Atmospheric Deposition of Sulfur and Base Cations to European Forests

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W O R K I N G P A P E R

I

ATMOSPHERIC DEPOSITION OF SULFUR AND BASE CATIONS TO EUROPEAN FORESTS

October 1988 WP-88-101

I n t e r n a t i o n a l I n s t i t u t e for Applied Systems Analysis

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ATMOSPHERIC DEPOSITION OF SULFUR AND BASE CATIONS TO EUROPEAN FORESTS

Wilfried Ivens

October 1988 WP-88-101

Working Papers are interim reports on work of the International Institute for Applied Systems Analysis and have received only limited review. Views or opinions expressed herein do not necessarily represent those of the Institute or of its National Member Organizations.

INTERNATIONAL INSTITUTE FOR APPLIED SYSTEMS ANALYSIS A-2361 Laxenburg, Austria

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Author

Mr. Wilfried Ivens participated a t IIASA's Young Scientistss Summer Program in 1987, and this report i s an outcome of that working period. His present address is University of Utrecht, Department of Physical Geography, Heidelberglaan 2, 3508

TC

Utreoht, The Netherlands. In Utreaht, Wilfried Ivens oarrigs out research on atmospheric deposition into forests.

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Forests have a tendency

to

f i l t e r o u t a i r pollutants, t h e r e b y absorbing l a r g e r amounts of d r y deposition than a n equal area of open ground. Furthermore, although atmospherio deposition everywhere in t h e industrial world i s on t h e a v e r - a g e acidic, i t i s

w e l l

known t h a t some precipitation events are in f a c t alkaline.

Since 1983 IIASA's Acid Rain P r o j e c t h a s included work on t h e f o r e s t filtering ef- f e c t and alkaline deposition in mnnection with t h e development of t h e Regional Acidification Information and Simulation (RAINS) model.

Wilfried Ivens from t h e University of Utrecht, The Netherlands, h a s p r e p a r e d t h i s overview which analyzes measurements from f o r e s t e d s i t e s in different p a r t s of Europe. This working p a p e r r e p r e s e n t s t h e

m o s t

detailed examination t h a t h a s so f a r been c a r r i e d out

at

IIASA of t h e f o r e s t filtering e f f e c t a n d alkaline deposition.

Roderick W. Shaw Leader

Acid Rain P r o j e c t

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I would like

to

thank Pekka Kauppi, Joseph Alcamo, Egbert Matzner and Maximilian Posch f o r t h e i r vallrable comments, fruitful discussions and reviewing t h e paper. I also like

to

thank Jean-Paul Hettelingh f o r helping in computing t h e EMEP-model results and t h e forest coverage deta.

The research

w a s

partly funded by t h e Foundation IIASA-Netherlands and t h e Netherlands Organisation f o r t h e Advancement of

Pure

Research (Z.W.O.).

-

^vii

-

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To simulate acidification processes in f o r e s t s (soils), i t i s important

to

know as well as possible t h e atmospheric input. Large scale models have recently been im- proved

to

t a k e b e t t e r into acoount t h e differences in deposition between f o r e s t s and o t h e r surfaces.

In t h i s r e p o r t measurements of sulfur-fluxes onto t h e f o r e s t floor (54 case studies) are compared with deposition fluxes as calculated by t h e EMEP-model and by t h e RAINS modifications on t h i s model. The value of t h e filtering p a r a m e t e r used in RAINS

at

t h i s moment i s discussed. A new quantitative basis f o r t h e filtering e f f e c t of different

tree

species i s given.

Fluxes of base cations

are

compared

to

sulfur fluxes to quantify t h e neutralizing e f f e c t s of b a s e cations. There a p p e a r s

to

b e no d i r e c t proportional relationship between base cation and sulfur fluxes onto t h e f o r e s t floor.

I t

i s proposed t o study t h e possibility of linking, within t h e RAINS m o d e l . basic cation deposition with t h e amount and magnitude of s e v e r a l s o u r c e s of basic cations.

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Table of Content.

1. Introduction

2. The problem of estimating f o r e s t filtering of S-compounds 3. Measurements of fluxes onto t h e f o r e s t floor

4. Calculation of sulfur deposition 5. Comparison of sulfur fluxes

5.1 Difference between t h e fluxes onto t h e f o r e s t floor and bulk deposftion

5.2 "Observed" versus "calculated" deposition

5.2.1 Observed fluxes versus EMEP model results 5.2.2 Observed fluxes v e r s u s RAINS m o d e l r e s u l t s 6. Conclusions on t h e f o r e s t filtering of sulfur regarding RAINS 7 . Base cation deposition

8. Conclusions on base cation calculations of RAINS References

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Atmospheric Deposition of Sulfur and Base

Cations to

European Forests

1. Introduction

IIASA's Acid Rain P r o j e c t h a s developed a n acidification model, RAINS (Regional Acidification and INformation Simulation), which links atmospheric t r a n s p o r t and deposition of a i r pollutants with t h e ecological impacts of a i r pollutants, notably sulfur, on a European

scale

(Alcamo et

ul.,

1987).

The t r a n s p o r t and deposition of S-compounds in t h e RAINS model are based on t h e output of t h e EMEP long r a n g e t r a n s p o r t model f o r sulfur compounds (Eliassen and Saltbones, 1983: Lehmhaus et d . , 1986). The EMEP-model computes mean overall S 4 e p o s i t i o n

to

l a r g e a r e a s , including both forested and open a r e a s . The atmospheric sulfur input in forested ecosystems is very important

to

t h e impact models incorporated in RAINS. Because t h e deposition

to

f o r e s t s has been assumed

to

b e g r e a t e r than o t h e r areas (the f o r e s t filtering effect), some modifications of t h e EMEP-model r e s u l t s have been made

to

estimate t h e specific f o r e s t deposition within RAINS (e.g., Kamiiri, 1986). Besides this, t h e deposition of alkaline compounds h a s

to

b e taken into account within RAINS because of t h e i r neutralizing ef- f e c t on s u l f u r 4 r i v e n acidifying processes (e.g., Kauppi st d . , 1986). The objec- tives of t h i s study are

to

find a new basis for:

1. The quantification of t h e f o r e s t filtering effect with relation

to

sulfur, and 2. The estimation of deposition of basic cations.

