vertical profiles of air pollutants in a spruce forest — analysis of adherent water, throughfall...
TRANSCRIPT
Armspheric Environment Vol. 23, No. 8. pp. 1807 1814. 1989. Printed in Great Britain.
OOG-6981/89 $3.M)+O.W Pergamon Press plc
VERTICAL PROFILES OF AIR POLLUTANTS IN A SPRUCE FOREST - ANALYSIS OF ADHERENT WATER,
THROUGHFALL AND DEPOSITS ON SURROGATE SURFACES
ULRICH FRITSCHE, MICHAEL GERNERT and CHRISTINE SCHINDLER
Fraunhofer-Institut fiir Umweltchemie und ijkotoxikologie, 5948 Schmallenbcrg-Grafschaft. F.R.G.
(First received 17 October 1988 and received for publication 10 March 1989)
Abstract-Vertical profiles of air pollutants were studied in a spruce forest, which is damaged mainly in the upper parts of the canopy. The sampling included plant surface water, throughfall and surrogate surfaces at five different heights. For most components a low amount of precipitation after a dry period yielded in vertical profiles of the plant surface water shaped by the wind speed. If there was enough precipitation to start wash-off, the maximum of the concentrations shifted downwards. An exception was the acidity, which increased continuously from the top to the bottom of the trees. The deposition on vertically orientated surfaces and face up orientated surfaces were in the same order of magnitude, but the deposition on face down orientated surfaces was negligible.
Key word index: Spruce forest, damage, air pollutants, vertical profiles, plant surface water, throughfall, surrogate surfaces, wind speed, acidity.
1. INTRODUCTION 2. EXPERIMENTAL
Damage of forests due to pollution varies consider- ably on the mesoscale. For example, from the top to the lower parts of the canopies a gradient of damage can be observed. Possibly, this is the consequence of different burdens of pollutants. We conjecture that causal relationships exist between the damage and the dose of toxic substances at the interface of plant and atmosphere. One of the possible mechanisms is dam- age to the needles or leaves by pollutants, which are dissolved in water films and drops (Mengel et al., 1986; Fritsche, 1987; Fritsche et al., 1988).
2.1. Sampling site
The sampling was performed in a stand of Norway spruce about 42 years old, which is situated about 400 m above sea level in North-Rhine Westphalia, F.R.G. The trees were heavily damaged with different symptoms. Many needles were lost. Older needles showed a yellow color and even young needles were brown or had a pale color.
The main objective of the investigations described in this paper was the determination of vertical profiles of inorganic components in water which contacts spruce branches. The variability of these profiles was explained as far as possible. Wind speed and the amount of precipitation had to be considered because of their influence on the accumulation of air pollutants on the needle surface and in the cleaning of the surface (Draaijers et al., 1987).
The canopy was accessible by a tower. At a tree about 13 m high five sampling spots were established: 11.5, 9.0, 7.9, 5.8 and 3.0 m above ground, respectively, for the adherent water and the throughfall. The sampling heights were similar for the surrogate surfaces and the measurement of the wind speed. In the following sections the sampling heights are described as top, upper canopy, middle canopy, lower can- opy and zone of dead branches, respectively.
2.2. Sampling methods
The results for components of the plant surface water were compared with the results obtained by the usual sampling method for throughfall. Additionally, inert surfaces were used to assess the extent of uptake or leaching. Since the deposition depends on the orientation (Sehmel, 1973), these surfaces were fixed vertically and horizontally, respectively. The results were interpreted in view of the variation of the dam- age.
Throughfall was collected from 29 October to 12 Novem- ber 1987 using open cylindrical buckets (diameter 19 cm, polyethylene). On the first and the last day of this period plant surface water was sampled by wiping the water into small polyethylene bottles. In addition, cellulose surfaces were exposed to the air pollution. To avoid contamination. these surrogate surfaces were fixed in slide frames without glass. On the branches the frames themselves were fixed vertically, face up or face down. For the last two orientations one side was covered. Each type of sampling was done once at each sampling spot. The mean horizontal wind speeds were measured by cup anemometers.
The amount of rainfall prior to as well as during the investigation period was known by a recording rain collector operating in the open field, see Figs 1 and 2 (LIS, 1987). On 29 October 1987 there was only low rainfall after 4 dry days. 12 November 1987 was the 3rd successive rainy day after 8
1807
1808 ULRICH FRITSCHE et al.
PPECIPIWlON (Mu)
FIRST DAY
OF SAYPLING
Fig. 1. (From LIS, 1987.) baily rainfall for October 1987: low precipitation on 29 October 1987 after a dry period.
