E. Nemitz et al. Agricultural and Forest Meteorology 105 2000 427–445 439
can be ruled out, calculated according to Eq. 4. Since condensation is expected to take place to any
hydrophilic aerosol Wexler and Seinfeld, 1992, the limitation to NH
4 +
aerosol may overestimate τ
∞
. The aerosol was only analysed for NH
4 +
, Cl
−
, NO
3 −
and SO
4 2−
but not, for instance, for Na
+
, Ca
2+
or Mg
2+
. Thus, a best estimate of the mass of total hy- drophilic aerosol was obtained as follows: if NH
4 +
was more than balanced by the sum of the measured anions, any Cl
−
in excess was thought to represent NaCl. By contrast, excess NH
4 +
was interpreted as resulting from an uncertainty in the NO
3 −
measure- ment. Although, the concentration of additional Cl
−
interpreted as NaCl was high during some runs, it only contributed a minor fraction to the total aerosol
surface owing to its large MMD Table 1. Hence, the net effect was only marginally smaller values of
τ
∞
larger u
∗ crit
Table 3. For comparison, measured values of u
∗
ranged from 0.004 to 1.0 m s
− 1
. Values of u
∗
0.03 m s
− 1
, the largest values of u
∗ crit
derived here, were found for less than 2 of the time. How-
ever, the applicability of the aerodynamic gradient technique is restricted at such low turbulence and
fluxes are very small.
4. Discussion
4.1. Exchange of aerosol The aerosol V
d
measured at North Berwick showed considerable variability Table 2. In other studies V
d
has been shown to change with particle size, fric- tion velocity, atmospheric stability and boundary layer
height e.g. Wesley et al., 1985, and a certain range of values is therefore to be expected. Possible arte-
facts include gas–particle interactions on the pre-filter of filter-packs Andersen and Hovmand, 1994 and
errors due to sequential sampling at different heights during non-linear concentration changes Sutton et al.,
1993b. The latter problem could explain why the SJAC detected NH
4 +
aerosol emission for 50 of the time Table 2. However, the filter-pack measurements,
though often not significant at P = 0.05, showed the same feature. Aerosol emission has been reported in
several other studies Katen and Hubbe, 1985; Gal- lagher et al., 1997a and it could well be real, espe-
cially considering the possibility of aerosol sources within the canopy. As an average over all emission sit-
uations, NH
4 +
contributes 25 to the emission of total reduced nitrogen NH
4 +
+ NH
3
, which constitutes an upper limit of the effect of gas-to-particle conversion
at this site. In agreement with theory, the median V
d
of NH
4 +
measured with the SJAC increased with u
∗
, whereas substantial emission periods cause the over-
all arithmetic mean to be negative. The mean V
d
mea- sured with filter-packs increased with the particle size
of the different species cf. Tables 1 and 3, although the values are not significantly different from each
other, due to their large standard deviation. The val- ues of V
d
for NO
3 −
and Cl
−
are large compared with other measurements Duyzer, 1994; Gallagher et al.,
1997b. 4.2. Emission of HCl
Due to small HCl concentrations the calculated flux is somewhat uncertain. However, the diurnal pattern in
the scaling parameter χ
∗
of HCl and the coincidence of emission with periods of high HCl air concentra-
tions, strengthen the confidence in the measurements, indicating that the emissions are a real phenomenon.
