Full latitudinal marine atmospheric measurements of iodine monoxide

Iodine compounds destroy ozone (O3) in the global troposphere and form new aerosols, thereby affecting the global radiative balance. However, few reports have described the latitudinal distribution of atmospheric iodine compounds. This work reports iodine monoxide (IO) measurements over unprecedented sampling areas from Arctic to the Southern Hemisphere and spanning sea surface temperatures (SSTs) of approximately 0 °C to 31.5 °C. The highest IO concentrations were observed over the Western Pacific warm pool (WPWP), where O3 minima were also measured. There, negative correlation was found between O3 and IO mixing ratios at extremely low O3 concentrations. This correlation is not explained readily by the “O3-dependent” oceanic fluxes of photolabile inorganic iodine compounds, the dominant source in recent global-scale chemistry-transport models representing iodine chemistry, and rather implies that “O3-independent” pathways can be similarly important in the WPWP. The O3-independent fluxes result in a 15 % greater O3 loss than that estimated for O3-dependent processes alone. The daily O3 loss rate related to iodine over the WPWP is as high as approximately 2 ppbv despite low O3 concentrations of ~10 ppbv, with the loss being up to 100 % greater than that without iodine. This finding suggests that warming SST driven by climate change may affect the marine atmospheric chemical balance through iodine–ozone chemistry.

Mini ozone holes due to dust release of iodine

<p>Desert dust as a source of iron and other micronutrients is recognized to fertilize oceans, but little attention has been paid to dust as a source of iodine. Empirical observations find iodate on dust measured during ship cruises downwind of the Sahara desert. However, it remains unclear whether iodine in dust is the result of marine iodine uptake on dust during transport in the marine boundary layer, or whether such iodine accumulates over geological time scales, and is emitted together with dust. Significant enhancements of iodine have been observed in Sahara dust events in form of methyl iodide (CH<sub>3</sub>I) and iodine monoxide (IO) radicals, but atmospheric models currently do not consider dust as a source of iodine. Furthermore, dust plumes are often accompanied by significant ozone loss, which is commonly attributed to reactive uptake of O<sub>3</sub> and other odd oxygen species (i.e., N<sub>2</sub>O<sub>5</sub>, HNO<sub>3</sub>) on dust surfaces. However, laboratory experiments struggle to reproduce the large reactive uptake coefficients needed to explain field observations, and do not consider iodine chemistry. We present evidence that dust induced "mini ozone holes" in the remote (Southern Hemisphere) lower free troposphere west of South America (TORERO field campaign) are largely the result of gas-phase iodine chemistry in otherwise unpolluted (low NO<sub>x</sub>) dust layers that originate from the Atacama and Sechura Deserts. Ozone concentrations inside these elevated dust layers are often 10-20 ppb, and as low as 3 ppb, and influence entrainment of low ozone air from aloft into the marine boundary layer. Ozone depletion is found to be widespread, extending up to 6km altitude, and thousands of kilometers along the coast. Elevated IO radical concentrations inside decoupled dust layers are higher than in the marine boundary layer, and serve as a source of iodine, and vigorous ozone sink following entrainment to the marine boundary layer. The implications for our perception of iodine sources, surface air quality, oxidative capacity, and climate are briefly discussed.</p>

Analysing the retrieval quality and column densities of iodine monoxide from multiple satellite sensors

<p>Iodine compounds are emitted from the ocean and ice covered areas through organic and inorganic pathways involving macroalgae and microalgae as well as inorganic surface processes and volcanic eruptions. Iodine monoxide (IO) molecules are produced after photolysis of precursors and reaction with ozone. IO is thus an indicator of active iodine chemistry, and impacts on ozone levels, the NO/NO<sub>2</sub> ratio and particle formation. Rapid changes in Polar sea ice coverage and conditions may affect iodine levels in Polar Regions with respective consequences for tropospheric composition in the Arctic and Antarctic.</p><p>Remote sensing of IO faces the challenge that IO column densities are fairly small with a maximum absorption optical depth on the order of a few times 10<sup>-4</sup>, which is close to the detection limit of satellite instruments. IO column densities are retrieved from several satellite sensors including SCIAMACHY (2002 to 2012), GOME-2 (since 2006) and TROPOMI (since 2017) by using Differential Optical Absorption Spectroscopy. Previous studies have shown slightly enhanced IO column densities above the Antarctic Region and in a strong volcanic plume, while IO column densities in the Arctic remain mostly below the detection limit. These areas are in the focus of iodine measurements from space. Retrieval quality and resulting IO column densities are investigated and compared between the different sensors with a focus on the recent instrument TROPOMI.</p><p>The small IO absorption signal complicates the identification of optimal retrieval settings, such as the choice of an adequate wavelength window. Aspects for quality control are discussed. In addition to the immediate retrieval RMS, also the IO standard deviation in (reference) areas with expected low IO absorption, consistency checks with other retrieval parameters as well as plausibility of IO column density results are considered. Finally, the idea of an ensemble retrieval strategy is discussed, which is based on the fact that for small trace gas quantities, the retrieval result depends unfavourably on the fit settings. After selection of reasonable quality criteria, the remaining fit parameter sets are all used for the retrieval of IO. The selected ensemble of parameter sets yields a result for IO as well as uncertainty estimates induced by the choice of fit settings. Due to computational effort, application of this strategy is restricted to case studies.</p>