PRBCIPIWION (UN)
LAST DAT
OF SAMPLING
Fig. 2. (From LIS, 1987.) Daily rainfall for November 1987: medium precipita- tion on 12 November 1987 after a dry period.
dry days. So 29 October 1987 and the 12 November 1987 can be considered as representative for low rainfall and for medium rainfall, respectively, after a dry period.
2.3. Preparation and measurement
The adherent water and the throughfall samples were stored at 4” C until they were further analyzed. The cellulose surfaces were extracted with deionized water using an ultra- sonic bath. The residual cellulose matrix was digested in HNO,.
Measurements for characterization of the solutions were performed by conductometry, potentiometry, atomic absorp- tion spectrometry and ion chromatography.
3. RESULTS
All the results are listed in Tables 1 and 2. The changes of the concentrations and deposition rates, respectively, are shown in Tables 3 and 4. The signs between the lines for the sampling spots mean:
+ = increase, - = decrease.
More details are given below.
3.1. Components of plant surface water
3.1.1. Samples collected on 29 October 1987: low amount of precipitation. The vertical profiles for most components were similarly shaped. Concentrations decreased from ‘top’ to ‘lower canopy’, but there was an increase in the ‘zone of dead branches’. A typical
example is Mg. The SO:--concentration even de- creases in the ‘zone of dead branches’. In contrast, the H+-concentration increases, while for Mn differences are low except the increase in the ‘zone of dead branches’.
3.1.2. Samples collected on 12 November 1987: medium amount of precipitation. In contrast to the results for low amount of precipitation, in most cases an increase of the concentrations was observed down through the canopy. Exceptions were a decrease from the top to the upper canopy in case ofNHf and Cl- as well as a decrease for Mg from the middle to the lower canopy and for Mn from the lower canopy to the dead branches.
3.2. Results for the throughfall
Surprisingly, the amount of the precipitation in-
creased from the top to the bottom with a distinct
Tab
le
1.
Con
cent
ratio
ns
of d
isso
lved
co
mpo
nent
s
Tab
le
la.
Adh
eren
t w
ater
: lo
w
prec
ipita
tion
(sam
plin
g on
29
Oct
ober
19
87)
Hei
ght
Con
duct
ivity
H
+ N
H:
Na
K
Mg
Ca
Mn
(m)
(@
cm-‘
) (m
g e-
r)
Top
11
.5
691
0.03
6 9.
06
7.18
39
.6
5.31
43
.9
0.50
U
pper
ca
nopy
9.
0 40
8 0.
068
3.72
6.
55
30.3
2.
70
21.2
0.
37
Mid
dle
cano
py
7.9
470
0.25
0 3.
01
1.73
20
.1
1.89
18
.2
0.44
L
ower
ca
nopy
5.
8 28
8 0.
196
2.18
1.
05
9.3
1.08
9.
96
0.37
D
ead
bran
ches
3.
0 12
00
0.77
7 10
.4
6.57
21
.3
6.39
53
.4
1.54
Tab
le
lb.
Adh
eren
t w
ater
: m
ediu
m
prec
ipita
tion
(sam
plin
g on
12
Nov
embe
r 19
87)
Pb
Cl-
N
O;
SO:-
<O.O
Ol
17.2
10
4 17
3 <O
.OO
l 12
.6
34.1
10
3 <O
.OO
l 6.
3 16
.6
12.4
<O
.OO
l 2.
8 16
.9
70.8
<O
.OO
l 14
.5
52
29.6
Hei
ght
Con
duct
ivity
H
+ N
H:
Na
K
Mg
Ca
Mn
Pb
Cl_
N
O;
so:-
(m)
($S
cm
-‘)
(mg
e-‘)
Top
11
.5
59
0.06
1 1.
32
1.59
6.
0 0.
43
0.62
0.
016
<O.O
Ol
7.2
7.1
12.3
U
pper
ca
nopy
9.
0 10
6 0.
076
0.76
3 1.
38
6.8
0.66
1.
84
0.06
3 <O
.OO
l 4.
7 6.
5 18
.0
Mid
dle
cano
py
7.9
202
0.12
2 2.
12
1.88
8.
6 1.
46
5.35
0.
175
<O.O
Ol
5.2
11.8
37
.7
Low
er
cano
py
5.8
221
0.14
3 2.
13
2.19
12
.3
1.27
4.
38
0.32
9 <O
.OO
l 6.
