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A note on the consumption of acid through cation exchange with clay minerals in atmospheric precipitation
AtmosphericEnvironmentVol.25A,No.2, pp. 487~.90,1991
000~6981/91$3.00+0.00 ~ 1991PergamonPresspie
Printedin GreatBritain.
A NOTE ON THE CONSUMPTION OF ACID THROUGH CATION EXCHANGE WITH CLAY MINERALS IN ATMOSPHERIC PRECIPITATION RONALD SEQUEIRA Department of Applied Sciences, The University of Newcastle, P.O. Box 84, Waratah West, New South Wales 2298, Australia (First received 2 June 1988 and in final form 8 August 1990) Abstract--The first estimates of the consumption of potentially available strong acid in atmospheric
precipitation through the mechanism of cation exchange with clay minerals have been obtained for three climatically different regions of the world: (1) Antarctica (South Pole); (2) continental Europe (north of the Alps); and (3) the continental Mediterranean. From the calculations involving the ranges of the concentrations of potentially available strong acid and of clay minerals of low-to-mediumcation exchange capacity for each of the three regions it is suggested that the above process of acid consumption may almost always be insignificant in liquid (or liquified) precipitation over Antarctica and continental Europe. On the other hand, it is only under the most favourable conditions--involving the maximum stipulated cation exchangecapacity of the clay minerals--thatthe same process could possiblylead to a maximum acid consumption in precipitation water up to ~ 30% in the eastern Mediterranean region. The cation exchange process is also considered briefly in relation to the probable recycling of clay minerals of aeolian origin through evaporating clouds. Application of the first principles of cloud physics to the qualitative discussion suggests that clay minerals acting as freezing nuclei may have the best chance of participating in cation exchange, and hence, in acid consumption in clouds. Key word index: Precipitation chemistry, cation exchange, clay minerals, acid precipitation, acid consumption, cloud droplets, cloud processes.
l. INTRODUCTION The insoluble mineral component of precipitation water could react with the aqueous phase, leading to: (i) variable contributions to the observed trace element concentrations in solution and (ii) the so-called 'buffer capacity', which apparently leads to the observed discrepancy between the measured and calculated bicarbonate alkalinity (cf. Granat, 1972). A possible mechanism that could simultaneously explain the above features is the exchange of H+-ions with clay minerals in the insoluble material found in precipitation. Preliminary estimates of the ranges of acid consumption by this mechanism have been obtained for three regions of the world, where the present-day impact of aeolian dust could be categorized as 'negligible', 'low', and 'high', respectively. The three regions are, sequentially, I--Antarctica (South Pole); II---continental Europe (north of the Alps); and III----continentaleastern Mediterranean. While the South Pole is an area virtually unaffected by continental dust at the present time (Legrand, 1987), the eastern Mediterranean region is a major receptor of aeolian dust of Saharan origin transported by the Sirocco winds across the Mediterranean Sea (Chester et al., 1977; Yaalon and Ganor, 1979; Glavas, 1988).
Dan and Yaalon (1966) conclude that the clay-rich soils developed on sandy parent materials in the coastal plains of Israel owe their characteristics primarily to the addition of fine-grained aeolian dusts which are rich in kaolinite and mica, as shown by the studies of Chester et al. (1977).
