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Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 1 Halogen Oxides in the Troposphere measurements, distributions, and impacts Lecture at the.

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1 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 1 Halogen Oxides in the Troposphere measurements, distributions, and impacts Lecture at the ERCA 2008 Grenoble, January 14, 2008 Andreas Richter Institute of Environmental Physics University of Bremen Bremen, Germany ( richter@iup.physik.uni-bremen.de )richter@iup.physik.uni-bremen.de

2 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 2 Overview 1.Introduction 2.Reminder: Stratospheric Halogen Chemistry 3.Some Tropospheric Halogen Chemistry 4.Polar Ozone Depletion Events 5.Salt Lakes 6.The marine Boundary Layer 7.Halogen Emissions from Volcanoes 8.BrO in the free Troposphere 9.Iodine and Particle Formation 10.Iodine in the Polar Troposphere 11.Summary

3 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 3 Halogen Oxides in the Troposphere www.gdargaud.net/Climbing/AntarClimb.html http://seaweed.ucg.ie

4 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 4 Stratospheric Ozone Chemistry Chapman cycle O 2 + hν → O( 3 P) + O( 3 P) O + O 2 + M → O 3 + M O 3 + hν → O 2 + O( 1 D) O + O 3 → 2 O 2 => too large ozone columns! Catalytic ozone loss (HO x, NO x ) XO + O 3 → XO 2 + O 2 XO 2 + O → XO + O 2 net: O 3 + O → 2 O 2 => good agreement with measurements Halogens can also act as catalysts sources: –anthropogenic (CFCs, CH 3 Br) –natural (CH 3 Br) sinks: –reservoir species (e.g. HCl, ClONO 2 ) –sedimentation => slow ozone depletion at high altitudes, mainly through Cl F all bound in HF Br and I more reactive but much lower concentrations Cl y ~ 3.5ppb, Br y ~ 16ppt, I y < 1ppt

5 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 5 Stratospheric Ozone Hole at very low temperatures. PSCs can form they consist of ice, HNO 3, H 2 SO 4 at their surface heterogeneous reactions take place inactive halogen reservoirs are converted into reactive forms rapid non-linear ozone loss in the lower stratosphere in polar spring at low temperatures ClO dimer cycle ClO + ClO + M → ClOOCl + M ClOOCl + hν → ClOO + Cl ClOO + M → Cl + O 2 2(O 3 + Cl → ClO + O 2 ) net: 2O 3 →3O 2 ClO - BrO cycle O 3 + Cl → ClO + O 2 O 3 + Br → BrO + O 2 ClO + BrO → Cl + Br + O 2 net: 2O 3 →3O 2

6 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 6 Halogens and tropospheric Ozone O 3 + X → XO + O 2 XO + hν (+O 2 + M) → X + O 3 X, Y = Cl, Br, I Ozone loss with HO 2 and CO: O 3 + X → XO + O 2 XO + HO 2 → HOX + O 2 HOX + hν →X + OH OH + CO ( + O 2 ) →CO 2 + HO 2 net: CO + O 3 →CO 2 + O 2 Ozone loss at high BrO: 2(Br + O 3 → BrO + O 2 ) BrO + BrO →2 Br + O 2 BrO + BrO → Br 2 + O 2 Br 2 + hν → 2Br net: 2O 3 → 3O 2 Ozone loss with cross-reactions: X + O 3 → XO + O 2 Y + O 3 → YO + O 2 XO + YO → X + Y + O 2 net: 2O 3 → 3O 2 Release: by photolysis of X 2 or XY HOX, XONO 2, XNO 2 organic halogens Main Sinks: XO + NO 2 → XONO 2 XONO 2 → XO + NO 2 XONO 2 + hν → X + NO 3 XONO 2 + H 2 O →HOX + HNO 3 Cl + RH → HCl + R BrO + ClO is faster than BrO + BrO IO + IO also possible

7 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 7 Release of Halogens from Salt Deposits / Salt Aerosols I Release with Br only: Br 2 + hν → 2Br 2(Br + O 3 → BrO + O 2 ) 2(BrO + HO 2 → HOBr + O 2 ) 2 HOBr → 2HOBr aq 2(HOBr aq + H + + Br - →Br 2, aq + H 2 O) 2 Br 2, aq → 2 Br 2 net: 2O 3 + 2HO 2 + 2Br - + 2H + → Br 2 + 4O 2 + 2H 2 O Release with Br and Cl: BrCl + hν → Br + Cl Br + O 3 → BrO + O 2 BrO + HO 2 → HOBr + O 2 HOBr → HOBr aq HOBr aq + H + + Cl - →BrCl aq + H 2 O BrCl aq + Br - ↔ Br 2 Cl - Br 2 Cl - ↔ Br 2,aq + Cl - Br 2, aq ↔ Br 2 net: BrCl + O 3 + HO 2 + Br - + H + → Cl + Br 2 + 2O 2 + H 2 O (also works with BrCl instead of Br 2 ) Autocatalytic Release – one Br less into condensed phase than out! => once started, Br 2 will increase exponentially until reservoir is empty, O 3 is gone or loss processes exceed production: Bromine Explosion O 3 and HO x needed! acid catalyzed

