Photocatalytic chloride to chlorine conversion by ionic iron in aqueous aerosols: A combined experimental, quantum chemical and chemical equilibrium model study

. Prior aerosol chamber experiments show that the ligand-to-metal charge transfer absorption in iron(III) chlorides can lead to the production of chlorine (Cl 2 /Cl). Based on this mechanism, the photocatalytic oxidation of chloride (Cl − ) in mineral dust-sea spray aerosols was recently shown to be the largest source of chlorine over the North Atlantic. However, there has not been a detailed analysis of the mechanism including the aqueous formation equilibria and the absorption spectra of the 5 iron chlorides; neither has there been an analysis of which iron chloride is the main chromophore. Here we present the results of experiments measuring the photolysis of FeCl 3 · 6H 2 O in specific wavelength bands, an analysis of the absorption spectra of FeCl 3 − n n ( n = 1 .. 4) made using density functional theory, and the results of an aqueous phase model that predicts the abundance of the iron chlorides with changes in pH and iron concentrations. Transition state analysis is used to determine the energy thresholds of the dissociations of the species. Based on a speciation model with conditions extending from dilute water 10 droplet to acidic seawater droplet to brine to salty crust, and the absorption rates and dissociation thresholds, we find that FeCl +2 is the most important species for chlorine production for a wide range of conditions. The mechanism was found to be active in the range of 400 to 530 nm with a maximum around 440 nm. We conclude that iron chlorides will form in atmospheric aerosols


