the aerosol surface area concentration, and nX is the gas phase concentration of X.

B2 Aerosols

We consider halogen reactions on sulfate aerosols, sea-salt aerosols, and liquid and ice cloud droplets. The implementation of sulfate type aerosols in GEOS-Chem is described by Park et al. (2004) and Pye et al. (2009). Sulfate aerosols are assumed to be acidic with pH = 0.

The GEOS-Chem sea-salt aerosol simulation is as described by Jaegl et al. (2011). The transport and deposition of sea-salt bromide follows that of the parent aerosol. Oxidation of bromide on sea salt produces volatile forms of bromine that are released to the gas phase. Sea-salt aerosol is emitted alkaline, but the alkalinity can be titrated in GEOSChem by uptake of HNO3, SO2, and H2SO4 (Alexander, 2005). Sea-salt aerosol with no remaining alkalinity is assumed to have pH = 5. We assume no halide oxidation on

alkaline sea-salt aerosol.

The liquid cloud droplet surface area is modelled using cloud liquid water content from GEOS-FP (Lucchesi, 2013) and assuming effective cloud droplet radii of 10 and 6 m for marine and continental clouds, respectively. The ice cloud droplet surface area is modelled in a similar manner assuming effective ice droplet radii of 75 m. We assume that ice cloud chemistry is conned to an unfrozen overlayer surrounding the ice crystal (see Schmidt et al. (2016) for details). Cloud water pH (typically between 4 and 6) is cal-

culated locally in GEOS-Chem following Alexander et al. (2012).

The reactive uptake coefcients depend on the aerosol halide concentration. For sea-salt aerosol, the bromide concentration is calculated directly from the bromide content and the aerosol mass. Sea-salt aerosol chloride is assumed to be in excess (see below). For clouds and sulfate aerosol, the bromide and chloride concentration is estimated by assuming equilibrium between gas phase HX and aerosol phase X.

B3 Reactive uptake coefcients

B3.1 HOBr + Cl / Br

The reactive uptake coefcient is calculated as

= [Gamma1]1 + 1[parenrightBig]1, (B2)

where the mass accommodation coefcient for HOBr is =

0.6, and

[Gamma1] =

r

Dg +

4 c

lr

r , (B4)

with r being the radius of the aerosol. For sea-salt aerosol, HOBr + Cl is assumed to be limited by mass accommoda

tion, i.e. [Gamma1] , due to high concentration of Cl in sea-salt

aerosol. The reacto-diffusive length scale is

lr =

[radicalBigg]

r lr

Dl kHOBr+X [X][H+]

, (B5)

where Dl = 1.4 [notdef] 105 cm2 s1 is the aqueous phase diffu

sion coefcient for HOBr. The listed parameters are taken from Ammann et al. (2013), and kHOBr+Br is from Beck-

with et al. (1996).

B3.2 ClNO3 + Br

The reactive uptake coefcient is calculated as

= [Gamma1]1 + 1[parenrightBig]1, (B6)

where the mass accommodation coefcient for ClNO3 is =

0.108, and

[Gamma1] =

p[Br]Dl

c , (B7)

where c is the average thermal velocity of ClNO3, Dl = 5.0[notdef]

106 cm2 s1 is the aqueous phase diffusion coefcient for ClNO3, and W = 106 pMsbar1.

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4WRT

T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem 12261

Table B1. Photolysis reactions of halogens included in scheme. Photolysis is described in Eastham et al. (2014) (ClNO2, ClNO3, and ClOO), Sherwen et al. (2016a) (I2, HOI, IO, OIO, INO, INO2, INO3, I2O2, I2O3, I2O4, CH3I, CH2I2, CH2ICl, and CH2IBr), and Schmidt et al.

(2016) (BrCl, Cl2, ClO, HOCl, ClNO2, ClNO3, ClOO, Cl2O2, CH3Cl, CH3Cl2, and CHCl3). As stated in Sect. 2, we have used the I2O2 cross section for I2O4.

