Halogen oxides strongly influence atmospheric composition. Catalytic reaction cycles involving chlorine, bromine or iodine, lead to ozone depletion in the troposphere. In the stratosphere, the role of chlorine and bromine, which are released predominantly as a consequence of anthropogenic emissions of chlorofluorocarbon compounds, is well established . The potential importance of iodine reactions for stratospheric ozone depletion is discussed in , , and .

Stratospheric concentrations of iodine species are much lower than those of chlorine and bromine compounds . From balloon-borne observations, an upper limit for stratospheric iodine monoxide, IO, of 0.1 parts per trillion by volume (pptv) was determined in the tropics , while upper limits for IO of 0.2 pptv at 20 km, or 0.1 pptv at 15 km, were derived in the mid- and high latitudes . estimate upper limits of total gaseous iodine of about 0.09 to 0.16 pptv in the tropical lower stratosphere (21.0 to 16.5 km) and 0.17 to 0.35 pptv in the tropical upper troposphere (16.5 to 13.5 km). A recent study by estimates that stratospheric iodine may range between 0.25–0.7 pptv. This is based on, for example, new aircraft observations in the tropics from which volume mixing ratios (VMRs) of IO between 0.1–0.2 pptv at altitudes up to 14 km were retrieved .

The ozone destruction potential of stratospheric iodine is significantly higher than that of the other halogens. Bromine is about 60 times more effective in destroying ozone than chlorine and, for iodine, the factor is about 150 to 300 . The effective chain length of the catalytic cycles involving iodine and IO is larger than those involving the other halogens. This is in part because the temporary reservoir species containing iodine are photolysed and/or react more rapidly with stratospheric free radicals than their chlorine or bromine analogues. As a result, reactive iodine may impact stratospheric ozone chemistry even at sub-pptv levels.

IO is formed by the reaction of iodine radicals with ozone, O3. Catalytic cycles including IO, by which tropospheric O3 is effectively destroyed, were already proposed in the 1980s . As a result of self reactions, iodine oxides may lead to particle formation and thereby affect atmospheric radiation balance . These effects of iodine motivate the scientific interest in the assessment of sources, amounts and distributions of iodine species in the atmosphere.

Atmospheric iodine is of organic as well as inorganic origin, e.g. from emissions of I2 and of halogenated organic compounds such as CH3I and CH2I2 and references therein. The largest iodine sources in general are the world's oceans. Iodine compounds are emitted into the marine boundary layer, e.g. from algae or via inorganic pathways involving the ocean surface . In the polar troposphere, bromine and iodine oxides are both observed predominantly during spring time. Release mechanisms of iodine and bromine above sea ice areas, however, are considerably different. Bromine monoxide, BrO, is released following an autocatalytic Br activation , also known as the bromine explosion mechanism. Iodine most probably takes different pathways involving the release of organoiodine compounds , while inorganic reactions cannot be excluded.

Volcanic eruptions are an important source of halogens in the atmosphere, especially for the free and upper troposphere and the lower stratosphere . Volcanic plumes are known to contain halogen species initially in acidic form, e.g. HF, HCl, HBr and HI . Bromine oxides as well as chlorine oxides have been previously observed in volcanic plumes. Volcanic BrO was first detected by , who applied the well-established Differential Optical Absorption Spectroscopy (DOAS) technique with a ground-based Multi-Axis DOAS (MAX-DOAS) system. Volcanic chlorine oxides, ClO and OClO, were also measured, e.g. by and , using ground-based DOAS instruments. From space, volcanic BrO was detected for the first time from the Kasatochi eruption in 2008 , followed by volcanic OClO from the Puyehue eruption in 2011 . Several further observations using ground-based measurements , airborne instrumentation as well as satellites have confirmed and further quantified the abundances of bromine oxides injected into the atmosphere following volcanic eruptions. The release mechanism of volcanic BrO is believed to be similar to polar tropospheric BrO and is based on an autocatalytic reaction cycle involving volcanic aerosols . Ozone depletion was observed within volcanic plumes and is attributed to reactive halogen chemistry and references therein.

