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
( )

2. Overview Introduction Reminder: Stratospheric Halogen Chemistry
Some Tropospheric Halogen Chemistry
Polar Ozone Depletion Events
Salt Lakes
The marine Boundary Layer
Halogen Emissions from Volcanoes
BrO in the free Troposphere
Iodine and Particle Formation
Iodine in the Polar Troposphere
Summary

3. Halogen Oxides in the Troposphere

4. Stratospheric Ozone Chemistry
Chapman cycle
O2 + hν → O(3P) + O(3P)
O + O2 + M → O3 + M
O3 + hν → O2 + O(1D)
O + O3 → 2 O2
=> too large ozone columns!
Catalytic ozone loss (HOx, NOx)
XO + O3 → XO2 + O2
XO2 + O → XO + O2
net: O3 + O → 2 O2
=> good agreement with measurements
Halogens can also act as catalysts
sources:
anthropogenic (CFCs, CH3Br)
natural (CH3Br)
sinks:
reservoir species (e.g. HCl, ClONO2)
sedimentation
=> slow ozone depletion at high altitudes, mainly through Cl
F all bound in HF
Br and I more reactive but much lower concentrations Cly ~ 3.5ppb, Bry ~ 16ppt, Iy < 1ppt

5. Stratospheric Ozone Hole
at very low temperatures. PSCs can form
they consist of ice, HNO3, H2SO4
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 + O2
2(O3 + Cl → ClO + O2)
net: 2O3 → 3O2
ClO - BrO cycle
O3 + Cl → ClO + O2
O3 + Br → BrO + O2
ClO + BrO → Cl + Br + O2
net: 2O3 → 3O2

6. Halogens and tropospheric Ozone
X, Y = Cl, Br, I
Release:
by photolysis of
X2 or XY
HOX, XONO2, XNO2
organic halogens
Main Sinks:
XO + NO2 → XONO2
XONO2 → XO + NO2 XONO2 + hν → X + NO3 XONO2 + H2O → HOX + HNO3
Cl + RH → HCl + R
O3 + X → XO + O2
XO + hν (+O2 + M) → X + O3
Ozone loss with HO2 and CO:
O3 + X → XO + O2
XO + HO2 → HOX + O2
HOX + hν → X + OH
OH + CO ( + O2) → CO2 + HO2
net: CO + O3 → CO2 + O2
Ozone loss at high BrO:
2(Br + O3 → BrO + O2)
BrO + BrO → 2 Br + O2
BrO + BrO → Br2+ O2
Br2 + hν → 2Br
net: 2O3 → 3O2
Ozone loss with cross-reactions:
X + O3 → XO + O2
Y + O3 → YO + O2
XO + YO → X + Y + O2
net: 2O3 → 3O2
IO + IO also possible
BrO + ClO is faster than BrO + BrO

7. Release of Halogens from Salt Deposits / Salt Aerosols I
Release with Br only:
Br2 + hν → 2Br
2(Br + O3 → BrO + O2)
2(BrO + HO2 → HOBr + O2)
2 HOBr → 2HOBraq
2(HOBraq + H+ + Br - → Br2, aq + H2O)
2 Br2, aq → 2 Br2
net: 2O3 + 2HO2 + 2Br - + 2H → Br2 + 4O2 + 2H2O
O3 and HOx needed!
Release with Br and Cl:
BrCl + hν → Br + Cl
Br + O3 → BrO + O2
BrO + HO2 → HOBr + O2
HOBr → HOBraq
HOBraq + H+ + Cl - → BrClaq + H2O
BrClaq + Br - ↔ Br2Cl -
Br2Cl - ↔ Br2,aq + Cl -
Br2, aq ↔ Br2
net: BrCl + O3 + HO2 + Br - + H → Cl + Br2 + 2O2 + H2O
acid catalyzed
(also works with BrCl instead of Br2)
Autocatalytic Release – one Br less into condensed phase than out!
=> once started, Br2 will increase exponentially until reservoir is empty, O3 is gone or loss processes exceed production: Bromine Explosion

8. Release of Halogens from Salt Deposits / Salt Aerosols II
Comments:
autocatalytic release works only for BrO
uptake of HOBr and release of Br2 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 Br2 release
in the condensed phase, HOI can also have similar reactions as HOBr
in the presence of NOx, N2O5 can be taken up and halogen nitrites (XNO2) be released (works in the dark)
in the presence of NOx, halogen nitrates (XONO2) can also be taken up and lead to release of X2 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. 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-Ålesund) 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. The link between Ozone Depletion and Bromine
During ODEs, filterable bromine is much enhanced
Barrie L. A., et al., , Ozone destruction and photochemical reactions at polar sunrise in the lower Arctic atmosphere. Nature 334, 138–141, 1988
with long-path DOAS, up to 20 ppt of BrO could be detected close to the ground during ozone depletion events in Ny-Ålesund
Hausmann M. and Platt U. Spectroscopic measurement
of bromine oxide and ozone in the high Arctic during Polar Sunrise Experiment J. Geophys. Res. 99, 25399–25413., 1994

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 O3 depletion without change in wind direction

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 – )
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. Time Series of BrO Columns April, NH
GOME
SCIA

14. Time Series of BrO Columns September, SH
GOME
SCIA

15. SCIAMACHY: Arctic Boundary Layer BrO Events

16. SCIAMACHY: Antarctic Boundary Layer BrO Events

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. 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. The Link to Sea Ice
zenith-sky DOAS geometry
from backward trajectories, it is evident that enhanced BrO and reduced O3 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
stratosphere
free troposphere boundary layer
Frieß, U. et al., Dynamics and chemistry of tropospheric bromine explosion events in the Antarctic coastal region, J. Geophys. Res., 109, D06305, doi: /2003JD004133, 2004

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: /2004GL020655, 2004.

