Increasing tropospheric ozone levels over the past 150 years have led to a significant climate perturbation; the prediction of future trends in tropospheric ozone will require a full understanding of both its precursor emissions and its destruction processes. A large proportion of tropospheric ozone loss occurs in the tropical marine boundary layer and is thought to be driven primarily by high ozone photolysis rates in the presence of high concentrations of water vapour. A further reduction in the tropospheric ozone burden through bromine and iodine emitted from open-ocean marine sources has been postulated by numerical models, but thus far has not been verified by observations. Here we report eight months of spectroscopic measurements at the Cape Verde Observatory indicative of the ubiquitous daytime presence of bromine monoxide and iodine monoxide in the tropical marine boundary layer. A year-round data set of co-located in situ surface trace gas measurements made in conjunction with low-level aircraft observations shows that the mean daily observed ozone loss is approximately 50 per cent greater than that simulated by a global chemistry model using a classical photochemistry scheme that excludes halogen chemistry. We perform box model calculations that indicate that the observed halogen concentrations induce the extra ozone loss required for the models to match observations. Our results show that halogen chemistry has a significant and extensive influence on photochemical ozone loss in the tropical Atlantic Ocean boundary layer. The omission of halogen sources and their chemistry in atmospheric models may lead to significant errors in calculations of global ozone budgets, tropospheric oxidizing capacity and methane oxidation rates, both historically and in the future.

Halogens influence the oxidizing capacity of Earth's troposphere, and iodine oxides form ultrafine aerosols, which may have an impact on climate. We report year-round measurements of boundary layer iodine oxide and bromine oxide at the near-coastal site of Halley Station, Antarctica. Surprisingly, both species are present throughout the sunlit period and exhibit similar seasonal cycles and concentrations. The springtime peak of iodine oxide (20 parts per trillion) is the highest concentration recorded anywhere in the atmosphere. These levels of halogens cause substantial ozone depletion, as well as the rapid oxidation of dimethyl sulfide and mercury in the Antarctic boundary layer.

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 (O 3 ) concentrations, except in polar regions where the model now underestimates O 3 concentrations. Halogen chemistry reduces the global tropospheric O 3 burden by 18.6 %, with the O 3 lifetime reducing from 26 to 22 days. Global mean OH concentrations of 1.28 × 10 6 molecules cm −3 are 8.2 % lower than in a simulation without halogens, leading to an increase in the CH 4 lifetime (10.8 %) due to OH oxidation from 7.47 to 8.28 years. Oxidation of CH 4 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.

Although tropospheric reactive halogen chemistry is well studied in coastal and polar environments, the presence of halogens over the open ocean environment has not been widely reported. The impacts of halogens on the tropical open ocean marine boundary layer (MBL), in particular, are not well characterised. This paper describes observations of iodine monoxide (IO) and bromine oxide (BrO) over eight months in the tropical open ocean MBL, on the north-eastern side of São Vicente (Cape Verde Islands, 16.85° N, 24.87° W). The highest BrO mixing ratio observed was 5.6±1 pmol mol⁻¹, while the maximum observed IO mixing ratio was 3.1±0.4 pmol mol⁻¹. The average values seen between 09:00–17:00 GMT were ~2.8 pmol mol⁻¹ for BrO and ~1.5 pmol mol⁻¹ for IO; these averages showed little variability over the entire campaign from November 2006 to June 2007. A 1-dimensional chemistry and transport model is used to study the evolution of iodine species and quantify the combined impact of iodine and bromine chemistry on the oxidising capacity of the MBL. It appears that the measured fluxes of iodocarbons are insufficient to account for the observed levels of IO, and that an additional I atom source is required, possibly caused by the deposition of O₃ onto the ocean surface in the presence of solar radiation. Modelling results also show that the O₃ depletion observed at Cape Verde cannot be explained in the absence of halogen chemistry, which contributes ~45% of the observed O₃ depletion at the height of measurements (10 m) during summer. The model also predicts that halogens decrease the hydroperoxy radical (HO₂) concentration by ~14% and increase the hydroxyl radical (OH) concentration by ~13% near the ocean surface. The oxidation of dimethyl sulphide (DMS) by BrO takes place at a comparable rate to oxidation by OH in this environment. Finally, the potential of iodine chemistry to form new particles is explored and conditions under which particle formation could be important in the remote MBL are discussed

