Modelling base cations in Europe—sources, transport and deposition of calcium

Atmospheric Environment 33 (1999) 2241—2256

Modelling base cations in Europe—sources, transport and deposition of calcium D.S. Lee *, R.D. Kingdon , J.M. Pacyna
, A.F. Bouwman, I. Tegen AEA Technology Environment, National Environmental Technology Centre, Culham Laboratory, Culham, Oxfordshire OX14 3DB, UK
Norwegian Institute for Air Research (NILU), N-2007 Kjeller, Norway  National Institute of Public Health and the Environment (RIVM), P. O. Box 1, NL-3720 BA Bilthoven, Netherlands  NASA Goddard Institute for Space Studies, 2880 Broadway, NY, 10025, USA Received 30 October 1997; accepted 29 April 1998

Abstract The deposition of the base cations calcium, magnesium and potassium from the atmosphere needs to be quantified in the calculation of the total deposited acidity in the critical loads approach. Of these base cations, calcium has been found to be the most important in terms of mass deposited. However, the sources of calcium to the atmosphere are not well understood. Recently, the first spatially disaggregated inventory of industrial calcium emissions for Europe was presented by Lee and Pacyna (1998) who estimated a total European emission of 0.7—0.8 Mt yr\. However, it is thought that wind blown dust from soils contributes a substantial fraction to the deposition of calcium. In this work, the source strength of calcium from arid regions within the EMEP modelling domain was estimated using the global mineral dust emission data base of Tegen and Fung (1994) and an estimation of the calcium content of soils. This results in a ‘‘natural’’ calcium emission of 6 Mt yr\. A long-range transport model, TRACK, was used to calculate the wet and dry deposition of calcium arising from these industrial and natural sources to the UK which resulted in a total deposition of 29—30 kt yr\. Of this annual deposition, 0.6—0.7 kt arises from cement manufacturing, 0.02—0.03 kt from iron and steel manufacturing, 0.8—0.83 kt from a large point source power generation, and 28 kt from power generation from a small boiler plant. The natural emissions of calcium from arid regions result in a deposition of calcium to the UK of 0.5 kt yr\. The measured wet deposition of calcium to the UK is 89 kt yr\ and the estimated dry deposition 14 kt yr\. The short-fall in the modelled deposition of calcium is thus of the order of 70 kt yr\, which is suggested to arise from wind-blown dust from agricultural land in the UK and mainland Europe. The estimated emissions, and thus modelled deposition are rather uncertain, such that estimating deposition of calcium attributable to agricultural soil emissions by differencing has a large uncertainty. However, this is the first such study of its kind for Europe and represents a first step towards understanding the sources of calcium and their contribution to mitigating deposited acidity from acidifying pollutants such as sulphur dioxide, nitrogen oxides and ammonia.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Base cations; Calcium; Sources; Modelling

1. Introduction *Corresponding author. Now at DERA Pyestock, Propulsion Department, Farnborough, Hampshire, GU14 0LS, UK.

The atmospheric deposition of the base cations calcium (Ca), magnesium (Mg) and potassium (K) has recently become of interest within the acidification debate.

1352-2310/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 1 6 9 - 1

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The acidifying emissions of sulphur dioxide (SO ), nitro gen oxides (NO ) and ammonia (NH ) are the subject of V  ongoing discussions with respect to emissions control within the UK and the rest of Europe. However, declines of base cations in air and precipitation have been documented for mainland Europe, North America and the United Kingdom (Hedin et al., 1994; Lee et al., 1998). Atmospheric inputs of base cations to some remote receptors can be of ecological significance (Driscoll et al., 1989; Miller et al., 1993) such that if a significant fraction of the base cations deposited are attributable to industrial emissions, and that these are also declining, then reductions of acidifying emissions will not necessarily result in the recovery of these ecosystems (Gorham, 1994). The importance of base cations has now been recognised within the United Nations Economic Commission for Europe’s Convention on the Long-Range Transboundary Air Pollution (UNECE-CLRTAP) in the negotiations of the forthcoming Second NO Protocol. V This is being negotiated under the ‘‘Critical Loads’’ approach (Bull, 1991, 1995), which is multi-pollutant and multi-effects based. Thus, quantifying inputs of base cations becomes important. Measurements of Ca wet deposition are reasonably well quantified, both at the UK scale (RGAR, 1997) and the European scale (Draaijers et al., 1997a, b). However, dry deposition is only poorly quantified with very few measurements of Ca in aerosol available. Dry deposition is usually calculated from back-calculating aerosol concentrations from Ca in precipitation and scavenging ratios (RGAR, 1997; Draaijers et al., 1997a). Very little is known about the relative source strengths of these base cations to air. It is clear from observations of the composition of precipitation and the atmospheric aerosol that Ca is the most important in terms of mass (Lee et al., 1998). Natural sources include wind soils from arid regions (Arimoto et al., 1995), forest fires and volcanic eruptions. Sea salt is also a source of Ca (Keene et al., 1986). Various man-made sources have been identified: unpaved roads are important in North America (Gillette et al., 1992) as are some open sources such as quarrying (Gillette et al., 1992; Raper and Lee, 1996). Construction and general urban activities have been found to be a strong local influence on the Ca content of urban precipitation composition (Gatz, 1991; Lee, 1993). Agricultural tilling and wind blown soil dust are likely to be an important source. Industrial sources of Ca have been considered in Europe as a result of declining Ca in precipitation and air (Hedin et al., 1994; Lee et al., 1998). Only two estimates of the source strength of Ca from industrial emissions in Europe have been made: a preliminary estimate by Semb et al. (1995) of 1.4 Mt yr\; and a more refined estimate by Lee and Pacyna (1997) of 0.75—0.8 Mt yr\. Soils have been long known as a source of Ca to the atmosphere, although no emission estimates have been made for Europe.

