Quantification of volcanic reactive halogen emissions - Volcanism and Global Environmental Change - page 121

uenced by
halogens released from volcanic eruptions, which reach the stratosphere (see also
Chapter 16 ). Speci

It is well known that stratospheric chemistry can be strongly in

cally, halogen species can lead to stratospheric ozone destruc-
tion for periods of months to years after an eruption (e.g. Coffey, 1996 ).
However, tropospheric chemistry is also influenced by volcanic emission of
RHS in various ways:

(1) RHS catalyse O 3 destruction (see Section 8.2.3 );
(2) RHS inhibit the O 3 production in the troposphere (Stutz et al ., 1999 );
(3) RHS reactions interfere with the NO x reaction cycle (Stutz et al ., 1999 );
(4) RHS reactions enhance the OH/HO 2 ratio;
(5) Hypohalous acids (HOBr, HOCl) accelerate the formation of sulfate in aque-
ous particles (e.g. von Glasow and Crutzen, 2007 );
(6) Br can oxidise mercury and therefore reduce its atmospheric lifetime (e.g. von
Glasow, 2010 );
(7) Chlorine also plays a key role in trace-metal transport from the magma to the
atmosphere (Symonds et al ., 1994 );
(8) Chlorine atoms released in volcanic plumes may take part in the degradation
of atmospheric methane (CH 4 ). For instance, Platt et al .( 2004 ) conclude
from 13 C/ 12 C ratio measurements in atmospheric CH 4 that 3% of the CH 4

in the extra tropical southern hemisphere is removed by reaction with Cl
atoms.

>

Points 1 - 4 above directly or indirectly influence tropospheric ozone, which is a
key atmospheric constituent. Therefore, it is important to find experimental evi-
dence for ozone reduction in tropospheric volcanic plumes by volcanic RHS and to
evaluate and quantify related model predictions. Recent field studies on this issue

(see, e.g., Vance et al ., 2010 ; Schumann et al ., 2011 ; Surl et al. , 2014 ) still largely
suffered from instrumental limitation and cross-sensitivities of O 3 measurements
with the abundant SO 2 in volcanic plumes.

8.3 In situ measurements and remote sensing of halogens
8.3.1 In situ measurements
In situ measurements have been the basis for monitoring volcanic-gas emissions
for many years and they still play an important role. Today, they are comple-
mented by remote-sensing techniques (see Section 8.3.2 ). Traditionally, several
techniques for collecting gas samples have been in use (e.g. Finlayson, 1970 ;
Carroll and Holloway, 1994 ); evacuated sampling flasks were introduced by

Robert W. Bunsen and later re ned by Walter Giggenbach ( 1975 , now known