408 E. Nemitz et al. Agricultural and Forest Meteorology 105 2000 405–425
Fig. 2. The dependence of the canopy compensation point χ
c
and the net flux F
t
on the air concentration χ
a
for constant values of the total atmospheric resistance R
a
+ R
b
= 30 s m
− 1
, cuticular resistance R
w
= 100 s m
− 1
, stomatal resistance R
s
= 50 s m
− 1
and stomatal compensation point χ
s
= 1.1 mg m
− 3
. The true canopy compensation point χ
c
is the value for χ
c
at F
t
= 0.
where χ
c
is here termed a ‘canopy compensation point’, although unlike the stomatal compensation
point, it is not only a function of the physiological state of the canopy but depends also on χ
a
Fig. 2. Strictly, the true canopy compensation point χ
c
denotes the mean concentrations when all emission and deposition processes within the canopy balance
in such a way that the net flux vanishes Sutton et al., 1998. For the model of Fig. 1 this is the case when
stomatal emission is balanced by cuticular adsorption. Nevertheless, χ
c
remains the central term in resolving the net effect of within-canopy component emission
and deposition fluxes Eq. 2. As a further modification of this single-layer
model, Sutton et al. 1998 treated cuticular adsorp- tiondesorption fluxes of NH
3
in a dynamic approach as an electric capacitor with a surface charge χ
d
which may be released when water layers evaporate in the morning. Flechard et al. 1999 predicted χ
d
by modelling the full leaf surface chemistry. The present
study, however, concentrates on multi-layer modelling and is restricted to resistance calculations that change
only with micrometeorological conditions, but not according to the emissiondeposition history of the
canopy.
3. Summary of the measurement results at North Berwick
An overview over the measurements carried out dur- ing the North Berwick campaign 6–27 June 1995 was
given by Sutton et al. 2000a. The NH
3
net exchange flux, measured with the aerodynamic gradient tech-
nique using a three-point continuous denuder analyser, was presented by Sutton et al. 2000b. These measure-
ments show the diurnal cycle typical for agricultural canopies with emission of up to 150 ng m
− 2
s
− 1
dur- ing the day, consistent with stomatal emissions. How-
ever, as exemplified in Fig. 3, night-time emission was also observed which indicated a source in addition to
leaf tissues, since R
s
is very large during night. One possible non-stomatal source is the evaporation of wa-
ter layers on leaf cuticles Sutton et al., 1998. How- ever, this would not be able to explain the persistence
of these emissions, nor the positive relationship to the relative humidity h in the canopy, e.g. nights from
11 to 12, and from 12 to 13 June see also Fig. 9b.
Measurements of vertical concentration gradients inside the canopy showed the highest NH
3
concentra- tion close to the ground, indicating that decomposition
of fallen leaf litter contributed to these emissions. The
E. Nemitz et al. Agricultural and Forest Meteorology 105 2000 405–425 409
Fig. 3. Example results of the application of the 1-layer χ
s
–R
w
model to measurement data from North Berwick for 10–13 June 1995. The measured flux F
t
is shown together with the modelled fluxes for a measured apoplastic ratio [NH
4 +
][H
+
] Γ
s
of 390 and an arbitrary value of Γ
s
= 1200. Also shown is the component stomatal flux F
s
for Γ
s
= 1200. The flux to the leaf cuticle F
w
is given as the difference between F
t
Γ
s
= 1200 and F
s
.
application of an inverse Lagrangian technique ILT to infer the vertical distribution of sources and sinks
in the canopy showed that during the day the ground level emission was recaptured at mid canopy Nemitz
et al., 2000a. With a mid-canopy flux close to zero the sourcesink analysis predicted daytime NH
3
net emis- sion to originate from the top of the canopy, where
the siliques seed cases are located. However, dur- ing night there was a strong dependency of the sign
of the net flux on the value of the friction velocity u
∗
Fig. 4. This suggests that, during high turbu- lence conditions, the ground level leaf litter emission
Fig. 4. The ratio of the number of observed nocturnal NH
3
emis- sions 10 min values to total observations for different classes of
u
∗
20:00–4:30 GMT.
reported by Nemitz et al. 2000a penetrated through the canopy, whereas it was captured by the canopy for
small values of u
∗
. Likely reasons are i that the turbu- lent resistance within the canopy R
ac
, and hence the time-scale for the transport, decreases with increasing
u
∗
, ii that high turbulence is often associated with lower h and less dewfall Monteith and Unsworth,
1990, reducing recapture within the canopy, and iii that an increased NH
3
flux through the canopy as a re- sult of i and ii leads to saturation of the water layers.
Although rather uncertain in this respect, the ILT esti- mated an average leaf litter emission of 32 ng m
− 2
s
− 1
with peaks of up to 150 ng m
− 2
s
− 1
, while indepen- dent measurements with a soil chamber ranged from
10 to 50 ng m
− 2
s
− 1
. Husted et al. 2000 estimated the stomatal com-
pensation points χ
s
of leaves at different heights, from the ratio of [NH
4 +
][H
+
] Γ
s
in the apoplas- tic fluid, extracted by vacuum infiltration in a mobile
laboratory in the field. These values were consistently lower than the air concentrations at which emission
occurred, suggesting that during daytime leaf stom- ata acted as a sink for the ground level emission.
Unless the values of Γ
s
are severely underestimated, the only possible source of the net emission during day
are therefore the siliques. Unfortunately, the apoplas- tic extraction technique has not yet been extended to
seed cases. However, in an independent study the bulk NH
4 +
concentrations of oilseed rape siliques were
410 E. Nemitz et al. Agricultural and Forest Meteorology 105 2000 405–425
found to be two to five times higher than those of the foliage of the same plants Husted et al., 2000. Direct
daytime measurements of bulk tissue [NH
4 +
] and pH of the litter leaves by the same authors showed a high
temporal and spatial variability. Estimated pH values of leaf litter ranged from 5.1 to 5.4 and [NH
4 +
] from 5.0 to 56.5 mM, probably due to differences in de-
composition stage and humidity. These variations are reflected in the value of the ratio [NH
4 +
][H
+
] of the litter Γ
l
. In the following sections, the single-layer χ
s
–R
w
model is used as the basis for the development of more detailed models including exchange with leaf
litter and siliques, capable of reproducing these mea- surements. Both the original model and the extended
models are then applied to the NH
3
fluxes measured over oilseed rape.
4. Model development
