The prediction of rain acidity and SO2 scavenging in Eastern North America

Atmospheric Enrironmunr. Vol. 15, pp. 3 I-41. Pergamon Press Lid 1981. Printed in Great Britam

0

THE PREDICTION OF RAIN ACIDITY AND SO, SCAVENGING IN EASTERN NORTH AMERICA L. A.

BARRIE

Air Quality and Inter-Environmental Research Branch, Atmospheric Environment Service, 4905 Dufferin Street, City of North York, Downsview, Ontario, Canada (First received 1 November 1979 and

in,finnf firm

17 March 1980)

Abstract - The eastern half of the North American continent experiences acidic rain. An approach to

predicting that acidity from SO, emission rates and meteorological data is suggested. It is based on modelling the atmospheric sulphur cycle and on the strong correlation between hydrogen and sulphate ions in rain collected by the CANSAPand MAP3S precipitation networks. The molar ratio [H’]/[SO:-] in rain is commonly 1.6-2 in the northeastern U.S.A. and - 1 in acid sensitive areas of eastern Canada. Using the solution chemistry of sulphur dioxide, it is concluded that dissolved SOz can contribute substantially to the acidification ofa receptor by rain especially at low temperatures in areas having particulate matter with a low hydrogen ion content. An expression is derived relating SO2 washout ratio to two parameters, rain water pH and temperature. Ways of incorporating SO? washout into sulphur cycle models are discussed.

Most of the sulphur entering the atmosphere from anthropogenic sources is in the form of sulphur dioxide gas, SO,. Typically, 95-99x of the total sulphur in the effluent of oil or coal-fired thermal generating stations is in the form of SOa (Forrest and Newman, 1977). The remainder consists of “primary” particulate sulphate, mainly sulphuric acid. While being transported through the atmospheric boundary layer (ca O-2 km), SOz is chemically transformed to “secondary” particulate sulphate, absorbed at the earth’s surface and scavenged by precipitation. It has been observed that dissolved SO2 can contribute up to 30% (on a molar basis) to the total sulphur content of rain in the northeastern United States (Hales and Dana, 1979) and 14% on the average in rural England (Davies, 1979). In this paper, the role of SOz as a rainwater acidifying agent is examined. A quantitative relationship between the S(IV) content of rain, rainwater pH and temperature is derived which can be used in long-range transport models to simulate the SO2 scavenging process mathematically. Methods of calculating precipitation acidity and hence SO, washout are demonstrated for various rainwater chemistry data sets.

1. INTRODUCTION

In the last decade, serious effects of acidic precipitation associated with anthropogenic emissions of sulphur and nitrogen oxides have been observed in the biosphere (Ambio, 1976; NATO, 1978). Consequently, efforts have been initiated in many countries to develop models that predict the temporal and spatial distribution of the hydrogen ion content of precipitation from known or projected pollutant emission rates and meteorological data (Voldner ef at., 1981; Shannon, 1979). A predictive capability is required in order to assess the impact of long-term energy strategies. For instance, it is necessary to anticipate the increase in precipitation acidity in eastern North America if power plants were to switch to burning coal of a higher sulphur content. In theory, the hydrogen ion content of rain can be predicted by accurately modelling the cycles of all acidic and basic compounds scavenged by rain and then solving the ion balance equation for hydrogen ion concentration. However, in practice this is not feasible. This is, in part, because the cycles of important basic constituents such as ammonia and calcareous windblown dust are poorly understood. In the following discussion, an alternative empirical approach to predicting the hydrogen ion content of rain is suggested for eastern North America. It involves modelling the atmospheric sutphur cycle to predict the sulphate content of rain and then using an empirical relationship between the hydrogen and sulphate ion content of rain revealed by recent data from the Canadian CANSAP and American MAP3S precipitation networks to produce an estimate of rainfall acidity.

2. THEORY OF SO, WASHOUT

In models, the SOz concentration in rain is often calculated from predicted, atmospheric concentrations of the gas using a dimensionless washout ratio Wso, (Slinn et al., 1978) defined as

CSOJr wso2= m’ 31

(1)

32

L. A. BARRE

Subscripts r and a denote rain and air, respectively. The wet removal rate F (dimension ML-’ T- ‘)is then given by F = Wso, I(SO&,.