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

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
???
O3 depletion in turn also effects radiative forcing locally with a potential for positive feedback

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), , 2002

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. 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. 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. BrO Observations at Dead Sea
OMI BrO July 2005
BrO [ppt]
pollution peak
O3 [ppb]
NOx[ppb]
SO2 [ppb]
wind speed
figure courtesy of T. Kurosu, SAO
wind dir.
Matveev V. et al., Bromine oxide–ozone interaction over the Dead Sea. J. Geophys. Res. 106, 10375–10387, 2001

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-Sulphur Interactions
cloud droplet formation depends critically on the availability of cloud condensation nuclei (CCN)
sulphate aerosols formed by SO2 can act as CCN
one major source of SO2 is biogenic dimethyl sulphate (DMS)
BrO can interfere with sulphate production in two ways:
A: removal of DMS
BrO + DMS → Br + DMSO
Br + O3 → BrO + O2
net: DMS + O3 → DMSO + O2
=> reduction in DMS
=> reduction in SO2 available for new particle formation
B: increased uptake of SO2
oxidation of SO2 by HOCl (mainly aerosols) and HOBr (mainly cloud droplets)
=> reduction in SO2 available for new
particle formation

30. Halogen-Sulphur Interactions
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
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.

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
SO2
BrO
Bobrowski, N. et al., (2003), Detection of bromine monoxide from a volcanic plume Nature, 423,

32. BrO and ClO Release from Volcanoes
ground-based MAX-DOAS measurements at Soufriere Hills volcano (and others) show tight correlation between SO2 and BrO emissions
the ratio SO2 / 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!
SO2
BrO
ClO
SO2
Bobrowski, N. et al., (2003), Detection of bromine monoxide from a volcanic plume Nature, 423,
Lee et al., High ClO and ozone depletion observed in the plume of Sakurajima volcano, Japan , Geophys. Res. Lett., 32, L21809, doi: /2005GL023785

33. BrO Release from Volcanoes: Satellite View
Nyamuragira: 1.41°S, 29.20°E
Summit elevation 3,058m
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: /2004GL020994, 2004

34. BrO Release from Volcanoes: Satellite View
GOME
SCIAMACHY
=> no indication for 1:1000 BrO / SO2 emissions during large volcanic eruptions
=> BrO emission / production limited to degassing
Afe, O. T., et al., BrO Emission from Volcanoes - a Survey using GOME and SCIAMACHY Measurements, Geophys. Res.Lett., 31, L24113, doi: /2004GL020994, 2004

35. The Mystery of BrO in the free Troposphere
satellite geometry
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
stratosphere
free troposphere boundary layer
van Roozendael, M.et al., : Intercomparison of BrO Measurements from ERS-2 GOME, ground-based and Balloon Platforms, Adv. Space Res., 29(11), , 2002

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
MAX-DOAS geometry
stratosphere
free troposphere boundary layer
with tropospheric BrO
no tropospheric BrO
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. Iodine in the marine Boundary Layer
long-path DOAS geometry
phytoplankton and macroalgae emit alkyl iodides (CH3I, C3H7I, CH2Cl I, CH2I2) and I2
photolysis produces I which then attacks O3
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 I2 mainly at night
more recent measurements in the Bay of Maine found IO, OIO and I2 during daytime also IO
Carpenter L. J. et al., Coastal zone production of IO precursors: a
2-dimensional study. Atmos. Chem. Phys. 1, 9–18, 2001

38. Iodine and Particle Formation
during periods of high IO, large particle concentrations are observed
low tide and solar illumination needed, I2 or CH2I2 as precursors
new particles formed through iodine oxides (IO, OIO, I2O2,I2O3, ...)
very large concentrations needed => hot spots?
this could be a relevant mechanisms for new particle formation in addition to sulphate CCN
particle concentration
time of day
low tide
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

39. Polar Iodine Oxide
MAX-DOAS measurements in Ny-Ålesund (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?
at these concentrations, IO dominates O3 loss
uptake of IO could also trigger Br release and start bromine explosion
A. Saiz-Lopez et al.,Boundary Layer Halogens in Coastal Antarctica, 317, Science, 348, 2007

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,  , 2007

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 H2SO4
iodine can form new particles at high concentrations
=> many of the mechanisms and explanations shown are still highly uncertain!

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 , 2007

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 , 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, , 2007 (
Wayne et al., Halogen oxides: radicals, sources and reservoirs in the laboratory and atmosphere, Atmos. Environ., 29, , 1995