The influence of clouds and rain on the vertical distribution of sulfur dioxide in a one-dimensional steady-state model

The influence of clouds and rain on the vertical distribution of sulfur dioxide in a one-dimensional steady-state model

THE INFLUENCE OF CLOUDS AND RAIN ON THE VERTICAL DISTRIBUTION OF SULFUR DIOXIDE IN A ONE-DIMENSIONAL STEADY-STATE MODEL G.GRAVENHORST,TH. J~~-~H~DT a...

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THE INFLUENCE OF CLOUDS AND RAIN ON THE VERTICAL DISTRIBUTION OF SULFUR DIOXIDE IN A ONE-DIMENSIONAL STEADY-STATE MODEL G.GRAVENHORST,TH. J~~-~H~DT

and D II. EHHMT

Institut fur Atmosphiirische Chemie der Kemforschungsanlage Jitlich. D-5170 Jtilich, West Germany and E. P. Rtjnr Universitiit Essen, GHS (Firsr receiued 26 August 1977 and in find form 27 Seprrmber 1977) Abstract-It was attempted to include wet chemical removal rates for sulfur dioxide in a one-dimensional steady-state model. The interactions with liquid water were separated intd the removal of absorbed sulfur dioxide by ram and into the formation of sulfate in cloud- and rain-water. The chemical reaction rates and the gas-phase ditTusion are fast compared to cloud- and min-formation so that equilibrium conditions between gas-phase and liquid-phase were assumed. The most sensitive parameters affecting the wet chemical removal of SO2 seem to be the pH value of rain-water and the formation rate of sulfate in atmospheric water. The gas-phase destruction proceeds predominantly through the oxidation by OH radicals The calculated SO2 volume mixing ratio deerears from I ppb at the ground level to ea. 0.01 ppb in 15 km altitude. integrated over a vertical column the gas-phase destruction is about 2 times larger than wet chemical removal. The relative proportions, however, depend strongly on the chosen parameters for rain- and cloud-water. The direct SO1 deposition onto the ground seems to be larger than the sum of the removal rates within the atmosphere.

of SO2 at any altitude is given by:

INTRODUCIION In order to describe the vertical dist~bution of SOIuble components of the atmosphere, wet removai

mechanisms have to be considered This paper rep resents an attempt to include wet chemical interactions of SO2 in an existing one-dim~sional steadystate model. The basic reason for using a full one-dimensional chemical model was to generate vertical profiles of the gaseous species with which SO2 reacts in a consistent manner. The I-D model also includes a water cycle to generate a water-vapour profile and precipitation rates in a self-consistent way. Since very few data exist on the vertical ~st~bution of SO, in the troposphere, such an attempt may appear premature. On the other hand, as we shall see below, such a calculation can identify the parameters which dominate the vertical SO,-distribution and thus guide the design of the experiments, so that all the data required for an explanation are indeed collected simultaneously. In the following, the aspects of the model pertinent to the vertical S02-distribution are presented and discussed. The SO,

balance

equation

In the model, the SO,-concentration profile is derived by solving the steady-state balance equation of all SO, Formation and destruction processes In a vertical one-dimensional steady-state model the balance

p: air density, K: vertical eddy diffusion coefficient, [SO,]: SO, gas-phase conflation, z: height, k,: effective rate coefhcient of component i having a volume mixing ratio of X, for the gas-phase reaction with S02, ri: effective first-order rate coefficient for wet chemical removal mechanism j. In Equation (1) the divergence of the vertical eddy flux of SO2 (first-term) is balanced by the net loss due to gas-phase reactions (second term) and by the net loss due to wet chemical reactions (third term) Production of SO2 is assumed to occur only in the layer close to the ground It is, therefore, included in the boundary condition at 1 km and a production term of SO2 does not appear in Equation (I). This implicitly assumes that oxidation of HsS, if important at all, takes place close to the ground. It is characteristic of a 1-D steady-state model, and therefore of Equation (I) accordingly, that it describes average conditions and all the processes; transport, gas-phase and wet removal always take place simultaneously. In the mode1 the sun is shining and it rains at the same time. This is, of course, also true in the real atmosphere but there these conditions are separated spatially, whereas in the model, due to its average nature, all processes go on simultaneously at any