8 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 8 Release of Halogens from Salt Deposits / Salt Aerosols II Comments: autocatalytic release works only for BrO uptake of HOBr and release of Br 2 and BrCl have been observed in the lab from dry salt crystals, aqueous salt solutions, aerosols, and halide ice surfaces over ice, release does not depend on pH but on temperature start-up needed for initial Br 2 release in the condensed phase, HOI can also have similar reactions as HOBr in the presence of NO x, N 2 O 5 can be taken up and halogen nitrites (XNO 2 ) be released (works in the dark) in the presence of NO x, halogen nitrates (XONO 2 ) can also be taken up and lead to release of X 2 and XY without acid catalysis higher oxides are of atmospheric relevance for Iodine only sea water content: ratios[Cl] / [Br] = 660, [Br] / [ I] = 15000

9 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 9 Ozone Depletion in the Polar Boundary Layer Each spring, episodes of severe ozone depletion (ODE) are observed in both hemispheres at the surface: Over sea-ice, full depletion is the standard situation, at some coastal stations (e.g. Barrow) they are frequent, at others (e.g. Ny- Å lesund) rare. At coastal stations, ODEs are linked to air parcel origins over sea-ice ODEs are usually correlated with low temperatures. Snow No snow Figure courtesy W. Simpson

10 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 10 The link between Ozone Depletion and Bromine During ODEs, filterable bromine is much enhanced with long-path DOAS, up to 20 ppt of BrO could be detected close to the ground during ozone depletion events in Ny- Å lesund Hausmann M. and Platt U. Spectroscopic measurement of bromine oxide and ozone in the high Arctic during Polar Sunrise Experiment 1992. J. Geophys. Res. 99, 25399–25413., 1994 Barrie L. A., et al.,, Ozone destruction and photochemical reactions at polar sunrise in the lower Arctic atmosphere. Nature 334, 138–141, 1988

11 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 11 Ozone Depletion in the Polar Boundary Layer Vertical extension: the vertical extent of the ozone depletion varies but usually coincides with the arctic inversion layer over Antarctica, elevated layers of depleted ozone have been observed as result of upward transport of depleted air masses from the ice BrO is often located close to the ground, but some DOAS measurements of BrO layers at larger altitude have also been reported Time scales: ozone depletion at coastal sites often occurs very rapidly but this is frequently linked to change in wind directions models predict ozone depletion to take many hours recently, observations on the Polarstern showed much faster O 3 depletion without change in wind direction

12 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 12 Satellite Observations of Polar BrO UV absorption spectroscopy of scattered sun-light Differential Optical Absorption Spectroscopy (DOAS) measurements several satellite instruments: –Global Ozone Monitoring Experiment (GOME) (1996 – 2002 -...) –and Scanning Imaging Absorption Spectrometer for Atmospheric CartograpHY (SCIAMACHY) (2002 – today) –OMI (2004 – today) –GOME-2 (March 2006 – today) BrO events cover large area each spring in both hemispheres individual events can be followed for several days largest values are observed along the coasts and over sea-ice the same areas are covered each year high altitude snow covered regions (Antarctica, Greenland are not affected

13 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 13 Time Series of BrO Columns April, NH GOME SCIA

14 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 14 Time Series of BrO Columns September, SH GOME SCIA

15 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 15 SCIAMACHY: Arctic Boundary Layer BrO Events

16 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 16 SCIAMACHY: Antarctic Boundary Layer BrO Events

17 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 17 A BrO Explosion Event March /April 2007 Case Study: GOME-2 BrO columns large plume of elevated BrO forms plume “lives” for more than 10 days long-range transport from the Arctic towards Hudson Bay plume evolution not in line with general wind pattern => is this transport or chemical evolution? => is this typical or an unusual event?

