Ammonia emissions from a grazed field estimated by miniDOAS measurements and inverse dispersion modelling
2016
Ammonia (NH3) fluxes were estimated from a field being
grazed by dairy cattle during spring by applying a backward Lagrangian
stochastic model (bLS) model combined with horizontal concentration gradients
measured across the field. Continuous concentration measurements at field
boundaries were made by open-path miniDOAS (differential optical absorption
spectroscopy) instruments while the cattle were
present and for 6 subsequent
days. The deposition of emitted NH3 to clean patches on the field
was also simulated, allowing both net and gross emission estimates,
where the dry deposition velocity (vd) was predicted by a canopy
resistance (Rc) model developed from local NH3 flux and
meteorological measurements. Estimated emissions peaked during grazing and
decreased after the cattle had left the field, while control on emissions was
observed from covariance with temperature, wind speed and humidity and wetness
measurements made on the field, revealing a diurnal emission profile. Large
concentration differences were observed between downwind receptors, due to
spatially heterogeneous emission patterns. This was likely caused by uneven
cattle distribution and a low grazing density, where hotspots of
emissions would arise as the cattle grouped in certain areas, such as around
the water trough. The spatial complexity was accounted for by separating the
model source area into sub-sections and optimising individual source area
coefficients to measured concentrations. The background concentration was the
greatest source of uncertainty, and based on a sensitivity/uncertainty
analysis the overall uncertainty associated with derived emission factors
from this study is at least 30–40 %.Emission factors can be expressed as 6 ± 2 g NH3 cow−1 day−1, or 9 ± 3 %
of excreted urine-N emitted as NH3, when deposition is not simulated and 7 ± 2 g NH3 cow−1 day−1, or 10 ± 3 % of excreted urine-N emitted as
NH3,
when deposition is included in the gross emission model. The results suggest
that around 14 ± 4 % of emitted NH3 was deposited to patches
within the field that were not affected by urine or dung.
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