(21

I is precipitation intensity (dimension LT- ‘). It has been common practice to use a constant “representative” washout ratio to calculate the rate of removal of SO2 by pr~ipitation (e.g. Henmi and Reiter, 1978). However, Wso2 is not constant, but rather strongly dependent on rain pH and temperature. The relationship is governed by the equilibrium solution chemistry of SOZ which is described by the following equations

Table 1. The temperature dependence of the Henry‘s constant K,,. the first dissociation constant I<, and their produc! K 1K, for an SO, atr- water system

Temperature,

P.2

__.___~ 0 15 25

k‘,, (mol I; ’ molt;‘)

XI Imol I; I)

73.5 41‘5 30.3

3.1 r( IO-’ 2.’ x lo--” 17x 10-L

~..

K!hI,

(tnol i ! ’ (rnt,i ; ,I i; 24 0 91 0 5.3

A graphical illustration of the dependence of Wsoz on pH and temperature is given in Fig. 1. Wso2 values that are likely to occur in the atmosphere are denoted by the hatched area. They fall within a range 10’. 10”. The strong dependence on pH and temperature illusES02HzOlr If fW trated in Fig. 1 emphasizes the need to account for it in w21. * long-range transport models rather than neglecting it CH+lrWO;Ir (3b) as is often done by using a constant “representative” ’ [SO, .H,O], ’ Wso2 value (W,,, increases lo-fold for unit increase in pH and 4.3-fold for a change in temperature from 25 to K = W+ldX’:-lr (3c) OC). 2 CHWI, Another type of SO,-washout parameterization is K,, K, and K2 are equilibrium constants. Square sometimes used in models. A scavenging coefficient i. brackets represent molar concentrations. SOZ exists in has been utilized to calculate the SOz-removal rate R solution as physically-dissolved SO;! (SO2 . H,O), due to precipitation scavenging in simple box or layer bisulphite ion (HSO;) and sulphite ion (SO:-). models (e.g. McMahon et al, 1976). R is assumed to be Barrie (1978) has shown that for the range of pre- directly proportional to the ambient SO, concencipitation pHs commonly observed in the atmosphere tration where 3, is the proportionality constant. If (3-6), over 95% of total-dissolved SO2 is in the form of washout is the only sink for SO,, i, is given by bisulphite ion (i.e. [SO,ll = [HSO;],). Thus, under equilibrium conditions Equations (I), (3a) and (3b) i, = - (” 1n~~021a) -during rain event. (6) yield K

K

=

=

ii_“”

W

lWO;lr s02=

[S02],

K,K, =

[H+l,’

(44

I

_._,

___.__~.__.__._

____.~

~-

oc /’

or, in terms of pH log,, Wso, = log,,(K,K,)

+ PH.

14b)

In deriving Equations (4a) and (4b), the assumption was made that SOZ in the air is in equilibrium with SOZ in the rain. Hales (1978) introduced an objective criterion which must be satisfied in order for “equilibrium scavenging” to occur. He points out that beyond 20 stack heights of a point source the criterion is usually satisfied. Thus, for the purposes of regional and global transport modelling, Equation (4a) for W,,, is valid. One must, however, be careful to avoid using it in situations in which the Hales criterion is not met. (Non-equilibrium scavenging is more complex requiring, among other things, a knowledge of the raindrop spectra - Barrie, 1978). The equifibrium constants K, and K, are temperature-dependent (Table 1) and their product K,K,, determines the temperature dependence of the SOZ washout ratio. Over the range 273.1-303.1 K (0- 30”(Z),K,K, is related to temperature T(K) by the following empirical equation K,KH = 6.22 x 10e8 exp (47555/T).

(5)

Fig. 1. The dependence of the washout ratio of SO, by rain on pH for two temperatures, 0 and WC, under equilibrium scavenging conditions (the hatched area represents those situations commonly encountered in the atmosphere).

