Halogen Activation Research Articles

Influence of wave activity on the composition of the polar stratosphere

The planetary wave impact on the polar vortex stability, polar stratosphere temperature, and content of ozone and other gases was simulated with the global chemical–climatic model of the lower and middle atmosphere. It was found that the planetary waves propagating from the troposphere into the stratosphere differently affect the gas content of the Arctic and Antarctic stratosphere. In the Arctic region, the degree of wave activity critically affects the polar vortex formation, the appearance of polar stratospheric clouds, the halogen activation on their surface, and ozone anomaly formation. Ozone anomalies in the Arctic region as a rule are not formed at high wave activity and can be registered at low activity. In the Antarctic Regions, wave activity affects the stability of polar vortex and the depth of ozone holes, which are formed at almost any wave activity, and the minimal ozone values depend on the strong or weak wave activity that is registered in specific years.

Report on the Photochemical Induced Halogen Activation of Fecontaining Aerosols

Iron (Fe) occurs in minerals and highly saline media (e.g., sea salt aerosol, salt brines etc.) and induces photochemical processes. High salinity, low pH, and organic constituents promote the dissolution of iron, forming photosensitive complexes that are responsible for a production of gaseous Cl atoms when irradiated by sunlight. We studied the production of atomic Cl, Br and OH radicals from modeled salt pans and artificial sea-salt aerosols containing Fe(III) ions or pyrogenic Fe2O3 particles (Sicotrans Orange, BASF) at various compositions in a Teflon smog-chamber. The samples were either spread on a Teflon sheet or they were nebulized from dilute brines (most abundant particle diameter: ~0.4 μm, initial surface area: up to 3·10-2cm2cm-3) and exposed to simulated sunlight at 60-90% relative humidity. The photochemical formation of Cl, OH (and Br if possible) in the gas phase was quantified by the radical clock method resulting in time profiles of the radical-production rates and total productions. Simultaneous monitoring of the aerosol surface area enabled us to determine the initial Cl production rate per cm2 aerosol surface. Whereas no significant Cl production was detected employing a molar Cl-/Fe(III) ratio of 955, the rate increased to ~2.8·1017 atoms cm-2 s-1 (at a ratio of 101), ~4.1·1017 atoms cm-2 s-1 (at a ratio of 53) and ~1.9·1018 atoms cm-2 s-1 (at a ratio of 13). The presence of NO2 (~20 ppb) or O3 (630 ppb) in the gas phase additionally increased the Fe(III)-induced chloride activation to ~7·1018 atoms cm-2 s-1 and ~9·1018 atoms cm-2 s-1 (at a Cl-/Fe(III) ratio of 13), respectively. SO2 slightly restrained the Cl formation. Artificial sea salt aerosols doped with Fe2O3 particles (Cl-/Fetot~13) did not result in detectable Cl concentrations. However, decreasing the pH below 2 favored the dissolution of Fe and led to Cl production rates comparable to the Fe(III) experiments (~7·1018 atoms cm-2 s-1) whereas accelerating the erosion of the particles by freezing and thawing did not lead to detectable results. These observations are in accord with the speciation of the photolabile, aqueous Fe(III) complexes obtained from an equilibrium model (PHREEQC). The aerosol experiments result in much higher effective Cl production rates with ~50% of active Fe(III) in zero air, as compared to the previous salt-pan experiments in the same Teflon smog-chamber with a portion of 0.05-0.07% of active Fe(III)). This is caused by the larger active surface area of the homogeneously distributed aerosols.

SYNTHESIS OF SUBSTITUTED QUINOLINE-3-CARBALDEHYDES

Dehalogenation of а series substituted 2-chloroquinoline-3-carbaldehydes was investigated. It was found that zinc dust in alkali ethanol practically do not react with 2-chloro-3-(1,3-dioxolan)-2-yl-7-methylquinolines for a 5 day at ambient temperature. Increase temperature to boiling point of reaction mixture lead to increase yield 7-methylquinoline-3-carbaldehyde to 12.5%. This reaction with 2-chloro-3-dimetoxymetyl-7-methylquinoline give 26.5 above aldehyde. Replacement of ethanol to methanol give only traces of desired aldehyde. Neither 2-chloro-7-methylquinoline-3-carbaldehyde, nor his acetals do not react with sodium dithionite. Furthermore for activation of halogen we replaced chloraldehydes to iodoaldehydes by Finkelstein reaction, then protect aldehyde function by formation of dimethylacetals and treatment his by sodium dithionite in mixture pyridine-water. Desired aldehydes was obtained with poor to moderate yields. Increasing yields of this reaction is goal our further investigations.