In t h i s study t h e observed atmospheric deposition in different f o r e s t stands around Europe i s compared

to

t h e calculated deposition. Both t h e original EMEP estimates and t h e estimates modified in RAINS

are

used

to

obtain t h e "calculated"

deposition.

2. The

Problem

in Emtimating Forest Filtering of S-Compounds

The EMEP-model computes t h e annual

w e t

and d r y S-deposition throughout Europe on grid s q u a r e s of 150 % 150 km. Estimates of t h e EMEP-model f o r t h e annual mean S-concentrations in a i r and in precipitation have been compared with measurements (Lehmhaus st d., 1986). The correlation between calculated and observed values was 0.87 f o r sulfur dioxide. The particulate sulfate concentrations in a i r and in precipitation

were

less

w e l l

predicted, t h e correlation between predicted and calculated observations being 0.59 and 0.65, respectively. The calculated overall annual mean

air

concentrations of SO2 and sulfate

were

v e r y close

to

t h e observed values. However, t h e model appeared

to

underestimate t h e overall annual mean sulfate concentrations in precipitation by about 152. Similar model valida-

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tions have not been done f o r t h e deposition estimates. The deposition of S- compounds will depend not only on a i r and precipitation concentrations, but also on t h e aerodynamic conditions, and on t h e physical, chemical and physiological c h a r a c t e r i s t i c s of t h e r e c e p t o r s u r f a c e (cf. Fowler, 1980). Observations confirm t h a t t h e deposition velocity of S-uompounds depends on t h e r e c e p t o r s u r f a c e (land use), although t h e r a n g e i s wide and just a few estimates are available f o r f o r e s t s (IlhbLe I ).

Table

I.

Deposition velocities of SO2 and SO:' above different c a t e g o r i e s of land use, cm es (from Voldner at

at.,

1986).

~ r r r u i

use

SO:- SO2

0.4 (0.0

-

^1.2)

Water 0.5 (0.16

-

^4.0)

n = 7 n = 5

Snow 0.13 (0.04

-

^<.20) 0.05 (0.005

-

^.17)

n = 4 n

=

5

Soil

+

^urban

-

0.9 (0.04

-

^2.5)

n

=

9 G r a s s

+

c r o p s

Forest

The most r e c e n t EMEP-model (Lehmhaus e t

at.,

1986) calculates t h e d r y deposition velocity of SO2 as a function of windspeed and s u r f a c e roughness (Ilhble II ).

A "mean" s u r f a c e roughness i s assigned

to

e v e r y grid square. For particulate sul- f a t e , t h e d r y deposition velocity i s set

to

t h e constant value of 0.1 ms" f o r all re- c e p t o r s u r f a c e s , which i s lower than t h e mean values indicated in Ilhble I. Because of t h e coarse resolution of t h e model, i t i s not possible

to

calculate t h e deposition at a specific s i t e within t h e grid. A s a r e s u l t , t h e model will probably underesti-

mate

deposition

at

s i t e s with h i g h e r s u r f a c e roughness, such as f o r e s t s .

IlhbLe 111indicates t h a t f o r e s t s tend t o "filter" sulfur compounds, i.e., in- c r e a s e t h e d r y deposition flux from t h e a i r

to

t h e soil s u r f a c e . Between t h e dif- f e r e n t kinds of f o r e s t , t h e r e can b e l a r g e differences. However, "filtering" will not only depend on

tree

species. The s t r u c t u r e of t h e stand i s probably a l s o of g r e a t importance.

A method h a s been developed in RAINS

to

modify t h e output of t h e EMEP-model in o r d e r

to

obtain realistic estimates f o r f o r e s t deposition (Kauppi et d., 1986).

The method i s based on t h e rationale t h a t t h e sum of f o r e s t deposition, d f , and open land deposition, d o , equals t h e

total

deposition as estimated by t h e EMEP- model, titot, in grid element f t h a t is

where jt is the f r a c t i o n of f o r e s t land in grid element i . Data

to

d e s c r i b e j f o r e a c h grid s q u a r e were collected from the World F o r e s t r y A t l a s (1975). A coeffi- cient, q,

w a s

^defined

to

denote the f a c t o r by which open land deposition is multi- plied

to

obtain a n

estimate

f o r t h e deposition

on

a nearby f o r e s t stand t h a t i s

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Table

II.

Surface roughness assumed in EMEP-model.

hrjhCB t ~ l ~ e Sur- r o u g h n e s s (m)

Sea 10

"

Desert, snow lo4

Grass 3

x

10"

Countryside 0.25

Suburbia, cities 0.8

W o o d s 1.0

Sourae: Ellasaen and Saltbones, 1883.

Table III. Total sulfur deposition in f o r e s t compared

to

deposition in adjacent

ter-

r a i n with low vegetation.

Vegetation

Forest Low Veg.

Quercus Betula P i n u s P t e r i d i u m robur p e n d u l a s y l u e s t r i s a q u i l i u m

Deposition ( g / m 4.45 4.21 10.12 1.60

f o r e s t deposition

The ratio: low vegetation deposition 2.8 2.6 6.3

-

Source: Skeffington, 1983.

Forest Low Veg.

Quercus Fbgus P t n u s Picea C h l l u n a Vegetation robur s y l v a t i c a s y l v e s t r i s a b i e s u u l g a r i s

-

Deposition ( g / m 2/year) 3.31 5.17 3.53 8.76 1.88 f o r e s t deposition

The ratio: 1.8 2.8 1.9 4.7

--

low vegetation deposition Sourae: Matzner, 1983.

On this basis i t i s straightforward t o calculate d f , i [= f o r e s t deposition p e r unit of land area in grid s q u a r e i ( g m 4 y r

-*)I

as a function of dtOt,*, f" and 9:

Posch et al. (1985) demonstrated t h a t d f i s estimated to b e v e r y similar t o dtot if 1

<

^S^1.2

^or

^if^0.7^S^f

<

^1.0t h a t is, when f o r e s t deposition does not differ substantially from t h e deposition

to

open land

or

when f o r e s t s w v e r 70

to

100% of t h e

area

of t h e grid square. A s a n example, RAINS would estimate f o r e s t deposition 50% higher than t h e corresponding EMEP grid average deposition in conditions where Q i s 2.0 and t h e fraction of f o r e s t land i s 30% ( f i g u r e 1).