Prediction of rain acidity and SO2 scavenging Attempts have been made to extract I-values from time series measurements of ambient SO, concentrations during precipitation events (e.g. Maul, 1978). It is, however, difficult to rule out the possibility that phenomena other than SOz washout such as SO1 oxidation are also contributing to the change in atmospheric concentration thus placing the calculated i,-values in a somewhat uncertain light. 1, is related to the washout ratio by the following equation

i, = -wso2 H

’

where H is the depth of the box in a one-layer model, or of a layer in a multilayer model, and I is the precipitation intensity. Modelers should use the washout ratio approach rather than the 3, approach to calculate precipitation scavenging since the former contains precipitation intensity I and washout depth H in an explicit form whereas the latter does not.

3. THE CONTRIBUTION OF DISSOLVED SO, TO THE ACIDITY OF RAIN

The fraction of total hydrogen ions in wet deposition contributed by dissolved SO,,f, is an indicator of the importance of precipitation scavenging of SO2 in the acidification of a receptor. Dissolved SO2 is eventually oxidized to sulphate in most natural receptors (oceans, lakes and calcareous soil). Hence, two hydrogen ions are released for each SO1 molecule deposited in rain. Assuming that, as the net result of pollutant scavenging and aqueous phase SO1 and nitrogen oxide oxidation, there are, in addition to hydrogen ions contributed by dissolved SOz, D hydrogen ions associated with each sulphate ion in rain reaching the ground, f, is given by f=

2CHSO;Ir D[SO:-1,

+ 2[HSO;],’

(8)

f can be related to parameters commonly predicted in long-range transport models by defining a washout ratio for sulphate Wso4, similar to that for SO, in Equation (4a). Using Equations (3a), (3b) and (4a) and the following expression for the hydrogen ion concentration in rain as it reaches the ground [H+], = D[SO:-1,

+ [HSO;],,

In this derivation, the use of the D formulation is based on the assumption that the net result of atmospheric scavenging and aqueous transformation processes is a correlation between the hydrogen and sulphate ion content of rain. It will be demonstrated in the next section that this is indeed the case at many locations in eastern North America. There are a number of processes that acidify cloud and raindrops. The most important are: (i) scavenging of sulphate aerosols (e.g. H,SO.+, NH,HSOJ; (ii) scavenging of nitric acid ; (iii) oxidation of dissolved SO1 to sulphate; (iv) oxidation of dissolved nitrogen oxides to nitrate. Not all hydrogen ions entering rain are accompanied by sulphate; for instance those taken up during process (ii) above. Consequently, D values greater than 2 are possible. For example, if H,S04 aerosol and HNO, gas were the only substances scavenged and if they were taken up in equal molar amounts D would be 3. That the sulphate content and hydrogen ion content of rain are proportional means one of two things: either nitrogen oxides contribute little to variations in the acidity, or they contribute substantially and are mixed in the atmosphere and scavenged by rain in relatively constant proportions with respect to sulphates. The spatial variation of D will reflect the geographical dependence of the chemical composition of the atmosphere and of the nature of rain scavenging processes. In areas where sulphate and hydrogen ion concentrations in rain are not correlated due to, for instance, the strong influence of non-acidic sulphates, acidic non-sulphates or calcareous wind blown dust, formulations using D are not appropriate. The dependence of f on [SO,], [SO:-], and temperature is shown in Figs 2 and 3 for typical D values of 2 and 1 (see Section 4), respectively. A sulphate washout ratio of 5 x lo5 was chosen as representative for eastern Canada by examining data obtained in the Canadian Intensive Sulphate Study of August 1976 (Intensive Sulphate Study, 1976). During that study sulphate washout ratios ranged from 3 x lo5 to 8 x 105. In order to demonstrate the importance of dissolved SO2 as a contributor of hydrogen ions to wet deposition, consider the following typical situations : (1) A power plant plume 100 km from the source. Typically

(9)

one obtains an equation for f in terms of commonly measured variables

W-M,= 200 pgw3 (70ppb), [SO:-],

= 10pgme3, D = 2 (sulphuric acid),

2ws,,c3321a f= 2Wso,CSO21, + DWso,[SO:-I,’ (10)

f = 0.20 at 25°C (Fig. 2), 0.50 at 0°C. (2) A polluted maritime tropical air mass in the

where

wso,=

33

K,Kn

; [SO:-I,Wso,+ . ;[sO:-1. Wso,’ + K~&,Wzla

(11)

34

L.