Development of a chlorine chemistry module for the Master Chemical Mechanism

Abstract. The chlorine atom (Cl·) has a high potential to perturb atmospheric photochemistry by oxidizing volatile organic compounds (VOCs), but the exact role it plays in the polluted troposphere remains unclear. The Master Chemical Mechanism (MCM) is a near-explicit mechanism that has been widely applied in the atmospheric chemistry research. While it addresses comprehensively the chemistry initiated by the OH, O3 and NO3 radicals, its representation of the Cl· chemistry is incomplete as it only considers the reactions for alkanes. In this paper, we develop a more comprehensive Cl· chemistry module that can be directly incorporated within the MCM framework. A suite of 205 chemical reactions describes the Cl·-initiated degradation of alkenes, aromatics, alkynes, aldehydes, ketones, alcohols, and some organic acids and nitrates, along with the inorganic chemistry involving Cl· and its precursors. To demonstrate the potential influence of the new chemistry module, it was incorporated into a MCM box model to evaluate the impacts of nitryl chloride (ClNO2), a product of nocturnal halogen activation by nitrogen oxides (NOX), on the following day's atmospheric photochemistry. With constraints of recent observations collected at a coastal site in Hong Kong, southern China, the modeling analyses suggest that the Cl· produced from ClNO2 photolysis may substantially enhance the atmospheric oxidative capacity, VOC oxidation and O3 formation, particularly in the early morning period. The results demonstrate the critical need for photochemical models to include more detailed chlorine chemistry in order to better understand the atmospheric photochemistry in polluted environments subject to intense emissions of NOX, VOCs and chlorine-containing constituents.

Element mobility patterns in magnetite-group IOCG systems: The Fab IOCG system, Northwest Territories, Canada

This paper documents element mobility patterns from a magnetite-group Iron Oxide Copper–Gold (IOCG) prospect in the Northwest Territories of Canada and explores implications for space–time chemical evolution of metasomatic systems hosting IOCG deposits. The Fab system, located in the Great Bear magmatic zone (GBMZ) of the Northwest Territories, Canada, contains numerous Fe–Cu–U showings associated with high temperature (HT) potassic–iron alteration overprinting extensive zones of sodic to HT calcic–iron alteration. Each hydrothermal alteration assemblage is associated with distinct element mobility patterns that record evolving physico-chemical properties of the hydrothermal fluids. New geochronological data constrain the alteration and IOCG mineralization in the Fab system to a 3m.y. window between 1870–1867Ma, which is broadly contemporaneous with extensive high-level intrusive activity across the GBMZ. Regional- to local-scale element mobility patterns characteristic of the sodic and sodic–calcic–iron alteration type record leaching combined with weak to strong mass losses. Pure sodic alteration depleted the rocks in Ca, Co, Cu, Fe, Mg, Th, U and V. Conversely, sodic–calcic–iron alteration records significant depletions of Nb, REE, Ta, Ti, Th and U. These element mobility patterns differ from intense HT calcic–iron alteration that is enriched in Ca, Co, F, Fe, Mg, Mn, Ni and V with modest enrichments to locally significant mineralization in Th, U and REE. HT calcic–iron alteration is also characterized by substantial mass gains that translate into volume gains in stockwork zones and mass/volume gains in zones of intense host rock replacement. HT potassic–iron alteration is characterized by enrichments in Ba, K, Ni, U and V, along with locally Co and Cu. The temporal and spatial association of the Fab system alteration and the emplacement of the porphyritic dacite are indicative of the predominant involvement of magmatic–hydrothermal fluids. The high F- and Cl- contents of the porphyritic dacite and of the HT calcic–iron alteration zones as well as Nb, REE, Ta, Th, and Ti mobility provide strong evidences of high halogen activities (F and Cl) in the hydrothermal fluids. High F- and Cl-activities in the hydrothermal fluid are interpreted to have facilitated the mobilization of normally immobile (Nb, Ta, Ti, Th) or weakly mobile elements as well as some metals (e.g., V, Ni, Co). The formation of REE fluorocarbonates and calcite in the early and incipient HT calcic–iron alteration zones indicates the presence of CO2 in the hydrothermal fluids. Weaker HFSE, HREE and Ti mobility during later HT potassic–iron alteration is interpreted to reflect decreasing temperatures, pressures, F-activities and increasing fO2 as the fluids evolved and interacted with the host rocks.