The HCl emission was on average 20 ng m
− 2
s
− 1
, with peaks of up to 120 ng m
− 2
s
− 1
Fig. 3, and HCl emission was also indicated by the in-canopy profiles
Figs. 5d and 6d. Due to the scatter in the gradients measured above the canopy and the limited number
of filter-pack runs within the canopy, each observation on its own is rather uncertain. However, combined,
they make a strong case for the occurrence of HCl emission from the oilseed rape canopy. To this may be
added the measurement of high Cl
−
concentrations of 12.7 mM in the apoplastic liquid of the oilseed rape
leaves Husted et al., 2000. The average Cl
−
aerosol concentration measured with the cascade impactor at
this coastal site was 1.1 mg m
− 3
, but this average does not cover maritime wind directions, during which the
canopy should be expected to receive a much larger Cl
−
loading. Although the Cl
−
within the canopy was captured as Cl
−
aerosol by the filter-pack it is doubtful that it was released as aerosol from the plant for several reasons:
i the only filter-pack run during which gaseous HCl was measured alongside Cl
−
aerosol showed higher concentrations of gaseous HCl than of Cl
−
aerosol, ii the batch denuders detected upward gradients of HCl,
440 E. Nemitz et al. Agricultural and Forest Meteorology 105 2000 427–445
and iii at usually small u 1.4–2.2 m s
− 1
there was no efficient mechanism by which particles could have
been released from the canopy. By contrast, potential sources of gaseous HCl emission or apparent emission
include.
a Emission of HCl through the leaf stomata. Even using a high T 25
◦
C and the lowest measured pH 4.5 the large apoplastic Cl
−
concentration would have resulted in χ
s
HCl 4 ng m
− 3
Brimble- combe and Clegg, 1990, two orders of magnitude
smaller than χ HCl at which emission was ob- served. Stomatal emission of HCl from live leaves can
therefore be ruled out as the cause of the emission of Cl compounds observed, unless there were localized
sites at which the Cl
−
concentration was much larger or the pH smaller.
b Evaporation of HCl from the leaf cuticle, leaf surface chemistry. Unpublished field data obtained by
the authors indicate that emission peaks of HCl and HNO
3
due to cuticular desorption can occur when concentrations rise in evaporating water-layers, al-
though this process should be expected to be limited to the morning hours Sutton et al., 1997. Neftel et al.
1996 attributed HNO
3
emission gradients to the re-evaporation of NH
4
NO
3
from leaf cuticles when leaves heat up in the morning. Another source of HCl
could be reactions on leaf surfaces, e.g. Cl
−
in de- posited sea salt could be replaced by HNO
3
or H
2
SO
4
; both substitution reactions have been shown to be ef-
ficient Rossi et al., 1995; Clegg and Brimblecombe, 1985. However, since the HNO
3
concentration was of the same order of magnitude as HCl Fig. 2, all
deposited HNO
3
would have had to be converted into HCl to yield the emission flux observed.
c Emission of CH
3
Cl. Rather than representing HCl, the gaseous Cl detected could have represented
organic Cl compounds: Saini et al. 1995 reported biogenic emissions of halomethanes from 87 plant
species and showed that Brassica oleracea has one of the highest activities of the methyl-transferase in-
volved in the production of halomethanes. High emis- sions of CH
3
Cl have also been found to originate from wood-rotting fungi Harper, 1985 and many oilseed
rape leaves showed significant fungal decomposition. However, here the maximum collection efficiency of
the batch denuder for CH
3
Cl was estimated to be 12, based on kinetic data for the hydrolysis of CH
3
Cl at T = 100
◦
C Fells and Moelwyn-Hughes, 1959, and it is probably much by a factor of 10–100 less at
ambient T. Other biogenic chlorinated hydrocarbons appear to be even less water soluble. The CH
3
Cl emis- sion flux would have had to be by a factor of 10–100
larger than the HCl flux, derived for Fig. 3 under the assumption of perfect adsorption, to mask a HCl depo-
sition flux efficiently. From CH
3
Cl production rates of Brassica oleracea plants Saini et al., 1995, emission
densities of up to 2 ng Cl m
− 2
s
− 1
may be estimated. Therefore, CH
3
Cl fluxes of 200–2000 ng Cl m
− 2
s
− 1
required to cause the measured emission gradients are highly improbable, and the Cl emission fluxes almost
certainly represented HCl. d Liberation of gaseous Cl compounds during
decomposition. In the oilseed rape canopy the Cl con- centration was largest at the height of the attached
senescing leaves. It is possible that part of the Cl
−
contained in the leaves was released during the decom- position. This could explain why the concentrations
increased close to the ground surface, as this was cov- ered with leaf litter that also emitted large quantities
of NH
3
Nemitz et al., 2000a. e Counter-gradient fluxes close to the ground.