A. BARRIF: [SO& [SO:-],

= 80 pg m 1 ’ (28 ppb), = 10~(gm-2. D = 1 (CANSAP data) (see Section 4), f= 0.30 at 25°C (Fig. 3),0.57 at 0°C.

Dissolved SO, can contribute a sizeable fraction of the total hydrogen ions entering a receptor during a rain event (in the above examples,fvalues ranged from 11 to 307; at 25°C and from 34. 57% at OC). If longrange transport models neglected this source ofacidity considerable error would be introduced into deposition estimates, especially : (i) during weather having near-zero temperatures, and (ii) in areas having an 100 10’ 107 IO’ IO4 aerosol with a low hydrogen ion content. Northeastern SULPHUR DIOXIDE CONCENTRATION tug mm3) Canada during autumn fulfils both these conditions. Cold rains scavenge air containing what was originally Fig. 2. The computed fraction of total hydrogen ions in wet sulphuric acid aerosol that has been at least partially deposition that are contributed by dissolved SO,,f, as a neturaiized by ammonia on its 2-6 day journey from function of SO, concentration for various aerosol sulphate the source. SO, washout will therefore be important as concentrations (I, II and III correspond to 1, 10 and 100 ng SOi- rne3, respectively) and for two tem~ratur~ a source of hydrogen ions. fpC (solid line) and 25°C (dashed line). &VW4= 5 x 10’. 4. OBSERVED CORRELATIONS BETWEEN HYDROGEN AND SULPHATE IONS IN RAIN

American industrial northeast. Typically [SO&

= 100pgm-3 (35 ppb),

[SO:-&

= 10pgme3, D = 2 (MAP3S data) (see Section 4), f= 0.11 at 25°C (Fig, 2),0.34atO”C.

(3) A polluted maritime tropical air mass at a rural site in southeastern Canada (e.g. Ottawa region). Typically 10

0.8

06

04

0.i

0.0

100 SULPHUR

10' DIOXIDE

101 CONCENTRATION

103

$04 hg

n-i-3)

Fig. 3. The computed fraction of total hydrogen ions in wet deposition that are contributed by dissolved S02, J as a function of SO, concentration for various aerosol sulphate concentrations (I, II and III correspond to 1, IO and 1OOrg SO:- rnm3, respectively) and for two temperatures 0°C (solid line) and 25°C (dashed line). Wso, = 5x 10’. D=

1.

The composition of monthly precipitation samples collected at 25 CANSAP (Canadian Network For Sampling Precipitation) sites in eastern Canada between May and September 1977 and 1978 (Berry, 1979) as well as of event samples collected at four MAP3S (Multistate Atmospheric Power Production Pollution Study) stations in the northeastern United States during the same period (Hales and Dana, 1979) was examined (site locations Fig 4, site names Tables 2 and 3). A linear regression of observed hydrogen and sulphate ion concentrations, [H+lob” and [SO~-~bs, revealed significant correlations between the two parameters at many sites. All four MAP3S sites (Table 3) have very significant correlations (R values for 40-60 samples greater than 0.88). Of the 25 CANSAP sites in eastern Canada (Table 2), the correlation was significant at the lo/, level for 13. Of the 12 sites having no correlation at the l”/, level, five (Trout Lake, Pickle Lake, Atikokan, Moosonee, Fort Chimo) were remote ones having pHs above 5, and six (Windsor, Simcoe, Mt Forest, Kingston, St Hubert, Quebec City) had elevated calcium levels. The remaining site, Charlo, showed a correlation bordering on significant but only seven monthly samples were available. A plot of the correlation coefficient R for both US and Canadian sites on a map (Fig. 5) reveals a pattern (R values are underlined if they show a significant correlation at the 1% level). Downwind of major source regions (Fig. 6), hydrogen and sulphate ions are strongly related at sites whose local surface geology is non-calcareous. Locations in the Limestone-based St Lawrence lowlands have poor correlations. This is probably due to alkaline calcium particles in the rain that originate either from wind-blown dust scavenged by the rain or