Kinetic method for assaying the halogenating activity of myeloperoxidase based on reaction of celestine blue B with taurine halogenamines

Myeloperoxidase (MPO) is a challenging molecular target which, if put under control, may allow regulating the development of inflammatory reactions associated with oxidative/halogenative stress. In this paper, a new kinetic method for assaying the halogenating activity of MPO is described. The method is based on measuring the rate of iodide-catalyzed oxidation of celestine blue B (CB) by oxygen and taurine N-chloramine (bromamine). The latter is produced in a reaction of taurine with HOCl (HOBr). CB is not a substrate for the peroxidase activity of MPO and does not react with hydrogen peroxide and superoxide anion radical. Taurine N-chloramine (bromamine) reacts with CB in molar ratio of 1:2. Using the new method, we studied the dependence of MPO activity on concentration of substrates and inhibitors. The specificity of MPO inhibition by non-proteolyzed ceruloplasmin is characterized. The inhibition of taurine N-chloramine production by neutrophils and HL-60 cells in the presence of MPO-affecting substances is demonstrated. The new method allows determining the kinetic parameters of MPO halogenating activity and studying its inhibition by various substances, as well as screening for potential inhibitors of the enzyme.

Dependence of the vertical distribution of bromine monoxide in the lower troposphere on meteorological factors such as wind speed and stability

Abstract. Multiple axis differential absorption spectroscopy (MAX-DOAS) measurements of bromine monoxide (BrO) probed the vertical structure of halogen activation events during March–May 2012 at Barrow, Alaska. An analysis of the BrO averaging kernels and degrees of freedom obtained by optimal-estimation-based inversions from raw MAX-DOAS measurements reveals the information is best represented by reducing the retrieved BrO profile to two quantities: the integrated column from the surface through 200 m (VCD200 m), and the lower tropospheric vertical column density (LT-VCD), which represents the integrated column of BrO from the surface through 2 km. The percentage of lower tropospheric BrO in the lowest 200 m was found to be highly variable ranging from shallow layer events, where BrO is present primarily in the lowest 200 m, to distributed column events where BrO is observed at higher altitudes. The highest observed LT-VCD events occurred when BrO was distributed throughout the lower troposphere, rather than concentrated near the surface. Atmospheric stability in the lowest 200 m influenced the percentage of LT-VCD that is in the lowest 200 m, with inverted temperature structures having a first-to-third quartile range (Q1–Q3) of VCD200 m/LT-VCD from 15–39%, while near-neutral-temperature structures had a Q1–Q3 range of 7–13%. Data from this campaign show no clear influence of wind speed on either lower tropospheric bromine activation (LT-VCD) or the vertical distribution of BrO, while examination of seasonal trends and the temperature dependence of the vertical distribution supported the conclusion that the atmospheric stability affects the vertical distribution of BrO.

Rapid and reliable determination of the halogenating peroxidase activity in blood samples

By combining easy and fast leukocyte enrichment with aminophenyl-fluorescein (APF) staining we developed a method to quickly and specifically address the halogenating activity of the immunological relevant blood heme peroxidases myeloperoxidase and eosinophil peroxidase, respectively. For leukocyte enrichment a two-fold hypotonic lysis procedure of the blood with Millipore water was chosen which represents a cheap, fast and reliable method to diminish the amount of erythrocytes in the samples. This procedure is shown to be suitable both to human and murine blood micro-samples, making it also applicable to small animal experiments with recurring blood sampling. As all types of leukocytes are kept in the sample during the preparation, they can be analysed separately after discrimination during the flow cytometry analysis. This also holds for all heme peroxidase-containing cells, namely neutrophils, eosinophils and monocytes. Moreover additional parameters (e.g. antibody staining) can be combined with the heme peroxidase activity determination to gain additional information about the different immune cell types. Based on previous results we applied APF for specifically addressing the halogenating activity of leukocyte peroxidases in blood samples. This dye is selectively oxidized by the MPO and EPO halogenation products hypochlorous and hypobromous acid. This approach may provide a suitable tool to gain more insights into the immune-physiological role of the halogenating activity of heme peroxidases.