691

G. GRAVENHORST. TH. JANSSM-S~ZHMIDT and D. H. EHHALT

692

given point. In the followmg we discuss the three terms of Equatton (1) separately Divergence of the eddy SO2 jiux

To calculate the eddy flux, the vertical profile of the eddy diffusion coefficient has to be known. We assumed an eddy diffusion coefficient constant at 2 x 10’ cm’ s- ’ throughout the troposphere decreasing to a stratospheric value of 2 x lo3 cm* s- ’ at the tropopause (Ehhalt et al.. 1975). A larger eddy diffusion coefficient in the troposphere would increase the SO2 delivery from the ground and thus maintain a higher SO2 concentration. To start the iterative solution of Equation (1) an initial profile of the SOI concentration is prescribed (naturally this estimated profile has no influence on the final solution). Gus-phase reactions The major homogeneous

gas-phase reactions are:

Rl:SO,+OH+M+-+HSO,+M k = 1.8 x 10-14exp(l104/T)Mcm_‘s_i 1 1.9 x IO’s + M R2: SOa + HO2 --.+

SOS + HO k2 = 9 x 10-16cm3 s-i

followed by

phase. The uptake of SO, by the liquid-phase proceeds in several steps The first part of the SOz simply dissolves in the cloud droplets. This dissolved SO, then further reacts with water to form bisulfite ions HSO; which in turn partly dissociate to form sulfite ions, SO:-. These fast steps are eventually followed by a slow oxidation to sulfate ions (Beilke and Gravenhorst, 1978). All these steps incorporate SOz into cloud-droplets which partly coagulate to precipitation and deposit their sulfur content on the ground. The calculation of wet removal of SO, from the atmosphere is greatly aided by the observation that the first steps in the dissolution of SO1 are m fact so fast that we can assume thermodynamic equilibrium between the gas-phase and the droplet phase m a cloud. The reaction of physically dissolved SO2 to bisulfite (Eigen et al.. 1961) and the pure ionic reaction of HSO; to SOi- proceed very fast. so that gas-phase diffusion of SOz to a cloud-droplet becomes the rate-limiting step in the initial absorption. This step is still reasonably fast. equilibrium between gas-phase SOI and dissolved SOz, HSO; and SOi- is reached within seconds. This time is quite short compared to the dynamical processes and mixing within the cloud. Thus the concentration of the dissolved SOI, HSO; and SOi- are governed by the following thermodynamic equilibria: [SO, x H20] = SCSO,].

R3: SO3 + Hz0 -*-

H,SO, k, = 9 x lo-i3 cm3 s-i.

The effective bimolecular reaction rate constant k, was estimated from data given by Castleman et al. (1975) and Cox (1976) for 220 K and 295 K. k, and k, were taken from Hampson and Garvin (1975). As mentioned above the required concentrations of the reactions. OH, HO* and H,O are calculated in the model for annual average conditions. The destruction rate of SO, due to R2 amounts to less than 1% of the rate of RI. Oxidation by CHsO, was estimated to be less effective than R2. Since the rate constant is not well known this SOz removal was not consldered. The rate constant estimate of 5.3 k 2.5 x lo-’ cited by Calvert et al. (1978). however. makes this oxidation rate nearly as important as the OH oxidation. The fate of HSOS was not followed because exact reaction mechanisms are unknown. As the model is not Intended to simulate polluted atmospheres. most of the reactions discussed by Sander and Seinfeld (1976) are not included. Finally, the absorption of SO, on aerosol surfaces was neglected. One part of the aerosol surfaces in the free atmosphere consists of H&SO4 which hardly absorbs S02, and the absorption efficiency for SO2 molecules of the other part is not known. Wet removal of SOI The moment water vapour begins to condense

in ascending and cooling air mass, the SO, present will be distributed between the gas- and the condensed

S = 7.1 x IO-‘exp(3145iT) [HSO;][H+J

= K,[S02

cm3 gas-phase cm3 aqueous-phase

(2)

x H,O],

K, = 1.9 x lO-5 exp(2022/T)mol/l [SO:-][H+]

(3)

= KnCHSO;],

Kt, = 2.4 x 10-‘“exp(1671/T)mol/l.