3 10
.8
47.2
D
ead
bran
ches
3.
0 35
8 0.
285
4.18
2.
35
15.9
1.
82
8.88
0.
297
<O.O
Ol
8.5
15.8
63
.7
Tab
le
lc.
Thr
ough
fall
Hei
ght
Con
duct
ivity
H
+ N
H:
Na
K
Mg
Ca
Mn
Pb
cl-
NO
; so
:-
Prec
ipita
tion
(m)
(DS
cm
-‘)
(mg
P-r)
(m
m)
Top
11
.5
93
0.03
3 0.
68
0.87
1.
3 1.
32
4.63
0.
092
0.02
0 2.
8 12
.9
20.8
20
U
pper
ca
nopy
9.
0 15
9 0.
110
1.23
1.
06
1.4
1.38
5.
55
0.21
6 0.
006
2.8
14.5
23
.9
23
Mid
dle
cano
py
7.9
330
0.26
9 2.
57
2.15
3.
5 2.
62
10.8
0.
337
0.01
0 4.
7 24
.8
58.5
47
L
ower
ca
nopy
5.
8 93
0.
07 1
0.
74
0.50
0.
8 0.
72
2.92
0.
117
0.00
4 1.
0 6.
2 12
.4
23
Dea
d br
anch
es
3.0
393
0.49
0 2.
30
1.75
3.
3 2.
22
10.9
0.
566
0.02
5 4.
6 20
.7
76.4
26
Tab
le
2.
Dep
ositi
on
rate
s of
sev
eral
ai
r po
lluta
nts
Tab
le
2a.
Dep
ositi
on
on
vert
ical
su
rfac
es
Hei
ght
Con
duct
ivity
H
+ N
Hf
Na
Mg
Ca
Mn
Pb
NO
; SO
:-
(m)
(pts
cm
-‘)
(mg
e-r)
Top
11
.5
4.0
4 1.
9 1.
5 1.
4 4.
0 0.
0348
0.
0092
6
6.5
Upp
er
cano
py
9.0
5.5
9 2.
0 1.
5 1.
1 4.
5 0.
1520
0.
0030
5
5.5
Mid
dle
cano
py
7.9
4.0
7 0.
8 1.
0 0.
4 2.
0 0.
0276
0.
0010
3
4.5
Low
er
cano
py
5.8
6.5
13
0.5
1.5
0.8
4.0
0.08
24
0.00
59
4 9.
5 D
ead
bran
ches
3.
0 8.
5 14
1.
2 2.
0 1.
3 6.
5 0.
1670
0.
0150
6
14
Tab
le
2b.
Dep
ositi
on
on
face
up
su
rfac
es
Hei
ght
Con
duct
ivity
H
+ N
H:
Na
Mg
Ca
Mn
Pb
NO
,- SO
,*-
(m)
(j&
cm-‘
) (m
g t-
‘)
Top
11
.5
1 <2
10
.3
3 1.
2 4
0.15
90
0.06
2 <2
3
Upp
er
cano
py
9.0
2 3
<0.3
1
1.0
6 0.
1910
0.
004
<2
4 M
iddl
e ca
nopy
7.
9 8
13
1.1
3 1.
7 9
0.18
80
0.02
6 7
15
Low
er
cano
py
5.8
10
16
<0.3
3
1.7
12
0.22
60
0.00
6 3
5 D
ead
bran
ches
3.
0 10
16
<0
.3
4 2.
3 13
0.
3000
0.
017
<2
21
Tab
le
2c.
Dep
ositi
on
on
face
do
wn
surf
aces
Hei
ght
(m)
Con
duct
ivity
H
+ N
H:
Na
(PC
S cm
-‘).
Mg
Ca
Mn
Pb
NO
; so
:-
b-w
2 ~-‘I
Top
11
.5
1 <2
<0
.3
< 1
<0.4
<1
0.
0202
0.
002
<2
<1
Upp
er
cano
py
9.0
2 3
<0.3
<l
<0
.4
<1
0.01
44
0.00
2 <2
<1
M
iddl
e ca
nopy
7.
9 1
3 <0
.3
<1
< 0.
4 <l
0.
0329
0.
002
42
<l
Low
er
cano
py
5.8
1 4
<0.3
<
I <0
.4
<l
< 0.
0005
0.
003
<2
2 D
ead
bran
ches
3.