The concentration increase close to the ground could have purely physical reasons. Here turbulent diffusion
might become a less important transport mechanism than near field effects which have been shown to al-
low for counter-gradient transport Raupach, 1989. The application of the ILT Fig. 6, in which near field
effects are accounted for, shows that for the chosen division of the canopy into sourcesink heights, the
lowest level did not act as a net source, despite the con- centration increase close to the ground Nemitz et al.,
2000a.
While the exact mechanism of the emission of gaseous Cl compounds could not be identified, it al-
most certainly represented HCl, probably originating from senescingdecomposing leaf material or from
leaf surface reactions.
4.3. Sources and sinks within the canopy The series of in-canopy measurements as exempli-
fied in Figs. 5 and 6 indicated a strong mid-canopy source of Cl
−
. During the night of June 22 a signif- icant concentration of Cl
−
developed at a height of 0.7 m, which slowly declined in the morning hours.
Due to analytical restrictions, only a single run Run
E. Nemitz et al. Agricultural and Forest Meteorology 105 2000 427–445 441
29 was available for gaseous HCl. However, it also showed the highest concentration in the middle of
the canopy. Filter-packs, in particular for NH
3
and NH
4 +
, are liable to contamination and this might well be the reason why some single point high con-
centrations of NH
3
and NH
4 +
were found during these measurements. In contrast, the in-canopy peaks
of Cl
−
aerosol and HCl gas were indicated by more than one filter-pack and occurred in several sequential
runs. Since the humid conditions at night-time within the canopy favour the aerosol phase, it is probable
that the Cl
−
aerosol captured by the filter-packs was generated from emitted gaseous Cl compounds in the
canopy air by GTPC or on the surface of the particle filter. Unlike Cl
−
, NH
4 +
was measured to be con- tinuously deposited to the canopy during these runs.
The ion balance shows that during the concentration build-up, NH
4 +
could balance only a fraction of the Cl
−
aerosol and seemed to represent mainly NH
4
NO
3
or NH
4 2
SO
4
. Therefore, a part of the captured Cl
−
aerosol was not formed by reaction with gaseous NH
3
. 4.4. Attainment of phase equilibrium at North
Berwick Concentrations of acidic gases were small at
this coastal Scottish site, with average values of 10 neq m
− 3
HNO
3
and 8.8 neq m
− 3
HCl Fig. 2. The comparison of measured concentration products K
m
with those predicted for thermodynamic equilibrium Figs. 7 and 8 shows that, at a height of 1 m, gas
phase concentrations were generally not in equilib- rium with the aerosol phase. Many comparisons of
K
m
with K
e
have been presented, with contradicting results. Harrison and Pio 1983 reported that thermo-
dynamic equilibrium was best attained for conditions of low T and high h, while most authors, such as
Allen et al. 1989, stated that under these condi- tions dis-equilibrium was most marked. As with the
measurements by Erisman et al. 1988 and Zhang et al. 1995, those studies indicated positive depar-
tures from equilibrium, reflecting the relatively high pollution at their measurement sites in England and
the Netherlands. By contrast, Matsumoto and Tanaka 1996 found K
m
K
e
for an urban environment without major local sources, which is similar to the
present study. For aqueous aerosol, the coexistence of SO
4 2−
can considerably reduce K
e
of these species compared with pure solutions Stelson and Seinfeld, 1982;
Matsumoto and Tanaka, 1996. As information about the full aerosol composition, necessary for the ap-
plication of thermodynamic aerosol models, was not available, K
e
had to be calculated for pure NH
4
Cl and NH
4
NO
3
and might therefore be overestimated for h above the deliquescence point. The formulations of
K
e
applied here are, nevertheless, applicable to solid aerosol, which is usually found for h 60, even
for mixed aerosols Stelson and Seinfeld, 1982. The fact that low values of h coincided with some of the
smallest values of K
m
Fig. 8a,b, indicates that mod- ification of the thermodynamic equilibrium by SO
4 2−
cannot be the only reason for the observation of K
m
K
e
. For T 10
◦
C, K
m
NH
4
NO
3
increased with h, in contradiction to theory. One possible explanation
is that, in the absence of local sources of NH
4
NO
3
, advected aerosol became increasingly depleted when
the potential for volatilization was high K
e
large. The potential for NH
4
NO
3
formation was limited to periods of h 85, in agreement with measurements
at another Scottish site Flechard and Fowler, 1998. Equilibrium was most closely attained for T 10
◦
C supporting the conclusions of Harrison and Pio 1983.
