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

Introduction Halogens: very reactive radicals Tropospheric Halogens Influence the oxidation power of the atmosphere Direct way: O 3, OH, NOx ( NO + NO 2 ) Indirect way: Cl + RH ( e.g. CH 4 ) Play an important role in stratosphere chemistry CFCsOzone depletion (Molina and Rowland, 1974) Cl, ClO Why important

Main reaction mechanisms Formation of halogen radicals O 3 + X Salt deposits / Sea salt aerosol XO + hvX + O 3 Photolysis of 1) dihalogens (X 2 or XY) 2) inorganic species ( HOX, XONO 2, XNO 2 ) 3) organic halogen precursors XO + O2 Heterogeneous processes No O 3 depletion

O 3 destruction paths Main reaction mechanisms O 3 + XXO + O 2 XO + HO 2 HOX + O 2 HOX + hvX + OH OH + CO, O 3 or VOCHO 2 + products → Net reaction : 2O 3 → 3O 2

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

Sinks of Halogens Main reaction mechanisms Reactions with RH → Cl + RH → HCl + R XO + NOx HOX + HNO 3 hv H2OH2O Reactions with NOx → XO + NO 2 → XONO 2 → XO + NO → XONO (Deposition)

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

Sources of reactive halogens Industry and fossil fuel burning Fossil fuel burning: 4.6 Tg (Cl) a -1 in 1990 Industrial CHCl 3 : 62 Gg (Cl) a -1 (Aucott et al, 1999) Swimming pools and cooling towers: ~1 Tg (Cl) a -1 Pulp and paper manufacturing Water treatment

Sources of reactive halogens Biomass Burning and dust plumes CH 3 OH + HClCH 3 Cl + H 2 O Inefficient combustion: Global production in the late 1990sCH 3 Cl 450 Gg (Cl) a -1 CH 3 Br 24 Gg (Br) a -1 CH 3 I 12 Gg (I) a -1 Dust as an important reactive surface ( Andreae & Merlet, 2001) 25% 20% Biomass burning --- a source of Methylhalides

Sources of reactive halogens Ocean ConcentrationLifetime CH 3 Cl~ 630 ppt~ 1yr CH 2 Cl 2 ~ 32 ppt83 days CH 3 Br ppt1 - 2 yrs CHBr 3 Days CH 3 I ppt3 - 4 days CH 2 I 2 up to 1 ppt5 min CH 2 ClIup to 1 ppt10 h CH 2 BrIup to 1 ppt45 min Main Sources Organohalogen compound Terrestrial plants Fungi Biomass burning Anthropogenic emissions

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

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 Tg/year

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)).

1. Sea salt aerosol ionCl - Na + Mg 2+ SO 4 2- K+K+ Ca 2+ HCO 3 - Br - I-I- Conc.(mmol/l) 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

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

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.

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:

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*10 4 atoms/cm 3, 6.5*10 4 atoms/cm 3

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.

3. Reactive bromine

When sufficient Br - is available:

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 times in rain compared to sear water -> a major additional iodine source Biogenic?Anthropogenic?

4. Reactive iodine Main source of iodine in the MBL: emission of biogenic alkyl iodides like CH 3 I, C 3 H 7 I, CH 2 Cl I or CH 2 I 2 and inorganic iodine like I 2 by various types of macro- algae and phytoplankton that live in the upper ocean and in tidal areas along the coast. Other sources

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

5. Halogen – sulfur interactions

Ozone Depletion Event in Polar Region 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. Discovery Why? (Possible reason) 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 ODEs event happen? BrO and ozone time series measured at Ny AAlesund,Spitsbergen during ARCTOC96 by Tuckermann et al. (1997) /doas/scia_data_browser.htm SCIAMACHY

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

Major Chemcial mechanism of polar ODEs XO XXY HOX XNO2 N2O5HNO3 XONO2 NO2 XO,YO,NO O3 Gas phase Aqueous phase HO2 hv NO2 hv H2O X-X- XY(aq) HOX(aq) X-,Y-,H+ hv

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

The different roles of Bromine and Chlorine in Polar ODEs Time series of O3, Br2, BrCl, and global irradiance at Alert for 10 – 11March 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)

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

Halogen chemistry in Salt lake 1.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 ) 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)

BrO, O3 and NO2 levels at the Dead Sea southern site, 5 August 2001.(Tas et al.,2005) 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

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.