Introduction
Common components of atmospheric mineral dust including TiO 2 and Fe 2 O 3 are photocatalytically active (Ponczek and George, 2018) and yet evidence of this playing a meaningful role in the atmospheric radical budget has been elusive (Abou-Ghanem et al., 2020;Chen et al., 2012).Recently, a large new source of chlorine atoms was discovered resulting from the combination of Sahara dust with sea spray aerosol over the North Atlantic (van Herpen et al., 2023).The mechanism is triggered when Sahara dust mixes with sea spray aerosol in the marine boundary layer.Iron from the Sahara dust is released and forms iron chlorides with chloride from the sea spray.Iron chlorides can absorb sunlight, releasing a chlorine atom.The chlorine is emitted from the aerosol as molecular chlorine (Cl 2 ), which is then photolysed by sunlight to yield atomic Cl in the gas phase (Wittmer et al., 2015(Wittmer et al., , 2016;;Wittmer and Zetzsch, 2017).The chlorine produced by mineral dust-sea spray aerosols is estimated to produce 41 % of the chlorine over the Atlantic, impacting methane directly (Cl + CH 4 ) and indirectly (reduction in [O 3 ] from Cl + O 3 reduces OH source).Oeste proposed a method for intentionally increasing the production of chlorine using iron salt aerosol to achieve atmospheric methane removal (AMR) (Oeste, 2009;Meyer-Oeste, 2014).The use of chlorine from any source as a climate intervention was recently evaluated by Li et al. (2023).
Traditionally, the tropospheric chlorine cycle (Saiz-Lopez and von Glasow, 2012;Simpson et al., 2015) begins with the formation of sea spray aerosol (Nielsen and Bilde, 2020), which are known to be particles with high acidity (Angle et al., 2021).Acids such as HNO 3 and H 2 SO 4 deposit forcing HCl into the gas phase which can react with OH to produce chlorine atoms, HCl + OH → Cl + H 2 O (Young et al., 2014).Cl reacts with ozone, impacting the formation of hydroxyl radicals, and it reacts with methane and other hydrocarbons, reforming HCl (Chang and Allen, 2006;Knipping and Dabdub, 2003;Badia et al., 2019).
Chlorine production is poorly constrained and as a result, Cl is estimated to remove between 0.8 and 3.3 % of tropospheric methane, depending on the model (Allan et al., 2007;Hossaini et al., 2016;Sherwen et al., 2016;Gromov et al., 2018;Li et al., 2022).Multiple lines of evidence show chlorine concentrations in the troposphere exceed what can be explained with existing mechanisms.These include 1. 13 C depletion in CO in air samples from Barbados (Mak et al., 2003), a signature of methane oxidation by chlorine.2. Anomalies in the CO:ethane ratio seen at Cape Verde (Read et al., 2009).3. Observations of elevations in the concentration of HOCl above what can be explained with standard chemistry (Lawler et al., 2011).4.
A comprehensive simulation of tropospheric chlorine using the GEOS-Chem global 3-D model of oxidant-aerosol-halogen atmospheric chemistry could not explain the elevated Cl 2 mixing ratios measured in the boundary layer in the WINTER aircraft campaign (Wang et al., 2019).
Additional chlorine production impacts our understanding of the methane budget because the abundance of 13 C in atmospheric methane is used to constrain emissions sources, and because Cl + CH 4 has a large kinetic isotope effect, while the main atmospheric methane sink reaction OH + CH 4 does not.The reaction of CH 4 with Cl has a carbon kinetic isotope effect (KIE) of 13C KIE Cl = 1.066 (±0.002) at 297 K, which is around 17 times more fractionating than methane oxidation with OH radicals (Saueressig et al., 2001;Cantrell et al., 1990;Saueressig et al., 1995).The discovery of a new chlorine source means that methane sources must be more depleted than had been recognized, leading to the conclusion that previous methane emissions budgets, which did not include the new chlorine source, likely underestimate biogenic methane (e.g.wetlands and agriculture), and overestimate the fossil fuel source (van Herpen et al., 2023).To understand the methane budget it is imperative to fully characterize the chlorine production mechanism and to see how it will vary with chemical conditions such as pH, chloride concentration and the concentrations of possibly interfering ions.
Historically, iron(III) chloride has been believed to form four complexes: FeCl 2+ , FeCl + 2 , FeCl 3 , and FeCl − 4 (Gamlen and Jordan, 1953).Uchikoshi et al. (2022) presented a model of iron(III) chloride species, which shows the most plausible species to be FeCl + 2 , FeCl 3 , FeCl − 4 , and FeCl 3− 6 .With the use of a theoretical mathematical decomposition model called the "Multivariate Curve Resolution Alternative Least Squares (MCRALS)" and a "5 complex model", they determined that the combination of Cl coordination numbers are n = 0, 2, 3, 4, and 6.That study indicates that FeCl 2+ will not be formed and the highest chlorinated complex, forming at the highest chloride concentrations, will be FeCl 3− 6 .The research by Uchikoshi et al. (2022) shows FeCl + 2 , FeCl 3 , and FeCl − 4 forming at a lower chloride concentration than previously expected.However, FeCl 3− 6 has not been implemented in this study.
In this study, we present a detailed description of the photocatalytic oxidation of chloride to chlorine-based on four iron(III) V) (Harnung and Johnson, 2012).Given the presence of chloride and Fe(III), the iron(III) chlorides will form.
The central Fe(III)Cl n 3−n reactions that occur in an aerosol or the marine boundary layer are shown in Reaction R1 for n=0..3 (Lindén et al., 1993).
Iron(III) chloride formation may be inhibited by the presence of other ions such as sulfate or fluoride, or by organic compounds such as oxalate that bind with Fe(III).
The photolysis of the iron(III) chlorides is shown in general form in Reaction R2 for n ≥ 1.In this ligand to metal electron transfer absorption Fe(III) is reduced to Fe(II) and chloride is oxidised yielding a chlorine atom (Lindén et al., 1993;Nadtochenko and Kiwi, 1998).
For example for n =1, reaction R2 has a quantum yield of 0.5 ± 0.1 at a wavelength of 347 nm.At higher pH the formation of iron(III) chloride complexes competes with iron hydroxide complexes (also depending on [Cl − ]), Reaction R3 and R4.
Iron(III) hydroxide complexes can be photolysed, similar to iron(III) chlorides, however only some of the iron is photoreduced; Reaction R5 produces OH radicals with a quantum yield of 0.21 ± 0.04 at a wavelength of 347 nm (Nadtochenko and Kiwi, 1998).Where reaction R6 shows the photolysis of Fe(OH) + 2 does not grant a reduction of iron (Loures et al., 2013;Korte et al., 2011).