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3. Yea-enta of nnxw onto the Forest Floor

The

focus

of this study

was

on element fluxes

to

t h e forest floor by throughfall and stemflow. Throughfall is t h e

water

drfpping through t h e a o p y during rainfall and sternflow i s t h e water running along t h e trunk. These fluxes include both w e t and dry deposition onto t h e

tree

surface.

A total of 22 publications

of

experimental studies were screened describing t h e S-flux a t 54 sites, t h e Ca-flux

at

⁴⁷sites and t h e Mg-flux a t 38 sites. All measurements

were

done between 1967 and 1986. The duration of measurements varied from a few months

to more

than 10 years. If t h e measurement period was s h o r t e r than one year, t h e fluxes

were

interpolated

to

annual flux by multiplying by 365/measurement period (dap/days).

M o s t of t h e stands (38 oases) w e r e formed by conifers (IMLe N). Stand age

was

reported in 29

cases

and stand height in 8 cases. For t h e location of t h e sites as

w e l l

as references

see

M l e Vand IXgure 2.

Table

W.

Tree species in

case studies

(total

=

^54).

Sitka spruce 6 Beech sp.

Douglas f i r 1 Oak sp.

Norway spruce 22 Birch sp.

Scots pine 9 Maple sp.

- Mixed deciduous

Conifers 38 Deciduous

4 Mixed

3 Conif ./Decid. 2

3 1

-

3 14

The aim w a s

to

include both throughfall and stemflow fluxes into t h e "observed" deposition. In 31 cases only t h e throughfall

w a s

given. The stemflow flux

w a s

estimated in these

cases

based on the ratios between stemflow and throughfall fluxes reported elsewhere (Verstraten et al., 1983; van Breemen et al., 1982; Mill- er et

&.,

1980; ~ i h l g g r d , 1970; Johnson et al., 1986). Stemflow contributions between 0 and 20% were assigned depending on t r e e species and stand age. Bulk deposition, being t h e precipitation collected in the open land by means of continuously opened funnels, w a s available on all sites. The fluxes were corrected f o r t h e contribution of

sea-salt

particles, using sodium and chloride as sea-salt

tracers

(Asman et d., 1981).

4. Calculation of Sulfur Deposition

To obtain model deposition estimates, sulfur emissions of each European country were computed by means of RAINS-model (Alcamo et al., 1987) during t h e y e a r s of t h e throughfall and stemflow measurements.

The impact of these emissions

on

t h e deposition

to

t h e forest

sites

was computed by means of two methods. Pirst, t h e deposition estimates

were

calculated f r o m t h e average results of rpns f o r t h e y e a r s 1979,1080, 1983 and 1984 of t h e

m o s t

re- cent EMEP-model (cf. Lehmhaus et d., 1986). A second

set

of "calculated" deposition estimates

w a s

prepared using t h e RAINS modification of EMEP output given in

m.

^(3).

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Table

V.

Description of study s i t e s . NAME

K6nigstein Grebenau Witzenhausen Wintersw. 1 Wintersw. 2 Hackfort Campina G m b b t o r p 1 G r a b b t o r p 2 Tillingb. 1 Tillingb. 2 Tillingb. 3 Kilmichael Leanachan S t r a t h y r e Kershope E l i b m k F e t t e r e s s o Birkenes 1 Birkenes 2 Birkenes 3 Dividal 1 Dividal 2 Dividal 3 Solling 1 Solling 2 Luneb H. 1 Luneb

H.

2

&rdsjBn Kongalund 1 Kongalund 2 Alptal

LBgern Davos

SchBnbuch 1 SchBnbuch2 Feldberg 1 Feldberg 2 Gribskov

J B ~ & S

Delamere Waroneu Robinette Kootwi jk Ispina Edinburgh Wingst H a n Hils H a r s t e Spanbeck

REF 13 13 13 34 34 7 7 31 31 33 33 33 26 26 26 26 26 26 17 17 17 17 17 17 5 5 5 5 14 27 27 3 3 3 8 8 B 8 11 2 32 9 9 18 21 28 6 6 6 6 6

TlME

83-85 83-85 83-85 81-82 81-82

81 81 748 758 81 81 81 75-77 75-77 75-77 75-77 75-77 75-77 778 778 778 778 778 778 69-83 69-83 80-84 80-84 80-81 67-688 67-688 86*

868 868 79-83 79-83

86 86 84*

ca. 77 77-788 ca. 82 ca. 82 85-86 73-748

79 83 83 84-85 82-85 82-85

LNG 8.28 9.29 9.51 6.44 6.44 6.14 5.15 18.20 18.20 -0.20 -0.20 -0.20 -5.28 -4.50 -4.19 -2.50 -2.50 -2.20 7.10 7.10 7.10 19.40 19.40 19.40 9.25 9.25 10.00 10.00 11.30 13.10 13.10

8.45 8.22 9.50 9.10 9.10 8.02 8.02 12.19 16.23 -2.40 6.00 6.00 5.46 20.13 -3.30 9.02 10.25 9.40 9.50 10.50

continued o n next p a g e

. . .

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Rouquet 30 68-70 3.40 43.50

Oberwarm. 1 15 84-86 11.47 49.59

Oberwarm. 2 15 85-86 11.47 49.59

Weulfersreuth 15 84-86 11.46 50.04

REF = referen-, see literature list.

TIME = year(8) of meaeurennent.

*

= part of the year.

LNG

=

longitude.

Ll'T

=

latitude.

PC = foreat ooverage in EMEP grid (2).

SP

=

tree speaies: 2 = Douglas f i r (Paeudotsuga spp.); 3 = Spmoe

moea

^spp.);

4 = Plne (Pinus epp.); 5 = Beeoh Qagus spp.); 6

-

^Oak(Queroua spp.);

7 = Biroh W u l a epp.); 8 = Maple (Aoer epp.); 8 = Linden (Tillia spp.).