ID Reaction Cross-section reference

J1 I2

!2I Sander et al. (2011)

J2 HOI h

!I + OH Sander et al. (2011) J3 IO (+O2)

h

!I (+O3) Sander et al. (2011)

J4 OIO h

!I + O2 Sander et al. (2011) J5 INO h

!I + NO Sander et al. (2011) J6 INO2

h

!I + NO2 Sander et al. (2011) J7 INO3

h

!I + NO3 Sander et al. (2011) J8 I2O2

h

!I + OIO Gmez Martn et al. (2005); Spietz et al. (2005) J9 CH3I

h

!I Sander et al. (2011)

J10 CH2I2

h

!2I Sander et al. (2011)

J11 CH2ICl

h

!I + Cl Sander et al. (2011) J12 CH2IBr

h

!I + Br Sander et al. (2011) J13 I2O4

!2OIO see caption

h

!OIO + IO Gmez Martn et al. (2005); Spietz et al. (2005) J15 CHBr3

h

!3Br Sander et al. (2011)

!2Br Sander et al. (2011)

J17 BrO (+O2)

h

!Br (+O3) Sander et al. (2011)

J18 HOBr h

!Br + OH Sander et al. (2011) J19 BrNO2

h

!Br + NO2 Sander et al. (2011) J20 BrNO3

h

!Br + NO3 Sander et al. (2011) J21 BrNO3

h

!BrO + NO2 Sander et al. (2011) J22 CH2Br2

h

!2Br Sander et al. (2011)

!2Cl Sander et al. (2011)

h

!Cl (+O3) Sander et al. (2011)

J26 OClO (+O2)

J25 ClO (+O2)

h

!Cl + ClOO Sander et al. (2011) J28 ClNO2

h

!Cl + NO2 Sander et al. (2011) J29 ClNO3

h

!Cl + NO3 Sander et al. (2011) J30 ClNO3

h

!ClO + NO2 Sander et al. (2011) J31 HOCl h

!Cl + OH Sander et al. (2011) J32 ClOO h

!Cl Sander et al. (2011)

J33 CH3Cl

h

!Cl + CH3O2, Sander et al. (2011) J34 CH3Cl2

h

!2Cl Sander et al. (2011)

h

h

J14 I2O3

J16 Br2

h

J23 BrCl h

!Br + Cl Sander et al. (2011) J24 Cl2

h

h

!ClO (+O3) Sander et al. (2011)

J27 Cl2O2

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12262 T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem

Table B2. Bimolecular halogen reactions included in scheme. This includes reactions from previous updates to descriptions of halogen chemistry in GEOS-Chem (Eastham et al., 2014; Parrella et al., 2012; Schmidt et al., 2016; Sherwen et al., 2016a) and those described in Sect. 2. These are given in the Arrhenius form with the rate equal to A [notdef] exp(

Ea

RT ). Unknown values are represented by a dash and these set to 0 in the model, reducing the exponent to 1. The bi-molecular reactions with an M above the arrow represent termolecular reactions where the pressure dependence is not known or are uni-molecular decomposition reactions. Abbreviations for tracers are expanded in Appendix C.

Rxn ID Reaction A Ea/R Citation

cm3 molecules1 s1 K

M1 I + O3 ! IO + O2 2.10 [notdef] 10

11

11

10 Atkinson et al. (2007)

M4 HI + OH ! I + H2O 1.60 [notdef] 10

11 440 Atkinson et al. (2007)

M5 HOI + OH ! IO + H2O 5.00 [notdef] 10

12 Riffault et al. (2005)

M6 IO + HO2 ! HOI + O2 1.40 [notdef] 10

11 540 Atkinson et al. (2007)

M7 IO + NO ! I + NO2 7.15 [notdef] 10

12 300 Atkinson et al. (2007)

M8 HO + CH3I ! H2O + I 4.30 [notdef] 10

12

11

12

12 Atkinson et al. (2007)

M12 INO3 + I ! I2 + NO3 9.10 [notdef] 10

11

11 Sander et al. (2011)

M14 IO + Br ! I + BrO 2.70 [notdef] 10

11 Bedjanian et al. (1997)

M15 IO + BrO ! Br + I + O2 3.00 [notdef] 10

12 510 Atkinson et al. (2007)

M16 IO + BrO ! Br +OIO 1.20 [notdef] 10

11 510 Atkinson et al. (2007)

M17 OIO + OIO ! I2O4 1.50 [notdef] 10

10 Gmez Martn et al. (2007)

M18 OIO + NO ! NO2 + IO 1.10 [notdef] 10

12 542 Atkinson et al. (2007)

