Agriculture Reference
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(McGinn et al ., 2009) to be sufficiently precise
to allow evaluation of the differences in CH 4
emission from cattle under different dietary
treatments. There are also disadvantages, which
include the need for expensive instrumentation
to measure: (i) wind statistics needed to estimate
atmospheric stability; and (ii) background con-
centration and concentration downwind from the
source. Another limitation of the BLS technique
is the loss of emission data when atmospheric
conditions are not conducive to the assumptions
in the BLS model (Flesch et al ., 2007).
For the BLS dispersion technique, McGinn
et al . (2009) found that the BLS underestimated
CH 4 emissions from penned cattle by 7% relative
to the SF 6 tracer technique. In this application,
the gas plumes from the individual animals were
estimated, and the animal position was meas-
ured using global positioning devices. Laubach
et al . (2008) reported a good fit of CH 4 emissions
from confined cattle over a period of days
between the BLS, FG and IHF techniques, with
the provision that a separation distance of cattle
and sensors be at least 20 m for the FG and IHF
techniques. In this application, the emissions
were treated as a uniform area source.
A good fit of emission data existed using the
BLS dispersion and the IHF techniques for NH 3
emissions from stocked beef cattle manure
(Sommer et al ., 2004). However, the use of static
chambers (NFT-NSS) was shown in this same
study to underestimate greatly the emissions of
several gases including CH 4 by at least 78% rela-
tive to the micrometeorological techniques. The
cause of this underestimation may be related to
obstruction to airflow from the pile, which vented
in a similar fashion to a series of chimneys.
Shah et al . (2006) reported on several
attributes of enclosures and three micro-
meteorological techniques, the IHF, FG and EC,
all used to estimate NH 3 emissions. They rated
enclosures to have low-medium reliability while
the IHF had high reliability (FG and EC not rated
on this attribute).
Comparison of Techniques
Differences in emissions between treatments can
result from instrumentation error in making the
emission measurements, and error in the emis-
sion technique (e.g. invalid assumptions). It is rec-
ommended where possible that more than one
technique be used to evaluate emissions in order
to develop an appreciation for variability attributed
to the measurement system. Alternatively, com-
parison of emission data by different studies may
aid in understanding variability due to technique,
but clearly these comparisons will be confounded
by differences in the studies such as animal and
diet factors. For example, Boadi et al . (2004) com-
pared CH 4 emissions from dairy cows in different
studies that also used different techniques and
reported a wide range in CH 4 emissions varying
from 286 to 433 g per animal per day.
Methane emissions for beef cattle and sheep
have been compared using different techniques
within a single study (Table 15.1). The SF 6
tracer technique was reported to be 93-95% of
that measured using whole-animal chambers
(Johnson et al ., 1994; Ulyatt et al ., 1999; McGinn,
2006) and 105% of that measured using hood
chambers (Boadi et al ., 2002). Grainger et al .
(2007) found the SF 6 tracer technique to overes-
timate the chamber technique by 2%. Pinares-
PatiƱo et al . (2008) also compared the two
techniques, and found that the SF 6 tracer tech-
nique (coefficient of variation, CV, ranged
from 7.8 to 18.4) was more variable compared
with the chamber technique (CV 4.3-7.7). The
higher CV of the SF 6 tracer technique was
inferred to be caused by a change in the SF 6
tracer release rate.
Conclusions
Before initiating any emission measurement
study, the possible quantitative measurement
systems need to be scrutinized to avoid known
limitations associated with each technique. This
involves understanding the assumptions under-
lying the techniques, and the advantages and
disadvantages for a given technique within the
context of the study parameters. Of key consid-
eration is a needed knowledge of the accuracy
and precision of each technique to ensure the
results will be applicable to the treatments (usu-
ally looking at mitigation strategies) being used
in mitigation studies. In this case, the precision
must be greater than the differences in emis-
sions imposed by the treatments.
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