5. Caaparison of Sulfur ?'luxes

5.1. Difference between the fluxes onto the f o r d floor and bulk deposition K h k i (1986) calculated

an

approximation f o r t h e *parameter by comparing t h e measured t o t a l stemflow and throughfall flux of sulfur

to

t h e f o r e s t soil (TD) with t h e deposition measured on bulk collectors in t h e open field (bulk deposition

=

BD):

He proposed a n a v e r a g e value of (p

=

2 f o r whole Europe, which i s used

at

t h e moment in RAINS. A l l t h e 1 4 c a s e study s i t e s forming t h e d a t a investigated by K W r i (1986) were included also in t h i s study. An additional number of 40 s i t e s

were

found in l i t e r a t u r e so t h a t t h e d a t a base of t h i s investigation i s somewhat l a r g e r and more suitable f o r a statistical analysis.

The deposition t o t h e f o r e s t floor a p p e a r e d

to

b e significantly r e l a t e d t o t h e bulk deposition ( f i g u r e 3). The mean (p of all s i t e s w a s 2.9 1.5. The (p-value was not equal f o r all f o r e s t types, coniferous f o r e s t having t h e highest (p and deciduous f o r e s t t h e lowest ( a b l e

W.

There are two reasons why i t i s somewhat uncertain t o approximate t h e t r u e filtering e f f e c t (atmospheric deposition

to

f o r e s t vs. t h a t t o a nearby c r o p or g r a s s field) by Eq. (5). First, p a r t of t h e flux of sulfur measured in stemflow and throughfall could b e due

to

t h e internal nutrient cycle of t h e ecosystem. Lindberg et al. (1986) and Bredemeier (1987) a r g u e , however, t h a t in case of S t h e internal flux i s insignificant

(<

5%) in conditions found in Central Eu- r o p e where t h e total deposition

to

t h e canopy i s high. Secondly, bulk deposition probably underestimates t h e deposition

to

g r a s s and crops. Low vegetation f i l t e r s d r y deposition

to

some e x t e n t and t h e r e b y tends

to

a b s o r b SO2 and SO;- more effi- ciently

than

t h e bulk collector. Deposition on bulk aollectors i s predominantly comprised of gravitational deposition. A small amount i s contributed by c a p t u r e from t h e atmosphere through turbulent t r a n s f e r , impact and diffusion. Little i s known

about

t h e r a t i o of bulk deposition to deposition on low vegetation. Skeffing- ton (1983) found about 10% higher S-deposition on g r a s s (Pteridium aquilinum) com- p a r e d

to

bulk deposition. Heil at d . (1988) showed t h a t sulfur deposition

to

g r a s s is closely related

to

t h e leaf' area index of t h e grass. Deposition to unrnown grassland a p p e a r e d

to

b e 3 times bulk deposition in summer and about equal

to

bulk deposition during t h e o t h e r S~RISOTLS. Both of t h e s o u r c e s of uncertainty

act

in t h e

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way t h a t Eq. (5) tends

to

overestimate t h e t r u e (p, i.e. t h e filtering of f o r e s t as compared with t h a t of grasslands and c r o p s .

T a b l e

VL

Ratio between t h e flux of S onto t h e f o r e s t floor (TD) and bulk deposition (BD).

Forest type ALL * r e s t s ConiJbrous D e c i d u o u s

n 54 38 13

TD/BD 2.89

*

^1.50 ^3.15

*

^1.61 ^2.05 ^0.73

5.2. "Obnemed" r- "calculated" deporition 5.2.1. Obmemed fluxes re- EHEP model results

The a o r r e l a t i o n between t h e measured fluxes

to

the f o r e s t floor and t h e deposition calculations done by the EMEP-model was 0.70. This c o r r e l a t i o n i s less t h a n t h e c o r r e l a t i o n r e p o r t e d between calculated and measured S O p i r concentrations but higher t h a n t h e c o r r e l a t i o n between calculated and measured concentrations of sulfate in a i r and precipitation (Lehmhaus, 1986).

T h e r e a p p e a r e d . however,

to

b e a n obvious bias. Measured fluxes generally were higher t h a n model estimates. This bias w a s not t h e same f o r all f o r e s t types (TbbLe

WI

and f i g u r e s 4a,b). Coniferous f o r e s t s had t h e highest r a t i o between ob- s e r v e d and calculated deposition. Only in coniferous f o r e s t s w a s t h e difference between the flux

to

t h e f o r e s t floor and t h e EMEP-model estimate statistically significant (paired t-test, a

=

0.05).

T a b l e VlI. Ratio of o b s e r v e d and calculated

total

S-deposition.

Forest t y p e ALL forests C o n W r o u s D e c i d u o u s

5.2.2. Obaerved flux-

v -

RAMS m o d e l

d t s

F k g u r e 5 shows the comparison of observed d a t a w i t h RAINS predictions. The value 2.0

w a s

used f o r (p

as

usual. The c o r r e l a t i o n between the o b s e r v e d fluxes and t h e RAINS

estimates

w a s 0.70. The RAINS-estimates were significantly higher t h a n the o b s e r v e d fluxes (paired t-test, a

=

0.05), indicating that t h e o v e r a l l value of t h e f o r e s t filtering p a r a m e t e r of 2.0 is too high.

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6. Conclariona on

the

Forest F i l t m of S u l f u r Regadbg RAINS

I t can b e concluded t h a t f o r e s t filtering i s a significant f a c t o r determining t h e fate of atmospheric sulfur deposition. In p a r t i c u l a r , s p r u c e f o r e s t s a b s o r b high deposition loads. The t o t a l deposition estimates of t h e EMEP-model a g r e e v e r y w e l l with t h e deposition measured in deciduous f o r e s t s , but i t underestimates deposition in coniferous f o r e s t s on t h e a v e r a g e by 30-402. Therefore, i t i s proposed

to

estimate f o r e s t deposition in RAINS applying a value of 1.0 f o r t h e f o r e s t filtering parame-

ter

p f o r deciduous f o r e s t s and

a

value of 1.6 f o r coniferous f o r e s t s . Such a transformation a p p e a r s

to

r e s u l t in a n overall mean deposition estimate which equals the o v e r a l l mean observed throughfall and stemflow flux (FEgure 6).

