354 M.A. Sutton et al. Agricultural and Forest Meteorology 105 2000 351–369
the flow rate through the tube in relation to windspeed as constrained by the hole at one end determined at
0.77 by wind tunnel studies, Schjoerring et al., 1992. Since the denuders sample directly in proportion to
windspeed, division of F
hz
by the mean windspeed for a sampling period, provides the windspeed weighted
concentration χ
u
χ
u
= F
hz
¯ u
= uχ
¯ u
7 Net vertical fluxes of NH
3
are then estimated using the mean profiles of χ
u
z − d and uz − d substi-
tuted into Eqs. 1 and 3, respectively. In order for M to be sufficiently accurate, sampling periods are typi-
cally 3 days, and the stability terms ψ
H
and ψ
M
are therefore neglected. Stability effects are partly dealt
with, however, by preferentially sampling during peri- ods of higher windspeeds, when the error of neglecting
the stability correction is smallest. It is acknowledged that this approach contains large approximation and
empiricism. Nevertheless, it is of interest since it pro- vides a much cheaper means of estimating long-term
fluxes than the classical approaches.
3. Methodology
The following sections describe briefly the sam- pling systems applied to determine NH
3
concentra- tions, windspeed, temperature and humidity, from
which the turbulent exchange parameters and fluxes of NH
3
, sensible and latent heat were calculated. 3.1. Active ammonia sampling systems
The core measurement technique applied for deter- mining concentration gradients of NH
3
in this study was the AMANDA ammonia measurement by annu-
lar denuder sampling with on-line analysis continuous wet denuder system Wyers et al., 1993. Two indepen-
dent systems were operated CEH, UPM, each pro- viding determination of NH
3
concentrations with three separate inlets. This allowed for replicate sampling
by the two systems independently at three heights, or having established agreement or inter-calibrated be-
tween the systems, at six heights. The latter approach was applied for parallel measurements of above- and
within-canopy concentration profiles as reported by Nemitz et al. 2000a.
Each of the AMANDA systems used consists of three annular wet rotating denuders sampling air
at approximately 25 l min
− 1
, connected to a com- mon detection and switching system for analysis and
logging of the amount of NH
3
trapped into acidic solution as NH
4 +
. Air flow rates through the de- nuders are controlled by critical flow orifices, with
flow rates checked regularly by dry gas meters. The wet denuders are oriented horizontally, and supplied
with a solution of 0.5 g l
− 1
NaHSO
4
· H
2
O containing 0.2 ml l
− 1
38 vv HCHO as a biocide. A con- stant liquid level in the denuder is maintained by the
use of two peristaltic pumps, one of which empties the denuder at a constant rate, while the other one,
controlled by electrodes determining conductivity along the denuder, refills the denuder to a constant
level. The collection solution is typically abstracted from the denuders at a rate of 1.5 ml min
− 1
, and NaOH is added to the sample, thereby converting
the NH
4 +
to NH
3
in solution. The sample is then passed over a Teflon membrane with NH
3
diffusion into a counter flow of deionized water; the NH
3
re-ionizes to NH
4 +
and is detected by conductivity. As it has been established that the sampling efficiency
of the denuder inlets is close to 100 Wyers et al., 1993, calibration is performed using aqueous NH
4 +
standards. The filter pack system operated by CEH followed
the design applied by Harrison et al. 1989, Sutton 1990 and Sutton et al. 1993a. Up to 10 separate fil-
ter packs using 90 mm diameter filters were used both above and within the canopy Nemitz et al., 2000a.
The filter packs consisted of three PTFE stages, sam- pling first aerosol on 1 mm pore size PTFE Teflon
membrane filters Micro Filtration Systems, Dublin, CA; Cole-Parmer, Hanwell, London, then, option-
ally, acid gases on NaF impregnated Whatman 41 fil- ters, and finally NH
3
on Whatman 42 filters impreg- nated with 2 H
3
PO
4
. Paper filters were extracted by shaking for 30 min in 10 ml of deionized H
2
O and analysed by a laboratory NH
4 +
analysis system AMFIA: ammonium by flow injection analysis op-
erating on a similar principle to that described for the AMANDA. The extraction of the PTFE filters
and analysis of anions is described by Nemitz et al. 2000c.
M.A. Sutton et al. Agricultural and Forest Meteorology 105 2000 351–369 355
3.2. Passive ammonia sampling systems Two types of passive horizontal flux samplers were
applied: the oriented denuder system of Schjoerring 1995 operated by RVAU and a wind-vane mounted
‘shuttle’ system following the design of Leuning et al. 1985 operated by ADAS. The denuder system pro-
vided fluxes using four double denuders angled at 90
◦
to each other, each 2 × 0.1 m
2
long and 6 mm i.d. at each height, with the profile measured at four heights
16 samplers per profile. In these samplers NH
3
is captured onto the inner wall of the denuders which is
coated with 3 wv oxalic acid in acetone. For the shuttles one shuttle at each of eight heights, a sim-
ilar coating is used, but in this case the surface is a mesh of oriented stainless steel foil. Three consecu-
tive runs were made with the RVAU passive denuders and one run with the ADAS shuttles, with each group
using two replicate masts per run.
