Halogens in the Troposphere

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Halogens in the Troposphere AOSC 637 Atmospheric
Chemistry Russell R. Dickerson Finlayson-Pitts
Chapt. 4,6 Seinfeld Chapt. 6 OUTLINE History
Importance Detection Techniques Sources and
Sinks Global Chemistry Remaining
Challenges References
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History

• Duce (1963) measured sea salt aerosols and found
  depletions of Cl and Br but enrichments of I.
• Molina and Rowland (1974) showed that chlorine is
  important in the strat but it was generally
  thought that there was no halogen chemistry in
  the trop.
• Chameides and Davis (1980) hypothesized that
  iodine chemistry could be an ozone sink in the
  marine boundary layer.
• Barrie et al. (1988) observed rapid ozone
  destruction in polar sunrise and attributed this
  to Br chemistry mediated by ice.
• Finlayson-Pitts et al. (1989) observed the
  formation of ClNO2 in laboratory reactions on
  salt-containing aerosols.

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Importance

• Ozone destruction at polar sunrise
• Ozone destruction in the marine boundary layer
• Ozone production in polluted atmospheres
• Conversion of elemental mercury to reactive
  mercury.
• Removal of Methane.

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Detection Techniques

• Tandem mass spectroscopy (e.g., Spicer et al.
  1998).
• H3O generated and it reacts with Cl2
• Chemical ionization Mass Spec (CIMS)
• Mist chamber
• DOAS
• Resonance fluorescence

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VonGlasow and Crutzen (2007)
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Sea salt aerosol production.
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Halogen atoms can destroy ozone in the unpolluted
trop.
X O3 ? XO O2 XO hv (O2) ? X
O3 -----------------------------------------------
------ Do nothing X O3 ? XO O2 XO HO2 ?
HOX O2 HOX hv ? X OH OH CO (O2) ? CO2
OH -----------------------------------------------
----------- O3 CO ? CO2 O2 net
Where X represents Cl, Br, or I, but not F. F
H2O ? HF OH And HF is stable.
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Destruction can also proceed through a halogen
dimer.
2(X O3 ? XO O2) XO XO ? 2X O2
? X2 O2 X2 hv ? 2X -----------------------
----------------------------------- 2O3 ? 3O2 net
This cycle proceeds in the Arctic where X
represents Br, and BrO concentrations are high.
It can also proceed with one Br and one Cl.
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Chlorine atoms can initiate ozone production in
the polluted trop. as OH does.
R-H Cl ? HCl R? R? O2 M ? RO2 M RO2
NO ? NO2 Etc.
HCl is pretty much dead in the troposphere. It
is lost by dry wet deposition. Bromine atoms,
in contrast, do not react with hydrocarbons.
They do react with aldehydes though.
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Bromine atoms destroy ozonein the absence of NOx
Barrie et al. (1988) Used these reactions to
explain rapid ozone loss in the Arctic spring
(polar sunrise). Keene et al. (1990) and Vogt et
al. (1996) proposed this mechanism as a path to
ozone destruction in the marine boundary layer.
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Proof?

• The concentrations of Br (and Cl) are so small as
  to be nearly impossible to detect. Is it
  possible to infer the existence of halogens from
  their effect on the chemistry of ozone?

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Diel cycle of ozone over the Indian Ocean
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The vertical structure of ozone shows that the
upper trop and strat are sources and the MBL is a
sink for ozone.
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The Model of Chemistry Considering Aerosols
(MOCCA) evaluates trace gas concentrations in an
environment with clouds or aerosols. For the
marine boundary layer the rate of change in
concentration is derived from gas-phase
reactions, input from the free troposphere and
ocean surface, and exchange with aerosols.
Where Cg is the gas-phase concentration, Pg and
Lg are the gas-phase photochemical production and
loss terms, E is the emission or entrainment
rate, Z is the MBL height, Vd is the deposition
velocity L is the liquid water content (LWC), kt
is the gas-aerosol exchange coefficient, and
Cg,eq is the gas-phase concentration in
equilibrium with the aqueous phase (Henrys Law).
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• The vapor-phase concentrations of the molecules
  Br2 and BrCl reach ppt levels at night.