It is under such circumstances of poor quantification of sources that modelling the fate of substances released to the atmosphere becomes an important part of the process of understanding sources. In some respects, this is not too formidable a task as Ca is released as a primary particle, and does not have any significant atmospheric chemistry that will change its transport and deposition properties, and wet deposition of Ca is reasonably well quantified. However, there remain significant uncertainties regarding its source strengths and their geographical distributions, and the size spectrum of Ca particles emitted. In this paper, we attempt to quantify the importance of two particular sources of Ca: industrial emissions of Ca (Lee and Pacyna, 1998) and a preliminary estimate of soil emissions of Ca from desert and arid regions. This is done by treating these sources in a long-range transport model, TRACK (Lee et al., 1997a), to investigate sources and fates of Ca in the atmosphere. Unpaved roads are not considered as these are not commonly found in northern Europe and are therefore, unlikely to be of great importance in contrast to the eastern United States. Wind-blown emissions from agricultural tillage practices, whilst potentially of great importance are also not considered in this study as no emissions data were available. Sea-salt emissions of Ca are also ignored as this is a relatively small component of Ca in precipitation and air, even in a maritime dominated environment such as the UK (Lee et al., 1998) and furthermore, such salts (sulphates and chlorides) have only a small capacity to neutralise acidic species.

2. Sources of calcium and transport modelling 2.1. Industrial sources Two estimates of Ca sources were used in this study: industrial emissions and soil emissions from arid regions. The industrial emissions of Ca were taken from the inventory of Lee and Pacyna (1998) which includes cement manufacturing, iron and steel manufacturing and power production (see Table 1). Emissions from cement manufacturing, iron and steel production, and power generation were calculated from production statistics (United Nations, 1995) and distributed by point source locations. Emissions from unabated small boilers for power production are much more uncertain, and were calculated using emission factors published in CORINAIR/EMEP Guidebook (1996) and an assumed Ca content of 2%. Emissions from these small point sources were distributed by a European population distribution kindly provided by the National Institute of Public Health and the Environment (RIVM), Netherlands (van Veldhuizen et al., 1997), on a 10 min;10 min spatial

Ca cement (low)

Totals

213

Former Czechoslovakia 8892 Poland 12,888 Former GDR 7839 Former Soviet Union 147,130 Albania 333 Bulgaria 5754 Hungary 2710 Romania 7170 Former Yugoslavia 6429 Austria 362 Switzerland 337 Norway 82 Sweden 357 Finland 96 Denmark 144 Iceland 8 Belgium 513 France 1792 Former FRG 2175 Greece 843 Ireland 114 Italy 2879 Luxembourg 49 Netherlands 253 Portugal 524 Spain 1970 United Kingdom 857

Country

266

11,115 16,110 9798 183,912 417 7192 3387 8963 8036 453 421 103 446 120 180 10 642 2240 2719 1054 143 3598 61 317 656 2462 1072 2

208 66 45 1425 0 7 16 44 14 9 2 1 10 6 1 0 24 40 83 2 1 51 7 11 1 28 36

Ca cement Ca iron (high) and steel (low)

7

283 230 165 5195 0 18 56 145 45 11 3 2 15 8 3 0 30 53 110 3 1 83 8 13 2 46 51

Ca iron and steel (high)

36

1177 1906 2611 19,190 44 479 290 609 568 176 0 0 87 76 295 0 295 790 4018 334 68 416 21 254 0 584 1832

Ca power point sources (low)

47

1655 2681 3673 26,992 62 674 408 856 798 176 0 0 87 76 295 0 295 790 4018 334 68 416 21 254 0 584 1832

Ca power point sources (high)

42

1416 2294 3142 23,091 53 577 349 732 683 176 0 0 87 76 295 0 295 790 4018 334 68 416 21 254 0 584 1832

Ca power point sources (mean)

638

45,136 88,504 60,262 151,502 430 3994 5474 5347 6989 4411 659 1648 2753 9330 9790 127 12,549 24,149 56,498 7698 6061 16,978 1554 10,379 5631 28,364 71,647

Ca power area pulverised

Table 1 Emissions of calcium by country and source sector (t Ca yr\), sector totals in kt Ca yr\ (from Lee and Pacyna, 1998)

69

8465 16,598 11,302 28,413 81 749 1027 1003 1311 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Ca power area cyclone