3.3. Determination of micrometeorological parameters
Windspeed and turbulent exchange parameters were measured using both a combination of wind and tem-
perature profiles, and several ultrasonic anemometers. The wind profile CEH was measured at five heights
above the canopy using Vector A100R cup anemome- ters Vector Instruments, Clywd, UK. Two point pro-
files of temperature above the canopy were obtained using a Campbell system Campbell Scientific, Lough-
borough, UK, which also provided profile measure- ments of water vapour pressure from a cooled mirror
sensor. This system operated by CEH was also used to measure net radiation R
n
and soil heat flux G, with data recorded on a Campbell 21X datalogger.
Three different ultrasonic anemometers type So- lent Research 1012RA, Gill Instruments, Lymington,
Hants., UK were available, which because of different logging systems provided four estimates of windspeed,
u
∗
and H. Two systems CEH were logged digitally using the eddysol software Moncrieff et al., 1997,
while the signal of one of these was also processed by an analogue data acquisition system UMIST. The
third anemometer was recorded by ECN software.
In addition to the profile estimates of λE a fast-response Krypton hygrometer KH
2
O, Campbell Scientific, Loughborough, UK was connected to the
ECN ultrasonic anemometer and referenced against a Rhotronic combined temperature and humidity sen-
sor ECN. Further independent estimates of λE were calculated assuming closure of the energy balance
λE = R
n
− G − H
. Additional cup anemometer wind profiles using
four heights were made in parallel to the passive flux samplers RVAU, ADAS, and these provided the time
averaged windspeed over the duration of each passive sampling run.
3.4. Field site and micrometeorological restrictions Measurements were carried out over an extensive
ca. 30 ha oilseed rape field B. napus — cv. Ex- press, which had received 285 kg N ha
− 1
, mainly as ammonium nitrate, since the previous autumn. A
plan of the measurement site has been provided by Sutton et al. 2000, indicating that the main wind
sectors for the measurements were in the SW–NW and NE–SE. This resulted in insufficient fetch for
either N or S winds, although these only occurred for a small fraction of the experiment. During the first
measurement period 7–25 June 1995 the crop was rather even at 1.38 m tall measured using a plastic
plate resting lightly on the canopy. The microme- teorological measurements were mostly made in the
range 0.2–2 m above canopy or 1.6–3.4 m above ground. The canopy during this period was at the fi-
nal stages of flowering, with seed filling progressing. Following cutting on 22 July, the field was banded
with E–W oriented rows of alternately 0.8 m high cut crop 1.8 m wide and 0.25 m high 1.5 m wide
standing stems see Sutton et al., 1996, cover pho- tograph. To ensure measurements were above the
roughness sublayer, profiles were determined in the range 1.8–3.9 m above ground. The sensors for the
gradient measurements were spaced exponentially within the height ranges stated to maximize the ability
to determine the expected log-linear profiles see e.g. Fig. 4.
A number of restrictions and corrections need to be recognized in analysing micrometeorological mea-
surements, and the data were corrected for: a chang- ing air concentrations and consequent storage errors
Fowler and Duyzer, 1989 and b density correc- tions due to parallel fluxes of sensible and latent heat
Webb et al., 1980. A further restriction and correc-
356 M.A. Sutton et al. Agricultural and Forest Meteorology 105 2000 351–369
tion would apply if fluxes are not conserved due to gas–particle reactions above the canopy. This issue is
addressed separately by Nemitz et al. 2000c, where it is shown that above-canopy reactions would have
had little effect on the calculated NH
3
fluxes during the North Berwick experiment. For the analysis of the
measured fluxes, three levels of data filtration and cor- rection were considered:
1. A general filtering of data to remove major prob-
lems, such as fetch interruptions and denuder mal- functioning.
2. A more rigorous filtering to exclude conditions where the flux is likely to be estimated with less
certainty, such as very stable conditions or where the contribution of the rape field to the flux at the
top height was less than a defined value, according to a foot-print analysis.
3. Application of correction procedures to the remain- ing data to account for storage errors and density
corrections. For the foot-print analysis Schuepp et al., 1990
data were accepted if the field contributed to the top NH
3
sampling height by 65 or more, equivalent to almost 100 at the middle and bottom sampling
heights.
Fig. 1. Comparison of friction velocity u
∗
as measured with three ultrasonic anemometers logged digitally and an anemometer profile during the main campaign 20–21 June after filtering. For clarity, the mean of estimates is not shown. Heights are shown above ground.
4. Results
