Agriculture Reference
In-Depth Information
where z/
L
is the Monin-Obukhov parameter and
L
(Monin-Obukhov length) can be determined
using the sensible heat flux (
H
) and friction
velocity (
u
*
) as derived from the output from
a three-dimensional anemometer at height
z
(
L
<0 unstable conditions;
L
>0 stable conditions).
The FG technique has been used to esti-
mate enteric CH
4
emissions from livestock by
Judd
et al
. (1999) and Laubach
et al
. (2008),
and NH
3
from beef feedlot manure by Todd
et al
. (2005).
regressed wind speed and concentration against
ln(z) and expressed
F
G
as a function of the regres-
sion coefficients.
An alternative approach to the IHF tech-
nique is to use a line-averaged concentration
along each perimeter (not just at a single point),
known as the mass difference (MD) technique.
This technique was used by Denmead
et al
.
(1998) and Harper
et al
. (1999) as a direct meas-
ure of CH
4
from confined cattle. The calculation
of
F
G
is as follows:
(
)
+
(
)
å
2
z
ë
û
D
Mass balance (IHF)
FX
=
UC C VCC
-
-
z
(15.8)
G
4
2
3
1
z
In a similar way that the difference in mass of
the gas entering and leaving a chamber is used
to calculate (along with the airflow in the ducts)
the emission from the enclosed source, so too
can the emission from a source in the open
(non-enclosure) be estimated. In this case, the
incoming and outgoing air over the source is
characterized by dividing it into layers, where
the wind speed and gas concentration is meas-
ured in each layer. The difference between
incoming and outgoing gas concentration in
each layer is then multiplied by the correspond-
ing wind speed - this horizontal flux is then
summed for all layers to yield the integrated
horizontal flux (IHF):
where
X
is the perimeter length (m) of the square
source area,
-
and
-
are the wind speeds (m s
−1
)
perpendicular to the perimeter orientation, and
-
4
and
-
3
are the line-averaged concentrations
(g m
−3
) along the downwind perimeters, while
-
2
and
-
1
the concentrations along the upwind
perimeters.
The advantage of the mass balance (MB)
technique is that it is simple to set up and does
not require a great deal of expensive instru-
mentation. For example, acid traps can be used
for NH
3
sampling along with cup anemometers
to yield net NH
3
emission over an extended
time frame (e.g. 24 h). This technique as well
does not require atmospheric stability correc-
tions. One disadvantage of the technique is the
need to use multiple sampling measurements
to correctly characterize the vertical concen-
tration and wind speed profiles associated with
the source.
1
(
)
å
ë
û
D
F
=
z
2
u
CC
z
(15.7)
-
x
O
I
G
z
1
where
x
(m) is the distance between the incom-
ing and outgoing perimeters (fetch),
−
is the
time-averaged wind speed (m s
−1
; at mid-point of
a perimeter) in a layer, and
-
O
and
-
I
are the
outgoing and incoming time-averaged concen-
tration (g m
−3
; at a single point mid-way along
the perimeter).
Typically, at least four measurement
heights are used covering the complete internal
boundary layer (the lowest height where
C
O
is
equal to
C
I
). The IHF technique was employed by
Laubach
et al
. (2008) to measure CH
4
emission
from a small number of penned cattle. This
technique was used for CH
4
emissions from
circular (
c
.11 m diameter) open-top tanks filled
with dairy manure (Vanderzaag
et al
., 2011).
However, for larger area sources, with a high
internal boundary layer, it may not be feasible to
measure over the complete internal boundary
layer. In this case, Ryden and McNeill (1984)
Energy balance (EB)
The energy balance approach makes use of the
concept that the transfer of a gas is equated to
that for water vapour and heat in a specific layer
above a uniform surface (within the internal
boundary layer). Denmead
et al
. (1974) deter-
mined this transfer coefficient (
h
; m s
−1
) using
the relationship
(
)
RG
CT
p
-
(15.9)
h
=
r
D
e
where
R
is the net radiation at the surface
(W m
−2
),
G
is the soil heat exchange (W m
−2
),
r
is
the air density (kg m
−3
),
C
p
is the specific heat of
air (kJ kg
−1
°C
−1
) and D
T
e
is the equivalent tem-
perature gradient (°C; equivalent temperature is