The difference between coniferous f o r e s t s and o t h e r f o r e s t s might b e caused by t h e f a c t

that

conifers are g r e e n throughout t h e y e a r and t h e i r canopies pro- vide

a

l a r g e r e c e p t o r surface continuously. Also t h e specific (micro) s t r u c t u r a l c h a r n c t e r i s t i c s of t h e conifer canopy may play

a

role, involving

a

higher

aero-

dynamic s u r f a c e roughness than o t h e r forests. In t h e next phase, i t would b e in- t e r e s t i n g

to

collect m o r e

material

of t h i s kind.

to

s u b t r a c t

w e t

deposition from

to-

taldeposition estimates, and

to

investigate f o r e s t filtering specifically related

to

d r y deposition.

In addition, r e s e a r c h i s needed on t h e physical and meteorological mechan- isms of f o r e s t deposition.

7. Base Cation Deposition

A s discussed by K h a r i (1986) two r a t h e r different a p p r o a c h e s have been con- sidered in RAINS on how

to

t a k e into account t h e neutralizing effect of base cation deposition. One assumes t h a t base cation deposition i s proportional

to

sulfur deposition. If sulfur emissions are reduced and sulfur deposition d e c r e a s e s , base cation deposition according t o t h i s assumption will also d e c r e a s e in a proportional way. The reduction of sulfur emissions i s thus assumed t o r e d u c e base cation emissions as well.

The o t h e r approach would b e

to

assume t h a t t h e base cation deposition i s independent of t h e sulfur deposition. This would be c o r r e c t , if t h e s o u r c e s of emissions

are

not t h e same f o r base cations and f o r sulfur. The estimated neutralizing effect of base cations could then be subtracted from t h e estimated acidifying ef- f e c t of s u l f u r deposition. In f i g u r e s 7a and b t h e fluxes of Ca and Mg onto t h e f o r e s t f l o o r

at

s e v e r a l European s i t e s

are

compared

to

t h e flux of sulfur onto t h e f o r e s t floor.

Calcium flux in t h e s e d a t a tended

to

b e low

at

s i t e s where sulfur flux i s also low ( f i g u r e 7a). The relationship seems

to

b e curvilinear, although t h e wide

scatter

especially w n n e c t e d

to

high values of sulfur flux does not allow firm conclusions. Magnesium flux (Flqurs 76)

w a s

r a t h e r constant

at

40-50 meqm2-yr and thus independent of sulfur flux.

The relationship between calcium and sulfur fluxes i s most probably coin- cidental and does not indicate t h a t calcium and sulfur would originate from t h e

same sources.

Power plants, f o r example, emit considerable amounts of sulfur dioxide but v e r y little calcium compounds. Wind erosion, r o a d dust and agricultural liming p r a c t i c e s

are

sources of calcium emission into t h e a i r but

are

insignifi- c a n t

sources

of atmospheric sulfur. The relationship may r e f l e c t t h e simple f a c t t h a t power plants and o t h e r

sources

of sulfur

are

located in t h e same regions as calcium s o u r c e s (agricultural fields and roads). Both s o u r c e s are concentrated in

(16)

c e n t r a l E u r o p e where population density, industrial development and agricultural production have a stronger e f f e c t on t h e environment than, f o r example, in Scandi- navia.

The magnesium flux d a t a indicated hardly any gradient between industrialized regions and remote areas. This suggests t h a t

m o s t

of t h e magnesium falling onto t h e f o r e s t f l o o r h a s i t s origin e i t h e r within t h e f o r e s t stand or in other non- anthropogenic processes.

Possible sources of Ca and Mg are:

1. Soil dust (mainly from a g r i c u l t u r a l land);

2. Agricultural f e r t i l i z e r s (liming);

3. Road dust (mainly from unpaved roads);

4. Limestone q u a r r i e s ;

5. Burning of fuels containing Ca and Mg;

6. Sea-spray.

To assess t h e influence of e a c h of t h e s e s o u r c e s on t h e deposition of basic cations in European f o r e s t s , t h e s e s o u r c e s should b e parameterized. Some possibili- t i e s are:

a d l . A r e a of a g r i c u l t u r a l land on c a l c a r e o u s soils (km ');

ad2. Consumption of limestone f e r t i l i z e r s (kg);

ad3. Length of unpaved r o a d s on c a l c a r e o u s soils (km);

ad4. Number of q u a r r i e s ;

ad5. Ca

+

Mg emission (kg), calculated from fuel use;

ad6. Distance

to

sea (km).

To estimate t h e r e l a t i v e importance of t h e s e variables, a multiple regression according

to

t h e following model could b e done:

where BC

=

Ca

+

Mg deposition

to

European f o r e s t s estimated from throughfall and stemflow fluxes and bulk precipitation.

A more demanding scientific t a s k would b e

to

d e s c r i b e t h e a c t u a l magnitude of Ca and Mg emissions and

to

develop atmospheric long r a n g e tmnsport models f o r t h e s e elements. Even if t h e

transport

of Ca a n d Mg

oacurs

o v e r s h o r t e r distances t h a n sulfur compounds, a description of t h e physics t h a t connect sources and re- c e p t o r s would b e v e r y useful f o r ecological assessment purposes.

Further work is needed b e f o r e these ideas c a n b e introduced into

RAINS.

A c a r e f u l l i t e r a t u r e study should b e carried o u t

to

examine how

to

take into account t h e internal c y c l e of Ca and Mg

ions

wlthin f o r e s t

ecosystems.

Unlike wlth sulfur, i t is not clear whether how significant i s t h e leaching of t h e internally circulating b a s e mtions. The contribution of all t h e d i f f e r e n t

sources

to t h e basic cation deposition in f o r e s t s should be studied. Also. i t would need

to

b e studied what is t h e filtering e f f e c t of f o r e s t s r e g a r d i n g b a s e cations.

