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Halogen Chemistry in the troposphere EAS 6410 Xiaolu Zhang, Bo Yao, Jin Liao

Halogen Chemistry in the troposphere EAS 6410 Xiaolu Zhang, Bo Yao, Jin Liao. CFCs. Cl, ClO. Ozone depletion. (Molina and Rowland , 1974). Introduction. Halogens: very reactive radicals . Play an important role in stratosphere chemistry. Why important. Tropospheric Halogens.

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Halogen Chemistry in the troposphere EAS 6410 Xiaolu Zhang, Bo Yao, Jin Liao

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  1. Halogen Chemistry in the troposphere EAS 6410 Xiaolu Zhang, Bo Yao, Jin Liao

  2. CFCs Cl, ClO Ozone depletion (Molina and Rowland, 1974) Introduction Halogens: very reactive radicals Play an important role in stratosphere chemistry Why important Tropospheric Halogens Influence the oxidation power of the atmosphere Direct way: O3, OH, NOx ( NO + NO2 ) Indirect way: Cl + RH ( e.g. CH4 )

  3. Photolysis of 1) dihalogens (X2 or XY) 2) inorganic species ( HOX, XONO2, XNO2) 3) organic halogen precursors Main reaction mechanisms Formation of halogen radicals O3 + X XO + O2 No O3 depletion XO + hv X + O3 Salt deposits / Sea salt aerosol Heterogeneous processes

  4. O3 + X XO + O2 XO + HO2 HOX + O2 HOX + hv X + OH OH + CO, O3 or VOC HO2 + products Main reaction mechanisms O3 destruction paths Net reaction : 2O3→ 3O2

  5. Main reaction mechanisms O3 destruction paths BrO + ClO 4 times faster than BrO + BrO X + O3 → XO + O2 Halogen oxide cross reactions Y + O3 → YO + O2 XO + YO → X + Y + O2 ( X, Y = Cl, Br, I ) Net reaction : 2O3→ 3O2

  6. Main reaction mechanisms Sinks of Halogens Reactions with RH Cl + RH → HCl + R Reactions with NOx hv XO + NO → XONO XO + NOx H2O XO + NO2→ XONO2 HOX + HNO3 (Deposition)

  7. Additional sources Volcanoes Stratosphere ~0.01% Up to hundreds Tg of HCl Large Eruption Troposphere precipitation

  8. Sources of reactive halogens Industry and fossil fuel burning Industrial CHCl3: 62 Gg (Cl) a-1 (Aucott et al, 1999) Pulp and paper manufacturing Water treatment Fossil fuel burning: 4.6 Tg (Cl) a-1 in 1990 Swimming pools and cooling towers: ~1 Tg (Cl) a-1

  9. Sources of reactive halogens Biomass Burning and dust plumes Biomass burning --- a source of Methylhalides Inefficient combustion: CH3OH + HCl CH3Cl + H2O Global production in the late 1990s CH3Cl 450 Gg (Cl) a-1 25% ( Andreae & Merlet, 2001) CH3Br 24 Gg (Br) a-1 20% CH3I 12 Gg (I) a-1 Dust as an important reactive surface

  10. Sources of reactive halogens Organohalogen compound Main Sources Ocean Terrestrial plants Fungi Biomass burning Anthropogenic emissions

  11. Marine Boundary Layer • MBL: the lowest, 500-1,000m deep part of the troposphere that is in direct contact with the sea surface • Separated from the free troposphere by a temperature and humidity inversion and is generally well mixed • Halogens are very abundant in the form of sea salt aerosols which contain chloride and bromide

  12. 1. Sea salt aerosol • Produced at the sea surface by the bursting of air bubbles • Bubble bursting produces small droplets from the film of the air bubbles as well as large jet droplets. • Even larger spray droplets are produced by strong winds blowing over wind crests. • Global flux of sea salt: 1500Tg/year-104Tg/year

  13. 1. Sea salt aerosol Figure 10: Four stages in the production of sea salt aerosol by the bubble-burst mechanism. (a) A bubble rises to the ocean surface thereby forming a thin film at the interface which begins to thin. (b) Flow of water down the sides of the cavity further thins the film which eventually ruptures into many small sea spray particles. (c) An unstable jet, produced from water flowing down the sides of the cavity, releases a few large sea spray drops. (d) Tiny salt particles remain airborne as drops evaporate; a new bubble is formed. Note the scale change between Figures (a) to (c) and Figure (d) (after Pruppacher and Klett (1997)).

  14. 1. Sea salt aerosol Ionic composition of sea water pH of ocean surface water is around 8.2, buffered by HCO3- Uptake of acids from the gas phase leads to acidification of the particles. Keene and Savoie(1998,1999): pH values for moderately polluted conditions at Bermuda were in mid-3s to mid-4s

  15. 1. Sea salt aerosol • Major differences between reactions on sea salt aerosol and in free troposphere: • Acidity • Semi-liquid layer on the surface

  16. 2. Reactive chlorine • Reactive chlorine in the MBL is important for its roles in the acidity budget (HCl), the aqueous phase oxidation of S(IV) by HOCl, and the oxidation of organics and DMS by the chlorine atom.