Fe(OH)
2+ + hν → Fe 2+ + OH • (R5) In the aerosol process, the Fe(II) product is oxidised back to Fe(III) by one half of the Fenton process.In one example, for marine mineral aerosol, Zhu et al. found the oxidation rate to be 0.19 min −1 (Zhu et al., 1993).The Fenton process describes how Fe(II) and Fe(III) act as a catalyst pair, breaking down hydrogen peroxide and generating radicals (Fenton, 1894).The Fenton reactions are shown in Reactions R7 to R9. Hydroxyl may react or escape to the gas phase.Wittmer and Zetzsch (2017) proposed that the production of OH will enhance Cl 2 production due to the production of a chlorine radical, shown in reactions R10 to R12.This route could be enhanced in chloride-rich environments.Several sources of OH are known including photocatalytic minerals (Chen et al., 2012) and Fenton degradation of H 2 O 2 reaction R7.
Reactions R10 to R12 take place in the aqueous phase.For the chloride produced in Reaction R2 and the chlorine radical in R12 to impact gas phase chemistry, it must escape the particle.The following Reactions, R13 and R14, lead to the production of Cl 2 (aq) in Reaction R15.
Chlorine atoms may be lost before escaping as molecular chlorine in the gas phase.Possible mechanisms include failure of the chlorine atom produced by photolysis to escape the solvent cage or diffusion back into contact leading to reformation of the iron chloride, and reaction of chlorine with condensed phase hydrocarbons forming HCl/Cl − .

Methods
This section will include method details of the study divided into three main parts: Aqueous Equilibrium Model (AEM) (modelling the concentrations of FeCl 3−n n species), Ab Initio Calculations (estimating absorption rates), and Laboratory Experiments (prove the formation of chloride from photolysis of FeCl 3−n n species).

Aqueous Equilibrium Model Methods
Visual MinteQ is a chemical equilibrium model that calculates the equilibrium speciation for the input species and predicts their concentration (J.P. Gustafsson, (released 2014).The program has been used to evaluate species with direct or indirect effects on iron(III) induced chloride production.
Three AEM scenarios shown in Table 1, called Simple, Sulphate, and Seawater.The species FeCl 2+ , FeCl + 2 , and FeCl 3 were manually added to the database, shown in the Appendix Table A1 (Tagirov et al., 2000), however, FeCl − 4 could not be added because the thermodynamic data is not available.Two different iron concentrations correlating to seawater and aerosol concentrations are used in all models, where the ratio between Fe 2+ and Fe 3+ is estimated to be 7.5 to 92.5 % (Achterberg et al., 2001;Hsu et al., 2010;Zhu et al., 1993).The Simple model shows the relation between iron chloride species, the Sulphate model evaluates the potential effect of SO 2− 4 on iron chlorides, and the Seawater model includes all major ions found in seawater (Harnung and Johnson, 2012;Stumm and Morgan, 2012).The results from each of the three models are shown as species concentrations and iron species as a fraction of total iron, as a function of pH in Section 3.1.Figure 2 describes the computational method, initialized by the generation of the molecules in Gaussview followed by geometry optimizations in vacuum (Frisch et al., 2016).The density functional theory method CAM-B3LYP/6-311++G(d,p) was used for all calculations (Yanai et al., 2004;Francl et al., 1982;McLean and Chandler, 1980;Spitznagel et al., 1987).

Ab Initio Calculations
Solvents were modelled using the PCM model (Tomasi et al., 1999).The relative permittivity, ϵ r , for the solvents is in Table 2.
See Appendix A2 for the geometries of the iron(III) chlorides.
A transition state scan was made to evaluate the bond dissociation energy.In a few cases (mainly FeCl − 4 ) in some solvents, the TS optimizations did not converge.TD-DFT calculations were used to explore the excitation energies of the molecules.Photolysis rates j A were calculated using Equation 1.