18 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 18 What are the Active Surfaces? The source of halogens are sea salt, but how are they released? photolysis of biogenic precursors (too slow, not enough ?) sea salt aerosol (not enough Br ?) from first year ice (still salty) from snow-pack (salt deposited by aerosols) from frost flowers (highly saline quasi-liquid layer) from Br enriched aerosols formed from frost flowers or wind blown snow

19 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 19 The Link to Sea Ice from backward trajectories, it is evident that enhanced BrO and reduced O 3 is found when air masses had contact with new sea ice this has been shown for both hemispheres correlation is best if first year sea-ice is used Frieß, U. et al., Dynamics and chemistry of tropospheric bromine explosion events in the Antarctic coastal region, J. Geophys. Res., 109, D06305, doi:10.1029/2003JD004133, 2004 stratosphere free troposphere boundary layer zenith-sky DOAS geometry

20 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 20 The Link to Frost Flowers Frost flowers form on fresh ice at very low temperatures They offer large surface area and high salinity Areas of potential frost flower coverage (PFA) can be computed from sea-ice measurements, meteorological data and a simple model PFA correlates very well with areas of enhanced BrO if 1 day of transport is accounted for and enough light is available This indicates correlation between frost flowers or conditions necessary for frost flowers and BrO BUT: So far no direct evidence for a role of frost flowers in Br release has been produced! Kaleschke, L. et al., Frost flowers on sea ice as a source of sea salt and their influence on tropospheric halogen chemistry, Geophys. Res. Lett., 31, L16114, doi:10.1029/2004GL020655, 2004.

21 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 21 Observed Changes in Sea Ice Extent Sea ice extent in the NH has been declining since start of measurements 2007 has set another dramatic record this mostly affects summer, but also other seasons the trend is expected to continue as the Arctic gets warmer http://nsidc.org/news/press/2007_seaiceminimum/20071001_pressrelease.html

22 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 22 Link to Climate Change? Ingredients needed for BrO explosion: low temperatures –as temperatures increase in the Arctic, the region and time favourable for bromine explosions will change fresh ice –as multi-year ice coverage decreases, the probability for frost flowers / first year ice will increase sunlight –as cloud cover changes, so will photochemistry a trigger –??? O 3 depletion in turn also effects radiative forcing locally with a potential for positive feedback

23 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 23 The Link between Bromine and Mercury total gaseous mercury (TGM) is also depleted when ozone is depleted this has been observed in both hemispheres (above: Neumayer Station) reaction with BrO and HOBr is thought to be the main pathway Ebinghaus, R., et al., Antarctic Springtime Depletion of Atmospheric Mercury, Environ. Sci. Technol., 36(6), 1238-1244, 2002

24 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 24 Effect on Mercury Chemistry Mercury in the atmosphere: most mercury is of anthropogenic origin (> 70%, mainly combustion) it is in the form of gaseous mercury Hg(0) Link between mercury and bromine: reaction with BrO or HOBr can oxidize it to Hg(I) and Hg(II) which can deposit onto snow and ice => depletion of gaseous mercury importance of GEM depletion: re-evaporation of Hg(0) is possible => no net effect if not, mercury is injected into polar ecosystem during snow melt => bio-accumulation

25 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 25 Polar Mercury Chemistry Steffen et al., A synthesis of atmospheric mercury depletion event chemistry linking atmosphere, snow and water, Atmos. Chem. Phys. Discuss., 7, 10837–10931, 2007

26 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 26 Salt Lakes salt lakes provide liquid and solid surfaces with very high salinity measurements found extremely high BrO concentrations (up to 200 ppt) at the Dead Sea and also high BrO (up to 6 ppt) and ClO (5 – 15 ppt) at the Great Salt Lake in Utah and at Salar de Uyuni (Bolivia) it is thought that the same heterogeneous release processes act for Br as in the polar BrO explosion (uptake of HOBr) IO was also found at the Dead Sea, possibly related to bacterial emissions rapid and strong ozone depletion is observed in air masses with high BrO as saline soils cover more than 2% of the Earth’s surface, this might be a significant source of halogens in the troposphere climate change induced desertification might even increase that

27 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 27 BrO Observations at Dead Sea OMI BrO July 2005 figure courtesy of T. Kurosu, SAO BrO [ppt] O 3 [ppb] NO x [ppb] SO 2 [ppb] wind speed wind dir. Matveev V. et al., Bromine oxide–ozone interaction over the Dead Sea. J. Geophys. Res. 106, 10375–10387, 2001 pollution peak