2. CLAY-SIZEDPARTICLESAND CLAYMINERALSIN PRECIPITATION WATER
Of the total mass/volume concentration of the insoluble material present in precipitation water, a concentration which will henceforth be denoted by I (t), an important portion lies in the clay size range defined by the limits (cf. Jenny, 1980): 0.1 ~ r c < 1 pm, where rc is the radius of the particle. The mass/volume concentration of clay-sized particles in precipitation is given by:
487
[c] = I (t).f~
(1)
wheref~ is the clay size fraction of the total insoluble material. On the other hand, it is to be noted that the socalled clay minerals per se represent a specific group of layered silicates, including illite, montmorillonite, kaolinite, etc. which account for an important fraction
488
RONALD SEQUEIRA
of the total concentration in precipitation of clay-sized particles. The mass/volume concentration of clay minerals in precipitation may therefore be written as: I-c- m] = [c] "f~_,
(2)
where, f~-m is the clay mineral fraction of [c]. A conservative value off~_~,=0.5 is used as the upper limit for Regions II and III (continental Europe and the eastern Mediterranean, respectively). This value is based on the reasonable assumption that the insoluble clay-sized material in these regions would not only consist of the typical clay minerals specified earlier, but also other inorganic (and organic) matter, such as colloidal silica, feldspars, and a variety of combustion and condensation products, e.g. soot, tenacious metal oxides, etc. As for the South Pole, it was considered safe to employ anf~_,,= 1, as suggested by the upper limit of the relative frequency of clay minerals among the total ice nuclei reported by Kumai (1976). 3. ACID CONSUMPTION THROUGH CATION EXCHANGE WITH CLAY MINERALSIN PRECIPITATIONWATER A major feature of clay minerals is that they possess 'exchangeable' cations, e.g. Na ÷, Mg 2 ÷, etc. The general equation for H ÷-ion consumption through cation exchange could be written as: X-clay(s) + n H ÷(aq) ~- (H)n-clay(s) + X" ÷(aq) (3)
where, X n÷ is the exchangeable cation with charge +n. The cation exchange capacities (CECs) of clay minerals are well known and are grouped together in Table 1 (cf. Bohn et al., 1979; Tan, 1982). Since the three regions considered are known to be influenced only by aerosol material rich in clay minerals possessing low-to-medium CECs, viz.: (i) the illite-chlorite or (ii) the kaolinite-montmorillonite combinations (Kumai, 1976; Goudichet et al., 1986; Yaalon and Ginzbourg, 1966), the use of the range of CEC=0.050.5/~eq : - ~ is justified in the present work. This range is indicated in Table 2, row 5. Since Table 2 will be referred to frequently in the remaining text of this paper, only the pertinent row number (e.g. row 1, 2, etc.) will be cited henceforth, with a few exceptions, where specific reference to Table 2 will also be made.
4. DATAON INSOLUBLEPARTICULATEMATTERAND POTENTIALLY AVAILABLESTRONG ACID CONCENTRATION IN PRECIPITATION Region I (South Pole) Several measurements indicate that the I (t)-value of 15 ppbw (parts per billion by weight) is appropriate for this area of the Antarctica (Kumai, 1976; Legrand, 1987), and that almost all the micro-particles constitute the clay size range (cf. Ram et al., 1988). Hence, fc,~ 1, and from Equation 1, [cl ~ I (t). Since
Table 1, Cation exchange capacities of clay minerals Mineral Name Mica* Illite~" Kaolinite* Vermiculite* Montmorillonite (smectite)* Chlorite*
Type
Cation exchange capacity, CEC meq/100 g #eq/10 mg
Primary Clay Clay Clay Clay Clay
20--40 10--40 1-10 120-150
2-4 1-4 0.1-1 12-15
80-120 20--40
8-12 2-4
Sources: * Bohn et al. (1979). ? Tan 0982).
Table 2. Estimates of the range of acid consumption by ion exchange with clay minerals in precipitation Item or parameter (1) (2) (3) (4) (5) (6) (7) (8)
Total insoluble matter, I (t) (mg : - 1) Clay fraction, f~ Clay mineral fraction, f~_,, Estimated clay mineral concentration, [c-m], (mg: -1) Range of CEC of clay minerals (#eq E- 1) Estimated total CEC (/~eq: - 1) Potentially available strong acid (/~eq: - 1) Extreme (possible) range for acid consumption (%)
I
Region II
1.5 x 10-2
III
1.5 x 10-2
1-20 0.5 0.5 0.25-5.0
40-180 0.3 0.5 6-27
0.05-0.5 7.5 x I0-5-7.5x lO * 24i* 1.3 x 10-3-1.9 x 10 -2
0.05-0.5 0.0125-2.5 I00-200t~ 0.0062-2.5
0.05-0.5 0.3-13.5 50-150~ 0.2-27
1 1
Data source: *Legrand (1987); ?Wallen (1980); :~Granat (1972); §EDS (1980-present).