Experimental Method
The experimental system is shown in Figure 3. Gasses are introduced with a 4-port valve ("4V"), which was used to choose a flow-through or loop pattern.The average flow was measured to be 125 mL/min.A sample of FeCl At the beginning of every experiment, the active carbon trap and cold trap are flushed and a new sample of 20 mg FeCl 3 •6H 2 O in a 10 mL beaker is placed in a cleaned Sample Chamber.Methane is introduced to achieve a nominal mole fraction of 95 ppm and the system is switched to loop mode.The concentrations of methane and carbon dioxide are monitored and a leak test of the system is performed after the concentrations have stabilized.System stability is monitored for 10 minutes, the xenon lamp is turned on for 15 minutes, and finally, an additional 10 minutes of measurement are taken to verify system stability.
The duration of an experiment is from 1.5 to 2.5 hours.Three repetitions were made for each bandpass filter.The experimental procedure is described in greater detail in Appendix Figure A1.
Two light sources were used: a Xenon lamp from Eimac, see Figure 4, and a UV-LED (Luminus SST-10-UV Surface Mount LED lamp) see Appendix Figure A2.The spectrum of the xenon lamp is shown with a dashed line in Figure 4; the light that enters the Sample Chamber is illustrated with a solid line.The legend name refers to the centre wavelength of the bandpass filter in nm.The system and measurements of the Sample Chamber, bandpass filters, and the xenon lamp are illustrated in Appendix Figure A3.

Results
The results are presented in three sections: Aqueous Equilibrium Modelling, Ab Initio Calculations, and Laboratory Experiments.According to this study, sulphates are the main seawater anion, competing for iron(III) with chloride.Increasing the sulphate and iron concentration has a small impact on the speciation of iron fluorides as seen in Appendix section B2, where all species above 0.1 % for all models are shown.The fraction of iron fluorides is below 5 % and they will not be discussed further.

Aqueous Equilibrium Modelling
The temperature dependence of FeCl 2+ and FeCl + 2 was modelled with the Seawater scenario of the AEM illustrated in Figure 6 from 0 to 100 • C. The fraction of FeCl 2+ increases with increasing temperature and the opposite trend is seen for FeCl + 2 .With varying temperatures, a change of 70 and 45 % is found for FeCl 2+ and FeCl + 2 , respectively.Thus, temperature is an important parameter when calculating the rate of chlorine production from iron chlorides, as a change of 20 • C will significantly change iron speciation.