28 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 28 Halogens in the marine Boundary Layer oceans cover most of our planet large concentrations of sea-salt aerosols are present in the MBL they are depleted in Cl and large aerosols also in Br while small particles are often enriched in Br there is indication for reactive chlorine from different measurements (mist chamber measurements, hydrocarbon clock) but results are highly variable there is indication for reactive bromine but again results are variable and point at rather low concentrations (< 1ppt) in some regions, biological sources might be more relevant (=> see discussion on iodine later) even at low concentrations, reactive halogens have an impact on ozone levels in the MBL and also on hydrocarbons => everything is still highly uncertain

29 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 29 Halogen-Sulphur Interactions cloud droplet formation depends critically on the availability of cloud condensation nuclei (CCN) sulphate aerosols formed by SO 2 can act as CCN one major source of SO 2 is biogenic dimethyl sulphate (DMS) BrO can interfere with sulphate production in two ways: A: removal of DMS BrO + DMS → Br + DMSO Br + O 3 → BrO + O 2 net: DMS + O 3 → DMSO + O 2 => reduction in DMS => reduction in SO 2 available for new particle formation B: increased uptake of SO 2 oxidation of SO 2 by HOCl (mainly aerosols) and HOBr (mainly cloud droplets) => reduction in SO 2 available for new particle formation

30 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 30 Halogen-Sulphur Interactions von Glasow R. and Crutzen P. J. (2004) Model study of multiphase DMS oxidation with a focus on halogens. Atmos. Chem. Phys., 4, 589–608. BrO in the marine boundary layer can have an impact on cloud processes and thus climate large uncertainties remain in the details of the chemistry

31 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 31 Halogen Oxides and Volcanoes volcanoes emit large quantities of hydrogen halides eruptive emissions can reach the stratosphere, but no indication for enhanced Cl or Br degassing emissions remain in the troposphere and appear to have very large BrO concentrations (up to 1 ppb), probably formed in the plume by photochemistry and recycling on aerosols Bobrowski, N. et al., (2003), Detection of bromine monoxide from a volcanic plume Nature, 423, 273-276. BrO SO 2

32 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 32 ground-based MAX-DOAS measurements at Soufriere Hills volcano (and others) show tight correlation between SO 2 and BrO emissions the ratio SO 2 / BrO was about 1000 at Soufriere Hills, smaller elsewhere this corresponds to a global estimate of 14  6 Tg yr –1 of sulphur emissions corresponding to a source strength of the order of 30,000 t Br yr -1 volcanoes could be a significant source of BrO in the troposphere! BrO and ClO Release from Volcanoes Bobrowski, N. et al., (2003), Detection of bromine monoxide from a volcanic plume Nature, 423, 273-276. Lee et al., High ClO and ozone depletion observed in the plume of Sakurajima volcano, Japan, Geophys. Res. Lett., 32, L21809, doi:10.1029/2005GL023785 BrO ClO SO 2

33 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 33 Nyamuragira: 1.41°S, 29.20°E Summit elevation 3,058m BrO Release from Volcanoes: Satellite View satellite measurements from GOME and SCIAMACHY offer global coverage sensitivity is decreasing towards surface but large at plume altitude large eruptions can be measured with good relative accuracy no evidence for BrO enhancements found in any case Afe, O. T., et al., BrO Emission from Volcanoes - a Survey using GOME and SCIAMACHY Measurements, Geophys. Res.Lett., 31, L24113, doi:10.1029/2004GL020994, 2004

34 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 34 GOME SCIAMACHY Afe, O. T., et al., BrO Emission from Volcanoes - a Survey using GOME and SCIAMACHY Measurements, Geophys. Res.Lett., 31, L24113, doi:10.1029/2004GL020994, 2004 BrO Release from Volcanoes: Satellite View => no indication for 1:1000 BrO / SO 2 emissions during large volcanic eruptions => BrO emission / production limited to degassing

35 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 35 The Mystery of BrO in the free Troposphere since GOME satellite measurements of BrO columns became available in 1995, it was clear that they are larger everywhere than expected from stratospheric BrO alone this is interpreted as indication for about 0.5 – 2 ppt of BrO in the free troposphere balloon borne and ground- based measurements confirmed this finding some SH DOAS measurements show less BrO van Roozendael, M.et al., : Intercomparison of BrO Measurements from ERS-2 GOME, ground-based and Balloon Platforms, Adv. Space Res., 29(11), 1661-1666, 2002 stratosphere free troposphere boundary layer satellite geometry

36 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 36 BrO in the free Troposphere ground-based and satellite measurements can be made consistent if a tropospheric background is assumed sources are not clear but could include –photolysis of organohalogens –release from sea salt aerosols –emissions from volcanoes –export of polar BrO –import from stratosphere even such small BrO amounts have a large (5 – 40%) effect on ozone levels in the free troposphere => still under investigation with tropospheric BrO no tropospheric BrO stratosphere free troposphere boundary layer MAX-DOAS geometry N. Theys et al., Retrieval of stratospheric and tropospheric BrO columns from multi-axis DOAS measurements at Reunion Island (21 S, 56 E), Atmos. Chem. Phys., 7, 4733–4749, 2007