Cation exchange with clay minerals f~_,, assumed is 1.0 (row 3), we have the estimate: [ e - m] = 1.5 x 10- 2 mg d - l (row 4). The potentially available strong acid concentration (PASA-concentration) for the region at the present time could be considered to be 2-6 #eq d - ~, as indicated by the data of Legrand (1987). This includes the N O 3 and the two non-sea salt components, SO 2-* and CI-* (Legrand, loc. cir.). Of course, the PASA in Antarctica is of the latent type since the precipitation accumulates as ice. Hence the above discussion is only of theoretical interest.
Region II (continental Europe) Data on I (t) for this region are very limited. Recently, Schutz and Kramer (1987) report l ( t ) = l 4.1 mgE-1 for particles in the size range, 0.1/~m ~
Region III (eastern Mediterranean) The rough l(t)-range could be obtained using the mean monthly dust fallout data of Yaalon and Ginzbourg (1966) and a mean monthly precipitation rate of 10 cm (e.g. Kolton-Shapira et al., 1984), which lead to a range of I (t) = 40-180 mg d 1. This range appears to be quite reasonable, especially with respect to the upper limit. However, the same continental region is bound to have some contribution to l(t) from a resuspended local soil component. Accordingly, the aerosol size distribution is likely to be skewed towards a relatively larger size as compared with continental Europe (Region II). Thus, in effect, a part of the I (t) could be considered to be non-cyclic in character. Accordingly, theft value used for the eastern Mediterranean region is 0.3, much lower than 0.5, considered in the case of continental Europe (Region II). A PASA-concentration range of 50-150 #eq d - l is suggested by the BAPMoN W M O data for the general Mediterranean region (EDS, 198(~-present), and this value has been employed (row 7).
5. RESULTS The possible extreme values of the maxima for the consumption of PASA by clay minerals through the process of ion exchange in precipitation water in the three regions considered are deduced as follows (cf. Table 2). The maxima and minima given in row 4 are considered in combination with the common range of the
489
CEC=0.05-0.5/aeqf -1 (row 5) to yield the total cation exchange possible (row 6). On the other hand, the minima in row 6 are logically associated with the maxima in row 7, i.e. the PASAconcentration maxima. Similarly, the maxima in row 6 are associated with the minima in row 7 to obtain the final row of values in Table 2, viz. the extreme range of per cent consumption of the total available strong acid.
6. DISCUSSIONSAND CONCLUSION The available acidity in precipitation in geographical regions affected by aeolian arid dust is bound to be neutralized by such dust. This neutralization has been somewhat vaguely referred to by a number of workers as 'buffering', 'consumption', etc. of acid. The calculations shown in this note indicate that the mechanism of cation exchange with clay minerals in precipitation water is not an important process for the consumption of PASA in Regions I and II (Antarctica and continental Europe). On the other hand, appreciable H ÷-ion consumption could occur in the case of the dusty eastern Mediterranean region, where under the most favourable combination of conditions--involving high l(t) and high CEC of clay minerals in precipitation--a maximum of up to ~ 3 0 % of the PASA might be consumed through the ion exchange of H + ions with clay minerals. The acid consumption figures (row 8) would most probably bracket extreme values that one may experimentally observe. This is further supported by: (i) the constraints imposed by the inefficiency of the ion exchange process in a precipitation collector (or storage container) as opposed to that in well-drained soil, and (ii) the fact that a part of the PASA may not be present initially in the acid form, e.g. H2SO4, as suggested by this author in his previous works (cf. Sequeira, 1982a, b). The process of ion exchange may indeed explain the low concentrations of some of the trace metals in precipitation (cf. Table 1 of Granat, 1972), although small-scale leaching by acid of the various minerals could easily explain the trace concentrations observed. A relevant question that needs to be addressed at this point is: how important is the process of cation exchange--for example, in the coastal region of the eastern Mediterranean--in the acid consumption in cloud drops, during the history of cycling of an aeolian clay mineral particle through its participation in one or more evaporating clouds? Due to a complete lack of data on the ion exchange process in cloud-borne droplets, one can only speculate on such a question as follows. The two important criteria that appear to be important to the ion exchange involving clay minerals in clouds are: (i) the mineral particle should at some stage
