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: O3, OH, NOx ( NO + NO2 )

Indirect way: Cl + RH ( e.g. CH4 )

Play an important role in stratosphere chemistry

CFCs Ozone depletion (Molina and Rowland, 1974)Cl, ClO

Why important

Main reaction mechanisms

Formation of halogen radicals

O3 + X

Salt deposits / Sea salt aerosol

XO + hv X + O3

Photolysis of 1) dihalogens (X2 or XY)

2) inorganic species ( HOX, XONO2, XNO2)

3) organic halogen precursors

XO + O2

Heterogeneous processes

No O3 depletion

O3 destruction paths

Main reaction mechanisms

O3 + X XO + O2

XO + HO2 HOX + O2

HOX + hv X + OH

OH + CO, O3 or VOC HO2 + products

Net reaction : 2O3 →→ 3O2

O3 destruction paths

Halogen

oxide

cross

reactions

X + O3 →→ XO + O2

XO + YO →→ X + Y + O2

Y + O3 →→ YO + O2

Main reaction mechanisms

Net reaction : 2O3 →→ 3O2

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 + HNO3

hv

H2O

Reactions with NOx

XO + NO2 →→ XONO2

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 CHCl3: 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

CH3OH + HCl CH3Cl + H2O

Inefficient combustion:

Global production in the late 1990s CH3Cl 450 Gg (Cl) a-1

CH3Br 24 Gg (Br) a-1

CH3I 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

OceanConcentration Lifetime

CH3Cl ~ 630 ppt ~ 1yr

CH2Cl2 ~ 32 ppt 83 days

CH3Br 20 - 40 ppt 1 - 2 yrs

CHBr3 Days

CH3I 1 - 30 ppt 3 - 4 days

CH2I2 up to 1 ppt 5 min

CH2ClI up to 1 ppt 10 h

CH2BrI up to 1 ppt 45 min

Main SourcesOrganohalogen 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-104Tg/year

1. Sea salt aerosol

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

1. Sea salt aerosol

ion Cl- Na+ Mg2+ SO42- K+ Ca2+ HCO3

- Br- I-

Conc.(mmol/l) 550 470 53 28 10 10 2 0.85 10-3

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*104atoms/cm3, 6.5*104atoms/cm3

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

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.

• 500-1000 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 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

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)

http://www.iup.uni-bremen.de/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

X XY

HOX

XNO2 XNO2

N2O5 HNO3

XONO2

NO2

XO,YO,NOO3

Gas phase

Aqueous phase

HO2

hv

NO2

hv

H2O

X-

XY(aq)

HOX(aq)

X-,Y-,H+

hv

hv

Sources of active bromine

Less than

One-year-old

Sea ice

Frost FlowerN2O5 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 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)

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 lake1. 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)

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

Chemical mechanismMatveev 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.