The surface exchange of the participating species is known to perturb the thermodynamic equilibrium.
At 1 m K
m
for HCl was consistently below K
e
. Be- cause of the emission of both HCl Fig. 3 and NH
3
Sutton et al., 2000b, however, K
m
for NH
4
Cl tended to exceed K
e
at z
′
for h 85, especially at higher T. Again bearing in mind the reducing effect of SO
4 2−
on K
e
, there is even stronger indication that at high h NH
4
Cl was potentially produced within the canopy, whereas it ought to have evaporated above the canopy.
As a consequence of the low acid gas concen- trations, the phase partitioning of total NH
x
was shifted towards the gas phase, with median NH
3
gas and NH
4 +
aerosol concentrations of 0.85 and 0.32 mg N m
− 3
, respectively. Coincidence factors Ta- ble 1 were similar to those obtained by Wexler and
Seinfeld 1992, with higher values of C
SO
4 2−
than for C
NO
3 −
. Wexler and Seinfeld 1992 suggested the following explanation: both acidic sulphate and ni-
trate condense according to the surface area provided by the aerosol, but NH
4
NO
3
, unlike NH
4 2
SO
4
, constantly evaporates and re-condenses. Thus, as the
442 E. Nemitz et al. Agricultural and Forest Meteorology 105 2000 427–445
pH of the aerosol drops, the sulphate remains fixed by the smaller particles whereas the nitrate is forced
to condense, together with NH
3
, onto bigger parti- cles or to react with sea salt NaCl also contained
in larger particles Matsumo and Tanaka, 1996. This process is more efficient at small τ
∞
. Consistently, the coincidence of NH
4 +
with SO
4 2−
increased with decreasing τ
∞
R = −0.93, N = 5, P 0.05, while C
NO
3 −
was positively correlated with τ
∞
R = −
0.89, N = 5, P 0.05, although the latter is in contradiction to the observations by Wexler and
Seinfeld 1992. Clearly, at North Berwick τ
∞
ac- cording to Eq. 3 was indeed a governing parameter
for the ageing process of the aerosol and is likely to have controlled the formation of secondary aerosol.
Having established that at North Berwick depar- ture from equilibrium was often sufficiently large to
drive GPIC, the magnitude of the effect on surface exchange fluxes depends critically on the time-scales
at which they would have occurred. Because of the height-dependence of K
m
, it is necessary to discuss the potential of GPIC above and within the canopy
separately. 4.5. Estimate of the potential effect of gas–particle
interconversions on NH
3
flux measurements As equilibrium was usually not attained above
the canopy, kinetic constraints must have prevented the outgassing from the aerosol. The main kinetic
constraint on both aerosol evaporation and vapour condensation was certainly imposed by the small con-
centrations of particles and the resulting small surface area on which chemical interactions could take place.
