Agriculture Reference
In-Depth Information
the procedure is to express the emission on a per
unit area of the surface basis (i.e. Eqn 15.2b).
The unique characteristic of using the
FT-SS chamber is that the monitored gas con-
centration in the air stream passing through
the chamber is associated with a known and
constant air volume contained by inlet and
exhaust ducts. When initially designing the
chamber, it is essential that the exhaust con-
centration (
C
O
) is appropriate for the gas
sensor concentration range; this yields the
optimum sensitivity of the instrument. This is
accomplished by assuming relevant values for
F
G
,
f
and
C
I
in Eqn 15.2. It is also useful to calculate
the time constant of the chamber, i.e. the time
it takes for the chamber air to reach 63% of a
step change in emission. A value of 3 times
the time constant is needed to reach 99% equi-
librium of the step change. For example, open-
ing the chamber door when accessing the
animal causes a drop in chamber CH
4
concen-
tration (as reflected in the exhaust concentra-
tion). Once the door is closed, there is a
build-up of CH
4
inside the chamber that will
require a time equal to 3 times the time con-
stant to reach near steady-state once again.
The time constant of a chamber acts as a
filtering function, in that it smoothes the
response time of the chamber to changes in
the source strength.
The FT-SS design is applied to measure CH
4
emissions from ruminants using: (i) face masks;
(ii) head hoods; (iii) whole-animal enclosures;
(iv) tunnels; and (v) barns treated as enclo-
sures. These enclosures are typically calibrated
by releasing a gas at a known rate, e.g. using a
mass flow controller, and verifying the emission
using Eqn 15.2. For example, Place
et al
. (2011)
developed a head hood for cattle that was
shown to recover 98-99% of a known release
rate of gas. The release-recovery data can be
used to remove the between-enclosure differ-
ences when using more than a single enclo-
sure. That will improve the sensitivity of the
experimental design to treatment differences.
The use of a Latin square experimental design
where more than one chamber is used (where
each enclosure is used in turn for each treat-
ment) can also be used to remove between-
enclosure differences, thus further improving
the sensitivity of enclosure measurements to
treatment differences.
Biogas production
A simple technique is a commonly used approach
to monitor gas production (e.g. biogases) from
anaerobic lagoons. In this design, the active gas
production is measured by directly monitoring
the total gas volume produced over time, and per-
centage composition of the gas (gases) in the pas-
sive off-venting from a chamber on the surface of
a lagoon. This approach was used by Craggs
et al
.
(2008) using a surface chamber to measure CH
4
production. A modified version of the technique
was used by Harper
et al
. (2004), who measured
the total volume of gases (e.g. NH
3
), and per-
centage composition of each gas over time, from
submerged open-bottom carboys. The simplicity
of the approach is a definite advantage; however,
concerns remain as with all chambers that
modify the temperature and airflow regimes
normally associated with open lagoons.
Tracer release techniques
The emission of a gas can be determined by
relating a target gas of interest to a second gas
(tracer) with a known release rate and a co-
located concentration measurement of the gases.
The underlying assumption is that the transport
of the tracer and targeted gas is the same
along the common dispersion path. As a result,
the relationship (Eqn 15.3) of the emission-to-
concentration ratio (where the concentration
terms are corrected for background concentra-
tion) along the dispersion path is identical for
both gases. Therefore, knowing the target and
tracer gas concentrations (
C
G
and
C
T
, respec-
tively) and the tracer release rate (
F
T
), the gas
emission (
F
G
) can be calculated as:
F
C
F
C
FF
C
C
G
=
T
rearranging
=
G
(15.3)
G
T
G
T
T
This technique has been applied to individual
ruminants to measure CH
4
emissions associated
with enteric fermentation (enteric tracer tech-
nique) where the tracer is released in the rumen.
A second approach is to release a tracer gas into
the air in proximity to the source of the emission
(atmospheric tracer technique). With few excep-
tions, the tracer gas commonly used is sulfur
hexafluoride (SF
6
). This is a straightforward
technique but assumes dispersion is the same
between gases.