Using filter techniques, measurements at Mt Etna in Italy and at the Masaya and Telica volcanos in Nicaragua , for example, showed that gaseous HI, I and HBr are relevant constituents in the degassing of these specific volcanos. Only a few studies are available that report on the analysed iodine content in samples of volcanic gases or volcanic fluids. investigated the 129I / I ratio in volcanic fluids in order to determine the ages of iodine species. The determined iodine ages are in agreement with the expected age of subducted sediments. An iodine accumulation takes place here, because marine sediments contain concentrated amounts of organic iodine. Iodine oxides have not been previously detected in the emission plumes of volcanos. report an upper limit for IO slant columns of 7.6 to 8.6 × 1012 moleccm-2 based on the detection limit of their ground-based DOAS observations at Mt Etna, Italy, during a stable quiescent degassing phase in September 2012.

The composition of volcanic gases is in general strongly variable, with individual characteristics changing from volcano to volcano as well as between eruption and degassing phases and references therein. While the gas-phase composition is individual for each volcanic eruption, there is also a general difference between iodine and other halogens in volcanic gases at high temperatures. At around 1000 ∘C, the main constituents are HF, HCl and HBr for the other halogens. For iodine, however, HI and atomic I may be present in equal amounts .

Up to the present, no detection of gaseous iodine oxides of volcanic origin has been reported, either by in-situ measurements or by remote sensing from ground or satellite. Iodine monoxide is retrieved from satellite measurements of backscattered solar radiation by the DOAS technique and is observed, for example, in the south polar region . In most cases, atmospheric amounts of IO are fairly small, so that usually temporal averages of the satellite data of at least 1 month are created in order to improve signal-to-noise ratio.

In August 2008, the eruption of the Kasatochi volcano took place . Kasatochi belongs to the volcanic arc of the Aleutian Islands, Alaska. The violent explosions started on 7 August 2008, in the afternoon. The Volcanic Explosivity Index (VEI) , which classifies the eruptive volume and eruption cloud height, was VEI 3–4. Large amounts of ash and sulfur dioxide, SO2, were released into the atmosphere, reaching the lower stratosphere . In total about 1.7 Tg of SO2 were emitted and spread over large parts of the globe.

In the following, the detection of IO from the eruption of the Kasatochi volcano using observations of the SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY) and GOME-2A (Global Ozone Monitoring Experiment-2A) satellite instruments is presented and discussed. The applied instruments and retrieval settings are briefly described, and the IO spectral fit quality is investigated. The IO results are analysed in terms of spatial distribution, temporal evolution and integral amount. In addition, IO and BrO distributions in the volcanic plume are compared among each other and to those of SO2.

The only satellite-borne spectrometer for which an IO product has been reported so far is the SCIAMACHY instrument onboard the European Environmental Satellite (ENVISAT) . The mission operated between March 2002 and April 2012. In the present study, data from the GOME-2A onboard MetOp-A (Meteorological Operational Satellite A) have also been successfully analysed, and the signatures of IO and BrO absorption have been retrieved. IO detection by GOME-2A is reported here for the first time. The DOAS method is used for the retrieval of trace gas amounts.

When comparing the integrated numbers of IO and BrO molecules in the volcanic plume, one important and interesting point is that the amount of iodine is only about 1 order of magnitude smaller than that of bromine. For the individual days from 9 to 12 August 2008, the ratio for the integrated number of BrO to IO molecules lies between 6.7 and 10.0 and amounts to 4.2 on 8 August 2008. The corresponding mass ratio for BrO to IO ranges between 4.0 and 6.7 and amounts to 2.8 on 8 August 2008, using data from Fig. .

Figure  shows a scatterplot between IO and BrO column amounts from the individual satellite observations. Data from the 4-day period, 9 to 12 August 2008, is included in the comparison. As in the correlation analysis described in Sect. , for each day those measurements that fall into a rectangular area enclosing the volcanic plume are used. The slope for all data of IO vs. BrO columns is 0.09, with a correlation coefficient of 0.74. This observation is consistent with findings by , who estimate a 1 order of magnitude difference between the volcanic abundances of the two halogen species iodine and bromine. In addition, estimate the annual fluxes of volcanic HBr and HI to be on the order of 5–15 Gg yr-1 for HBr and 0.5–2 Gg yr-1 for HI, respectively. The uncertainties in emission fluxes given by the latter study are rather large, but the results for the halogen flux ratios agree with the present satellite observations within their error bars.