Ab Initio Calculations
The four species FeCl 2+ , FeCl + 2 , FeCl 3 and FeCl − 4 were investigated computationally with the DFT functional CAM-B3LYP and basis set 6-311++G(d,p) (Yanai et al., 2004;Francl et al., 1982;McLean and Chandler, 1980;Spitznagel et al., 1987).UV-Vis spectra were extracted from TD-DFT calculations done with a variety of relative permittivities, shown in Table 2. UV-Vis spectra for FeCl 3 are displayed in Appendix Figure B1, showing an increasing red shift with increasing dielectric constant.This relative permittivity effect means that the solvation of iron chlorides in water increases the absorption cross-section at longer wavelengths where there is higher actinic flux, increasing the photolysis rate.The absorption spectra obtained for relative ).The value in parentheses shows the relative permittivity of the solute.The absorption is calculated using the CAM-B3LYP/6-311++G(d,p) basis set.The spectral actinic flux is calculated using the TUV model over the Atlantic Ocean, west of Cape Verde (18.97°N, 39.12°W, date 18/07/2022, 12:00) (Madronich et al., 2002).permittivities greater than 2 are seen to be virtually identical, thus, only the four environments vacuum, solid, seawater, and water, are discussed further.
Using the calculated UV-Vis spectra, the actinic flux and the quantum yield function, the photolysis spectra of FeCl 2+ , FeCl + 2 , FeCl 3 , and FeCl − 4 are calculated, see Figure 7.Each of these species absorb at visible wavelengths, which has not been described previously.The modelled result for Seawater (green) is almost identical to the result for water (blue); the two spectra overlap in the figure.The lowest absorption rates are calculated for FeCl 2+ where all rates are below 0.5 × 10 −3 s −1 nm −1 for all four solvents.The AEM scenarios found FeCl 2+ to be the second most abundant iron(III) chloride species.Its role becomes less important however when the absorption spectrum is considered, in addition to the chromophore's concentration.
FeCl + 2 in vacuum absorbs from 300 to 700 nm whereas solid, seawater, and aqueous FeCl + 2 absorb in the range of 400 to 700 nm.FeCl + 2 in water and seawater have the highest absorption rates of the investigated species, 3.5 × 10 −3 s −1 nm −1 .The AEM scenarios found FeCl + 2 to be the dominant iron(III) chloride species and when the absorption spectrum is considered in combination with concentration, we conclude that FeCl + 2 is the important chromophore for the catalytic, photo-oxidative conversion of chloride to chlorine in aqueous environments.
In the model, the absorption rates of FeCl 3 in seawater and water have a maximum of 2.6 × 10 −3 s −1 nm −1 at 402 nm.
Similar trends are calculated for FeCl − 4 where the solid has a maximum at 400 nm of 2.8 × 10 −3 s −1 nm −1 .The absorptions in vacuum and solid have similar trends for both FeCl 3 and FeCl − 4 .
Visible light does not necessarily provide enough energy for the iron(III) chlorides to dissociate, and a transition state analysis can assist in the estimation of the energy thresholds.In Figure 8 the energy thresholds for photodissociation yielding a chlorine radical are shown for the four iron(III) chloride species FeCl 2+ , FeCl + 2 , FeCl 3 , and FeCl − 4 .The photon energy is given in kJ/mol and converted to a photon wavelength in nm so it can be related to the solar spectrum.Near the Earth's surface, the actinic flux spectrum becomes negligible at wavelengths shorter than 300 nm (Harnung and Johnson, 2012).This threshold is shown in the figure with a yellow line.
Figure 8 shows FeCl 2+ as the only species to have an energy threshold corresponding to a wavelength shorter than 300 nm for solid, seawater, and water and so this species can not be dissociated by near-surface solar excitation.According to the model, FeCl + 2 has energy thresholds for solid, seawater, and water at 611, 603, and 605 nm, respectively.The highest absorption rates for FeCl + 2 are in the region of 500 to 700 nm (see Figure 7), thus these thresholds very likely impact the photolysis rate.According to the transition state model, FeCl 3 is the only species that can be photolysed by all visible wavelengths and even into the near-infrared, for any of the solvents.For FeCl − 4 the only sunlight-limiting energy threshold is in vacuum, at 573 nm.However, as FeCl − 4 absorbs at wavelengths shorter than this threshold, sunlight will dissociate this species under vacuum conditions.Sunlight at the surface has photons with enough energy for the dissociation of FeCl + 2 , FeCl 3 , and FeCl − 4 .
General trends in Figure 8 show vacuum to be an outlier that either has a higher or lower energy threshold than solid, seawater, and water.Therefore, solvent effects play an important role when estimating the energy thresholds and absorption rates of the dissociation of these iron(III) chloride species.Table 3. Integrated Absorption Rates, s −1 × 10 −3 ."Full" is integrated from 280 to 700 nm, as displayed in Figure 7. "Cut-off" is integrated from 280 nm to the dissociation energy threshold in Figure 8.The deviation is calculated between the Full and the Cut-off absorption rates.To relate the absorption rates and the energy dissociation threshold, the integrated absorption rates are listed in Table 3.The integrated absorption rate is integrated from 280 to 700 nm, as seen in Figure 7.The "cut-off" is integrated from 280 nm to the dissociation energy threshold regarding each species, as seen in Figure 8.The deviation between "Full" and "Cut-off" is listed in percentages for each species as a measure of the cut-off impact.
As listed in Table 3, FeCl 2+ has full absorption rates of 32, 4, 3, and 3 s −1 in a vacuum, solid, seawater, and water, respectively.This species generally has the lowest integrated absorption rates which decrease significantly from vacuum to solid, seawater, and water.When the energy dissociation threshold is included, the integrated absorption rates for solid, seawater, and water decrease to zero.
FeCl + 2 has full absorption rates of 177, 256, 312, and 312 s −1 in vacuum, solid, seawater, and water, respectively.Including the cut-off significantly decreases the absorption rates by 45, 58 and 57 % for solid, seawater and water, respectively.FeCl 3 has full absorption rates of 150, 222, 259, and 259 s −1 in vacuum, solid, seawater, and water, respectively.FeCl 3 is the only species that does not have sunlight-limiting energy thresholds; hence, the full and cut-off rates are identical.Furthermore, FeCl 3 has a higher cut-off absorption rate for seawater and water than the other species, which increases the importance of this species.
FeCl − 4 has full absorption rates of 220, 272, 164, and 164 s −1 in vacuum, solid, seawater, and water, respectively.The rates significantly decrease from vacuum to seawater and water.However, including the cut-off significantly decreases the rate in vacuum by 48 %, whereas the rate in water and seawater is unaffected and is consequently faster than the rate in vacuum.
Including the sunlight limiting energy thresholds, FeCl 3 is the species that has the fastest integrated absorption rates in seawater and water of 259 and 259 s −1 , respectively.Across all species, seawater and water have virtually identical rates, which further confirms that quantum calculations made in water can be used to evaluate behaviour in seawater.The change in measured methane concentration was used as a proxy for chlorine radical production, as shown in Reaction 250 R17.