(4)

The Henry constant S was taken from Gmehn (1963) and the first and second dtssociatton constants K, and K,, from Sillen (1964). They were extrapolated to negative temperatures for super-cooled cloud-droplets. The above absorbed sulfur compounds all possess the oxidation state IV. Moreover they qutckly interconvert into each other. so that they can be treated together. The sum of their concentrations can be expressed with the help of Equations (I), (2) and (3) as a function of the SO2 concentration in the gasphase, SO*: S(W) = S(1 + K,[H+]-’

+ K,K,,[H+]-*)S02.

(5)

During the condensation, the gas-phase concentration of SOI is somewhat reduced, which introduces a small correction into Equation (5). Allowing for the conservation of the total sulfur mass during condensatton we obtain S(IV) =

S(1 + K,[H+]-’ + K,K,,[H+]-*) 1 + S(1 + K,[H+]-’ + K,K,,[H+]-‘)F

so ” (6)

693

Vertical distribution of sulfur dioxide

for the amount of absorbed S (IV) per unit volume of water, where S02, is the gas-phase concentration of SO2 before condensation and F is the volume of liquid water per gas volume. F is not the actual liquid water content measured in a cloud but the annual average liquid water content present at one point in the atmosphere. For average conditions F is of the order of lo-‘. Thus in the case of SO2 and at pH values below 6 the second term in the denominator can be neglected compared to 1 and Equation (6) reduces essentially to Equation (5). To obtain the wet removal of SO* from the atmosphere in the form of absorbed S (IV), we simply have to multiply Equation (5) or Equation (6) by the rate, U, with which precipitation is formed in a given volume element. Unfortunately, at higher altitudes the situation is not so simple. There, condensed water may exist in the liquid (droplets) or solid-phase (ice crystals). Even if we assume that the absorption of SO2 in ice crystals is small, a fact also indicated by the low deposition velocity of SO1 on snow-covered ground and thus neglect the uptake of SO2 by the solid-phase, we still have to convert the precipitation formation for the presence of ice crystals. We do this by multiplying U, by the fraction of condensed water present as liquid, g. g is a dimensionless function of altitude g = g(z), with 0 < g -Z 1. At 6 km 90% of the condensed water. at 8 km 50% and at 10 km 0.3% are present as liquid. Accordingly, the effective firstorder rate coefficient for the wet removal of SO1 in the form of S (IV) absorbed in rain has the form: rl = S(l + K,[H+]-t

+ K&[H+]-*)g.U

where U, the rate of precipitation formation (molecules cm-3.s- ‘) is derived in the model, r-t represents only one path of wet removal of SO*. The other is the Irreversible wet oxidation of SO2 to SO:-. It is treated here in the sense that the sulfite ion SOiis the oxygen-carrier and not the bisulfite ion HSO; or both: so:- + l/2 02--Lso:(8) r2, the rate coefficient for the conversion

of SOiis not well known for atmospheric water containing a complex set of metal compounds, ions and dissolved oxidizing agents and inhibitors. An empirical relation r2 = 1.95 10” exp (- 11676/T)

(9)

can be deduced for sulfate formation per second from sulfite ions in natural rain water from data of Betz (1977) for the pH range 3-5. Betz (1977) measured the concentration decrease of S(W) with time at different temperatures of rain water samples. For calculations outside this range, the same value was used due to lack of other data. This value is about one hundred times larger than in distilled water. Since the measurements were made in a closed system, the continuous supply of oxidizing agents like H202 was not possible in contrast to the situation in the atmos-

phere. The true formation of sulfate from S (IV) in cloud- and rain-water may well be even faster. Since the oxidation to SO:- via reaction (8) is irreversible the wet removal rate of SO2 in form of SOicould be obtained simply by multiplying r2 by the SOi- concentration in liquid water and by the amount of liquid water present at any altitude. We would suggest that the oxidation to SOi- separates into two parts, one taking place in rain or droplets coagulating to rain and the other taking place in the non-precipitating liquid water. The reason for this is that rain- and cloud-droplets may have different pH values and different concentrations of catalysts which means that the overall oxidation, which depends on the pH, may proceed at different rates in these two cases. We therefore propose to use: R, = r2SKlKll[H+]-*(l