37 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 37 Iodine in the marine Boundary Layer phytoplankton and macroalgae emit alkyl iodides (CH 3 I, C 3 H 7 I, CH 2 Cl I, CH 2 I 2 ) and I 2 photolysis produces I which then attacks O 3 largest emissions are expected in biological active regions (coastal regions at low tide when seaweed is exposed, upwelling regions, polar regions) IO could be observed during day and OIO and I 2 mainly at night more recent measurements in the Bay of Maine found IO, OIO and I 2 during daytime also Carpenter L. J. et al., Coastal zone production of IO precursors: a 2-dimensional study. Atmos. Chem. Phys. 1, 9–18, 2001 long-path DOAS geometry IO

38 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 38 Iodine and Particle Formation during periods of high IO, large particle concentrations are observed low tide and solar illumination needed, I 2 or CH 2 I 2 as precursors new particles formed through iodine oxides (IO, OIO, I 2 O 2,I 2 O 3,...) very large concentrations needed => hot spots? this could be a relevant mechanisms for new particle formation in addition to sulphate CCN O’Dowd C. D. et al., A dedicated study of new particle formation and fate in the coastal environment (PARFORCE): overview of objectives and achievements. J. Geophys. Res. 107, 8108, 2002 particle concentration low tide time of day

39 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 39 Polar Iodine Oxide MAX-DOAS measurements in Ny- Å lesund (Arctic) and Neymayer station (Antarctica) indicate enhanced IO concentrations in Polar Regions long-path DOAS measurements at Halley Bay, Antarctica showed large IO concentrations highly correlated with BrO sources of IO could be photolysis of iodocarbons produced by phytoplankton recycling of IO on aerosols? A. Saiz-Lopez et al.,Boundary Layer Halogens in Coastal Antarctica, 317, Science, 348, 2007 at these concentrations, IO dominates O 3 loss uptake of IO could also trigger Br release and start bromine explosion

40 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 40 Satellite Observations of Antarctic IO satellite measurements confirm presence of IO in spring in Antarctica extended area close to Halley Bay seasonality agrees with ground- based measurements spatial distribution differs from that of BrO no detectable signal in the Arctic different release mechanisms must be involved (biological sources?) Halley Bay measurements may not be representative for Polar Regions in general IO BrO Schönhardt, A.et al., Observations of iodine monoxide (IO) columns from satellite, Atmos. Chem. Phys. Discuss., 7, 12959-12999, 2007

41 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 41 Summary halogen oxides play an important role in many aspects of tropospheric chemistry in contrast to the stratosphere, they are mainly of natural origin regions with large halogen oxide concentrations are the polar spring boundary layer, salt lakes, volcanoes and the marine boundary layer halogen oxides reduce ozone levels, at high concentrations dramatically, e.g. during ODEs in Polar Regions bromine converts gaseous mercury to particulate form which can lead to bioaccumulation bromine potentially reduces new particle formation by H 2 SO 4 iodine can form new particles at high concentrations => many of the mechanisms and explanations shown are still highly uncertain!

42 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 42 Overview on Tropospheric Halogen Chemistry von Glasow, R. and P. J. Crutzen, Tropospheric Halogen Chemistry, Holland H. D. and Turekian K. K. (eds), Treatise on Geochemistry Update1, vol. 4.02, pp 1 - 67, 2007

43 Halogen Oxides in the Troposphere, Andreas Richter, ERCA 2008 43 Some References von Glasow, R. and P. J. Crutzen, Tropospheric Halogen Chemistry, Holland H. D. and Turekian K. K. (eds), Treatise on Geochemistry Update1, vol. 4.02, pp 1 - 67, 2007 Platt U. and G. Hönninger, The role of halogen species in the troposphere. Chemosphere, 52, 325–338, 2002 Simpson et al., Halogens and their role in polar boundary-layer ozone depletion, Atmos. Chem. Phys., 7, 4375-4418, 2007 (http://www.atmos-chem-phys.net/7/4375/2007/acp-7-4375- 2007.pdf)http://www.atmos-chem-phys.net/7/4375/2007/acp-7-4375- 2007.pdf Wayne et al., Halogen oxides: radicals, sources and reservoirs in the laboratory and atmosphere, Atmos. Environ., 29, 2677-2884, 1995


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