The interesting point is that the seawater abundance yields a ratio of Br / I of 15 000, i.e. a 4 order of magnitude difference between I and Br. By considering singly the IO and BrO observations, the number ratio between iodine and bromine atoms is thus enhanced by about 3 orders of magnitude in the volcanic plume as compared to seawater. Modelling of the halogen chemistry within the volcanic plume would be required to calculate the iodine and bromine amounts from the IO and BrO column observations. These estimates are performed under the given restrictions, and the present observations of volcanic emissions of IO shall encourage including the chemistry of iodine and iodine oxides in volcanic halogen chemistry modelling in the future. Other studies have shown that while the Cl vs. Br ratio for volcanic condensates is in agreement with the seawater ratio of around 650 , the ratio of Cl vs. I is about 2 orders of magnitude lower in volcanic plumes than in seawater . Consequently, an enhancement of iodine species takes place in the processes which determine the release of halogens from volcanic activity.

Explanations for the observed enhancement of iodine in volcanic emissions relative to seawater are connected to the magma composition of the specific volcano. As Kasatochi is an oceanic arc volcano, marine sediments which are carried into the Earth mantle at the subduction zone directly influence the composition of the volcanic material. Marine sediments in turn are enriched in iodine compounds from organic material . In addition, volcanic emissions are influenced by the composition of the melts and fluids in the volcanic chamber. Results of hydrothermal experiments were used to analyse the compositions of hydrous fluids and silicate melts with respect to the different halogens . It was found that the partition coefficient between fluid and melt is clearly larger for iodine than for bromine and chlorine. The partitioning into the fluid phase is therefore stronger for iodine than for bromine, which is again stronger than for chlorine. Consequently, volcanic emissions to the atmosphere are expected to be enhanced in iodine relative to the other halogens.

It is also of interest to study the temporal evolution of the observed IO and BrO column amounts within the plume. The observations on the first day after the eruption, however, may be influenced by dust and clouds accompanying the eruption, especially close to the volcano . Consequently, trace gas amounts could be larger than quantified by the spectroscopic observations. Comparison of the temporal behaviour of IO and BrO shows that their evolution is similar, with maximum integral amounts detected 1 to 3 days after the eruption (cf. Fig. ). BrO reaches its highest values (around 5×1029 molecules) earlier than IO. The different chemical pathways and time constants for IO and BrO production and destruction also influence the temporal variation of the I / Br ratio. However, the temporal changes between 9 and 12 August 2008 are close to the limit of being significant. Considering only the 1σ standard deviation of IO on the order of 2.5 × 1012 moleccm-2, the uncertainty on the integrated IO molecule number within the volcanic plume lies between 0.8 and 1.2×1028, using the plume areas from 9 and 11 August, respectively. As a result, details in the temporal evolution need to be interpreted with care. Overall, it is interesting to note that the ratio of observed IO and BrO (Fig. ) shows little change during the aging of the plume within the 5 analysed days. This observation may imply that higher iodine oxides, which are formed more rapidly at larger IO concentrations (cf. estimation of IO mixing ratios below), are photochemically labile inside the volcanic plume. Thereby, the IO may persist in the plume for a longer time period than what would be expected from the atmospheric lifetime of IO. The evolution of iodine species in the volcanic plume may be further affected by particle formation and heterogeneous reactions. measured enhanced iodine content in aerosols in the upper troposphere and lower stratosphere (UTLS) region. This finding has two further implications. Particles may serve as a sink for iodine, reducing the availability of reactive iodine, and, on the other hand, they may provide pathways for heterogeneous reactions from which reactive iodine compounds may be released again.