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The presence of halogens can explain the
systematic destruction of ozone during the
daylight hours. Because much of the Earths
surface is oceanic, bromine multiphase reactions
may be a substantive sing on a global scale.
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Example from the Arctic
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New Paper by Thornton et al. (2010)

• Trop halogen chemistry had been thought to be a
  marine or coastal problem.
• Nitryl chloride (ClNO2) observed far from ocean.
• ClNO2 acts as a reservoir for NOx.
• ClNO2 hv ? Cl NO2
• See Finlayson page 120 for the absorption
  spectrum of ClNO2.

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Schematic of chlorine activation by night-time
NOx chemistry.
JA Thornton et al. Nature 464, 271-274 (2010)
doi10.1038/nature08905
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Time series of key quantities observed in
Boulder, Colorado, from 11 to 25 February 2009.
Three days showing high (left), moderate, and low
(right) RH.
JA Thornton et al. Nature 464, 271-274 (2010)
doi10.1038/nature08905
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Observed and modelled relationships of ClNO2 and
particulate chloride.
JA Thornton et al. Nature 464, 271-274 (2010)
doi10.1038/nature08905
Left Observed ClNO2 and particulate chloride.
Right Observed and modeled ClNO2 and particulate
chloride. Solid lines are model results
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Annual average components of PClNO over the US.
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a) Annual average NOx emissions over the US in
Kg/yr. b) Annual average fraction of total
nitrate (0-2km) formed via N2O5. c) Yield of
ClNO2. d) Production in g(Cl)/yr log scale
startng at 106.5 g(Cl)/yr.
JA Thornton et al. Nature 464, 271-274 (2010)
doi10.1038/nature08905
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Remaining Challenges

• Where does the Cl come from in the middle of a
  continent?
• What is the efficiency of ClNO2 production?
• Direct measurements of HOBr and HOCl in the trop.
• If Thornton et al are right it will explain why
  N2O5 does not seem such a major NOx sink, suggest
  that NOx is longer lived, and suggest Cl controls.

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References

• Barrie, L. A., J. W. Bottenheim, P. J. Crutzen,
  and R. A. Rasmussen (1988), Ozone destruction at
  polar sunrise in the lower Arctic atmosphere,
  Nature, 334, 138-141.
• Chameides, W. L. and D. D. Davis (1980), Iodine
  It's possible role in tropospheric
  photochemistry, J. Geophys. Res., 85, 7383-7398.
• Dickerson, R. R., K. P. Rhoads, T. P. Carsey, S.
  J. Oltmans, J. P. Burrows, and P. J. Crutzen
  (1999), Ozone in the remote marine boundary
  layer A possible role for halogens, Journal of
  Geophysical Research-Atmospheres, 104,
  21385-21395.
• Duce, R. A., J. T. Wasson, J. W. Winchester, and
  E. Burns (1963), Atmospheric Iodine, Bromine, and
  Chlorine, Journal of Geophysical Research, 68,
  3943.
• Finlaysonpitts, B. J., M. J. Ezell, and J. N.
  Pitts (1989), Formation of Chemically Active
  Chlorine Compounds by Reactions of Atmospheric
  NaCl Particles with Gaseous N2O5 and ClONO2,
  Nature, 337, 241-244.
• Keene, W. C., A. A. Pszenny, D. J. Jacob, R. A.
  Duce, J. N. Galloway, J. J. Schultz-Tokos, H.
  Sievering, and J. F. Boatman (1990), The
  geochemical cycling of reactive chlorine through
  the marine troposphere, Glob. Biogeochem. Cycles,
  4, 407-430.
• Spicer, C. W., E. G. Chapman, B. J.
  Finlayson-Pitts, R. A. Plastridge, J. M. Hubbe,
  J. D. Fast, and C. M. Berkowitz (1998),
  Unexpectedly high concentrations of molecular
  chlorine in coastal air, Nature, 394, 353-356.
• Thornton, J. A., J. P. Kercher, T. P. Riedel, N.
  L. Wagner, J. Cozic, J. S. Holloway, W. P. Dube,
  G. M. Wolfe, P. K. Quinn, A. M. Middlebrook, B.
  Alexander, and S. S. Brown (2010), A large atomic
  chlorine source inferred from mid-continental
  reactive nitrogen chemistry, Nature, 464,
  271-274.
• Vogt, R., P. J. Crutzen, and R. Sander (1996), A
  mechanism for halogen release from sea salt
  aerosol in the remote marine boundary layer,
  Nature, 383, 327-330.
• von Glasow, R. (2010), ATMOSPHERIC CHEMISTRY
  Wider role for airborne chlorine, Nature, 464,
  168-169.