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
1
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.
2
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.
3
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
4
VonGlasow and Crutzen (2007)
5
Sea salt aerosol production.
6
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.
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.
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.
Bromine atoms destroy ozone
in the absence of NOx
Barrie et al. (1988) Used these reactions to explain
rap[id 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.
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?
11
Diel cycle of ozone over the
Indian Ocean
12
13
The vertical structure of
ozone shows that the
upper trop and strat are
sources and the MBL is a
sink for ozone.
14
15
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 (Henry’s
Law).
16
17
18
The vapor-phase
concentrations of the
molecules Br2 and BrCl
reach ppt levels at
night.
19
The presence of halogens can
explain the systematic
destruction of ozone during the
daylight hours. Because much
of the Earth’s surface is
oceanic, bromine multiphase
reactions may be a substantive
sing on a global scale.
20
Example from the Arctic
21
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.
23
Schematic of chlorine activation by night-time NOx chemistry.
JA Thornton et al. Nature 464, 271-274 (2010) doi:10.1038/nature08905
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) doi:10.1038/nature08905
Observed and modelled relationships of ClNO2 and particulate chloride.
JA Thornton et al. Nature 464, 271-274 (2010) doi:10.1038/nature08905
Left: Observed ClNO2 and
particulate chloride.
Right: Observed and modeled ClNO2
and particulate chloride. Solid lines
are model results
Annual average components of PClNO over the US.
2
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) doi:10.1038/nature08905
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.
28
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.
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