  17. 2. Reactive chlorine • Many sea salt aerosol composition measurements found chlorine deficits • main reason: the release of HCl from sea salt aerosol by acid displacement:

  18. 2. Reactive chlorine • “Hydrocarbon clock” method for estimating Cl concentrations: by measuring changes in hydrocarbon relative abundances, the concentration of the Cl radical can be determined. • Wingenter et al. (1996): 3.3*104atoms/cm3, 6.5*104atoms/cm3

  19. 3. Reactive bromine • Many field measurements show not only a depletion of Cl- in aged sea salt but often even more so of Br- • On average at least 50% of the bromide is lost in the sampled aerosols. The effective solubility for bromide is about 600 times greater than for chloride (Brimblecombe and Clegg, 1989) so that HBr, unlike HCl, is not affected by acid displacement. Therefore, other mechanisms that involve photochemical processes are the reason for a release of bromine from the aerosol.

  20. 3. Reactive bromine

  21. 3. Reactive bromine • When sufficient Br- is available:

  22. 4. Reactive iodine • In sea water, iodide concentration is very low compared to chloride and bromide. • In sear salt aerosol, Cl and Br are usually depleted whereas I is strongly enriched. • 500-1000 times in rain compared to sear water -> a major additional iodine source Biogenic? Anthropogenic?

  23. 4. Reactive iodine • Main source of iodine in the MBL: emission of biogenic alkyl iodides like CH3I, C3H7I, CH2ClI or CH2I2 and inorganic iodine like I2 by various types of macro-algae and phytoplankton that live in the upper ocean and in tidal areas along the coast. • Other sources

  24. 5. Halogen – sulfur interactions • DMS and halogen • S(IV) and halogen

  25. 5. Halogen – sulfur interactions

  26. Ozone Depletion Event in Polar Region Discovery Low surface ozone level (below 10ppb,even reach zero value) in Arctic region in late winter/early spring were measured by (1)Oltmans(1981) at Barrow, Alaska. (2) Bottenheim(1986) at Alert, North Canada. 1.Polar Meterology: Stable, Stratified in vertical Prevent downward ozone from stratosphere 2.Less VOCs, NOx pollutants 3. Active halogen catalyzed ozone destruction chain. Why? (Possible reason)

  27. Why ODEs event happen? BrO and ozone time series measured at Ny AAlesund,Spitsbergen during ARCTOC96 by Tuckermann et al. (1997) http://www.iup.uni-bremen.de /doas/scia_data_browser.htm SCIAMACHY

  28. Meteorological analyses show that ODEs only occurred, when air masses have been in contact with the Arctic Ocean surface (Worthy et al. (1994)) Heterogenous reaction Bottenheim et al. (2002b) Transport: advection of an airmass in which O3 depletion had already occurred.

  29. HOX(aq) X-,Y-,H+ XY(aq) Major Chemcial mechanism of polar ODEs XONO2 H2O NO2 HO2 XO HOX hv O3 hv XO,YO,NO Aqueous phase hv X XY hv NO2 Gas phase XNO2 XNO2 X- N2O5 HNO3

  30. Sources of active bromine Less than One-year-old Sea ice Frost Flower N2O5 and sea Salt NaBr • When frozen • halide concentrated • on the surface • When melt, • lowered freezing • Point, greater density • Large surface areas • Potential frost flower • Area（PFF) region • Lead to regions with • enhanced BrO Do not need acidity during the reaction. Due to low NOx Concentration, It is not an important source

  31. The different roles of Bromine and Chlorine in Polar ODEs Time series of O3, Br2, BrCl, and global irradiance at Alert for 10 – 11March 2000. Spicer et al.(2002) 1.In the ARCTOC 1996 campaign, the time integrated concentration of Cl was a thousand times smaller than that of Br.(Ramacher et al.1999) 2.Ozone loss by ClO-BrO catalysis is much smaller than by the BrO-BrO. (Jobson et al 1994)

  32. The different roles of Bromine and Chlorine in Polar ODEs • 3.Fickert et al. (1999) • find: • The yield of Br2 and BrCl was • found to depend on the Cl− • to Br− ratio Iodine plays a more important role in ODEs in marine Boundary layer.

  33. Halogen chemistry in Salt lake • Measurement of high BrO concentration at a • site downwind of Dead sea area. • Hebestreit et al (1999) ,BrO up to 90pmol/mol • Matveev et al.(2001) ,BrO up to 200pmol/mol 2. Stutz et al.(2002) in 2000 detected ClO 5~15pmol/mol at the Great Salt Lake in Utah.(Br-/Cl- is only 0.0007) 3.In summer 2001 Zingler and Platt(2005) identified IO mixing ratio 0.5~6pmol/mol in the Dead Sea Basin. (Possible Oxidizing bacteria produce idoine)

  34. Chemical mechanism Matveev et al.(2001) Concluded: bromine release from salt deposit, autocatalytic reaction HOBr(aq)+ H++Br- Br2(aq) + H2O Salt lake gas and aerosol Phase cycling are similar to Polar region BrO, O3 and NO2 levels at the Dead Sea southern site, 5 August 2001.(Tas et al.,2005)

  35. Conclusion • 1.Halogen activation from aqueous phase to gas phase plays a critical role in Ozone depletion in polar region. • 2. ODEs in polar region will probably increase.

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