M19 IO + IO ! I + OIO 2.16 [notdef] 10

11 180 Atkinson et al. (2007)

M20 IO + IO ! I2O2 3.24 [notdef] 10

11 180 Atkinson et al. (2007)

10 Gmez Martn et al. (2007)

12

14

!2OIO 3.80 [notdef] 10

M

!I + NO2 9.94 [notdef] 10

M

!IO + NO2 2.10 [notdef] 10

12 280 Atkinson et al. (2007)

M28 IO + ClO ! I + Cl + O2 1.18 [notdef] 10

12 280 Atkinson et al. (2007)

M29 IO + ClO ! ICl + O2 9.40 [notdef] 10

13 280 Atkinson et al. (2007)

M30 Cl + HCOOH ! HCl + CO2 + H2O 2.00 [notdef] 10

13 Sander et al. (2011)

10 Sander et al. (2011)

M32 Cl + CH3OOH ! HCl + CH3O2 5.70 [notdef] 10

M31 Cl + CH3O2 ! ClO + CH2O + HO2

M34 Cl + C2H5O2 ! ClO + HO2 + ALD2

M35 Cl + EOH ! HCl + ALD2

80 Sander et al. (2011)

M38 Cl + C3H8 ! HCl + B3O2 6.54 [notdef] 10

11 Sander et al. (2011)

M39 Cl + ACET ! HCl + ATO2 7.70 [notdef] 10

11

11 500 Sander et al. (2011)

M41 Cl + MOH ! HCl + CH2O + HO2 5.50 [notdef] 10

11 Sander et al. (2011)

a Only rst channel from JPL considered. The second channel forms a Criegee (HCl + C2H4O2) and therefore cannot be represented by the reduced GEOS-Chem

chemistry scheme. b Reaction dened by JPL and interpreted as proceeding via hydrogen abstraction; therefore, the acetaldehyde product is assumed.

830 Atkinson et al. (2007)

M2 I + HO2 ! HI + O2 1.50 [notdef] 10

1090 Sander et al. (2011)

M3 I2 + OH ! HOI + I 2.10 [notdef] 10

1120 Atkinson et al. (2008)

M9 INO + INO ! I2 + 2NO 8.40 [notdef] 10

2620 Atkinson et al. (2007)

M10 INO2 + INO2 ! I2 + 2NO2 4.70 [notdef] 10

1670 Atkinson et al. (2007)

M11 I2 + NO3 ! I + INO3 1.50 [notdef] 10

146 Kaltsoyannis and Plane (2008)

M13 I + BrO ! IO + Br 1.20 [notdef] 10

M

!I2O3 1.50 [notdef] 10

M21 IO + OIO

M

!IO + IO 1.00 [notdef] 10

M

!OIO + I 2.50 [notdef] 10

M

M22 I2O2

9770 Ordez et al. (2012)

M23 I2O2

M24 I2O4

9770 Ordez et al. (2012)

2 Kaltsoyannis and Plane (2008)

M25 INO2

17 11 859 McFiggans et al. (2000)

15 13 670 Kaltsoyannis and Plane (2008)

M27 IO + ClO ! I + OClO 2.59 [notdef] 10

M26 INO3

a 1.60 [notdef] 10

11 Sander et al. (2011)

M33 Cl + C2H6 ! HCl + C2H5O2 7.20 [notdef] 10

11

11 Sander et al. (2011)

70 Sander et al. (2011)

a 7.40 [notdef] 10

b 9.60 [notdef] 10

11 Sander et al. (2011)

M36 Cl + CH3C(O)OH ! HCl + CH3O2, + CO2 2.80 [notdef] 10

14 Sander et al. (2011)

M37 Cl + C3H8 ! HCl + A3O2 7.85 [notdef] 10

11

1000 Sander et al. (2011)

M40 Cl + ISOP ! HCl + RIO2 7.70 [notdef] 10

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T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem 12263

Table B2. Continued.