35

Prediction of rain acidity and SO2 scavenging

I

I I

a23 \ \

i 9.-

5

\

.I,

Fig. 4. The location of CANSAP (circles) and MAP3S (squares) sites for which correlations between hydrogen and sulphate ions in rain were computed (Tables 2 and 3, respectively). The star marks the location

of an APN site at Chalk River, Ontario.

from wind-blown dust that entered the sample after collection (despite the use of wet-only samplers). A difference between the chemistry of rain collected at on-shield and off-shield sites in south-central Ontario has been observed previously and attributed to the greater abundance of alkaline soil particles at offshield sites (Dillon et al., 1977). The slope B of the linear regression equation [H+]fbS = A + BISO:-];bs

(12)

is plotted in Fig. 7 at those locations having a significant correlation. When the intercept A cannot be neglected, it is plotted in brackets. At most sites in the Canadian Shield, it can be neglected. Then, B represents the molar ratio of hydrogen ion to sulphate ions in a precipitation sample. B is close to 1 in the Canadian Shield. Its geographical variation is most easily explained by considering the spatial distribution of SO1 emissions (Fig. 6), and by bearing in mind that acidic sulphur and nitrogen oxides released by a source are gradually neutralized by ammonia gas as they are transported through the atmosphere. Consequently a

maximum B value of 2 observed at Penn State is to be expected since it lies immediately downwind of major sulphur sources in the PittsburgCincinnatti area. As the distance between source and sampler increases, B drops, attaining a value of about 1 in the Canadian Shield Region of Ontario and Quebec. The correlation between hydrogen and sulphate ions in monthly samples has also been observed in daily rain samples collected at another shield location, Chalk River (Fig. 4), at a somewhat later date. Twentynine rain samples were analyzed for hydrogen and sulphate ions between November 1978 and June 1979 (Barrie et al., 1980). A linear regression analysis yielded a correlation coefficient R of 0.9, a negligible A-value and a B-value of 0.77 [Equation (12)]. There was no correlation between hydrogen and nitrate ion concentrations. Caution should be exercised in attempting to relate B-values, representing the molar ratio of hydrogen ions to sulphate ions in rain samples reaching a laboratory, to the parameter D. D is defined in Section 3 as the number of hydrogen ions contributed per sulphate ion

36

L. A. B~RRI+

Table 2. The correlation between observed hydrogen and sulphate ion concentrations (~mol

I If in monthly ram sam&es ar

_

“.“. ,. 1 Trout Lake :! Pickle take 3 Atikokax

itr i’i

4 iMoosonee 5 wawa; 6 Windsor 7 Simcoc

8 Mount Furest 9 &%ZXhOFOtl~h I# 1

Kington

i Maniwaki

12 Saint Huben

13 Quebec City 14 ~hih~~uga~u

1S Nitch~~u(?n $6 17 18 I!? 20 21 22 23 24 25

f’ort Chime Se@-Her Goosr: Charlo Saint John, NR Shelburne Truro Sable isiand Step~e~~~~je Gander

entering ram either by a pollutant-s~vengin~ pathway or by aqueous phase oxidation of SO2 or NO, in cloud- or rain-drops. The reiationsh~p between B and D depends upon the way a sample is handled and analyzed for sulphate and on the molar fraction tn of total sulphur in rain that is dissolved SO,. In the MAP3S network, dissolved SO, is preserved in rain samples; suiphate and dissoived SC& are analyzed separately. Thus [SO:-]“b” in Equation (12) is the actual sulphate in rain. In the CANSAP network, dissolved SO2 is not preserved in a sampIe. It is oxidized to suiphate before analysis so that [SO;-t”“’ is the SUM of the actual sulphate in rain and dissofved

sop For CANSAP data B-2m

D=

‘!

(13)

i-~-----1 - ffl .,CAN&

For MAP% data

gearing in mind that the correlations betw~n hydrogen and sulphate ions reported above were for rainfall collected between May and September, TM-

Table 3. Thecorrelation between hydrogen am! sulphate inn Concentrations (pm01f ‘) far event rain samples at four MAP% sites for May--September 1977. 1978 (see Pig. 4 for site location using the number in the left hand column). [tf + J;“k= A I- B[So:-]:“’ __._. ._. _.. . . ...__,._,” .-..... -. ._ _._ . . .._,.,^^ ^_,... -... ---.-. --Corretatiox Number Intercept, of event Srope Goefficient {_ft” jr Tar@@ :i B Sili@FZS R &mol l “I ipmol i i I station “___ .._, ,_ -..