-f)gF

for the oxidation rate coefficient in cloud-water, and R2 = r2 S KtKtt[H+]-*fg

F

for the oxidation rate coefficient in rain water. f is the fraction of condensed water present as precipitation. As mentioned above, these rate coefficients are essentially given by the product of r2 with the SOi- concentration and the respective fractions of liquid water per volume of air. The three removal rates given by rt, Rt and R, represent our wet removal. They are annual averages covering fair weather and overcast conditions. It is obvious from the respective expressions that they all depend strongly on the pH value. In the model calculations the pH value of the liquid water was kept constant (although different for clouds and rain water). It is therefore not affected by the uptake of SO*, which greatly simplifies the calculations. Such an assumption can be justified, however, by noting that the pH value of cloud water is mainly determined by the chemical composition of incorporated aerosol particles and not by dissolution of gases other than CO*. The uptake of NH3 seems to counterbalance the effect of SO2 absorption and not to increase the pH value remarkably. Even if the total amount of ammonium found in rain water would be the result of gaseous NH3 uptake the average concentration of 0.5 x 10m5mole/l for the northern hemisphere (Bottger et al., 1977) could have changed the pH only slightly from 5.0 to 5.3 or from 4.5 to 4.6 when all other interactions are neglected. In fact considerations which do not include the chemical composition of cloud-active nuclei may lead to conclusions which are unrealistic for atmospheric situations (Scott and Hobbs, 1966; Easters and Hobbs, 1974). In order to calculate the pH value of atmospheric water the acidity and/or alkalinity of cloud-active nuclei and their buffer capacity have to be. known. Since only a little information is available (Junge and Scheich, 1969; Gravenhorst, 1978) we hope to circumvent that problem by fixing the pH value at an arbitrary value. Keeping the pH value constant has another conse-

694

G

GRAVENHORST.

TH. JANSSEN-SCHMIDT and D. H. EHHALI

quence for our considerations: the concentrations of the S (IV) compounds depend only on the SO2 concentration in the air and on the temperature. Thus any SOi- removed by oxidation to SO:- via reaction (8) is immediately replaced by absorbing more SO1 from the gas-phase. After a few seconds r2 controls the further uptake of SO2 by the droplets. and a constant flow of SO, molecules passes through the reactions (2), (3). (4) and (8). Choice oj parameters The pH value of ram water depends on the geographical location. In continental regions at middle latitudes, pH values around 4 are quite common (Likens er al., 1972). Under maritime influence, pH values are shifted to higher values because of the alkaline sea salt (Gravenhorst, 1975). The pH values in precipitating clouds show the same trend (Oddie, 1962; Petrenchuk and Drozdowa 1966; Fricke er al., in press) and a similar range of pH values as in rain. A pH value of 4.5 was chosen as the most likely average for ram water. The acidity of cloud- and raindroplets depends to some extent on how much soluble components of cloud-active nuclei are diluted by condensation. Non-precipitating clouds may therefore have a lower pH than rain water because they are expected to experience less condensation and therefore produce a smaller amount of liquid water than precipitating clouds. Hence their dilution effect would be smaller. Even over the ocean. a part of the cloud water could have lower pH values as predicted from HrO-CO2 interactions. since maritime aerosols smaller than 0.45pm radius seem to be acidic (Gravenhorst, 1975). A pH value of 3 was chosen for the cloud-droplets. This value is highly speculative. The parameters related to condensed water, F and U. are estimated in the following way: from data given by de Bary and Moller (1960), an average vertical frequency distribution can be calculated for the occurrence of condensed water as shown in Fig. 1. The distribution reflects the situation in Central Europe since the observations were made here. The cloud cover will be less in subtropical regions and larger in areas where low pressure systems occur more frequently. Liquid water contents for various clouds are given by Fletcher (1964). They range from 0.11 to 0.64 g m- 3 with most frequent, but not representative. data between 0.1 and 0.2 g rnm3. Assuming an average hqurd water content of 0.1 g m - 3 at all altitudes, for all clouds precipitating or not. a similar vertical distribution of the condensed water is obtained for annual average. The actual value for precipitating clouds is higher than 0.1 g rne3. Thts difference in liquid water content is taken into account in attributing a high fraction of lo”,, of the liquid water in the atmosphere to rain (/= 0.1). Formation rate of precipitation, U, was calculated in the model as the divergence of water vapour fluxes resulting from a standard Hz0 vapour profile and a constant eddy diffusion coefficient profile.