Rxn ID Reaction A Ea/R Citation

cm3 molecules1 s1 K

M42 CHBr3 + OH ! 3Br + CO 1.35 [notdef] 10

600 Sander et al. (2011)

M43 CH2Br2 + OH ! 2Br + CO 2.00 [notdef] 10

840 Sander et al. (2011)

M44 CH3Br + OH ! 3Br + CO 2.35 [notdef] 10

12 1300 Sander et al. (2011)

M45 Br + O3 ! BrO + O2 1.60 [notdef] 10

11

11

12

11

10 7000 King et al. (1970)

M50 Br + C2H6 ! C2H5OO 2.36 [notdef] 10

10 6411 Seakins et al. (1992)

M51 Br + C3H8 ! C3H7OO 8.77 [notdef] 10

11 4330 Seakins et al. (1992)

M52 Br + BrNO3 ! Br2 + NO3 4.90 [notdef] 10

11 0 Orlando and Tyndall (1996)

M53 Br + NO3 ! BrO + NO2 1.60 [notdef] 10

11 0 Sander et al. (2011)

M54 HBr + OH ! Br + H2O 5.50 [notdef] 10

12 200 Sander et al. (2011)

M55 BrO + NO ! Br + NO2 8.80 [notdef] 10

12 260 Sander et al. (2011)

M56 BrO + OH ! Br + HO2 1.70 [notdef] 10

11 250 Sander et al. (2011)

M57 BrO + BrO ! 2Br + O2 2.40 [notdef] 10

12 40 Sander et al. (2011)

M58 BrO + BrO ! Br2 + O2 2.80 [notdef] 10

14 860 Sander et al. (2011)

M59 BrO + HO2 ! HOBr + O2 4.50 [notdef] 10

12 460 Sander et al. (2011)

M60 Br2 + OH ! HOBr + Br 2.10 [notdef] 10

11 240 Sander et al. (2011)

M61 Cl + ALK4 ! HCl + R4O2 2.05 [notdef] 10

10 Atkinson et al. (2006)

M62 Cl + PRPE ! HCl + PO2 3.60 [notdef] 10

12 Atkinson et al. (2006)

M63 CH3Cl + Cl ! CO + 2HCl + HO2 2.17 [notdef] 10

11 1130 Sander et al. (2011)

M64 Cl + H2O2 ! HO2 + HCl 1.10 [notdef] 10

11 -980 Sander et al. (2011)

M65 Cl + HO2 ! O2 + HCl 1.40 [notdef] 10

11 270 Sander et al. (2011)

M66 Cl + HO2 ! OH + ClO 3.60 [notdef] 10

11

11

12 135 Sander et al. (2011)

M69 ClO + ClO ! Cl2 + O2 1.00 [notdef] 10

12 1590 Sander et al. (2011)

M70 ClO + ClO ! OClO + Cl 3.50 [notdef] 10

13 1370 Sander et al. (2011)

M71 ClO + ClO ! Cl + ClOO 3.00 [notdef] 10

11 2450 Sander et al. (2011)

M72 ClO + HO2 ! O2 + HOCl 2.60 [notdef] 10

12 290 Sander et al. (2011)

M73 ClO + NO ! Cl + NO2 6.40 [notdef] 10

12 290 Sander et al. (2011)

M74 ClOO + Cl ! 2ClO 1.20 [notdef] 10

11 Sander et al. (2011)

M75 ClOO + Cl ! Cl2 + O2 2.30 [notdef] 10

10 Sander et al. (2011)

M76 MO2 + ClO ! ClOO + HO2 + CH2O 3.30 [notdef] 10

12

12 1411 Sander et al. (2011)

M78 OH + Cl2 ! HOCl + Cl 2.60 [notdef] 10

12 1100 Sander et al. (2011)

M79 OH + Cl2O2 ! HOCl + ClOO 6.00 [notdef] 10

13 670 Sander et al. (2011)

M80 OH + ClNO2 ! HOCl + NO2 2.40 [notdef] 10

12 1250 Sander et al. (2011)

M81 OH + ClNO3 ! HOCl + NO3 1.20 [notdef] 10

12

13 230 Sander et al. (2011)

M83 OH + ClO ! HO2 + Cl 7.40 [notdef] 10

12 270 Sander et al. (2011)

M84 OH + HCl ! H2O + Cl 1.80 [notdef] 10

12

12

12 600 Sander et al. (2011)