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8. Conclllrions on

Ban

Cation Deposition in RAINS

Finally,

w e

may examine what conclusions can b e drawn from t h i s material with r e s p e c t

to

t h e way base cation deposition i s taken into

account

in t h e RAINS model.

Other methods have been m n s i d e r e d , but presently RAINS uses just one specific way of taking into account t h e deposition of base cations. Sulfur deposition, obtained f r o a energy-emissions and atmospheric mbmodels, i s transformed into a n

estimate

of a c i d load by assuming t h a t e a c h mole of sulfur produces

t w o

moles of protons. Base cation deposition is assumed

to

neutralize one-third of this acid load.

Did t h e s e data s u p p o r t t h e above method? Calcium plus magnesium flux, m e a s wed under the f o r e s t canopy, is presented as a function of t h e corresponding sul-

f u r

deposition in lsPgure 8. A non-linear c u r v e is fitted into t h e data (solid line y=100

+

⁶⁰ tanh (S/lOO-17); n=43; r2=0.66). The s h a p e of t h e c u r v e i s not derived from a n y theory. In f a c t ,

w e

believe t h a t t h e r e

are

v e r y few

causal

relationships between sulfur and base cation deposition and, t h e r e f o r e , t h e relationship may not follow any simple theory.

The dashed s t r a i g h t line i s t h e RAINS assumption t h a t one-third of t h e acidifying potential of S i s neutralized by base cations. The empirical relationship between base cations and S indicates higher values of b a s e cation deposition than t h e RAINS

estimate

o v e r

m o s t

of t h e r a n g e (light shading). Only with v e r y high sul- f u r deposition values RAINS seems

to

overestimate t h e neutralizing e f f e c t due

to

base cations (dark shading). However, t h e

scatter

of t h e d a t a in high deposition values is quite substantial. The only obvious conclusion i s t h a t in remote areas (where S deposition i s low) t h e base cation flux t h a t i s measured u n d e r forest canopy i s h i g h e r than t h a t assumed by t h e RAINS model.

These r e s u l t s , however, need not be i n t e r p r e t e d in t h e way t h a t t h e RAINS model would underestimate t h e neutralizing e f f e c t of base cation deposition falling porn the atmosphere onto the &rest canopy. A l a r g e fraction of b a s e cations in stemflow and throughfall samples can have t h e i r origin in t h e tree metabolism and ultimately fn t h e b a s e cation r e s e r v e s of t h e soil. Calcium and magnesium are effectively cycled within t h e ecosystem. Therefore basic cation deposition, meas- u r e d by means of collecting throughfall and stemflow, generally will overestimate atmospheric base cation deposition t o

some

extent. Sulfur, in t u r n , h a s long been known as a "mobile anion" t h a t effectively flows from t h e atmosphere through t h e t e r r e s t r i a l environment into aquatic ecosystems.

Estimates of t h e r e l a t i v e importance of internal cycling

to

t h e

total

b a s e cation deposition onto t h e f o r e s t floor could possibly b e gained by comparing bulk precipitation and throughfall and sternflow deposition both f o r base cations and o t h e r ions Uke sodium and chloride. This should be studied in future.

A t

t h e p r e s e n t time t h e r e

are

no European-wide quantitative estimates on t h e internal c y c l e of base cations. Fkgure 8, given t h e considerations above, en- courages

to

k e e p t h e c u r r e n t method within RAINS as i t is, as f a r as t h e time period 1978's and 1980's is concerned.

Although t h e method i s in a reasonable agreement with conditions of t h e 1970's and 19801s,

w e

must examine t h e question, urfU the relationship between M r a n d base catton deposition remain unchanged in t h e f i t u r e ? No, i s t h e o u r r e n t best answer. According

to

the c u r r e n t emission reduction plans, sulfur emissions w i l l b e 3 0

to

402

smaller in

1995 than they w e r e in 1980. Calcium and magnesium emissions

are

likely

to

remain

at

t h e i r c u r r e n t level;

at least

t h e r e are

no

major international plans

to

r e d u c e t h e i r emissions.

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If sulfur emissions decline and base cation emissions remain constant over time, base cation deposition will neutralize

a

l a r g e r fraction of sulfur deposition in t h e future than today. The stronger t h e sulfur emission reductions, t h e faster

w e

will approach t h e situation t h a t

m o s t or

all of t h e acidity due

to

sulfur deposition will be neutralized by base cation deposition. This i s a n important finding as re- gards t h e RAINS model. The treatment of Ca and Mg deposition should be changed as f a r as future acid deposition scenarios

are

concerned.

The default method f o r computing the neutralization effect of base cation deposition in RAINS f u t u r e projections should be t h e following. The model should be changed In such

a

way t h a t base cation d e w i t i o n is a l l o w e d

to

vary o v e r space but is kept

aonstant

o v e r the time between 1980-2040. The spatial variation could be described in a number of alternative ways. The ideal way would be

to

have in- ventories

of

the atmospheric emiseions of base catlons and

a

long range transport model

to

describe t h e source-receptor relationships. An alternative way is

to

develop

a

regression model (sue Eq. 6 ) with explanatory variables such that can be described o v e r all Europe. In t h e s h o r t term, however, t h e only option is

to

draw on the relationships of f i g u r e 8 t h a t is,

to

use t h e ooincidental relationship of base cation deposition

to

S deposition.

Sulfur deposition, after taking into account t h e f o r e s t filtering effect (Eq. 3) i s described f o r t h e y e a r 1975 into t h e forest land of each grid square of t h e RUNS model (the impact model grid). The y e a r 1975 is selected because t h e data of f i g u r e 8 r e p r e s e n t approximately t h a t period of time. Aoid load i s then computed, and t h e neutralizing effect of base cation deposition i s

estimated as usual

a s one-third of t h a t load. This spatial distribution of t h e neutralizing effect is then

stored

into RNNS and kept constant o v e r time in all RAINS scenarios.

The above procedure

seems to

be t h e most justified default method f o r RAINS calculations f o r t h e time being. The main impact of this change will be t h a t t h e ecological models (soil model and lake model) will respond

m o r e

strongly

to

a de- crease of sulfur emissions than they do in t h e i r present form. Soil acidification and lake acidification according

to

new calculations will be estimated

to

cease when sulfur emissions a r e reduced by 65-70% from t h e sulfur emission levels in 1975-1980. However, given t h e additional acld load due

to

nitrogen compounds, overcoming t h e soil and lake acidification problem in t h e most sensitive areas may require additional reductions in both sulfur and nitrogen emissions.