The characteristic times of the chemical conversion were estimated here to be ≫ 3 min, much longer
than that of vertical transfer Table 3. Gas–particle interactions are therefore unlikely to have affected
the above-canopy gradient measurements of NH
3
except for periods of very low friction velocities u
∗
0.03 m s
− 1
, but during these conditions the application of the AGM is also very uncertain for
other reasons. Because of the low turbulence, fluxes during these periods are small so that even large rela-
tive errors have little effect on the long-term estimate of the net exchange. In contrast, Seidl et al. 1996
frequently found values for τ
∞
of about 1.5 min at a heavily polluted site near Leipzig, Germany. At their
site τ
∞
was therefore often similar to the character- istic time of the vertical transport, and the constant
flux assumption was violated Kins et al., 1996. At North Berwick NH
3
concentrations were large compared with concentrations of acid gases and
aerosols. Consequently, GPIC would have had a rel- atively small effect on the NH
3
flux, even if it had occurred. During the evaporation of NH
4
Cl, e.g. the amount of volatilized NH
3
would have equalled that of HCl. A maximum of 25 of the observed NH
3
emission flux would have been produced by this aerosol evaporation mechanism, assuming that i all
HCl emission was caused by NH
4
Cl evaporation and ii at the surface HCl was deposited at V
max
. 4.6. Gas–particle interaction within the oilseed rape
canopy It can be concluded that at North Berwick the aero-
dynamic gradient technique was applicable to mea- sure NH
3
surface fluxes as chemical time-scales were long compared with diffusive time-scales. However, if
chemical transformations had taken place within the canopy air space, the measured flux, though valid,
would not have represented the direct exchange with the canopy and ground, but would also have included
these chemical transformations.
Diffusive transport through the canopy is much slower, providing more time for phase conversions
to take place. If R
a
z
1
–z
2
is the aerodynamic resis- tance between two heights z
1
and z
2
in s m
− 1
, then τ
transfer
= [z
2
− z
1
]R
a
z
1
, z
2
represents the approx- imate time a tracer takes to be transported from z
1
to z
2
. Assuming neutral conditions and u
∗
= 0.3 m s
− 1
, micrometeorological theory Sutton et al., 2000b
leads to τ
transfer
= 40 s for the transport from the no-
tional average height of the canopy exchange z
′
to the measurement height 1 m. In contrast, the expres-
sion for the turbulent in-canopy resistance derived by Nemitz et al. 2000b results in a transfer time
between the bottom and top of the canopy which is about 6 min. From considerations of the absolute time
in contrast to the characteristic time for a bi-modal gas–aerosol system to equilibrate, Meng and Sein-
feld 1996 demonstrated that condensation of HNO
3
and NH
3
to particles of 0.3-mm diameter takes about 20 min at high h. As discussed before, at h 85,
K
m
of NH
4
Cl tended to exceed K
e
at z
′
but not at
E. Nemitz et al. Agricultural and Forest Meteorology 105 2000 427–445 443
1 m. Thus, some condensation of NH
4
Cl might have occurred within the canopy, and the resulting emis-
sion would explain the upward gradients of NH
4 +
Table 2, whilst the Cl
−
emission would have been masked by the dominating sea salt deposition cf.
Section 3.2. The Cl
−
aerosol at North Berwick showed a bi- modal distribution not shown; the coarse mode typi-
cally represents sea salt, while the fine mode represents NH
4
Cl. If this NH
4
Cl was formed within the canopy, the fine mode Cl
−
concentration R
p
0.23 mm should increase at high h when K
m
K
e
. Indeed, de- spite the limited number of data points, a highly sig-
nificant positive correlation was found between fine Cl
−
and h R = 0.99, N = 5, P 0.01 sup- porting the concept of NH
4
Cl formation within the canopy. Furthermore, the sourcesink analysis showed
the canopy to be a source of Cl
−
aerosol, probably originating from precursor gases, although it cannot
represent NH
4
Cl alone Section 4.3.
5. Conclusions