12

12

780 Sander et al. (2011)

M46 Br + CH2O ! HO2 + CO 1.70 [notdef] 10

800 Sander et al. (2011)

M47 Br + HO2 ! HBr + O2 4.80 [notdef] 10

360 Atkinson et al. (2007)

M49 Br + (CH3)2CO ! CH3C(O)CH2OO 1.66 [notdef] 10

310 Sander et al. (2011)

M48 Br + CH3CHO ! CH3CO3 1.30 [notdef] 10

375 Sander et al. (2011)

M67 Cl + O3 ! ClO + O2 2.30 [notdef] 10

200 Sander et al. (2011)

M68 ClNO3 + Cl ! Cl2 + NO3 6.50 [notdef] 10

115 Sander et al. (2011)

M77 OH + CH3Cl ! Cl + HO2 + H2O 3.90 [notdef] 10

330 Sander et al. (2011)

M82 OH + ClO ! HCl + O2 6.00 [notdef] 10

250 Sander et al. (2011)

M85 OH + HOCl ! H2O + ClO 3.00 [notdef] 10

500 Sander et al. (2011)

M86 OH + OClO ! HOCl + O2 1.50 [notdef] 10

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12264 T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem

Table B3. Termolecular halogen reactions included in the scheme. This includes reactions from previous updates to halogen chemistry in GEOS-Chem (Eastham et al., 2014; Parrella et al., 2012; Schmidt et al., 2016; Sherwen et al., 2016a) and those detailed in Sect. 2. The lower pressure limit rate (k0) is given by A0 [notdef] (

300

T )x. The high pressure limit is given by k1. Fc characterises the fall off curve of the reaction as

described by Atkinson et al. (2007).

Rxn ID Reaction A0 x k1 Fc Citation

cm6 molecules2 s1 cm3 molecules1 s1

T1 I + NO

32 1 1.70 [notdef] 10

11 0.6 Atkinson et al. (2007)

M

!INO 1.80 [notdef] 10

T2 I + NO2

M

!INO2 3.00 [notdef] 10

31 1 6.60 [notdef] 10

11 0.63 Atkinson et al. (2007)

T3 IO + NO2

M

!INO3 7.70 [notdef] 10

31 5 1.60 [notdef] 10

11 0.4 Atkinson et al. (2007)

T4 Br + NO2

M

!BrNO2 4.20 [notdef] 10

31 2.4 2.70 [notdef] 10

11 0.6 Sander et al. (2011)

T5 BrO + NO2

M

!BrNO3 5.20 [notdef] 10

31 3.2 6.90 [notdef] 10

12 0.6 Sander et al. (2011)

T5 BrO + NO2

M

!BrNO3 5.20 [notdef] 10

31 3.2 6.90 [notdef] 10

12 0.6 Sander et al. (2011)

T6 Cl + PRPE

M

!HCl + R4O2 4.00 [notdef] 10

28 0 2.80 [notdef] 10

10 0.6 Atkinson et al. (2006)

T7 Cl + O2

M

!ClOO 2.20 [notdef] 10

33 0 1.80 [notdef] 10

10 0.6 Sander et al. (2011)

T8 Cl2O2

M

!2ClO 9.30 [notdef] 10

6 2 1.74 [notdef] 10

15 0.6 Sander et al. (2011)

T9 ClO + ClO

M

!Cl2O2 1.60 [notdef] 10

21 2 3.00 [notdef] 10

12 0.6 Sander et al. (2011)

T10 ClO + NO2

M

!ClNO3 1.80 [notdef] 10

31 1.9 1.50 [notdef] 10

11 0.6 Sander et al. (2011)

T11 ClOO M

!Cl + O2 3.30 [notdef] 10

9 0 2.73 [notdef] 10

14 0.6 Sander et al. (2011)

k1(T ) for Reactions (T7)(T11) have a form of k1(T ) = k1(

T

300 )

m, where m = 3.1, 4.5, 4.5, 3.4, and 3.1 respectively. Abbreviations for tracers are expanded in

Appendix C.