490
RONALDSEQUEIRA
or another, be in immediate contact with the liquid phase of a cloud particle, and (ii) the droplet surrounding the clay mineral particle must (originally) contain some acid, e.g. H2SO 4. Clay mineral particles are well-known ice nuclei (Kumai, 1976), acting either as freezing, contact, or deposition nuclei (cf. Wallace and Hobbs, 1977). When a freezing nucleus is involved, the mineral particle is inside the cloud drop, and the ice-embryo stage heralding the solidification of the drop is eventually reached at a specific temperature, depending on the size of the drop. In the case of contact nucleation, the mineral particle is in minimal contact with the liquid drop. In such a situation, the process of cation exchange may be necessarily limited, since drainage through the mineral structure is also limited. In the final case, a vapour-to-solid phase transfer of water is involved, and hence, there is no chance of any ion-exchange, unless the ice particle formed is evaporated only after an intermediate liquid phase, rather than through the direct route of sublimation. A further requirement is that some acid be incorporated into such a drop, either acquired during the deposition stage (by diffusiophoretic collection of fine particles of acid) or after the ice particle involved melts. In summary, cloud drops that are initially acidic and contain one or more clay mineral particles would appear to be the most likely candidates in the in-cloud consumption of acid through cation exchange. This would identify the freezing nuclei as the most important contenders for in-cloud ion exchange. The role of contact nuclei, is perhaps, very limited. On the other hand, the deposition nuclei could be either totally unimporant, or could be conditionally important, provided that the evaporation of the ice particle formed in the cloud proceeds through an intermediate stage involving the liquid phase (which inherits or acquires acid) and not by sublimation. A separate paper outlining the qualitative aspects related to these and other possible atmospheric processes that could involve acid consumption through cation exchange will be considered in the near future.
Acknowledoements--The original phase of this work was planned and prepared at the Univeristy of Arizona, Institute of Atmospheric Physics. I am thankful to Dr E. Philip Krider, Professor and Director for his constant support to my research. Dr Chris Eastoe of the Department of Geosciences took part in some fruitful discussions.In the present University, my thanks are due to Professors Keith Morgan and Michael Carter for their high commitment to research. Finally, I thank the referee who suggested that the question
of the importance of cation exchange in cloud drops be also addressed.
REFERENCES
Bohn H. L., McNeal B. L. and O'Connor G. A. (1979) Soil Chemistry. Wiley-Interscience,New York. Chester R., Baxter G. G., Behairy A. K. A., Connor K., Cross D., ElderfieldH. and Padgman R. C. (1977)Soil-sizeddusts from the lower troposphere of the eastern Mediterranean Sea. Mar. Geol. 24, 201-217. Dan J. and Yaalon D. H. (1966)Trends of soil developmentin the Mediterranean environment of Israel. Trans. Conf. Medit. Soils, Madrid, pp. 139-145. EDS (1980-present). Global Monitoring of the Environment for Selected Atmospheric Constituents. Environ. Data. Ser., National Clim. Center, Asbeville,North Carolina. Glavas S. (1988) A wet-only precipitation study in a Mediterranean site, Patras, Greece. Atmospheric Environment 22, 1505-1507. Goudichet A., Petit J. R., Lefevre R. and Lorius C. (1986) An investigation by analytical transmission electron microscopy of individual insoluble micro-particlesfrom Antarctic (Dome C) ice core samples. Tellus 36B, 250-261. Granat L. (1972) On the relation between pH and the chemical composition of atmospheric precipitation. Tellus 24, 550-560. Jenny H. (1980) The Soil Resource: Orioin and Behaviour. Springer-Verlag, New York. Kolton-Shapira R., Lakritz Y, and Luria M. (1984) Rainwater pH in the vicinity of Hadera Power Plant, Israel during the winter season of 1981/82. Atmospheric Environment 18, 1245-1248. Kumai M. (1976) Identification of nuclei and concentrations of chemical species in snow crystals sampled at the South Pole. J. atmos. Sci. 33, 833-841. Legrand M. (1987)Chemistry of Antarctic snow and ice. J. de Physique Colloque, Suppl. 