0

1

5 Annual

Fig. 1. Frequency of altitudes

over

IO

15

frequency

clouds

of

(annual

%

clouds

average)

Europe (calculated from by de Bary and Miiller. 1960). Central

20

at different data given

For the SO2 boundary value at 1 km altitude 1 ppb was chosen. In industrial areas annual averages of the order of 50 ppb are measured whereas in rural areas concentrations of a few ppb are found. 0.08 ppb were reported by Biichen and Georgii (1971) and Prahm et al. (1976) for maritime background concentrations. Over the North Atlantic preliminary results show a value of 0.1 ppb (Gravenhorst, 1975). The SOz boundary value allows implicitly for SO1 deposition on the ground and destruction in the layer close to the ground. RESULTS

The wet removal rate coefficients ri, R,, R2 as function of altitude calculated with the above choice of parameters is shown in Fig. 2. It shows that the incorporation of SO* as S (IV) into rain water is by far the major wet chemical removal mechanism whereas the oxidation rate to sulfate in cloud-water can be neglected. Oxidation to sulfate in rain water, R2. becomes significant at lower altitudes. The rate coefficient for SO2 removal as S (IV) has a pronounced maximum between 5 and 7 km altitude. Up to this altitude the increase in the rain formation rate, U. combines with an increase in uptake-capacity for S (IV) caused by the decrease in temperature. Above 7 km the rapid decrease in U determines the overall effect. The rate coefficients for the sulfite oxidation, R,, and R2 decrease rapidly with altitude. The activation energy of r2 is much larger than the negative activation energies in K, and K,, and causes a decrease with temperature and altitude [of Equations (3). (4). (9)l. The sum of all wet removal rates (curve 1) is compared with the gas-phase destruction of SOz+ (curve

695

Vertical distribution of sulfur dioxide

removal is greater by about a factor of 3. Integrated over all altitudes roughly one third of the SO1 is wetremoved, the bulk is destroyed by gas-phase reaction. The vertically-integrated conversion rates are 0.14:; h- ’ for liquid water reactions and 0.25:< h- ’ for gasphase reactions. For wet chemical conversion rates within clouds or fogs, values of 0.002-0.2”,/, h-’ were estimated by Beilke and Gravenhorst (1977) and between 0.1 and 33,; h-’ depending on a variety of cloud conditions (Barrie, 1975). In both these calculations, however. it was neither attempted to calculate average global conditions nor to consider the eddy diffusion transport of SO2 into the clouds.

kn1

5.

3-

5-

Other deriwtions

D-

I

I

1

I

do

IO-

IO-6

16

Rate oxfficlents

Fig 2. Effective first-order rate coefficients for reactions between SO, and the liquid phase in the atmosphere averaged over space and time. (rl :S (IV)-removal in rain water for pH 4.5; R,: sulfate formation in cloud-water for pH 3 and R’, for pH 4.5; R2: sulfate formation in rain water

for pH 4.5). 2) in Fig. 3. The OH concentration profile-which was generated within the model is also given in Fig. 3. Obviously the altitude dependence of the gas-phase destruction is dominated by the shape of the OH profile. The comparison between curves 1 and 2 suggests that gas-phase destruction dominates below 3 km and above 9 km altitude, in the region in between, wet OH - ccncentratlon 104

IO5

106

cm-’

I

km

3 g

I(

a

, C2

IO-’

IO0

%

SO2- removal Fig. 3. Removal rates of SOI in percent per hour for interactions with cloud- and rain water (I) and gas-phase reactions (2). [pH value for rain water 4.5, for cloud-water 3, r2 = 1.95 x IO’* exp (-11676/T)]. The OH concentration (upper scale) deermines the gas-phase destruction rate.

