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| Titel |
Is the 'Bromine Explosion' generated from the reaction BrO HO2 alone? |
| VerfasserIn |
Wolfgang Behnke, Cornelius Zetzsch |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250034293
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| Zusammenfassung |
| We observed bromine explosions (a fast production of atomic Br and Cl under
tropospheric conditions) in various smog chamber experiments in Teflon bags at room
temperature at a relative humidity of about 80% in the presence of NaCl/NaBr-aerosol,
simulated sunlight and ozone (200 - 400 ppb). Time profiles of ozone and hydrocarbons
(HCs: n-butane, 2,2-dimethylbutane, tetramethylbutane and toluene, initially about 2
ppb each) were monitored to determine concentrations and source strengths of OH
radicals, atomic Cl and Br and the corresponding time profiles of BrCl and Br2 as their
photolytic precursors. The number and size of aerosols are measured as well as
their chemical composition (Br-, Cl- and oxalic acid). Full records of raw data
from the smog chamber runs are available at www.eurochamp.org for potential
users.
Chemical box model calculations deliver concentrations of various intermediates,
such as aldehydes, HO2 and RO2 radicals and the inorganic halogen compounds
ClO, BrO, HOCl and HOBr, where HOBr from O3 + Br- => BrO- + O2 in the
aqueous/adsorbed phase induces the following gas-phase/ heterogeneous chain
reaction
Br + O3 => BrO + O2(1)
BrO + HO2 => HOBr + O2(2a)
HOBr + (Aerosol) => HOBrad(3)
Surface-adsorbed HOBr reacts with Br- or Cl- to produce Br2 or BrCl, both of which
are released and photolysed. Formation of Br2 should prevail up to Cl-/Br- -ratios of about
104 (Fickert, S., J.W. Adams, J.N. Crowley, J. Geophys. Res., D104, 23719-23727, 1999). A
maximum of this ratio is reached about 30 minutes after the beginning and decreases during
the next hours - probably by reaction of Br2 with oxalate and absorption of HBr, formed from
the reaction of Br with aldehydes.
Parallel to chain reaction (1)-(3) a chain reaction replacing Br by Cl seems
possible but can not be realized, since the main sink of atomic Cl is its reaction
with hydrocarbons - leading to chain termination - in contrast to atomic Br (ratio
of rates: kCl[O3]/kCl[HC] ~ 0.1; kBr[O3]/kBr[toluene] ~ 100). Formation of
aldehydes (R-CHO) interferes with the chain reaction (1) - (3) markedly, since kBr[O3]
-kBr[R-CHO].
The chain reaction is limited by availability of ozone (degradation of HCs by
atomic Cl stops completely with vanishing ozone), of HO2 (HCs are required to form
HO2) and of aerosol. The central question is: will sufficient HO2 be formed from
degradation of HCs to explain the magnitude of the formed Br2 and BrCl in our
experiments? We found that the formation of HO2 should be by a factor of 2-4 larger to
explain the formation of Br2 and BrCl. Which other sources for the formation of
HOBr besides reaction (2a) are then available? The rate of CH3O2with BrO is
25% of that with HO2 (Enami, S.; Yamanaka, T.; Nakayama, T.; Hashimoto, S.;
Kawasaki, M.; Shallcross, D.E.; Nakano, Y.; Ishiwata, T., J. Phys. Chem. A, 11,
3342 - 3348, 2007), suggesting that other RO2 radicals must contribute. In our
model calculations we use this rate constant for all RO2 radicals to obtain reasonable
agreement between the produced HOBr and the formed BrCl and Br2 necessary for our
experimental degradation results. So reaction scheme (1) - (3) should be completed
by:
BrO + RO2 => HOBr + products (2b)
The German Science Foundation (DFG) supported this research in unit 783
(HALOPROC). |
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