M.A. Sutton et al. Agricultural and Forest Meteorology 105 2000 351–369 353
from different sampling techniques. Fluxes measured before and after cutting are compared, and an initial
inter-comparison is also made between the classical flux estimates and those from the passive flux sam-
plers. The objective of this inter-comparison was to see whether the passive approach could reproduce
the net flux over the campaign as measured by the classical aerodynamic gradient method. The microm-
eteorological parameters and continuous NH
3
fluxes presented here form the basis for further analysis and
modelling as reported by Nemitz et al. 2000a–c.
2. Micrometeorological theory
2.1. Classical aerodynamic gradient method The principle of the aerodynamic gradient method
as applied here has been described in detail by Sutton et al. 1993a,b. The flux of NH
3
was calculated from vertical concentration profiles of NH
3
coupled with turbulence information from either combined wind
and temperature profiles or ultrasonic anemometers providing direct estimates of friction velocity u
∗
, sensible heat flux H. The NH
3
concentration profile was used to determine a friction concentration χ
∗
from which the micrometeorological flux estimate F was calculated as
χ z − d = χ
∗
k lnz − d−ψ
H
z − d L
− const.
1 F = −u
∗
χ
∗
2 where χ z − d is the air concentration at height z
above the zero plane displacement d, k the von Kár- man constant 0.41, L the Monin–Obukhov length,
and ψ
H
a semi-empirical correction for atmospheric stability being a function of z−dL. A similar func-
tion to Eq. 1 was applied for the wind profile esti- mate of u
∗
. uz − d =
u
∗
k lnz − d − ψ
M
z − d L
− u
∗
k lnz
3 where ψ
M
is the stability correction relevant for mo- mentum transfer and z
is the roughness length of the canopy. A combination approach to the aerodynamic
gradient method was applied when using the ultra- sonic anemometer estimates of u
∗
, which were calcu- lated by eddy covariance. This was found as the mean
of the product of the deviations of instantaneous net vertical w
′
and horizontal u
′
windspeed from the mean values
u
∗
= q
− u
′
w
′
4 The aerodynamic gradient method and eddy covari-
ance were similarly applied to determine fluxes of both sensible heat H and latent heat λE, from gra-
dient or fast-response measurements of temperature and vapour pressure, respectively. In the case of eddy
covariance measurements of entrained properties the flux was calculated as
F
c
= − q
′
w
′
5 where q is the scalar of the entrained property. For
further details, see Moncrieff et al. 1997. 2.2. Passive flux sampling approximation to the
aerodynamic gradient method In the classical gradient method, NH
3
is sampled at a constant rate, providing time-weighted aver-
age concentrations. This contrasts with the passive flux sampling approach, where the NH
3
denuders sample NH
3
in proportion to windspeed, resulting in windspeed-weighted averages. The method uses
hollow samplers, coated internally with an NH
3
ab- sorbing surface, which are placed horizontally at
several heights, either mounted in four fixed direc- tions from which average concentrations may be
calculated, Schjoerring, 1995 or mounted on a wind vane and thereby kept facing into the wind Hansen
et al., 1998. Given the tube dimensions, a known relationship is established between windspeed and
sampling rate allowing estimation of the horizontal flux at a particular height F
hz
. In the simplest case of a denuder directly oriented into the wind
F
hz
= M
π r
2
K
1
1t 6
where M is the mass of NH
3
trapped by the tube, r the radius of a constraining hole at the rear of the tube
typically 0.5 mm and K
1
a correction factor defining
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
