The role of ice in the formation of chemically active halogens in the environment requires a full understanding because of its role in atmospheric chemistry, including controlling the regional atmospheric oxidizing capacity in specific situations. In particular, ice and snow are important for facilitating multiphase oxidative chemistry and as media upon which marine algae live. This paper reviews the nature of environmental ice substrates that participate in halogen chemistry, describes the reactions that occur on such substrates, presents the field evidence for ice-mediated halogen activation, summarizes our best understanding of ice-halogen activation mechanisms, and describes the current state of modeling these processes at different scales. Given the rapid pace of developments in the field, this paper largely addresses advances made in the past five years, with emphasis given to the polar boundary layer. The integrative nature of this field is highlighted in the presentation of work from the molecular to the regional scale, with a focus on understanding fundamental processes. This is essential for developing realistic parameterizations and descriptions of these processes for inclusion in larger scale models that are used to determine their regional and global impacts

Volatile halogenated organic compounds containing bromine and iodine, which are naturally produced in the ocean, are involved in ozone depletion in both the troposphere and stratosphere. Three prominent compounds transporting large amounts of marine halogens into the atmosphere are bromoform (CHBr₃), dibromomethane (CH₂Br₂) and methyl iodide (CH₃I). The input of marine halogens to the stratosphere has been estimated from observations and modelling studies using low-resolution oceanic emission scenarios derived from top-down approaches. In order to improve emission inventory estimates, we calculate data-based high resolution global sea-to-air flux estimates of these compounds from surface observations within the HalOcAt (Halocarbons in the Ocean and Atmosphere) database (https://halocat.geomar.de/). Global maps of marine and atmospheric surface concentrations are derived from the data which are divided into coastal, shelf and open ocean regions. Considering physical and biogeochemical characteristics of ocean and atmosphere, the open ocean water and atmosphere data are classified into 21 regions. The available data are interpolated onto a 1°×1° grid while missing grid values are interpolated with latitudinal and longitudinal dependent regression techniques reflecting the compounds' distributions. With the generated surface concentration climatologies for the ocean and atmosphere, global sea-to-air concentration gradients and sea-to-air fluxes are calculated. Based on these calculations we estimate a total global flux of 1.5/2.5 Gmol Br yr⁻¹ for CHBr₃, 0.78/0.98 Gmol Br yr⁻¹ for CH₂Br₂ and 1.24/1.45 Gmol Br yr⁻¹ for CH₃I (robust fit/ordinary least squares regression techniques). Contrary to recent studies, negative fluxes occur in each sea-to-air flux climatology, mainly in the Arctic and Antarctic regions. "Hot spots" for global polybromomethane emissions are located in the equatorial region, whereas methyl iodide emissions are enhanced in the subtropical gyre regions. Inter-annual and seasonal variation is contained within our flux calculations for all three compounds. Compared to earlier studies, our global fluxes are at the lower end of estimates, especially for bromoform. An under-representation of coastal emissions and of extreme events in our estimate might explain the mismatch between our bottom-up emission estimate and top-down approaches

Hitherto unnoticed biogenic volatile iodated hydrocarbons have been determined in open ocean and coastal surface waters. The results indicate that iodated substances are produced by planktonic organisms rather than by macroalgae. It is shown that the diversity of iodated organic compounds released in the seawater is larger than was previously suggested. In the global cycling of iodine it is necessary to consider organic compounds other than methyl iodide.

Eucheuma denticulatum has been shown to release bromoform, diiodomethane, dibromochloromethane, perchloroethylene, chloroiodomethane, chloroform, sec-butyl iodide, methyl iodide, methylchloroform, carbon tetrachloride, trichloroethylene and butyl iodide into its growth media, with bromoform (310 4-25 #g kg DW -~ h -1) and diiodomethane (182 q-9 #g kg DW -~ h -I) being the dominant volatile hatocarbons (VHCs). The production of VHCs was always higher at a photon flux density of 1500 than at 400 #tool photon m -2 s -1. The influence of pH was minimal at 400 #tool photon m -2 s -1. The addition of azide decreased mean VHC production at pH 8"2 4-0"2 and 8"8. Algae kept in media where extracellular hydrogen peroxide was decomposed by addition of manganese dioxide showed a decrease in VHC production at pH 8"2 q-0"2 and an increase at pH 8"8. We suggest that high light intensity and carbon dioxide deficiency caused by high pH in E. denticulatum promote VHC production through induction of hydrogen peroxide synthesis.Key words: bromoform, Eucheuma denticulatum, halocarbons, hydrogen peroxide, red algae, sodium azide. IntroductionRecent Arctic field studies, especially during spring, have established that there are changes in the ozone composition of the lower Arctic atmosphere (0-2 km) resulting from an increased tropospheric halogen content, notably of bromine, with algal volatile halocarbons (VHCs) being suspected to be a contributory source (Barrie et al., 1988;Sturges et al., 1992). Depending upon the atmospheric lifetime of a particular VHC released into the troposphere, photolysis can produce halogen atoms which catalyse stratospheric ozone removal (Molina & Rowland, 1974). Bromine is considered to be a more efficient catalyst than chlorine in destroying ozone (Prather et al., 1984), and consequently the marine organisms contributing to the production of bromo-compounds are thought to have an important potential influence on the composition of the stratosphere.Halo-peroxidases are known to catalyse the synthesis of halogenated compounds (Krenn et al., 1989; Wever et al., 1991). The synthesis occurs in the presence of hydrogen peroxide (HzO2), the respective halide ion and a suitable acceptor (for example, fl-keto acids, cyclic fldiketones and substituted phenols: Hager et al., 1966 that VHCs are cleavage products of halogenated C3 or C4 ketones. Such polyhaloketones are widely accepted to be toxic, and their presence has also been reported in some prolific VHC producers (Fenical, 1974). The bromoperoxidase from the red alga Bonnemaisonia hamifera Harlot (Bonnemaisoniaceae) can, for instance, produce dibromomethane, bromoform and pentylbromide in vitro from bromide ions, H20 2 and 3-oxooctanoic acid (Theiler et aL, 1978).For a metabolite system to be effective for defence purposes or in an allelopathic interaction, the distribution of producer enzymes, metabolite precursors or the metabolites themselves should ensure that metabolites can easily reach the grazers, parasites or competitors. From the literature on alg...