(19)

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Kauppi, P. Kauppi, G. Kornai, and A. M5ikeN (1987). Acidification in Europe: a simulation model f o r evaluating control s t r a t e g i e s , Ambio, 1 6 , 232-245.

2. Andersson, F., T. Fagerstriim, and S. Ingvar Nilsson (1980). Forest ecosystems responses

to

acid deposition

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hydrogen ion budget and n i t r o g e n / t r e e growth model approaches. In T.C. Hutchinson and M. Havas (eds.),

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ofAcid Pre- c i p i t a t i o n o n Terrestrial Ecosystems. Plenum P r e s s , pp. 319-334.

3. Keller, H.M., P. Kliiti, and F. F o r s t e r (1987). Event studies and t h e i n t e r p r e t a - tion of

water

quality in f o r e s t e d basins. In Proceedings Intern. Symposium on acidification and

water

pathways, Bolkesjf5, Norway, May 1987, pp. 237-248.

4. Asman, W.A.H., J. SLanina, and J.H. Baard (1981). Meteorological interpretation of t h e chemical composition of rain-water

at

one measuring

site,

W.A.S.P., 1 6 , 159-175.

5. Bredermeier, M. (1987). Fbrest Cbnopg Transjbrmation of Atmospheric h p o - d f i o n . Presented

at

t h e IIASAflMGW Task Force Meeting on Atmospheric Com- putation

to

Assess Acidification in Europe, Warsaw, April 1987.

6. Bredermeier,

M.,

E. Matzner, and B. Ulrich (1986). A Simple and Appropriate Approach of Total Atmospheric Deposition i n Forest Ecosystem Monitoring.

Presented

at

NATO Advanced Research Workshop "Acid Deposition Processes

at

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7. Breemen, N. van, P.A. Burrough, E.J. Velthorst, H.F. van Dobben, Toke d e Wit, T.B. d e Ridder, and H.F.R. Reijnders (1982). Soil acidification from atmos- p h e r i c ammonium sulfate in f o r e s t canopy throughfall, Nature, 299, 548-550.

8. Biickling, W. and R. Steinle (1987). KLeinrbumliche Verteilungsmuster d e r Stojfdeposition i n n a t u r n a h e n Waldokosystmen. Symposium "Effects of Air Pollution on T e r r e s t r i a l and Aquatic Ecosystems", Grenoble (France), 18-22 May 1987.

9. Buldgen, P. and J. Remacle (1984). S u l p h u r Budget i n Fbrested Watersheds i n Haute Ardenne Region (Belgium). In SCOPE Belgium

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Proceedings "Acid Deposition and Sulphur Cycle", Brussels, 1984. pp. 163-169.

10. Eliassen, A. and J. Saltbones (1983). Modelling of long-range t r a n s p o r t of sul- p h u r o v e r Europe: a two-year model run and some model experiments, Atmos- pheric Environment, 1 7 ( 8 ) , 1457-1473.

11. Freiesleben, N.E. van, C. Ridder, and L. Rasmussen (1986). P a t t e r n s of acid deposition

to

a Danish s p r u c e f o r e s t , W.A.S.P., SO, 135-141.

12. Fowler, D. (1980). W e t and d r y deposition of sulphur and nitrogen compounds from atmosphere. In T.C. Hutchinson and M. Havas (eds.), W e c t s of Acid Pre- c i p t t a f i o n o n n r r e s t r i a l Ecosystems. Plenum P r e s s , pp. 9-27.

13. Georgii, H.W., S. Grosch, and G. Schmitt (1986). Feststellung d e r Schad- s t o m L a s t u n g won Waldgebieten i n d e r Bundesrepublik DwtschLand d u r c h trockene und n u s s e Dsposition. Abschlussbericht Teil A, Institut fiir Meteorolagie und Geophysik, J.W. Goethe Universitiit, Frankfurt.

14. Grennfelt, P., S. Larsson, P. Leyton, and B. Olsson (1985). Atmospheric deposition in t h e Lake ~ h j i i n a r e a , S W Sweden, EcoL. BULL.,

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15. Hantschel, R. (1987). Wasser- u n d E l m e n t b i l a n s w n gesch&digtm, gad4nqten FYcht&kosystmem i m F b h t e l g d r g e u n t e r B r i i c k d c h t i g u n g uon p h y s i k o l i s c h e r und c h m i s c h e r BodenheterogenitBt. Bayreut boden- kundliche Berichte, Band 3.

16. Heil, G.W., M.J.A. Werger, W. d e Mol, D.

van

Dam, and B. Heijna (1988). Capture of atmospheric ammonium by grassland canopies, Science, 239, 764-765.

17. Horntvedt, R., G.J. Dollard, and E. Joranger (1980). Effects of acid precipitation on soil and forest, 2. Atmospheric-vegetation interactions. In D. Drablos and A. Tollan (eds.), Ecologicd Impact of Acid P r e c i p i t a t i o n . Proceedings of t h e International Conferenoe on Ecological Impact Acid Precipitation, Nor- way, 1980, SNSF-Project, pp. 192-193.

18. Ivens, W.P. M.F., G .P. J. Draaijers, M.A. Brinksma, and W. Bleutsn (1987). Meting

van

atmosierische depositie in d e boswachterij Kootwijk door opvang

van

doorval en starnafvoer. Vakgroep Fys. Geogr., R.U. Utrecht AD 1987-1 (Publ.

r e e k s N a t . Prog. Zure Regen N r . 37-06).

19. Johnson, D.W., D. Richter, H. van Miegroet and J.M. Kelly (1986). Sulphur cycling in five f o r e s t ecosystems,

WASP.., SO.

965-979.

20. K h d i r i , J. (1986). Linkage between Atmosph.eric I n p u t s a n d Soil and Water A d d t l r c a t i o n . Summary of p a p e r presented

at

the International Technical Meeting on Atmospheric Computations f o r Assessment of Acidification in Eu- rope: Work in Progress. IIASA/IMGW, Warsaw, 4-5 September 1985.