The spatial distributions of IO, BrO and SO2 are described in Sect. , and some differences between the three species are observed. The chemical pathways of iodine and bromine within the plume are probably not independent from each other. Formation and loss processes may interfere with each other. Although the rate coefficients for the reactions of I and Br with O3 are similar, the smaller expected concentrations of I than Br imply that the time constant for IO production is larger than that for BrO. As a consequence, large amounts of Br that react with O3, thereby strongly reducing the O3 abundance, may prevent the build-up of IO. This results in spatially separated maximum values for the two halogen oxides. The reactions between IO and BrO, as well as self reactions of IO, also impact the spatial distributions and maximum amounts. Furthermore, the time of emission of the precursor substances may differ to some degree. Iodine and bromine have different solubility in volcanic fluids . For the two halogen species, degassing from the magma may therefore take place at different pressures, i.e. at different depth of the volcanic abyss. In addition, some clear differences between the spatial distributions of the halogen oxides and SO2 are found. In general, the comparison between the trace gas spatial distributions is interesting because it potentially yields information on the eruption process and chronology. Details of the plume composition and evolution need to be analysed in the future by chemical transport modelling to provide better insight into the complex reactions taking place within the plume.

For an estimate of the impact of volcanic iodine on atmospheric chemistry, the volume mixing ratio is a more relevant quantity than the column amount. For a rough estimate, the vertical plume extent derived by is used. They determine the major part of the plume to reside between 8 and 12 km altitude. The retrieved integrated number of IO molecules of about 5×1028molecules, for 10 and 11 August 2008, is used as lower limit of the emitted iodine amount. On both days, the plume extends horizontally over 5 × 105km2. Spreading the observed IO homogeneously within the 4 km thick layer and over the entire plume extent, the average VMR would be around 3 pptv at 10 km altitude using US standard atmosphere pressure and temperature values. Certainly, local VMR values will exceed this average VMR due to an inhomogeneous distribution within the plume. Iodine mixing ratios of 3 pptv may have a strong impact on ozone concentrations and constitute a large perturbation of stratospheric iodine, which is measured and estimated to be on the sub-pptv level.

Iodine from volcanic eruptions has several possible implications for atmospheric composition. The upper part of the Kasatochi plume may have reached into the lower stratosphere. Consequently, the presented satellite-based observations of iodine monoxide indicate that volcanic eruptions may have an impact on the iodine concentrations in the upper troposphere and lower stratosphere, at least regionally.

The above estimated IO VMR of 3 pptv in the Kasatochi plume will be diluted with time. Spreading the released trace gas amount over the area of the entire globe decreases the VMR at the given altitude by 3 orders of magnitude as compared to the plume area. Consequently, strong implications for ozone depletion through iodine from a single volcanic eruption are probably mainly regional and restricted in time. Primarily, the lower stratosphere or UTLS region is affected. However, the region impacted by the emitted iodine may be dislocated from the erupting volcano due to the quickly moving volcanic plume covering distances of typically around several hundred km per day.

Due to the larger chain length for the removal of O3 by BrOx and IOx than by ClOx, loss of O3 in the stratosphere can be significantly impacted by the BrO and IO in addition to ClO released from volcanic eruptions. In this case the lower stratosphere may become most affected. This could impact on ozone hole chemistry when volcanic eruptions enter the polar vortices, an issue recently raised by .

Background iodine amounts between 0.1 and 0.4 pptv in the free troposphere as observed recently are possibly also influenced by volcanic activity. Following a volcanic eruption, the iodine amount will directly influence the local and regional chemistry by reducing the ozone levels. The impact of the ability of volcanic IO to form aerosol condensation nuclei requires further study. In addition, volcanic plumes may be subject to long-range transport and therefore lead to effects also at larger distances.

The Kasatochi eruption was in some respect special as it was a major eruption; the plume altitude was relatively large and bromine amounts were also larger than for other investigated volcanic plumes . IO has not yet been detected for any other eruptions investigated, at least not at the Kasatochi levels. Scaling with the observed bromine amounts, iodine levels for the other eruptions could just be below or around the detection limit of current space-based instruments. Future satellite instruments with finer spatial resolution and improved signal-to-noise ratio may allow the observation and detailed investigation of iodine species in volcanic plumes more frequently.

