Environmental Engineering Reference
In-Depth Information
as Giggenbach bottles), providing comprehensive information on the sampled
gas composition, including halogens.
A clear disadvantage is that
in situ
sampling is dangerous, and sometimes access
to the emission source is impossible. A problem when sampling the frequently
more easily accessible fumaroles is the possible interaction of their gaseous
emissions with ground water and soil on their way to the surface. Therefore, direct
plume measurements at open vent volcanoes, if craters are accessible, are of
importance. However, in using the currently available
in situ
techniques, a funda-
mental problem is the high water solubility of some halogen compounds (e.g. HBr)
and how to distinguish the contributions of gas and particle phase. The techniques
developed to trap and determine volcanic halogen compounds are usually divided
into active and passive sampling techniques. The latter usually consist of a vessel
containing an alkaline solution or a
filter wetted by an alkaline solution that is
protected by a ventilated container and exposed near the vent of a volcano.
Originally devised by Noguchi and Kamiya (
1963
), passive alkaline traps are
small, cheap and easy to use. Since the sampled gas volume depends on diffusion,
measurement times are long and passive alkaline measurements often represent
mean values over periods of the order of weeks. Moreover, the success of sampling
also depends on the meteorological conditions.
Reduced collection periods and more
flexible reaction to local weather (e.g. the
wind direction) are enabled by active sampling methods that collect large volumes
of data in a short period of time. One possibility is active alkaline traps (
lter
packs, bubblers or Raschig-ring tubes), which generally consist of a solution
The method can be used to sample highly concentrated gases from fumaroles as
well as to probe strongly diluted plumes. Also in use are electro-chemical sensors,
which can be deployed in passive as well as active mode, but frequently suffer
from relatively high cross-sensitivity between different gaseous species.
8.3.2 Remote-sensing measurements
Remote-sensing measurements rely on the analysis of radiation emitted by plumes
or the determination of the attenuation of external radiation (e.g. from the Sun) to
determine remotely the concentration of species of interest inside the plume. In
either case the approach is based on Lambert
-
Beer
'
is law relating the attenuation of
an initial radiation intensity
I
0
(
λ
), (
λ ¼
wavelength) to the column density
S
of a
gaseous constituent of the plume:
I
(
λ
)
¼
I
0
(
λ
)
exp (
-
S
σ
(
λ
))
¼
I
0
(
λ
)
exp (
D
)
(8.10)