of parameters

The parameters determining the wet chemical removal were chosen to the best of our knowledge. Since only few experimental data are available, parameters which seem to control wet removal were varied in certain ranges. Most uncertain is the pH value of non-precipitating clouds which represent ca. 907; of liquid water in the atmosphere. The pH value influences the sulfite- and bisulfite-concentrations which in their turn determine the formation rate of sulfate and thus the SO2 uptake rate. The effective oxidation rate coefficient, RI is compared in Fig. 2 for cloud water pH values of 3 and 4.5. At the higher pH value R1 is faster than the rate coefficient Rz for rain water of the same pH value since more cloud water than rain water is present in the atmosphere. Whether in reality the SO2 uptake due to sulfate formation is higher in cloud water than in rain water depends on the pH values in the atmosphere. At similar pH values for rain and cloud water, the oxidation mechanism of absorbed SO1 to sulfate seems to be more important than S (IV) removal in precipitation in the lower few kilometers. The SO1 absorption in cloud and rain water can furthermore be altered by a change of the rate coefficient rZ for the sulfite oxidation. In Fig. 4, SO,-concentration profiles calculated for different r2 values are shown. A IOO-fold increase of r2 does not change the SO1 concentration drastically when pH values of 3 for cloud water and 4.5 for rain water are chosen. However if the pH value of cloud water is raised to 4.5 simultaneously, the SO2 concentration decreases rather steeply. Hydrogen peroxide and ozone oxidation of absorbed SO1 as suggested by Penkett er al. (1977) seems to justify an increase of r2. The synergistic efiect of oxidizing agents, catalysts and inhibitors is, however, not yet known. The calculations indicate that the SO, concentration, already remarkably below the tropopause level, decreases due to the sharp increase of OH concentrations (see Fig. 3) above 12 km altitude. The tropopause should therefore not generally represent a discontinuity for SO1. The SO, half-life at this altitude is ca. 20 days so that the SO2 distribution will be more determined by gas-phase reactions than by transport mechanisms.

696

G.

TH. JANSSEN-SCHMIDT and D. H. EHHAL~

GRAVENHORST.

kn

I:

9 E a

IC

C 105

IO6

IO'

IO8

109

10’0

IO"

105

SO, - concentration molecules, cm‘s Fig. 4. Vertical SO,-concentration profiles for different sulfit oxidation rate coefficients and pH values of cloudwater: I: r2 = 1.1 x IO-l6 exp (-1400/T) cm*s-’ (deduced from fktz’s (1977) data) and pH = 3.0; 2:

r; = 100 r2, pH = 3.0; 3: r; = lOOr,. pH = 4.5. The pH value of ram is always 4.5. The most effective wet chemical sink mechanism for SO1 above 2 km is for this model the removal of S (IV) in rain water (Fig. 2). A striking change in the SO2 profile 1s caused by a variation of the pH value of rain water (Fig. 5). It is therefore important to determine representative values for rain water. As appropriate values a pH of 4.5 for rain water and of 3 for cloud water were chosen to calculate vertical SO2 distribution for the case when wet and gas-phase removal were taken into account (Fig. 6). For compartson pure gas-phase removal was considered. In both situations the same boundary value (1 ppb) was used. The SO2 concentration has decreased at the tropopause level (15 km) due to gas-phase reactions by a factor of 250 (mixing ratio by a factor of 55-). The influence of liquid water reduces the SO2 concentration in additton, by a factor of 1.6 to 5.3 x IO’ molecules cm- 3 (cc. lo-’ ppb). This value is one order of magnitude lower than the one suggested by Junge (1974) discussing the formation of the stratospheric sulfate layer. The calculated vertical SO,-distribution can be related to a few measured data (Fig. 6). Until today. only preliminary results can be reported because of difficulties in measuring SO, in low concentrations in remote areas and in the middle and upper troposphere. In polluted continental air masses, the SOz concentration decreases more steeply with altitude (curve a: Georgii and Jost, 1964; curve b: Jost, 1974; curve c: Gravenhorst, 1975) because local sources increase the concentration near the ground. Over Colorado a distribution similar to the modelled one was found (Curve d: Georgii. 1970). At 3.5 km above sea-

106

IO’

100

109

104

IO"

SQ-c0nc8ntrat~on molecules, cmv3

Fig. 5. Vertical distributions of SO,. calculated for different pH values for rain water (pH value for cloud-water: 3.0).