21. Karkanis, M. (1976). The circulation of sulphur in t h e forest ecosystem Tilio- Ceu-pinetum in t h e northern part of Puszcra Niepolomicka n e a r Ispina, Frag- mentor FZoristica et Ceobotanica Ann. ;rWI, 3, 351-363.

22. Kauppi, P., J. K W r i , M. Posch, L. Kauppi, and E. MatPler (1986). Acidifica- tion of f o r e s t soils: model development and application f o r analyzing impacts of acidic deposition in Europe, Ecol. Modelling, 3, 231-253.

23. Lehmhaus, J., J. Saltbones, and A. Eliassen (1986). A Modwed L k l p h u r Budget for Europe for 1880. EMEP/MSC-W Report 1/86 (Draft), Norwegian Meteoro-

logical Institute.

24. Lindberg, S.E., G.M. Lovett, D.D. Richter, and D.W. Johnson (1986). Atmospher- ic deposition and canopy interactions of major ions in a forest, Science, 231, 141-145.

25. Matzner, E. (1983). Balances of element fluxes within different ecosystems im- pacted by acid rain. In B. Ulrich and J. Pankrath (eds.),

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ofAccumuLa- t i o n @ A i r P o l l u t a n t s i n Forest Ecosystems. D. Reidel Publ. Co., Dordrecht, pp. 147-156.

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@ A c i d P r e c i p i t a t i o n . Proceedings of t h e International Conference on Eco- logical Impact Acid Precipitation, Norway 1980, SNSF-Project, pp. 33-40.

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31. Richter, A. and L. Granat (1978). Pine p r e s t canopy throughJW1 measure- ments. MISUDMI, Stockholm. Report 1978-03-01.

32. Roberts, T.M., T.A. Clarke, P. Imeson, and T.R. Gray. Effects of sulphur deposition on l i t t e r decomposition and nutrient leaching

in

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o f A c i d P r a c i p i t a t t o n o n fir- r e s t r i d Ecosystsnrs. Plenum Press, pp. 381-393.

33. Skefffnqton, R.A. (1983). Soil properties under t h r e e species of tree in south-

ern

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Pankrath (eds.), m c t s of Accumulation of Air P o l l u t a n t s i n Forest Ecosys- tems. D . Reidel Publ. Co., pp. 219-231.

34. Verstmten, J.M., J.J.H.M. Duijsings, L.A. Bruynzeel, W. Bouten, and A.J. van Wijk (1984). Effecten van d e vegetatle (eiken/beuken/f i jnspar) o p de deposi- t i e van zuurvormende bestanddelen op d e bodem. In E.H. Adema and J. van Ham (eds.), Zure regen. oorzaken, m e c t e n en belstd. Proceedings van h e t Symposium gehoulden o p 17 en 1 8 November 1983. Is-Hertogenbosch. Pudoc, Wageningen.

35. Voldner, E.C., L.A. Barris, and A. Sirois (1986). A l i t e r a t u r e review of dry deposition of oxides of sulphur and nitrogen with emphasis on long-range transport modelling in North America, Atmos. Enu., 20 ( l l ) , 2101-2123.

36. World Forestry Atlas (1975). V e r h g Paul Parey, Hamburg and Berlin.

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f i g u r e 1. Deposition o n f o r e s t s as a function of t h e filtering f a c t o r p f o r various values of t h e f o r e s t c o v e r a g e f.

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Fkgure 2. Location of the measurement sites.

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A conifers

0 2 4 6

a

10

BULK DEPOSlllON

(g/m2-yr)

O deciduous forests

f i g u r e 3. Comparison of t h e S-flux o n t o t h e f o r e s t f l o o r (TD) a n d t h e S-flux in bulk deposition (BD). The d a t a point indicated with "0" r e f e r s to measurements t a k - e n in s o u t h e r n Poland ( K a r k a n i s , 1976). I t may or may n o t r e p r e s e n t east E u r o p e a n conditions m o r e b r o a d l y ; t h e o t h e r o b s e r v a t i o n s a r e from w e s t e r n E u r o p e (Figure 1 ) . Rejecting t h i s d a t a point t h e following r e g r e s s i o n i s obtained: TD

=

2 . 4 2 + ~ 0 ' . ~ ~ (n

=

53, r 2

=

0.70)

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MODEL ESllWE FOR DEWSlllON (g/m2-yr) A coniorws forest3

UOw ESllruTE FOR W%SlllOI( (q/m2-yr) 0 COCMS fwssh

F i g u r e 4. Throughfall and stemflow flux of sulfur onto the forest floor versus S- deposition calculated by the EMEP-model, (a) in coniferous forests, and (b) in deci- duous forests.

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(27)

0

0 2 4

6

8 10

"NEW'

RUNS

MODEL ESTUIATE (g/m2-yr)

f i g u r e 6. Throughfall a n d stemflow flux of s u l f u r o n t o t h e f o r e s t f l o o r v e r s u s S - deposition calculated by t h e RAINS-model applying gp

=

1.6 f o r c o n i f e r o u s f o r e s t s a n d p

=

1.0 for deciduous forests.

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n

¹²⁰

>

E ¹¹⁰

1

^loo

"

2M) 400

SVLRfVR FLUX (meq/m2-yr)

zoo 400

sURlUR FLUX (mq/m2-yr)

f i g u r e 7. Calcium (Ca) and magnesium (Mg) versus sulfur ( S ) in the flux onto the forest floor. Ca

=

4 . 9 9 * ~ ~ . ~ ~ meq/m -yr (n ²

=

47, r 2

=

0.66). Mg

=

26.20

+

0 . 0 5 7 6 meq/m2-yr (n

=

47, r 2

=

0.13).

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Fligure 8. The relationship between b a s e cation deposition and S deposition. Solid line (y=100+60

*

tanh(S/100-17); n=43; r 2 = 0 . 6 6 ) h a s been fitted into t h e data.

Dashed line (y=0.33 S ) i s t h e current assumption on this relationship within RAINS.