3, Tome 48, 77-86. Ram M. and Gayley R. I. (1988) Insoluble particles in Antarctic ice. J. geophys. Res. 93, 8378-8382. Schutz L. and Kramer M. (1987) Rainwater composition over a rural area with special emphasis on the sizedistribution of insoluble particulate matter. J. atmos. Chem. 5, 173-184. Sequeira R. (1982a) Acid rain: an assessment based on acid-base considerations. J. Air. Pollut. Control Ass. 32, 241-245. Sequeira R. (1982b)Chemistry of precipitation at high altitudes: interrelation of acid-base components. Atmospheric Environment 16, 329-335. Tan K. (1982) Principles of Soil Chemistry. Marcel-Dekker, New York. Wallace J. M. and Hobbs P. V. (1977) Atmospheric Science. Academic Press, New York. Wallen C. C. (1980) A Preliminary Evaluation of the WMO/ UNEP Precipitation Chemistry Data. MARC Report 22. Yaalon D. H. and Ganor D. (1979) East Mediterranean trajectories of dust-carrying storms from the Sahara and Sinai. In Saharan Dust (edited by Morales), pp. 187-193. Wiley, Chichester. Yaalon D. H. and Ginzbourg D. (1966) Sedimentary characteristics and climatic analysis of easterly dust storms in the Negev (Israel). Sedimentology 6, 315-332.
000~6981/91$3.00+0.00 ~ 1991PergamonPresspie
Printedin GreatBritain.
A NOTE ON THE CONSUMPTION OF ACID THROUGH CATION EXCHANGE WITH CLAY MINERALS IN ATMOSPHERIC PRECIPITATION RONALD SEQUEIRA Department of Applied Sciences, The University of Newcastle, P.O. Box 84, Waratah West, New South Wales 2298, Australia (First received 2 June 1988 and in final form 8 August 1990) Abstract--The first estimates of the consumption of potentially available strong acid in atmospheric
precipitation through the mechanism of cation exchange with clay minerals have been obtained for three climatically different regions of the world: (1) Antarctica (South Pole); (2) continental Europe (north of the Alps); and (3) the continental Mediterranean. From the calculations involving the ranges of the concentrations of potentially available strong acid and of clay minerals of low-to-mediumcation exchange capacity for each of the three regions it is suggested that the above process of acid consumption may almost always be insignificant in liquid (or liquified) precipitation over Antarctica and continental Europe. On the other hand, it is only under the most favourable conditions--involving the maximum stipulated cation exchangecapacity of the clay minerals--thatthe same process could possiblylead to a maximum acid consumption in precipitation water up to ~ 30% in the eastern Mediterranean region. The cation exchange process is also considered briefly in relation to the probable recycling of clay minerals of aeolian origin through evaporating clouds. Application of the first principles of cloud physics to the qualitative discussion suggests that clay minerals acting as freezing nuclei may have the best chance of participating in cation exchange, and hence, in acid consumption in clouds. Key word index: Precipitation chemistry, cation exchange, clay minerals, acid precipitation, acid consumption, cloud droplets, cloud processes.
l. INTRODUCTION The insoluble mineral component of precipitation water could react with the aqueous phase, leading to: (i) variable contributions to the observed trace element concentrations in solution and (ii) the so-called 'buffer capacity', which apparently leads to the observed discrepancy between the measured and calculated bicarbonate alkalinity (cf. Granat, 1972). A possible mechanism that could simultaneously explain the above features is the exchange of H+-ions with clay minerals in the insoluble material found in precipitation. Preliminary estimates of the ranges of acid consumption by this mechanism have been obtained for three regions of the world, where the present-day impact of aeolian dust could be categorized as 'negligible', 'low', and 'high', respectively. The three regions are, sequentially, I--Antarctica (South Pole); II---continental Europe (north of the Alps); and III----continentaleastern Mediterranean. While the South Pole is an area virtually unaffected by continental dust at the present time (Legrand, 1987), the eastern Mediterranean region is a major receptor of aeolian dust of Saharan origin transported by the Sirocco winds across the Mediterranean Sea (Chester et al., 1977; Yaalon and Ganor, 1979; Glavas, 1988).