0 1
3 <0
.3
<l
<0.4
<l
<0
.000
5 <O
.OO
l <2
<1
1811 Vertical profiles of air pollutants
Table 3. Vertical changes of the concentrations of dissolved components
Table 3a. Adherent water: low precipitation (sampling on 29 October 1987)
Conductivity H+ NH: Na K Mg Ca Mn Pb Cl- NO; SO:-
TOP _ + _ _ _ _ _ _ * _--
Upper canopy + + - _ - _ - + *__-
Middle canopy _ _ _ _ _ _ _ _ + - + +
Lower canopy + + + + + + + + * + + -
Dead branches
Table 3b. Adherent water: medium precipitation (sampling on I2 November 1987)
Top + + - - + + + + & - - +
Upper canopy + + + + + + + + * + + +
Middle canopy + + + + + - - + * + - +
Lower canopy + + + + + + + - f + + +
Dead branches
Table 3c. Throughfall
Top + + + + + + + + + i + +
Upper canopy + + + + + + + + + + + +
Middle canopy _ _ _ _ _ _ _ _ _
Lower canopy + + + + + + + + + + + +
Dead branches
maximum in the middle canopy. The concentrations of the measured components were also relatively high in the middle canopy. In general, concentrations of the components increased from the top to the bottom of the canopy except the transition from the middle to the lower canopy. The largest increases could be observed for H+ and Mn.
3.3. Deposition rates for surrogate surfaces
With regard to the following deposition rates sec- ondary transport occurring after deposition should be taken into consideration.
3.3.1. Vertical surfaces. As shown in Table 4, differ- ent changes of deposition rates of the components were observed from the top to the upper canopy. From there down to the middle there was a decrease in every case. The decrease was followed by an increase except for NH:.
3.3.2. Face up surfaces. The deposition rates ob- served for the surrogate surfaces are influenced by secondary transport processes, mainly wash-down.
Except for Mn, the deposition rates for all compo- nents were relatively high in the middle canopy. Probably this is a consequence of the amount of the precipitation. The profiles were quite different, but,
generally, an increase from the upper canopy to the middle canopy and-for most cases-an increase from the lower canopy to the zone of dead branches (com- pare Bytnerowicz et al., 1987) were observed.
3.3.3. Face down surfaces. No characteristic profiles
were obtained for the face downward orientated sur- face. All values were much lower compared with the other positions.
3.4. Wind speed
Figure 3 gives the vertical profile for the mean horizontal wind speed. The branches cause a decrease in the velocity, but in the zone of dead branches the wind speed increases a little. This is consistent with measurements of other authors (Item, 1974). It should be noted that the anemometers could not be placed as close to the branches as the surrogate surfaces.
4. DISCUSSION
4.1. General remarks
The sampling in this study does not allow one to distinguish between dry and wet deposition. A paper
1812 ULRICH FRITSCHE et al.
Table 4. Vertical changes of the deposition rates of several air pollutants
Table 4a. Deposition on vertical surfaces
Conductivity H+ NH: Na Mg Ca Mn Pb NO; SO;
Top + + + + - + + - - -
Upper canopy _ - _ _ _ _ _ _ _ _
Middle canopy + + - + + + + + + +
Lower canopy + + + + + + + + + +
Dead branches
Table 4b. Deposition on face up surfaces
Top + + k - - + + - * +
Upper canopy + + + + + + - + + +
Middle canopy + + - + + + + - - -
Lower canopy k * * + + + + + - +
Dead branches
Table 4c. Deposition on face down surfaces
Top + + _ *
Upper canopy _ * + k
Middle canopy k + _ +
Lower canopy + _ * -
Dead branches
14 Height Iml r
l2-
10 -
8-
4:. G
6-
2
0 I 0 0.5 I 1.5
Wiidspeed im. s-‘J
Fig. 3. Vertical profile for the horizontal wind speed.
dealing with measurements at the same sampling spots in a dry episode is in preparation.
The reproducibility of the analysis in the laboratory was about 5510%. The standard deviation of the sampling has not yet been determined, but is assumed to be significantly higher. Because the results represent single measurements, the variation of the values de-
pending on the height might be not significant in some cases. More extensive work is necessary to confirm the chemical differences of the vertical profiles, which indicate differences in the deposition processes, plant uptake and leaching, respectively.