It is interesting to speculate on the amount of halogens emitted to the atmosphere from past major eruptions which have severely impacted atmospheric composition prior to halogen observations from space. For the Pinatubo eruption in 1991, for example, a total mass of about 20 Tg of SO2 was emitted. The eruption injected gases and aerosols up to 25–30 km altitude, i.e. around the maximum stratospheric ozone mixing ratio. In relative terms, the IO VMR will be increased at these high altitudes due to much lower air density as compared to the Kasatochi estimates. Assuming a similar magma composition as that of Kasatochi, i.e. similar halogen to sulfur ratios, an amount of around 100 t of IO as well as 1 kt of BrO could have been emitted into the stratosphere from Pinatubo, with corresponding impact on stratospheric chemistry over extended horizontal distances and periods. A detailed assessment again requires better knowledge and studies of the loss of iodine and bromine into the stratospheric aerosol.

Following the major eruption of the Kasatochi volcano in August 2008, iodine monoxide is observed by satellite in the volcanic plume for several days. This is the first experimental evidence of IO emitted from a volcanic eruption. The satellite sensors SCIAMACHY and GOME-2A both detect slant column amounts of IO above 4×1013 molec cm-2 in the volcanic plume for several days following the Kasatochi eruption. Maximum vertical columns above 2 × 1013 moleccm-2 are derived. The presented observations also represent the first reported retrievals of IO from measurements of the GOME-2A instrument. In comparison to tropospheric IO observations in polar and mid-latitudinal regions, the observed column amounts are large, reducing the uncertainties and facilitating analysis of individual measurements. The IO data in the plume shows good fitting quality with fitting errors around 6 % for SCIAMACHY and below 15 % for GOME-2A retrievals.

Overall, the IO enhancements coincide in space with previously published observations of BrO and SO2. While the plumes of IO, BrO and SO2 are roughly found in the same area with similar shape, the maximum amounts of the individual species, however, do not always coincide. Differences between IO and BrO are smaller than those between the halogens and SO2. The emission chronology as well as chemical conversions are presumably individual for the three compounds and could probably lead to the observed differences in spatial distributions.

Correlating all observations of IO and BrO between 9 to 12 August 2008 yields a slope of 0.09, i.e. IO amounts are about 1 order of magnitude smaller than those of BrO. Judging from the IO and BrO column amounts alone, this volcanic ratio indicates a 3 order of magnitude difference with respect to the seawater ratio between iodine and bromine, in agreement with previous filter measurements of volcanic samples at arc volcanos. For this relative enhancement of iodine, two reasons play a role. Iodine shows a stronger preference than bromine for partitioning into volcanic fluid than melt in the volcanic chamber located underneath the volcano. This relative partitioning between fluid and melt determines the gas-phase composition of an eruption plume. In addition, iodine-enriched marine sediments are carried into the Earth's mantle in the subduction zone and directly influence the composition of the magma.

An integration of the observed IO amount within the emission plume results in a large mass of around 10 t (4 to 12 t) of IO emitted from the volcano. By comparing the integrated numbers of IO and BrO molecules found within the volcanic plume, the BrO / IO number ratio ranges between 6.7 and 10.0, while the BrO / IO mass ratio lies between 4.0 and 6.7. Together with the knowledge that the Kasatochi BrO plume predominantly reached the altitude between 8 and 12 km, it can be concluded that a substantial input of iodine to the lower stratosphere, UTLS and free troposphere has taken place following the Kasatochi eruption. If the IO amount is homogeneously spread over the plume area and within the main 4 km thick vertical layer, a VMR of 3 pptv at an altitude of 10 km results. The local VMR can be even higher due to inhomogeneous distribution in the volcanic plume. Iodine volume mixing ratios of around 3 pptv may have substantial impact on atmospheric composition, e.g. through regionally reducing the ozone concentrations.

The investigation of past and future volcanic eruptions with respect to their IO content and impact on tropospheric and stratospheric chemistry is subject to further work and will be facilitated by improved satellite instrumentation in the future.