Dan and Yaalon (1966) conclude that the clay-rich soils developed on sandy parent materials in the coastal plains of Israel owe their characteristics primarily to the addition of fine-grained aeolian dusts which are rich in kaolinite and mica, as shown by the studies of Chester et al. (1977).
2. CLAY-SIZEDPARTICLESAND CLAYMINERALSIN PRECIPITATION WATER
Of the total mass/volume concentration of the insoluble material present in precipitation water, a concentration which will henceforth be denoted by I (t), an important portion lies in the clay size range defined by the limits (cf. Jenny, 1980): 0.1 ~ r c < 1 pm, where rc is the radius of the particle. The mass/volume concentration of clay-sized particles in precipitation is given by:
487
[c] = I (t).f~
(1)
wheref~ is the clay size fraction of the total insoluble material. On the other hand, it is to be noted that the socalled clay minerals per se represent a specific group of layered silicates, including illite, montmorillonite, kaolinite, etc. which account for an important fraction
488
RONALD SEQUEIRA
of the total concentration in precipitation of clay-sized particles. The mass/volume concentration of clay minerals in precipitation may therefore be written as: I-c- m] = [c] "f~_,
(2)
where, f~-m is the clay mineral fraction of [c]. A conservative value off~_~,=0.5 is used as the upper limit for Regions II and III (continental Europe and the eastern Mediterranean, respectively). This value is based on the reasonable assumption that the insoluble clay-sized material in these regions would not only consist of the typical clay minerals specified earlier, but also other inorganic (and organic) matter, such as colloidal silica, feldspars, and a variety of combustion and condensation products, e.g. soot, tenacious metal oxides, etc. As for the South Pole, it was considered safe to employ anf~_,,= 1, as suggested by the upper limit of the relative frequency of clay minerals among the total ice nuclei reported by Kumai (1976). 3. ACID CONSUMPTION THROUGH CATION EXCHANGE WITH CLAY MINERALSIN PRECIPITATIONWATER A major feature of clay minerals is that they possess 'exchangeable' cations, e.g. Na ÷, Mg 2 ÷, etc. The general equation for H ÷-ion consumption through cation exchange could be written as: X-clay(s) + n H ÷(aq) ~- (H)n-clay(s) + X" ÷(aq) (3)
where, X n÷ is the exchangeable cation with charge +n. The cation exchange capacities (CECs) of clay minerals are well known and are grouped together in Table 1 (cf. Bohn et al., 1979; Tan, 1982). Since the three regions considered are known to be influenced only by aerosol material rich in clay minerals possessing low-to-medium CECs, viz.: (i) the illite-chlorite or (ii) the kaolinite-montmorillonite combinations (Kumai, 1976; Goudichet et al., 1986; Yaalon and Ginzbourg, 1966), the use of the range of CEC=0.050.5/~eq : - ~ is justified in the present work. This range is indicated in Table 2, row 5. Since Table 2 will be referred to frequently in the remaining text of this paper, only the pertinent row number (e.g. row 1, 2, etc.) will be cited henceforth, with a few exceptions, where specific reference to Table 2 will also be made.
