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14 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING
measurements tropospheric halogen species can have profound effects on many aspects of tropospheric
chemistry, these include:
(1) Reactive halogen species, and in particular reactive bromine and iodine can readily destroy tropospheric
ozone, which can have a multitude of consequences, both chemical and climatic (e.g. [Roscoe
et al. , 2001], [Platt & Hönninger, 2003]). Catalytic ozone destruction essentially occurs by two distinct
reaction cycles. Either (I) involving self reaction of halogen oxide radicals with other halogen oxide
radicals (XO+XO or XO+YO) or (II) reaction of halogen oxides with hydrogen radicals XO+HO2):
leading to the net result:
O3 + O3 → 3O2
Cycle (I) has been identified as the prime cause for polar boundary layer ozone destruction (e.g.
[Barrie & Platt, 1997]). Since the rate of ozone destruction is usually proportional to the square of
the halogen oxide concentration, cycle (I) ineffective at low XO levels usually found in mid-latitude
coastal areas. At low RHS levels, however O3 destruction takes place by reaction cycle (II), here the
rate of O3 destruction is linearly dependent on the XO concentration.
(2) An important side effect of cycle (II) the conversion of HO2 to OH and thus a reduction of the
HO2/OH ratio, in particular at low NOX levels. At a given photochemical situation (O3, H2O levels,
insolation) the presence of BrO or IO will therefore increase the OH concentration. It is interesting
to note that both effects have been observed in the upper troposphere (e.g. [Wennberg et al. , 1998]).
(3) The reaction of XO with NO leads to conversion of NO to NO2 and thus to an increase of the
Leighton ratio (L=[NO2]/[NO]). An increase in L is thus usually regarded as an indicator for photochemical
ozone production (due to the presence of RO2 (R = organic radical)) in the troposphere.
However, as already noted by [Platt & Janssen, 1996] an increase in L due to halogen oxides will not
lead to O3 production.
(4) A more subtle consequence of reactive halogen species for the ozone levels in the free troposphere
is due to the combination of the above effects (2) and (3) as pointed out by [Stutz et al. , 1999]:
Photochemical ozone production in the troposphere is limited by the reaction
(2.1)
NO + HO2 → NO2 + OH (2.2)
However, the presence of reactive bromine will reduce the concentrations of both educts of the above
reaction and thus reduce the NOX - catalysed ozone production.
(5) Heterogeneous reactions of bromine species (i.e. HOBr) with (sea salt) chloride can lead to the
release of Cl-atoms. This process constitutes a Br catalysed chlorine activation. Since Cl-atoms are
highly reactive, this process directly enhances the oxidation capacity of the troposphere, see e.g. [Platt
et al. , 2004].
(6) Gas-phase iodine species (like IO, OIO or HOI) may facilitate transport of I from the coast to
inland areas and thus contribute to our iodine supply [Cauer, 2004], [Cox et al. , 1999].
(7) Deposition of mercury was found to be enhanced by the presence of reactive bromine species in
particular in polar regions (e.g. [Barrie & Platt, 1997]), this process appears to be linked to the
oxidation of Hg0 to Hg(II), likely by reaction with BrO [Lindberg et al. , 2002].
(8) The reaction of BrO with dimethyl sulfide (DMS) might be important in the unpolluted remote
marine boundary layer where the only other sink for DMS is the reaction with OH radicals (e.g. [von
Glasow et al. , 2004], [von Glasow & Crutzen, 2004]).
(9) Iodine species have been shown to be involved in particle formation in the marine BL ([Leck &
Bigg, 1999], [Hoffmann et al. , 2001], [O’Dowd et al. , 2002], [Burkholder et al. , 2004]).
(10) Evidence is accumulating (see e.g. [Platt & Hönninger, 2003]) that there is a wide-spread ”background”
level of BrO radicals in the free troposphere, due to the processes described under (1)-(5)
and (8) above there might be an important effect of reactive halogens on the global tropospheric
chemistry as summarised by [von Glasow et al. , 2004].
Main methods
include the experimental determination of the halogen oxide distribution and the distribution of related
species in several compartments of the troposphere:
1) The marine boundary layer in coastal regions (Mace Head, Ireland).
2) The open ocean.
3) The free troposphere.
4) Volcanic emissions (contents of reactive halogen species, e.g. BrO, ClO, OClO in volcanic plumes).