Proatherogenic modification of LDL by surface-bound myeloperoxidase

One of the factors promoting oxidative/halogenating modification of low-density lipoproteins (LDL) is myeloperoxidase (MPO). We have shown previously that MPO binds to the LDL surfaces. The LDL–MPO complex is uncoupled in the presence of peptide EQIQDDCTGDED that corresponds to a fragment of apoB-100 (445–456). In this paper we studied how this peptide, as well as inhibitors and modulators of halogenating activity of MPO such as ceruloplasmin (CP), 4-aminobenzoic acid hydrazide (ABAH) and thiocyanate (SCN−) affect the accumulation of cholesterol and its esters in monocytes/macrophages after incubation with LDL subjected to different kinds of MPO-dependent oxidative/halogenating modification. In the presence of H2O2 and halides MPO causes stronger proatherogenic modification of LDL than exogenous reactive halogen species (HOCl and HOBr). Both monocytes, which differentiate into macrophages, and neutrophils secrete MPO in response to the presence of damaged LDL. The peptide EQIQDDCTGDED preventing interaction between MPO and LDL reduces the uptake of modified LDL and MPO by monocytes/macrophages and thus precludes the accumulation of intracellular cholesterol. Our results indicate that binding to MPO is important for LDL to become modified and acquire proatherogenic properties. The peptide EQIQDDCTGDED, CP, ABAH, and SCN− can play the role of anti-atherogenic factors reducing the deleterious effect of catalytically active MPO on LDL and accumulation of cholesterol in macrophages.

Presence of high nitryl chloride in Asian coastal environment and its impact on atmospheric photochemistry

Nitryl chloride (ClNO2) is a product of nocturnal halogen activation of nitrogen oxides and has potential to impact atmospheric photochemistry. Here we report the first observation result of ClNO2 in an Asian environment. The measurements were made with a thermal decomposition chemical ionization mass spectrometer at a coastal site in southern China during late summer of 2012. A clear diurnal pattern of ClNO2 was observed with nighttime 1-min average mixing ratio up to 1997 pptv, which is among the highest values ever reported in the world. The elevated ClNO2 mixing ratios were observed in plumes from Hong Kong and the Pearl River Delta region compared to the lower levels in maritime air. Calculations suggest that the production of chlorine atom from photolysis of ClNO2 in the early morning exceeds the production of hydroxyl radical via ozone photolysis by several factors. Therefore, ClNO2 plays an important role in jump-starting the atmospheric photochemistry in polluted marine boundary layer of southern China.

Potent Reversible Inhibition of Myeloperoxidase by Aromatic Hydroxamates

The neutrophil enzyme myeloperoxidase (MPO) promotes oxidative stress in numerous inflammatory pathologies by producing hypohalous acids. Its inadvertent activity is a prime target for pharmacological control. Previously, salicylhydroxamic acid was reported to be a weak reversible inhibitor of MPO. We aimed to identify related hydroxamates that are good inhibitors of the enzyme. We report on three hydroxamates as the first potent reversible inhibitors of MPO. The chlorination activity of purified MPO was inhibited by 50% by a 5 nm concentration of a trifluoromethyl-substituted aromatic hydroxamate, HX1. The hydroxamates were specific for MPO in neutrophils and more potent toward MPO compared with a broad range of redox enzymes and alternative targets. Surface plasmon resonance measurements showed that the strength of binding of hydroxamates to MPO correlated with the degree of enzyme inhibition. The crystal structure of MPO-HX1 revealed that the inhibitor was bound within the active site cavity above the heme and blocked the substrate channel. HX1 was a mixed-type inhibitor of the halogenation activity of MPO with respect to both hydrogen peroxide and halide. Spectral analyses demonstrated that hydroxamates can act variably as substrates for MPO and convert the enzyme to a nitrosyl ferrous intermediate. This property was unrelated to their ability to inhibit MPO. We propose that aromatic hydroxamates bind tightly to the active site of MPO and prevent it from producing hypohalous acids. This mode of reversible inhibition has potential for blocking the activity of MPO and limiting oxidative stress during inflammation.