4.2. Comparison of profiles for components of plant
surface water
The profiles for low amount of precipitation
(sampling on 29 October 1987) were similar to the profile of the wind speed. For these measurements the
dependency of the deposition rate on the wind speed (Sehmel, 1973; Bunch, 1966; Dasch, 1986) is seen as the decisive factor. The profiles for medium amount of
precipitation (sampling on 12 November 1987) show a shift of the maximum concentration from the top to the lower zones. The values of most components are considerably lower compared to the values for low amount of precipitation. Obviously, wash-off of dry deposit by precipitation was the decisive factor for the vertical profiles. Mass transfer caused by the dripping precipitation is not the same for all components. One explanation for this phenomenon is the different affin- ity to the branches, analogous to a chromatography
Vertical profiles of air pollutants 1813
effect. The concentrations of NH,* and NO; are influenced by needle uptake resulting in a decrease of the ~n~ntrations as the water flows downwards. Mn, K and-to some extent-Ca and Mg are leached, which results in an increase of their concentrations in the surface water downwards (Parker, 1983). The finding, that the H+-concentration increases strongly for low and medium pr~ipitation, could be explained by either leaching of acid or exchange of NH: for H + (Cronan, 1984).
4.3. Comparison of the adherent water to the through- fUli
The profiles of components of the throughfall water are more similar to the profiles of the adherent water for medium amount of precipitation (sampling on 12 November 1987) than for low amount of precipitation (sampling on 29 October 1987). For most components the concentrations in the throughfall are lower than the concentrations in the adherent water at the be- ginning of the sampling period, but they are higher compared to the adherent water at the end of this period. For NH:, Cl-, K and Ca the throughfaIl content was even lower compared to the adherent water at the beginning of the sampling period. Obvi- ously, the sampling technique throughfall is not very useful to get a real idea of the concentration gradients of components which are dissolved in water con- tacting the branches. The profiles of the throughfall are both temporally and spatially averaged, since the throughfall consists of drops from branches at differ- ent heights and, also, from drops which do not contact the branches.
Normally, the volume of throughfall is lower than the volume of the precipitation in an open area. Our results show an increase of the amount of precipitation from the top to the bottom of the canopy with a maximum in the middle. An explanation could be funneling of water by the branches. Another possibil- ity is the impaction of rain and fog drops with the branches which depends on the biomass (Jonas, 1984), having a maximum in the middle of the canopy.
Our results suggest that the amount of throughfall varies unexpectedly with the sampling height. More extensive investigations will be necessary in order to identify the factors influencing the vertical profile of the amount of precipitation and its measurement.
4.4. Significance of the position of the surrogate sur- faces
The profiles for the vertical surrogate surfaces were similar to the profile of the wind speed for most components. Similarities also exist for the adherent water at low amount of precipitation (sampling on 29 October 1987). However, differences were observed for H+ and the conductivity, and for NH:. This is caused by leaching and uptake, respectively.
In case of most components, the profiles for the face up surfaces were similar to the profile of the amount of
precipitation. Obviously, the profiles of some compo- nents, especially H+, Ca, Mn as well as the conduct- ivity, are also influenced by other factors.
Irrespective of direct stomata1 uptake of gases, the deposition on face down orientated surfaces appears to be relatively low for trees.
This agrees with the observation that the upper side of the twigs is often more damaged than the lower side.
The burdens on the vertical and face up surfaces are in the same order of magnitude (Sehmel, 1973), but there are remarkable differences in the profiles. This can be explained by domination of different deposition m~hanisms including sedimentation, impaction and diffusion as well as reaction with the biomass and transfer by the dripping precipitation.
5. CONCLLrsiONS
The vertical profiles of dry deposition for air pollu- tants in spruce forests are determined largely by the wind speed (cf. Sehmel, 1973). After a dry period a small amount of rain yields vertical profiles of the components of plant surface water which are similar to the profile of the wind speed. As the amount of rain increases sufficiently to cause wash-off, the location of the maximum of the concentrations shifts downward. The adherent water shows remarkable increases in acidity from top to bottom.
The vertical deposition and the face up deposition are in the same order of magnitude, but the first is more influenced by the wind speed and the second more by the precipitation. The face down deposition is relatively low. These findings agree with theory (Seh- mel, 1973).
High concentrations at the needle surface are to be expected after a dry period, when the accumulated deposit is dissolved in a small amount of water. In that case for several components the vertical profiles corre- spond to the extent of the damage. Exceptions are H+ and NH: because of leaching and uptake, respect- ively. The observation of more damage on the upper side of the branches compared with undersurface also corresponds to the variation of the deposition.
Acknowledgement-Financial support of this work by the German Umweltbundesamt, Berlin, contract-No. 1~07~6/01, is gratefulfy acknowledge.
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1814 ULRICH FRITSCHE et al.
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