._

__,._,

. . --.-- ..-. -.--

Whiteface Mountain, NY 27 Ithaca, NY 28 Penn State 29 Virginia __~_.__.____._____ ..,.--_--..--

26

* pH = - log,, pi+],

1.4-Xfl 21 -350 35 “-280 19. ?.xn

_ .~.” ..I-.., .-

;2 2ii.u 6.8 X.0

‘“”

._..

_,

.

._

. . . . . _. .__----

--.

I.7

0.89

59

I.6 2.1 1.x

0.95 0.98 0.96

39 53 44

.,.. I”_..._ _..-_- _,.... _“_““l.l ..I..--.

.^..“.._1_

___^I__ ..,, *._l.l._-_

Prediction

of rain acidity

and SO, scavenging

31

Fig. 5. The correlation coefficient R between hydrogen and sulphate ions in rain samples collected May-September, 1977, 1978. The number of samples are in brackets. The R-value is underlined if the correlation is significant at the 1% level.

values of 0 for the northeastern US (Hales and Dana, 1979) and 0.1 for the somewhat colder Canadian Shield Region are not unreasonable. Then D = B = 1.6-2 for the MAP3S and, for the Canadian Shield, D

there is a need to incorporate SO,-scavenging into models. In this section, ways of calculating SOZ washout and the pH of precipitation are suggested.

= B + 0.11 N 0.9.

Order of magnitude estimates

An important result of the above examination of the relationships between pH and sulphate in rain is that their concentrations are well correlated in those areas that are most sensitive to acidic precipitation, namely, in the bedrock shield areas of Canada, the Appalachian Mountain Region and the forested areas of eastern Canada. It is in these areas that an accurate prediction of rainwater acidity is especially valuable. In the following section, ways of utilizing observed correlations in models to calculate acidity and SO,-washout are outlined.

One of the cruder estimates of SO1 washout is achieved by using a “representative” Wso,-value and Equation (2) to obtain a deposition rate. To choose Wso2for a particular site one can reduce the number of possible values (Fig. 1) by considering the range of pH measured in rain at the site. For example, the pH of summer rain at four MAP3S sites in the eastern US (Table 3) has a range of about l.OpH unit (a tenfold range of [H+]). The corresponding uncertainty in a “representative” washout ratio obtained by inserting the precipitation-weighted mean pH into Equation (4b) is about a factor of 3. Thus for order of magnitude estimates in the MAP3S region this approach is acceptable. However if more accurate estimates are required this approach is inadequate.

5. INCORPORATING

SO2 WASHOUT INTO MODELS

In Section 2, it was shown that dissolved SO2 is an important contributor to the acidity of rain and that

38

0

SW KM

Fig. 6. The spatial distribution of the emission of oxides ofsulphur from power plants and smelters in eastern North America Galloway and Whelpdale, 1980).

Parameterization for long-range transport models

The correlation between hydrogen ion and sulphate in rain in eastern North America can be used in long-range transport models to predict the acidity and dissolved-SO, content of rain. Three cases will be considered and for each, expressions given for [H’), and Wsol in terms of modelpredicted parameters. Case 1. SO, is the main acidifying

agent. Then

Equation (2) yields [,+]fred

zz

[HSO;], =

$k,[SOJar

(15)

and ws.0, =

K,KH J CSOII.’

