Agriculture Reference
In-Depth Information
The use of a tracer like SF
6
allows the calcu-
lation of the herd's average CH
4
emission rate
continuously throughout the day. Although the
calculations are straightforward based on Eqn
15.3, it requires the use of a technically demand-
ing (and expensive) sensor to monitor multiple
diluted gases simultaneously.
A modification of the EC technique is the
relaxed eddy accumulation (REA) technique,
where the gas sampled at an inlet is parti-
tioned into two streams corresponding to
when the vertical wind is either moving
upwards or downwards. This requires quick
response switching of air streams in order to
isolate the upward and downward winds. The
gas in each of two air streams is accumulated
and the emiss
ion
(
F
G
;
g m
−2
s
−1
) is determined
as
F
G
Micrometeorological techniques
(
)
brs c c
(from Baum and Ham,
2009), where
b
is a dimensionless factor,
r
is
the air density (g m
−3
),
s
w
is the standard devi-
ation of the vertical wind speed (measured
with a sonic anemometer; m s
−1
), and
c
is the
mixing ratio (e.g. ppb) of the upward (
U
) and
downward (
D
) moving gas.
=
−
W
U
D
Micrometeorological techniques rely on the
estimation of the ventilation rate of the air
affected by the source. Unlike the chamber
technique (FT-SS design) where the ventilation
rate of the chamber is relatively easy to meas-
ure inside ducts, and in the trace technique
where it is characterized using a tracer gas, the
micrometeorological technique requires that
the ventilation be estimated from wind meas-
urements. The main advantage of micromete-
orological techniques is that they are relatively
less intrusive compared with chambers that
modify the environment, or the enteric SF
6
tracer technique that requires that the animals
be handled daily.
Flux gradient (FG)
The FG technique uses measurements of gas
concentration at two heights made within the
internal boundary layer - the layer of air above
the surface that characterizes the energy, mass
and momentum exchanges at the surface. The
emission from the source is the product of con-
centration gradient
(
)
C
/ and a transfer
coefficient, known as the eddy diffusivity (
K
),
given in the relationship:
DD
Eddy covariance (EC)
The EC technique has been used for monitoring
emissions of CH
4
from grazing livestock (Dengel
et al
., 2011), and has potential for determining
NH
3
and CH
4
losses from large uniform manure
storage facilities. It requires fast-response sens-
ing of the vertical wind speed and associated gas
concentration. Essentially the emission (
F
G
; g s
−1
)
is the cross-product of the instantaneous verti-
cal wind speed (
w
′; m s
−1
), measured with a sonic
anemometer, and the instantaneous gas con-
centration (
C
′
G
; gm
−3
), usually averaged over a
period of 10-30 min (
F
G
=
w
′
C
G
). The technique
is technically demanding since it requires fast-
response sensors (typically at 10 Hz) to capture
the characteristic of the vertical eddies. It also
requires a large homogeneous manure source
to create a thick internal boundary layer.
Because of these requirements, the EC technique
has had a limited application to CH
4
emissions
from livestock, and CH
4
and NH
3
from livestock
manure storage facilities. It would be more
applicable to nitrogen-amended soils, as used by
Famulari
et al
. (2004).
(
)
(
)
=
--
kz
d z
-
d
F
1
2
DD
Cu
(15.4)
G
2
(
)
Sz
f
2
-
z
cm
12
where
k
is a constant (0.4; Von Karmen),
z
1
(lowest) and
z
2
are the heights (m) above the
surface,
d
is the zero plane displacement (m) that
is determined using data collected with a three-
dimensional anemometer,
S
c
is the Schmidt
number assigned a value of 0.63 (Flesch
et al
.,
2002),
Ø
m
2
is the non-dimensional correction
for the effect of thermal stability on the wind
profile, D
C
is the gas concentration (mg m
−3
) dif-
ference between
z
2
and
z
1
, and D
u
is the hori-
zontal wind speed (m s
−1
) difference between
z
2
and
z
1
. The value of Ø
m
is calculated as a func-
tion of the atmospheric stability (stable or unsta-
ble) and is given as:
-
025
.
for unstable conditions
z
L
æ
ç
ö
÷
f
m
=-
119
(15.5)
z
L
f
m
=15.
for stable conditions
(15.6)
