Environmental Engineering Reference
In-Depth Information
along the path scanned (or traversed) multiplied by the plume speed (m s
1
), gives
an estimate of the SO
2
emission rate in g s
1
. Wind-speed estimation can often be
the limiting factor in the accuracy of emission-rate measurements. However, one
desirable aspect of these scanning systems is that multiple time-series of the SO
2
path concentration can be assembled as the plume passes overhead. By correlating
features between the time-series, the speed of movement of the features can
be estimated and hence the scanning mini-DOAS instrument itself can provide
all the necessary information to obtain emission rates. A good overview of the
developments in UV spectroscopy for applications to measuring SO
2
and SO
2
emission rates has been provided by McGonigle (
2005
). The NOVAC (Galle
et al
.,
2010
) and FLAME networks are now providing near continuous SO
2
measure-
ments at volcanoes around the world based on UV spectroscopy.
The SO
2
camera
The SO
2
camera is a natural extension of the compact spectrometer, where high
spectral resolution is sacri
ced in favour of high spatial sampling. Typically the
camera employs
filters (or uses two cameras with different
filters) to acquire
'
on-
band
'
and
off-band
'
images at
c
. 308 nm and
c
. 330 nm. The principle of the
method assumes only absorption by SO
2
and uses the Beer
'
Lambert law
to estimate the path concentration, given knowledge of the band-averaged absorp-
tion coef
-
Bouguer
-
cient. The mass loading,
m
i
is determined from,
1
k
a
ln
I
o
ðλÞ
IðλÞ
m
l
¼
,
ð
7
:
5
Þ
where
k
is a
n a
b
s
orption coef
cient averaged over the ba
nd
pass of the
lter withmean
wavelength
is the intensity outside
the plume. To reduce the effect of dark current,
4
the intensityobtainedwhenviewing a
blackened target is usually removed from all measurements. Under good conditions
of clear skies, camera
λ
,
I
ðλÞ
is the measured plume intensity and
I
o
ðλÞ
2 km), and few aerosols, this simple
retrieval can work well; however, these conditions are seldom met. Addition of a
second off-band
-
plume distance small (
<
filter can help to alleviate some of the problems associated with
interference from aerosols. With a second
filter,
Equation (7.5)
can be re-written as:
I
o
ðλÞ=
I
a
1
k
a
ln
ðλÞ
m
l
,
ð
7
:
6
Þ
I
o
ðλÞ=I
b
ðλÞ
where the superscripts refer to the on-band (
a
) and off-band (
b
)
filters and we have
omitted the overbar for notational convenience. The retrievals by this method have
been explained by Mori and Burton, (
2006
). Many re
nements, both technological
4
The residual current obtained when no light is falling on the detector.