B3.3 O3 + Br

The reactive uptake coefcient is calculated as

= [Gamma1]b + [Gamma1]s, (B8)

where [Gamma1]b is the bulk reaction coefcient,

[Gamma1]b =

4HO3RT kO3+Br [Br]lrf (r,lr)

c , (B9)

with kO3+Br = 6.8 [notdef] 108 exp(4450K/T ) M1 s1. In the

equation above c is the average thermal velocity of O3, and

f (lr,r) is a reacto-diffusive correction factor,

f (lr,r) = coth

[parenleftbigg]

where Dl = 8.9 [notdef] 106 cm2 s1 is the aqueous phase diffu

sion coefcient for O3.

The surface reaction coefcient is calculated as

[Gamma1]s =

4ks[Br(surf)]KLangCNmax

c(1 + KLangC[O3(g)])

, (B12)

where the surface reaction rate constant is ks =

1016 cm2 s1, the equilibrium constant for O3 is KLangC = 1013 cm3, and the maximum number of available sites is taken as Nmax = 3[notdef]1014 cm2. The surface bromide

concentration is estimated as

[Br(surf)] =min(3.41 [notdef] 1014 cm2 M1 [Br],Nmax). (B13)

lr

r , (B10)

with r being the radius of the aerosol. The reacto-diffusive length scale is

lr =

[radicalBigg]

r lr

Dl kO3+Br [Br]

, (B11)

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T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem 12265

Table B4. Halogen multiphase reactions and reactive uptake coefcients ( ).

ID Reaction Reactive uptake coefcient ( ) Note Reference

1 HCl ! Cl(SSA) 4.4 [notdef] 10

6 exp(2989 K/T ) Sea salt only Ammann et al. (2013)2 HBr ! Br(SSA) 1.3 [notdef] 10

8 exp(4290 K/T ) Sea salt only Ammann et al. (2013)3 HI ! I(aerosol) 0.1

4 ClNO3 ! HOCl + HNO3 0.024 Hydrolysis Deiber et al. (2004)

5 BrNO3 ! HOBr + HNO3 0.02 Hydrolysis Deiber et al. (2004)

6 INO3 ! 0.85ICl + 0.15IBr + HNO3 0.01 Sea salt only

7 INO2 ! 0.85ICl + 0.15IBr + HNO3 0.02 Sea salt only

8 HOBr + Cl(aq) ! BrCl See Sect. B3 in Appendix B Ammann et al. (2013)

9 HOBr + Br(aq) ! Br2 See Sect. B3 in Appendix B Ammann et al. (2013)

10 HOI ! 0.85ICl + 0.15IBr 0.01 Sea salt only

11 ClNO3 + Br(aq) ! BrCl + HNO3 See Sect. B3 in Appendix B Ammann et al. (2013)

12 O3 + Br(aq) ! HOBr See Sect. B3 in Appendix B Ammann et al. (2013)

13 I2O2 ! I(aerosol) 0.02

14 I2O3 ! I(aerosol) 0.02

15 I2O4 ! I(aerosol) 0.02

Table B5. Henrys law coefcients and molar heats of formation of iodine species. Where Henrys law constant equals innity a very large values is used within the model (1[notdef]10

20 Matm1). The INO2 Henrys law constant is assumed equal to that of BrNO3, from Sander (2015), by analogy. For I2Ox (X = 2, 3, 4) a Henrys law constant of innity is assumed by analogy with INO3.

No. Species Henrys law Reference d(lnH)/ Reference constant (H) d(1/T )at 298 K KMatm1

DX HOBr 6.1 [notdef] 10

3 Frenzel et al. (1998) 6.01 [notdef] 10

3 McGrath and Rowland (1994)

4 Schweitzer et al. (2000)

DX BrNO2 0.3 Frenzel et al. (1998)

DX BrNO3 1 Sander (2015)

DX Br2 0.76 Dean (1992) 3.72 [notdef] 10

DX HBr 7.1 [notdef] 10

DX HOCl 6.5 [notdef] 10

DX HCl 7.1 [notdef] 10

DX IBr 2.43 [notdef] 10

D1 HOI 1.53 [notdef] 10

D2 HI 7.43 [notdef] 10

4 Kaltsoyannis and Plane (2008)

D5 I2 2.63 Sander (2015) 7.51 [notdef] 10

3 Sander et al. (2006)

D6 INO2 0.3 see caption text 7.24 [notdef] 10

3 Sander et al. (2006)

D7 I2O3 1 see caption text 7.70 [notdef] 10

3 Kaltsoyannis and Plane (2008)

D8 I2O4 1 see caption text 1.34 [notdef] 10

4 Kaltsoyannis and Plane (2008)

Ka [H+]