Nickel‐catalyzed three‐component coupling reaction of terminal alkynes, dihalomethane and amines to propargylamines

Direct C―H and C―halogen activation is an important and practical task in C―C, C―N bond formation reactions using alkynes. Propargylic amines are synthetically versatile intermediates for the preparation of many nitrogen‐containing biologically active motifs. Herein, a 15 mol% Ni(py)4Cl2/bipyridine‐catalyzed three‐component coupling reaction of alkynes, halomethane and amines through C―H and C―halogen activation was developed for the facile synthesis of propargylic amines. Tetramethylguanidine shows excellent basicity in acetonitrile and works under mild conditions. The reaction has very good functional group tolerance to aliphatic and aromatic alkynes. Copyright © 2013 John Wiley & Sons, Ltd.

Photochemical chlorine and bromine activation from artificial saline snow

Abstract. The activation of reactive halogen species – particularly Cl2 – from sea ice and snow surfaces is not well understood. In this study, we used a photochemical snow reactor coupled to a chemical ionization mass spectrometer to investigate the production of Br2, BrCl and Cl2 from NaCl/NaBr-doped artificial snow samples. At temperatures above the NaCl-water eutectic, illumination of samples (λ > 310 nm) in the presence of gas phase O3 led to the accelerated release of Br2, BrCl and the release of Cl2 in a process that was significantly enhanced by acidity, high surface area and additional gas phase Br2. Cl2 production was only observed when both light and ozone were present. The total halogen release depended on [ozone] and pre-freezing [NaCl]. Our observations support a "halogen explosion" mechanism occurring within the snowpack, which is initiated by heterogeneous oxidation and propagated by Br2 or BrCl photolysis and by recycling of HOBr and HOCl into the snowpack. Our study implicates this important role of active chemistry occurring within the interstitial air of aged (i.e. acidic) snow for halogen activation at polar sunrise.

Heterogeneous formation of nitryl chloride and its role as a nocturnal NOx reservoir species during CalNex‐LA 2010

AbstractThe nocturnal conversion of dinitrogen pentoxide (N2O5) to nitryl chloride (ClNO2) on chloride‐containing aerosol can be a regionally important NOx (= NO + NO2) recycling and halogen activation pathway that affects oxidant photochemistry the following day. Here we present a comprehensive measurement data set acquired at Pasadena, California, during the CalNex‐LA campaign 2010 that included measurements of odd nitrogen and its major components (NOy = NOx + NO3 + 2N2O5 + ClNO2 + HNO3 + HONO + peroxyacyl, alkyl, and aerosol nitrates) and aerosol size distribution and composition. Nitryl chloride was present during every night of the study (median mixing ratio at sunrise 800 pptv) and was usually a more significant nocturnal NOx and odd oxygen (Ox = O3 + NO2 + 3N2O5 + ClNO2) reservoir species than N2O5 (whose concentrations were calculated from its equilibrium with NO2 and NO3). At sunrise, ClNO2 accounted for 21% of NOz (=NOy − NOx), 4% of NOy, and 2.5% of Ox, respectively (median values). Kinetic parameters for the N2O5 to ClNO2 conversion were estimated by relating ClNO2 concentrations to their time‐integrated heterogeneous production from N2O5 and were highly variable between nights. Production of ClNO2 required conversion of N2O5 on submicron aerosol with average yield (φ) and N2O5 reactive uptake probability (γ) of γφ = 0.008 (maximum 0.04), scaled with submicron aerosol chloride content, and was suppressed by aerosol organic matter and liquid water content. Not all of the observed variability of ClNO2 production efficiency could be rationalized using current literature parameterizations.

N2O5 uptake coefficients and nocturnal NO2 removal rates determined from ambient wintertime measurements