(16)

The relative contribution of SO2 and sulphate aerosols to precipitation pH was investigated in order to identify those situations in which SO2 is the main acidifying agent. The dependence of the pH of rain on sulphate aerosol concentrations was calculated for a range of atmospheric SO1 concentrations (Figs 8 and 9). A sulphate washout ratio of 5 x 105, the same as that used in Section 3, was chosen. Furthermore, it was

assumed that in addition to hydrogen ions from dissolved SOZ there are two (Fig. 8) and one (Fig. 9) hydrogen ions for each sulphate ion in rain (i.e. D = 2 and 1, respectively). As shown by observational evidence discussed in the previous sections, this assumption is valid for most acid-sensitive areas of eastern North America. Situations in which sulphur dioxide is pH-controlling are represented by a point falling on a horizontal segment of the curves in Figs 8 and 9. Those in which other acidic substances control pH fall on the sloping dashed line. Typically, in eastern North America [SO:-], is I-100~gmm3,[SO~],is 1-1000~gm-3andthemolar ratio [SO:-]&SO,], ranges from 0.01 to 1 (increasing away from the source). The acidity of rain is therefore usually controlled by both SOZ and acidic particulate matter. Case 2. [H+]zb8 and [S04]Fbs are correlated, sulphate is that actually occurring in rain reaching the ground and does not include that from oxidation of dissolved SO2 after sample collection. Results from the MAP3S network (Table 3) are particularly suitable for this treatment. If the observed correlation at a site is [H+];b’ = A + BISO;-];bs,

(17)

39

Prediction of rain acidity and SOz scavenging

Fig. 7. The slope B and intercept A if significant (in brackets) for the linear correlation [H’]$ = A + in rain samples May-September 1977, 1978. Values are plotted for those sites having a B[SO:-];“” significant correlation at the 1% level.

then [H+]yd

= A + BWso,[SO:--jr”,

(18)

and from Equation (3a),

K,Kn wsoz = A + BWso,[so:--j~d’

(19)

If all predicted input parameters are correct, [H’];“” and [H+]pled are equal and represent the actual pH of rain. Case 3. [H’]:” and [SO:-]:” are correlated,

d

IO’ SULPHATE

AEROSOL

101 CONCENTRATION

103 (Llg

m-3)

Fig. 8. Computed pH of rain as a function of sulphate aerosol concentration for various SO, air concentrations (pgme3) and for O”C, W,, = 5 x 10s and D = 2. The dashed line corresponds to the case with no SO, present.

sulphate consists of that in rain plus that from oxidation of dissolved SOz after sample collection. This situation is applicable to data from the Canadian CANSAP Network (Table 2) and Air and Precipitation Monitoring Network (APN) in eastern Canada as well as to that from the US NADP Network (Galloway et nl., 1978). In these networks dissolved SO2 is analyzed as sulphate.

40

L. A. BARRIE

(1) For the purposes of long-range

transport models SOI: scavenging by rain can be calculated using a washout ratio approach provided that the temperature and acidity ofrain are taken into account. (2) Dissolved SO2 can contribute a significant fraction of the total hydrogen ions deposited in rainfall particularly at near 0°C temperatures in air containing aerosols of low acidity such as ammonianeutralized sulphuric acid particles. If long-term trends in the acidity of rain arc to be explained, dissolved SO2 should be measured in

I

__-_L

IO0

II3 SULPHATE

10’

102

AEROSOL

CONCENTRATION

(kg

me3)

Fig. 9. Computed pH ofrain as a function of sulphate aerosol concentration for various SO2 air concentrations (pg m-‘) and for O”C, Wso, = 5 x 105, and D = 1. The dashed line corresponds to the case with no SOZ present.

In this case, the observed hydrogen ion concentration is related to the actual hydrogen ion concentration in rain by [H+]ybs = [H+]:” It is also empirically

related

+ [HSO;],. to observed

(20) sulphate

[H +]Fbs = A + B[SO;-]Fb”, = A + B [SO:-]:“’

(

(21a)

(21b)

1

Eliminating [H+]ybS from Equations and using Equation (3) to eliminate obtains

[H+];” = X + Jx2 + (B -

.

+ [HSO;],

1)~~

(20) and (21b) [HSO;],

&,[so,]yd ,

one

(22)

where X =

A + BWs,,[SO:-]~d

2,

(23)

and

If all predicted input parameters

are correct [H ‘1;” are equal and are related to [H+]:” by and [Ht]yd Equation (24). The washout ratio is given by

WSOJ =* and Equation

(22).