13 Frenzel et al. (1998) 1.02 [notdef] 10

3 Dean (1992)

3 Sander (2015)

3 Sander (2015) 5.9 [notdef] 10

3 Sander (2015)

DX ClNO3 1 Sander (2015)

DX BrCl 0.97 Sander (2015) DX ICl 1.11 [notdef] 10

2 Sander (2015) 2.11 [notdef] 10

3 Sander et al. (2006)

3 Sander et al. (2006)

15 Sander (2015) 5.9 [notdef] 10

1 Sander (2015) 4.92 [notdef] 10

4 Sander (2015) 8.37 [notdef] 10

3 Sander et al. (2006)

D3 INO3 1 Vogt et al. (1999) 3.98 [notdef] 10

3 Sander et al. (2006)

13 Sander (2015) 3.19 [notdef] 10

4 Kaltsoyannis and Plane (2008)

D4 I2O2 1 see caption text 1.89 [notdef] 10

Effective Henrys law of HX is calculated for acid conditions through K

typical cloud droplet.

H(T ) = KH(T ) [notdef] (1 +

). A pH of 4.5 is assumed for a

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12266 T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem

Appendix C

Table C1. Abbreviations used in the document. Abbreviated species names used here are dened in the GEOS-Chem manual (http://acmg.seas.harvard.edu/geos/doc/man/appendix_6.html

Web End =http://acmg. http://acmg.seas.harvard.edu/geos/doc/man/appendix_6.html

Web End =seas.harvard.edu/geos/doc/man/appendix_6.html ).

Abbreivation Expansion

PAN peroxyacetyl nitrate

PPN peroxypropionyl nitrateMPN methyl peroxy nitratePMN peroxymethacryloyl nitrateMOH methanolEOH ethanolALD2 acetaldehydeISOP isopreneALK4 C4 alkanes

CH3O2 methylperoxy radical

A3O2 primary RO2 from C3H8

B3O2 secondary RO2 from C3H8

ATO2 RO2 from acetone

R4O2 RO2 from C4 alkanes

RIO2 RO2 from acetone

HOx OH + HO2

NOx NO + NO2

SOx SO2 + SO4 + SO4 on sea salt

Iy I + 2I2 + HOI + IO + OIO + HI + INO + INO2 + INO3 + 2I2O2 + 2I2O3 + 2I2O4

Bry Br + 2Br2 + HOBr + BrO + HBr + BrNO2 + BrNO3 + IBr + BrCl

Cly Cl + 2Cl2 + HOCl + ClO + HCl + ClNO2 + ClNO3 + ICl + BrCl + ClOO + OClO + 2Cl2O2

Ox O3 + NO2 + 2NO3 + PAN + PMN + PPN + HNO4 + 3N2O5 + HNO3 + MPN + XO + HOX + XNO2 + 2XNO3+2OIO + 2I2O2 + 3I2O3 + 4I2O4 + 2Cl2O2 + 2OClO (where X = Cl, Br, I)

PRPE C3 alkenes

Atmos. Chem. Phys., 16, 1223912271, 2016 www.atmos-chem-phys.net/16/12239/2016/

T. Sherwen et al.: Global impacts of tropospheric halogens on oxidants and composition in GEOS-Chem 12267

Acknowledgements. This work was funded by NERC quota studentship NE/K500987/1 with support from the NERC BACCHUS and CAST projects NE/L01291X/1 and NE/J006165/1.J. A. Schmidt acknowledges funding through a Carlsberg Foundation post-doctoral fellowship (CF14-0519).R. Volkamer acknowledges funding from US National Science Foundation CAREER award ATM-0847793, AGS-1104104, and AGS-1452317. The involvement of the NSF-sponsored Lower Atmospheric Observing Facilities, managed and operated by the National Center for Atmospheric Research (NCAR) Earth Observing Laboratory (EOL), is acknowledged.T. Sherwen would like to acknowledge constructive comments and input from GEOS-Chem Support Team at Harvard Univeristy.We also acknowledge constructive input from Qianjie Chen and Becky Alexander of the University of Washington.

Edited by: R. SanderReviewed by: two anonymous referees

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