Heterogeneous N2O5 uptake onto aerosol is the primary nocturnal path for removal of NOx (= NO + NO2) from the atmosphere and can also result in halogen activation through production of ClNO2. The N2O5 uptake coefficient has been the subject of numerous laboratory studies; however, only a few studies have determined the uptake coefficient from ambient measurements, and none has been focused on winter conditions, when the portion of NOx removed by N2O5 uptake is the largest. In this work, N2O5 uptake coefficients are determined from ambient wintertime measurements of N2O5 and related species at the Boulder Atmospheric Observatory in Weld County, CO, a location that is highly impacted by urban pollution from Denver, as well as emissions from agricultural activities and oil and gas extraction. A box model is used to analyze the nocturnal nitrate radical chemistry and predict the N2O5 concentration. The uptake coefficient in the model is iterated until the predicted N2O5 concentration matches the measured concentration. The results suggest that during winter, the most important influence that might suppress N2O5 uptake is aerosol nitrate but that this effect does not suppress uptake coefficients enough to limit the rate of NOx loss through N2O5 hydrolysis. N2O5 hydrolysis was found to dominate the nocturnal chemistry during this study consuming ~80% of nocturnal gas phase nitrate radical production. Typically, less than 15% of the total nitrate radical production remained in the form of nocturnal species at sunrise when they are photolyzed and reform NO2.

Purification of chloroperoxidase from Musa paradisiaca stem juice

AbstractChloroperoxidase from Musa paradisiaca stem juice has been purified to homogeneity using a concentration obtained by ultrafiltration and anion exchange chromatography on diethylaminoethyl (DEAE) cellulose. The purified enzyme gave a single protein band in SDS‐PAGE analysis corresponding to molecular mass of 43 kDa. The native PAGE analysis result has also given a single protein band, confirming the purity of the enzyme. The purified enzyme was chlorinated and brominated with monochlorodimedone, the substrate used for measuring the halogenating activity of chloroperoxidases. The Km and kcat values using monochlorodimedone as the substrate were 20 μM and 1.64 s−1, respectively, giving a kcat/Km value of 8.2 × 104 M−1 s−1. The pH and temperature optima of the chlorinating activity were 3.0 and 25°C, respectively. The Km values for the peroxidase activity using pyragallol and H2O2 as the variable substrates were 89 and 120 μM, respectively. The pH and temperature optima of the peroxidase activity using pyrogalllol as the substrate were the same as the pH and temperature optima of the halogenating activity. The peroxidase activity of the enzyme is competitively inhibited by sodium azide, indicating that it is a hemeperoxidase different from nonheme peroxidases. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: 92–100, 2013

ChemInform Abstract: Quantifying Trace Gas Uptake to Tropospheric Aerosol: Recent Advances and Remaining Challenges

AbstractReview: 270 refs.

Dihaloiodates(I): synthesis with hydrogen peroxide and their halogenating activity

Tetraalkylammonium and pyridinium dichloro- and dibromoiodates(I) were efficiently prepared from iodine and tetralkylammonium chloride or pyridine by oxidation with hydrogen peroxide in the presence of an equimolar amount of a hydrogen halide. Their halogenating activity was tested on 1,3-dimethoxybenzene and styrene as model substrates. The results show that ICl2− salts acted as iodinating reagents, while IBr2− salts brominated both substrates.

The Fluorescein-derived Dye Aminophenyl Fluorescein Is a Suitable Tool to Detect Hypobromous Acid (HOBr)-producing Activity in Eosinophils

The specific detection of peroxidase activity in human granulocytes is essential to elucidate their role in innate immune responses, immune regulation, and inflammatory diseases. The halogenating activity of myeloperoxidase in neutrophils can be determined by the novel fluorescent probe aminophenyl fluorescein (APF). Thereby non-fluorescent APF is oxidized by HOCl to form fluorescein. We successfully verified that APF equally detects the hypobromous acid (HOBr)-producing activity of eosinophil granulocytes. This was revealed by three different approaches. First, we investigated the conversion of non-fluorescent APF into fluorescein by HOCl and HOBr by means of fluorescence and mass spectrometry approaches. Thereby comparable chemical mechanisms were observed for both acids. Furthermore in vitro kinetic studies were used to detect the halogenating activity of myeloperoxidase and eosinophil peroxidase by using APF. Here the dye well reflected the different substrate specificities of myeloperoxidase and eosinophil peroxidase regarding chloride and bromide. Finally, peroxidase activities were successfully detected in phorbol ester-stimulated neutrophils and eosinophils using flow cytometry. Thereby inhibitory studies confirmed the peroxidase-dependent oxidation of APF. To sum up, APF is a promising tool for further evaluation of the halogenating activity of peroxidases in both neutrophils and eosinophils.

Halogen activation via interactions with environmental ice and snow in the polar lower troposphere and other regions

Abstract. 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.