[H+] = B[SO:-]. where B is - 2 in the American industrial northeast and - 1 in southern Canada east of Lake Superior. SO2 washout can be calculated using the appropriate equation outlined in Section 5. In closing it should be emphasized that perhaps the greatest uncertainty in the acidity calculation outlined in this paper is the washout ratio for sulphate aerosols. More routine simultaneous observations of stormtype, sulphate aerosol size distribution, rain drop size distribution and precipitation sulphate concentrations are badly needed in order to parameterize sulphate aerosol washout. Acknowledgements -- The author thanks Mr R. Berry for his assistance in analyzing the CANSAP precipitation data, and Drs E. Voldner and D. M. Whelpdale for their comments.

6. CONCLUSIONS

From considerations based on the solution chemistry of S02, the following are drawn :

precipitation. Furthermore, in order to predict trends, the SO1 scavenging process has to be realistically simulated in models. To do this one must be able to predict precipitation acidity, an exercise which, by itself, yields valuable results. Since one cannot hope to model the cycles of all atmospheric substances that influence precipitation acidity accurately enough to predict rainwater acidity, another approach is needed. Fortunately, as illustrated by data from Canadian and US precipitation networks there is a strong relationship between the hydrogen ion and sulphate ion content of rain in acid sensitive areas of eastern North America. This enables one to predict precipitation acidity from models of the sulphur cycle which is probably understood quantitatively better than that of any other acidifying agent. The correlation between hydrogen and sulphate ions does not mean that nitrogen oxides are unimportant in the acidic rain problem. On the contrary, one finds that sulphate, nitrate and hydrogen ions are correlated in rain. However, until the cycles of nitrogen oxides and ammonia are well known, the best approach to pH prediction is to model the sulphate cycle, to calculate the sulphate content of rain and then to use observed empirical relationships between sulphate and hydrogen ions to predict pH. The relationships usually take the form

equilibrium conclusions

REFERENCES

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Dillon P. J., Jeffries D. S., Snyder, W., Reid R., Yan N. D., Evans D.. Moss J. and Schneider W. A. (19771 Acidic precipitation in south-central Ontario: recent’ observations. Ontario Min. Envir. Publication, Box 213, Rexdale, Ontario, Canada. Forrest J. and Newman L. (1977) Further studies on the oxidation of sulphur dioxide in coal-fired power plant plumes. Atmospheric Environment 11, 465-474. Galloway J. N., Cowling E. B., Gorham E. and McFee W. W. (1978) National Atmospheric Deposition Program NC-141, A Report to the Council on Environmental Quality. Galloway J. N. and Whelpdale D. M. (1980) An atmospheric sulphur budget for eastern North America. Atmospheric Environment 14, 409-417. Hales J. (1978) Wet removal of sulphur compounds from the atmosphere. Atmospheric Environment 12, 389-400. Hales J. M. and Dana M. T. (1979) Regional scale deposition of sulphur dioxide by precipitation scavenging. Atmospheric Environment 13, 1121-l 132. Henmi T. and Reiter E. R. (1978) Regional residence time of

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McMahon T. A., Denison P. J. and Fleming R. (1976) A longdistance air pollution transportation model incorporating washout and dry deposition components. Atmospheric Environment 10, 751-761. NATO (1978) Conference Series I, Effects of acid precipitation on terrestrial ecosystems. In Proc. Sympo&um (Edited bv T. C. Hutchinson and M. Harasl. Universitv of Toronto,- Canada, 21-27 May 1978. filenum Press, London. Shannon J. D. (1979) The advanced statistical trajectory regional air pollution model. Proc. 4th Symp. on Turbulence, Di$usion and Air Pollution, Reno, Nevada, January 1979. Sponsor Am. Met. Sot. Slinn W. G. N., Hasse L., Hicks B. B., Hogan A. W., La1 D., Liss P. S., Munnich K. O., Sehmel G. A. and Vittori 0. (1978) Some aspects of the transfer of atmospheric trace constituents past the air-sea interface. Atmospheric Environment 12, 2055-2089. Voldner E. C., Olson M. P., Oikawa K. and Loiselle M. (1981) Comparison between measured and computed concentrations of sulphur compounds in eastern North America. Proc. CAGP Symp. on the Budgets and Cycles of Trace Gases and Aerosols in the Atmosphere, 12-18 August, Boulder, Colorado, U.S.A. (to be published in J. geophys Res.)