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16 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING
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 [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
(French Brittany). 2) The open ocean. 3) The free troposphere 4) Volcanic emissions (contents of
reactive halogen species, e.g. BrO, ClO, OClO in volcanic plumes) The measurements are made by
Differential Optical Absorption Spectroscopy (DOAS), which allows the detection of many atmospheric
trace gases at a sensitivity in the ppt - range (i.e. at mixing ratios down to 10-12ppt). For studies
in the different compartments a series of variants of the technique were applied: 1) Studies in the
marine boundary layer were performed by active DOAS (i.e. using artificial light sources), which
allows also nighttime studies, as well as by passive ”Multi-Axis” DOAS (MAX-DOAS). 2) Shipborne
measurements in the open ocean relied on MAX-DOAS observations requiring relatively little
logistic prerequisites and allowed unattended operation. 3) Aircraft-based measurements in the free
troposphere are also based on passive MAX-DOAS observations, while ground based observations
(at the Zugspitze) employ active DOAS measurements. 4) Observation at volcanoes were made by
passive MAX-DOAS and by the novel Imaging-DOAS technique recently developed in our group. In
addition to the field campaigns a number of projects is aimed at technological improvements of the
DOAS technology. Activities included studies on the reduction of the optical noise [Platt, 1994] by the
use of optical fibers and mode mixers to connect light source and transmitting telescope, the use of