4. DATAON INSOLUBLEPARTICULATEMATTERAND POTENTIALLY AVAILABLESTRONG ACID CONCENTRATION IN PRECIPITATION Region I (South Pole) Several measurements indicate that the I (t)-value of 15 ppbw (parts per billion by weight) is appropriate for this area of the Antarctica (Kumai, 1976; Legrand, 1987), and that almost all the micro-particles constitute the clay size range (cf. Ram et al., 1988). Hence, fc,~ 1, and from Equation 1, [cl ~ I (t). Since
Table 1, Cation exchange capacities of clay minerals Mineral Name Mica* Illite~" Kaolinite* Vermiculite* Montmorillonite (smectite)* Chlorite*
Type
Cation exchange capacity, CEC meq/100 g #eq/10 mg
Primary Clay Clay Clay Clay Clay
20--40 10--40 1-10 120-150
2-4 1-4 0.1-1 12-15
80-120 20--40
8-12 2-4
Sources: * Bohn et al. (1979). ? Tan 0982).
Table 2. Estimates of the range of acid consumption by ion exchange with clay minerals in precipitation Item or parameter (1) (2) (3) (4) (5) (6) (7) (8)
Total insoluble matter, I (t) (mg : - 1) Clay fraction, f~ Clay mineral fraction, f~_,, Estimated clay mineral concentration, [c-m], (mg: -1) Range of CEC of clay minerals (#eq E- 1) Estimated total CEC (/~eq: - 1) Potentially available strong acid (/~eq: - 1) Extreme (possible) range for acid consumption (%)
I
Region II
1.5 x 10-2
III
1.5 x 10-2
1-20 0.5 0.5 0.25-5.0
40-180 0.3 0.5 6-27
0.05-0.5 7.5 x I0-5-7.5x lO * 24i* 1.3 x 10-3-1.9 x 10 -2
0.05-0.5 0.0125-2.5 I00-200t~ 0.0062-2.5
0.05-0.5 0.3-13.5 50-150~ 0.2-27
1 1
Data source: *Legrand (1987); ?Wallen (1980); :~Granat (1972); §EDS (1980-present).
Cation exchange with clay minerals f~_,, assumed is 1.0 (row 3), we have the estimate: [ e - m] = 1.5 x 10- 2 mg d - l (row 4). The potentially available strong acid concentration (PASA-concentration) for the region at the present time could be considered to be 2-6 #eq d - ~, as indicated by the data of Legrand (1987). This includes the N O 3 and the two non-sea salt components, SO 2-* and CI-* (Legrand, loc. cir.). Of course, the PASA in Antarctica is of the latent type since the precipitation accumulates as ice. Hence the above discussion is only of theoretical interest.
Region II (continental Europe) Data on I (t) for this region are very limited. Recently, Schutz and Kramer (1987) report l ( t ) = l 4.1 mgE-1 for particles in the size range, 0.1/~m ~
Region III (eastern Mediterranean) The rough l(t)-range could be obtained using the mean monthly dust fallout data of Yaalon and Ginzbourg (1966) and a mean monthly precipitation rate of 10 cm (e.g. Kolton-Shapira et al., 1984), which lead to a range of I (t) = 40-180 mg d 1. This range appears to be quite reasonable, especially with respect to the upper limit. However, the same continental region is bound to have some contribution to l(t) from a resuspended local soil component. Accordingly, the aerosol size distribution is likely to be skewed towards a relatively larger size as compared with continental Europe (Region II). Thus, in effect, a part of the I (t) could be considered to be non-cyclic in character. Accordingly, theft value used for the eastern Mediterranean region is 0.3, much lower than 0.5, considered in the case of continental Europe (Region II). A PASA-concentration range of 50-150 #eq d - l is suggested by the BAPMoN W M O data for the general Mediterranean region (EDS, 198(~-present), and this value has been employed (row 7).
5. RESULTS The possible extreme values of the maxima for the consumption of PASA by clay minerals through the process of ion exchange in precipitation water in the three regions considered are deduced as follows (cf. Table 2). The maxima and minima given in row 4 are considered in combination with the common range of the
489
CEC=0.05-0.5/aeqf -1 (row 5) to yield the total cation exchange possible (row 6). On the other hand, the minima in row 6 are logically associated with the maxima in row 7, i.e. the PASAconcentration maxima. Similarly, the maxima in row 6 are associated with the minima in row 7 to obtain the final row of values in Table 2, viz. the extreme range of per cent consumption of the total available strong acid.