Laboratory Study
The x-axis in Figure 9 is ordered according to the centre wavelength of the bandpass filter used in the experiments, where "none" indicates that no bandpass filter was used.The spectral irradiance for the bandpass filters is displayed in Figure 4.
The absolute and energy normalized methane removal is shown in Figure 9 (Left) in units of CH 4 molecules per FeCl 3 • 6H 2 O molecule.The energy normalization was calculated for the absolute removal at 408 nm and the integrated spectral irradiance for each bandpass filter.As the bandpass filters do not have the same wavelength width or energy throughput, Figure 4, the energy normalization contradicts this effect.

A3 Experimental Procedure
When the experimental procedure begins, the system is opened between the active carbon trap and the two measuring instruments.While the system is open, the flow is increased, and an overpressure vent is closed to flush the system.A FeCl 3 •6H 2 O sample of 20 mg is now measured in a 10 mL beaker on a fine scale.The Sample Chamber is cleaned with Milli-Q water on some paper and wiped again with dry paper.The beaker is placed in the Sample Chamber, and the chamber is closed.Two O-rings ensure a tight closure of the Sample Chamber.While the system is open, the cold trap is heated for 5 minutes to flush condensed species out of the system.The active carbon trap is heated for 20 minutes to release the captured CO 2 .The Sample Chamber is heated simultaneously with the active carbon trap to minimize the temperature difference in the Sample Chamber when the xenon lamp is turned on.After the heating processes, the flow is decreased by opening the over-pressure vent, and the system is closed.There is now flow through the whole system.During the process of closing the system, the CO 2 and H 2 O concentrations are recorded by the Picarro to ensure that the active carbon trap works properly.Methane is now introduced with a syringe to the nitrogen flow.The methane concentration in the system rises to 200 to 400 ppm, and the four-port valve is closed at around 150 ppm.Initiating the methane is controlled live with the Axetris instrument.After the looping process is started the methane concentration is monitored to ensure it stabilizes at around 95 ppm.After approximately five minutes, the stability test begins.Due to a small underpressure in the system, the presence of a possible leak is quantified by spraying pure methane from a syringe at joints, seals and so on, and observing whether the methane concentration in the system increases.
After the system has passed the stability test the cold trap, bandpass filter, fan, and the UV-LED are initiated.The two instruments are both slightly sensitive to temperature, 'leak rate 1' is therefore initiated five minutes after the cold trap is introduced, to measure stability.The next step is recording the leak rate over 10 minutes before illumination.The xenon lamp is turned on for exactly 15 minutes, and the second leak rate is measured over the following 10 minutes.One experiment has now ended, and the procedure is repeated with a new sample of FeCl 3 •6H 2 O.The different bandpass filters and light sources are discussed in methods (Section 2.3).Each of the 10 bandpass filters was used for three experiments.
Figure 1.The primary sources and sinks for iron(II) and iron(III) ions and chloride in the atmospheric aerosols and their influence on the formation of iron(III) chlorides in a low pH environment.