Abstract. We present a global simulation of tropospheric iodine chemistry within the GEOS-Chem chemical transport model. This includes organic and inorganic iodine sources, standard gas-phase iodine chemistry, and simplified higher iodine oxide (I 2 O X , X = 2, 3, 4) chemistry, photolysis, deposition, and parametrized heterogeneous reactions. In comparisons with recent iodine oxide (IO) observations, the simulation shows an average bias of ∼ +90 % with available surface observations in the marine boundary layer (outside of polar regions), and of ∼ +73 % within the free troposphere (350 hPa < p < 900 hPa) over the eastern Pacific. Iodine emissions (3.8 Tg yr −1 ) are overwhelmingly dominated by the inorganic ocean source, with 76 % of this emission from hypoiodous acid (HOI). HOI is also found to be the dominant iodine species in terms of global tropospheric I Y burden (contributing up to 70 %). The iodine chemistry leads to a significant global tropospheric O 3 burden decrease (9.0 %) compared to standard GEOS-Chem (v9-2). The iodine-driven O X loss rate 1 (748 Tg O X yr −1 ) is due to photolysis of HOI (78 %), photolysis of OIO (21 %), and re- action between IO and BrO (1 %). Increases in global mean OH concentrations (1.8 %) by increased conversion of hydroperoxy radicals exceeds the decrease in OH primary production from the reduced O 3 concentration. We perform sensitivity studies on a range of parameters and conclude that the simulation is sensitive to choices in parametrization of heterogeneous uptake, ocean surface iodide, and I 2 O X (X = 2, 3, 4) photolysis. The new iodine chemistry combines with previously implemented bromine chemistry to yield a total bromine-and iodine-driven tropospheric O 3 burden decrease of 14.4 % compared to a simulation without iodine and bromine chemistry in the model, and a small increase in OH (1.8 %). This is a significant impact and so halogen chemistry needs to be considered in both climate and air quality models.

Abstract.We have integrated observations of tropospheric ozone, very short-lived (VSL) halocarbons and reactive iodine and bromine species from a wide variety of tropical data sources with the global CAM-Chem chemistry-climate model and offline radiative transfer calculations to compute the contribution of halogen chemistry to ozone loss and associated radiative impact in the tropical marine troposphere. The inclusion of tropospheric halogen chemistry in CAM-Chem leads to an annually averaged depletion of around 10 % (∼2.5 Dobson units) of the tropical tropospheric ozone column, with largest effects in the middle to upper troposphere. This depletion contributes approximately −0.10 W m −2 to the radiative flux at the tropical tropopause. This negative flux is of similar magnitude to the ∼0.33 W m −2 contribution of tropospheric ozone to presentday radiative balance as recently estimated from satellite observations. We find that the implementation of oceanic halogen sources and chemistry in climate models is an important component of the natural background ozone budget and we suggest that it needs to be considered when estimating both preindustrial ozone baseline levels and long term changes in tropospheric ozone.

[1] Iodine chemistry in the marine boundary layer has attracted increasing attention over the last few years because iodine oxides cause ozone destruction, change the atmospheric oxidising capacity, and can form ultrafine particles. However, the chemistry of iodine in polluted environments is not well understood: its effects are assumed to be inhibited by reactions involving NO x (NO 2 & NO). This paper describes Differential Optical Absorption Spectroscopy (DOAS) observations of iodine species (I 2 , OIO and IO) and the nitrate radical (NO 3 ) at a semi-polluted coastal site in Roscoff, France. Significant concentrations of IO and I 2 were measured during daytime, indicating efficient recycling of iodine from iodine nitrate (IONO 2 ). I 2 , IO, OIO and NO 3 were observed at night. These observations are interpreted using a one dimensional model to demonstrate that iodine plays an important role even in polluted environments due to recycling mechanisms not considered previously.

Abstract. Fluorescence Assay by Gas Expansion (FAGE) has been used to detect ambient levels of OH and HO2 radicals at the Cape Verde Atmospheric Observatory, located in the tropical Atlantic marine boundary layer, during May and June 2007. Midday radical concentrations were high, with maximum concentrations of 9 ×106 molecule cm−3 and 6×108 molecule cm−3 observed for OH and HO2, respectively. A box model incorporating the detailed Master Chemical Mechanism, extended to include halogen chemistry, heterogeneous loss processes and constrained by all available measurements including halogen and nitrogen oxides, has been used to assess the chemical and physical parameters controlling the radical chemistry. The model was able to reproduce the daytime radical concentrations to within the 1 σ measurement uncertainty of 20% during the latter half of the measurement period but significantly under-predicted [HO2] by 39% during the first half of the project. Sensitivity analyses demonstrate that elevated [HCHO] (~2 ppbv) on specific days during the early part of the project, which were much greater than the mean [HCHO] (328 pptv) used to constrain the model, could account for a large portion of the discrepancy between modelled and measured [HO2] at this time. IO and BrO, although present only at a few pptv, constituted ~19% of the instantaneous sinks for HO2, whilst aerosol uptake and surface deposition to the ocean accounted for a further 23% of the HO2 loss at noon. Photolysis of HOI and HOBr accounted for ~13% of the instantaneous OH formation. Taking into account that halogen oxides increase the oxidation of NOx (NO → NO2), and in turn reduce the rate of formation of OH from the reaction of HO2 with NO, OH concentrations were estimated to be 9% higher overall due to the presence of halogens. The increase in modelled OH from halogen chemistry gives an estimated 9% shorter lifetime for methane in this region, and the inclusion of halogen chemistry is necessary to model the observed daily cycle of O3 destruction that is observed at the surface. Due to surface losses, we hypothesise that HO2 concentrations increase with height and therefore contribute a larger fraction of the O3 destruction than at the surface.