Retrieval quality and column densities of iodine monoxide from multiple satellite sensors – from SCIAMACHY to TROPOMI

<p>Iodine compounds are mainly emitted from the oceans through organic and inorganic pathways followed by photolysis and reaction with ozone to create iodine monoxide (IO) molecules. Emission sources of iodine species include the sea surface, phytoplankton and macroalgae as well as volcanic eruptions. IO is an indicator of active iodine chemistry, which may be relevant for tropospheric composition due to its impact on ozone levels, the NO/NO<sub>2</sub> ratio and potential particle formation. Rapid changes in Polar sea ice coverage and conditions may have an impact on iodine levels in Polar Regions with respective consequences for tropospheric composition in the Arctic and Antarctic.</p><p>Differential Optical Absorption Spectroscopy is used to retrieve IO column densities from various satellite sensors, including SCIAMACHY (2002 to 2012), GOME-2 (since 2006) and TROPOMI (since 2017). Case studies are presented with a focus on the intercomparison of the retrieval quality and IO column densities from the applied instruments. Previous satellite studies have shown slightly enhanced IO column densities mainly above the Antarctic Region and within one occasion of a strong volcanic plume, while IO column densities in the Arctic remain mostly below the detection limit of the applied instruments.</p><p>Reported column densities of tropospheric IO, as previously measured from ground and from space, are fairly small and close to the detection limits of current and former satellite sensors. Optical depth values of IO absorption are on the order of a few times 10<sup>-4</sup>. Individual satellite spectra allow trace gas retrievals with residual RMS values which lie around and often above the expected IO absorption optical depth. This is a challenge for the identification of optimal retrieval settings, especially the choice of an adequate wavelength window. Several aspects for quality control are discussed. In addition to the immediate retrieval RMS, the IO standard deviation in areas with expected low IO absorption, consistency checks with other retrieval parameters as well as plausibility of IO column density results are considered.</p>

Quantitative detection of iodine in the stratosphere

Oceanic emissions of iodine destroy ozone, modify oxidative capacity, and can form new particles in the troposphere. However, the impact of iodine in the stratosphere is highly uncertain due to the lack of previous quantitative measurements. Here, we report quantitative measurements of iodine monoxide radicals and particulate iodine (Iy,part) from aircraft in the stratosphere. These measurements support that 0.77 ± 0.10 parts per trillion by volume (pptv) total inorganic iodine (Iy) is injected to the stratosphere. These high Iy amounts are indicative of active iodine recycling on ice in the upper troposphere (UT), support the upper end of recent Iy estimates (0 to 0.8 pptv) by the World Meteorological Organization, and are incompatible with zero stratospheric iodine injection. Gas-phase iodine (Iy,gas) in the UT (0.67 ± 0.09 pptv) converts to Iy,part sharply near the tropopause. In the stratosphere, IO radicals remain detectable (0.06 ± 0.03 pptv), indicating persistent Iy,part recycling back to Iy,gas as a result of active multiphase chemistry. At the observed levels, iodine is responsible for 32% of the halogen-induced ozone loss (bromine 40%, chlorine 28%), due primarily to previously unconsidered heterogeneous chemistry. Anthropogenic (pollution) ozone has increased iodine emissions since preindustrial times (ca. factor of 3 since 1950) and could be partly responsible for the continued decrease of ozone in the lower stratosphere. Increasing iodine emissions have implications for ozone radiative forcing and possibly new particle formation near the tropopause.

Space-based observation of volcanic iodine monoxide

Volcanic eruptions inject substantial amounts of halogens into the atmosphere. Chlorine and bromine oxides have frequently been observed in volcanic plumes from different instrumental platforms such as from ground, aircraft and satellites. The present study is the first observational evidence that iodine oxides are also emitted into the atmosphere during volcanic eruptions. Large column amounts of iodine monoxide, IO, are observed in satellite measurements following the major eruption of the Kasatochi volcano, Alaska, in 2008. The IO signal is detected in measurements made both by SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY) on ENVISAT (Environmental Satellite) and GOME-2 (Global Ozone Monitoring Experiment-2) on MetOp-A (Meteorological Operational Satellite A). Following the eruption on 7 August 2008, strongly elevated levels of IO slant columns of more than 4 × 1013 molec cm−2 are retrieved along the volcanic plume trajectories for several days. The retrieved IO columns from the different instruments are consistent, and the spatial distribution of the IO plume is similar to that of bromine monoxide, BrO. Details in the spatial distribution, however, differ between IO, BrO and sulfur dioxide, SO2. The column amounts of IO are approximately 1 order of magnitude smaller than those of BrO. Using the GOME-2A observations, the total mass of IO in the volcanic plume injected into the atmosphere from the eruption of Kasatochi on 7 August 2008, is determined to be on the order of 10 Mg.