level in Switzerland, about 1 ppb was measured as an average over three years (single dot: Eidgeniissisches Amt fur Umweltschutz 1976). A rather homogeneous distribution was indicated in the lower 4 km in pure maritime air with a trend to increasing mixing ratios with altitude (curve e: Gravenhorst, 1975). But here also a comparison is restricted since the ocean does not act as a SO2 source. First preliminary kJ------

SO2-concentrabn molecules, cm3 Fig. 6. Vertical distributions of SO, calculated for appropriate average conditions with (1) and without (2) wet chemical reactiona Experimental values are added: (a) Georgii and Jest. 1964; (b) Jost, 1974; (c) Gravenhorst, 1975; (d) Georgii. 1970 (e) Gravenhorst, 1975; (f) Jaeschke et al., 1976; single dot l : Eidgeniissisches Amt ftiir Umweltschutx, 1976.

691

Vertical distribution of sulfur dioxide measurements in tropopause levels gave concentrations of similar magnitude as in the model (curve f: Jaeschke et al. 1976). Mass spectrometric observations of SO2 in the stratosphere (Sagawa and Itoh. 1977) show unrealistically high values in the lower stratosphere (ca. 20 ppm). To determine whether gas-phase or wet chemical mechanisms are more important in this model for the SO2 removal from the atmosphere, weighted rate coefficients for the troposphere were calculated. The SOz-removal of 0.14% h-i for wet chemical interaction (pH cloud-water 3, pH rain water 4.5) was smaller than the value of 0.259; h- 1 for gas-phase destruction. This means that during the same time ca. half as many SO2 molecules are absorbed in cloudand rain water than destroyed by gas-phase reactions. However, if the pH value of cloud water is raised to 4.5. the wet chemical removal increases to 0.339,; h-l whereas the gas-phase destruction remains essentially the same. namely 0.2604 h-‘. For this new condition interactions with the liquid-phase are more effective in the removal of SO* from the atmosphere than homogeneous gas-phase destruction. The relative importance of these removal mechanisms depends, therefore, quite sensitively on the chosen pH values. Since most SO1 is emitted into the atmosphere within the boundary layer. SO, can be deposited directly at the ground. Assuming a SO2 deposition velocity of 0.8 cm s-’ the SO2 removal from the troposphere by dry deposition amounts to co. 1.09; h-i. Compared with wet chemical and gas-phase reactions in this model. dry deposition on the ground seems, therefore, to be the most important SO2 sink mechanism. This model approach does not reflect conditions

in small scale phenomena such as urban plumes or precipitation systems. Due to its average nature the model should, however, represent the situation above the atmospheric mixing layer since there the influence of SO, sources and SO2 sinks do not vary so much with time and space as close to the ground. More confidence

could be placed

in the model

predictions

when better data. especially on chemical properties of cloud- and rain water and their climatological variations, become available.

REFERENCES

Barrle L A. (1975) An experimental investigation of the absorption of sulfur dioxide by cloud and ram drops containmg heavy metals, Promotionsarbeit, lnstttut fur Meteorologic und Geophysik der Universitat Frankfurt. Beilke S. and Gravenhorst G. (1977) A contribution to the formatton of atmospheric sulfate and its removal from the atmosphere. Seminar ‘Fine Particulate Air Pollution’ Villach.-Oct. 77. United Nations Economic and Soctal Counctl. Economic Commission for Europe, Genf. Beilke S. and Gravenhorst G. (1978) Heterogenous SO2 oxidation in the droplet phase. Armospheric Enrironment 12, 23 l-239.

Betz M. (1977) Untersuchungen

tiber die Absorption

und

Oxidation von Schwefeldioxid in natiirlichem Regenwasser, Diplomarbeit. lnstitut fur Meteorologic und Geophysik der Universitiit Frankfurt. Bijttger A., Gravenhorst G. and Ehhalt D. (1977) Deposition rates of ammonium and nitrate in the northern hemisphere. paper presented at 9th Inr. Con/. on Armospheric Aerosols. Ireland.

Condensation

und

Ice

Nuclei.

Galway.

Biichen M. and Georgii H. W. (1971) Ein Bcitrag zum atmospharischen Schwefelhaushalt iiber dem Atlantik. “Mefeor”

Forsch.

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