Figure 3 .
Figure3.Overview of the system used in the laboratory experiments.The sample is located in "Sample Chamber".The volume of the "Sample Chamber" and "Cl2 Photolysis Chamber" is 0.36 and 0.45 L, respectively.The four-port valve, "4V", changes the system between flow and loop mode.

Table 2 .a
The set of relative permittivities (relative permittivity, ϵr) used to represent various solvents with the PCM model.Value obtained from Gaussview, Frisch et al. (2016).b Values obtained from Midi et al. (2014).c Arbitrary values were only used for FeCl3.d Estimations.

Figure 4 .
Figure 4.The measured absolute spectral irradiance of the xenon lamp before and after the Sample Chamber, including bandpass filters.Recorded with the Ocean Optics spectrometer.

Figure 5 .
Figure 5. Three AEM scenarios with a seawater iron concentration of 9.76 ×10 −13 mol/kg (FeS) at 20 • C. All species below 5 % are shown in Appendix B2.See Appendix B3 and B4 for AEM scenarios with increased sulphate and iron concentration, e.g.Fe(OH)3.

Figure 5
Figure 5 displays the iron speciation from the three AEM scenarios as a function of pH for the seawater iron concentration of 9.76 ×10 −13 mol/kg.Iron(III) chlorides are stable at pH values less than 4. Above this, the dominant species are iron

Figure 8 .
Figure 8. Energy thresholds for photodissociation, yielding a chlorine radical calculated with CAM-B3LYP/6-311++G(d,p).The sunlight photolysis rate cut-off is illustrated in yellow.The relative permittivity for the solutes is given in parentheses.

Figure 9 .
Figure 9. (Left) Chlorine generation measured as methane removal for photoexcitation in a series of wavelength intervals defined by bandpass filters.Without a bandpass filter, the absolute CH4 removal was 10.4 × 10 −3 CH4 molecules per FeCl3 • 6H2O molecule.(Right) The change in the abundance of 13 C is shown as ∆δ 13 C − CH4."none" indicates that no bandpass filter was used during the experiment.Steady-state calculations of Cl are shown in Appendix section A6.The error bars represent the standard deviation.

Figure A1 .
Figure A1.Overview of the experimental procedure.Data is retrieved during the three grey highlighted steps.Abbreviations: Cold Trap (CT), Active Carbon Trap (AC), different bandpass filters.

Figure A2 .
Figure A2.Light source spectra.The UV-LED was a Luminus SST-10-UV Surface-Mount LED lamp.The spectrum is recorded by an OceanOptics Flame-S Miniature UV-Vis-NIR spectrometer, and a fibre optic cable with a diameter of 600 µm before and after the chamber.

Table 1 .
Species and initial concentrations listed for each AEM scenario.Two different iron concentrations are used for each: iron seawater concentration, marked S and iron aerosol concentration, marked A.
3 •6H 2 O was placed in the Sample Chamber (volume 0.36 L) illuminated by a Xenon lamp.For wavelength-controlled experiments, a bandpass filter is placed on top of the Sample Chamber.The "Cl 2 Photolysis Chamber" is a photolysis chamber with a volume of 0.45 L and a high-power UV LED light source, that ensures chlorine emitted by the sample in the Sample Chamber is photolysed.The cold trap and active carbon trap ensure that organic molecules and carbon dioxide are captured, which protects the instruments and removes possible interference.The airflow is divided for analysis by the Picarro "G2201-i Analyser for Isotopic CO 2 /CH 4 " and the Axetris "LGD Compact-A CH 4 " which measure CH 4 , δ 13 CH 4 , CO 2 , and H 2 O.The flow controller is set to 125 mL/min and the pump ensures flow through the system.

Table A2 .
Geometries of minima and TS for FeCl 2+ .TS and relative permittivities are given in parentheses.The TS is not optimized if not given otherwise.Lennard-Jones potential.Values given in units of Å.

Table A3 .
Geometries of minima and TS for FeCl + 2 .TS and relative permittivities are given in parentheses.The TS is not optimized if not otherwise indicated.Lennard-Jones potential.Values given in units of Å.

Table A5 .
Geometries of minima and TS for FeCl − 4 .TS and relative permittivities are given in parentheses.The TS is not optimized if not otherwise indicated.Values given in units of Å.