6. DISCUSSIONSAND CONCLUSION The available acidity in precipitation in geographical regions affected by aeolian arid dust is bound to be neutralized by such dust. This neutralization has been somewhat vaguely referred to by a number of workers as 'buffering', 'consumption', etc. of acid. The calculations shown in this note indicate that the mechanism of cation exchange with clay minerals in precipitation water is not an important process for the consumption of PASA in Regions I and II (Antarctica and continental Europe). On the other hand, appreciable H ÷-ion consumption could occur in the case of the dusty eastern Mediterranean region, where under the most favourable combination of conditions--involving high l(t) and high CEC of clay minerals in precipitation--a maximum of up to ~ 3 0 % of the PASA might be consumed through the ion exchange of H + ions with clay minerals. The acid consumption figures (row 8) would most probably bracket extreme values that one may experimentally observe. This is further supported by: (i) the constraints imposed by the inefficiency of the ion exchange process in a precipitation collector (or storage container) as opposed to that in well-drained soil, and (ii) the fact that a part of the PASA may not be present initially in the acid form, e.g. H2SO4, as suggested by this author in his previous works (cf. Sequeira, 1982a, b). The process of ion exchange may indeed explain the low concentrations of some of the trace metals in precipitation (cf. Table 1 of Granat, 1972), although small-scale leaching by acid of the various minerals could easily explain the trace concentrations observed. A relevant question that needs to be addressed at this point is: how important is the process of cation exchange--for example, in the coastal region of the eastern Mediterranean--in the acid consumption in cloud drops, during the history of cycling of an aeolian clay mineral particle through its participation in one or more evaporating clouds? Due to a complete lack of data on the ion exchange process in cloud-borne droplets, one can only speculate on such a question as follows. The two important criteria that appear to be important to the ion exchange involving clay minerals in clouds are: (i) the mineral particle should at some stage
490
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or another, be in immediate contact with the liquid phase of a cloud particle, and (ii) the droplet surrounding the clay mineral particle must (originally) contain some acid, e.g. H2SO 4. Clay mineral particles are well-known ice nuclei (Kumai, 1976), acting either as freezing, contact, or deposition nuclei (cf. Wallace and Hobbs, 1977). When a freezing nucleus is involved, the mineral particle is inside the cloud drop, and the ice-embryo stage heralding the solidification of the drop is eventually reached at a specific temperature, depending on the size of the drop. In the case of contact nucleation, the mineral particle is in minimal contact with the liquid drop. In such a situation, the process of cation exchange may be necessarily limited, since drainage through the mineral structure is also limited. In the final case, a vapour-to-solid phase transfer of water is involved, and hence, there is no chance of any ion-exchange, unless the ice particle formed is evaporated only after an intermediate liquid phase, rather than through the direct route of sublimation. A further requirement is that some acid be incorporated into such a drop, either acquired during the deposition stage (by diffusiophoretic collection of fine particles of acid) or after the ice particle involved melts. In summary, cloud drops that are initially acidic and contain one or more clay mineral particles would appear to be the most likely candidates in the in-cloud consumption of acid through cation exchange. This would identify the freezing nuclei as the most important contenders for in-cloud ion exchange. The role of contact nuclei, is perhaps, very limited. On the other hand, the deposition nuclei could be either totally unimporant, or could be conditionally important, provided that the evaporation of the ice particle formed in the cloud proceeds through an intermediate stage involving the liquid phase (which inherits or acquires acid) and not by sublimation. A separate paper outlining the qualitative aspects related to these and other possible atmospheric processes that could involve acid consumption through cation exchange will be considered in the near future.
Acknowledoements--The original phase of this work was planned and prepared at the Univeristy of Arizona, Institute of Atmospheric Physics. I am thankful to Dr E. Philip Krider, Professor and Director for his constant support to my research. Dr Chris Eastoe of the Department of Geosciences took part in some fruitful discussions.In the present University, my thanks are due to Professors Keith Morgan and Michael Carter for their high commitment to research. Finally, I thank the referee who suggested that the question
of the importance of cation exchange in cloud drops be also addressed.
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