489

26,800 52,551 35,782 89,958 255 2371 3250 3175 4150 4411 659 1648 2753 9330 9790 127 12,549 24,149 56,498 7698 6061 16,978 1554 10,379 5631 28,364 71,647

Ca power area average

256

10,516 15,248 11,026 171,646 387 6337 3075 7947 7125 547 339 83 453 179 441 8 832 2623 6276 1179 183 3346 77 519 526 2582 2726

Ca all point sources (low)

314

12,813 18,634 13,106 212,199 470 7786 3792 9840 8764 640 424 104 548 205 478 10 966 3084 6847 1391 213 4098 90 584 658 3092 2955

Ca all point sources (high)

745

37,316 67,799 46,808 261,603 642 8708 6325 11,121 11,275 4959 998 1731 3207 9509 10,231 135 13,382 26,771 62,774 8877 6244 20,325 1630 10,898 6156 30,946 74,373

Ca all sources point and area (low)

802

39,614 71,185 48,888 302,156 725 10,158 7042 13,014 12,914 5051 1083 1752 3302 9535 10,269 137 13,515 27,232 63,345 9089 6274 21,076 1644 10,964 6288 31,457 74,603

Ca all sources point and area (high)

D.S. Lee et al. / Atmospheric Environment 33 (1999) 2241–2256 2243

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D.S. Lee et al. / Atmospheric Environment 33 (1999) 2241–2256

Fig. 1. Calcium emissions from industrial sources (t yr\).

resolution and then bulked up to a 50;50 km resolution suitable for gridding on to the EMEP polar stereographic grid (see Fig. 1). 2.2. Soil sources from arid regions No inventory was available for soil sources of Ca from arid regions to the atmosphere. Therefore, an approach was devised by which an inventory could be created. The approach taken was to utilise the global emission inventory of mineral aerosols from disturbed and undisturbed land of Tegen and Fung (1994, 1995). The spatial resolution of this inventory is 1.125°;1.125° and was regridded to 50 km;50 km resolution across the EMEP domain. In order to determine the fraction of the aerosol which consisted of soluble calcium minerals (mostly CaCO and CaSO ), a further methodology was devised,   described below. Calcium is one of the major components in the lithosphere as a whole. After oxygen (46.6%), silicon (27.7%), aluminium (8.1%), iron (5.0%), the content of Ca in the Earth’s crust is 3.6% (Clarke and Washington, 1924). The composition in rocks varies strongly according to the type of mineral. In soils, the mineral composition is different to the lithosphere as a whole, because of the process of soil formation. Physical and chemical weathering processes depend upon climate, hydrology, and the time period over which weathering and soil formation

has taken place. The formation and accumulation of humus in itself causes a relative dilution. Apart from the Ca stored in the soil minerals Ca is present at the soils cation exchange complex, and in relatively soluble compounds such as calcium carbonate (CaCO ) and gypsum (CaSO ) 2H O). Although all    these forms can be potential sources of Ca after deposition, carbonates and sulphates are the most important sources. The Ca captured in minerals is released slowly after weathering, occurring at varying rates depending upon the type of mineral involved and the climate. Also, carbonate-rich and gypsiferous soils are generally located in the world’s major arid regions which are prone to wind erosion. Therefore, only Ca from carbonate and gypsum were included in this analysis. In very wet climates, soil leaching can lead to a complete loss of calcium, e.g. in the strongly leached Ferralsols. There are also soils that inherit their characteristics from their parent material: Podzols are soils formed in chemically poor sands; Andosols are formed in volcanic ash, usually poor in Ca; and Rendzinas are formed in materials rich in calcium carbonate. Formation of calcic or petrocalcic horizons occurs where the parent material contains cations and where evaporative losses are greater than those from precipitation. Important regions where the latter process occurs are arid regions, e.g. loess deposits in steppe climates, with major occurrences of Chernozems, Katanozems and Phaeozems. In loess

D.S. Lee et al. / Atmospheric Environment 33 (1999) 2241–2256

deposits in more humid climates in Western Europe, most of the CaCO has been leached out up to a cer tain depth. In deserts the process of carbonate accumulation (caliche) is an important sink for Ca (Schlesinger, 1982). The data presented below are based on Batjes (1997), from the global soil database presented by Batjes and Bridges (1994) and Batjes (1996). The database currently contains 4353 soil profile descriptions corresponding to 19 222 horizons, 32% of which contain analytical data for carbonate, and a much lower portion contained data on gypsum (Table 2). The Ca contents are calculated for the major soils and soil types of the FAO/UNESCO (1971—1981) legend of the soil map of the world. Major soils in this classification system are separated on the basis of genetic classification, and subdivided into soil types on the basis of diagnostic soil horizons (layers) or other diagnostic criteria. The major soils of the arid regions, Yermosols and Xerosols, are separated on the basis of climate, while Gleysols correlate with wet areas and soil with hydromorphic properties. As this study is concerned with airborne soil dust, the data are presented for the topsoils only (0—30 cm for most soils (10 cm for Lithosols and Rankers). The Ca content in CaCO and CaSO .2H O are from the mean for    major soils. For some major soils there is sufficient information to present estimates for each individual soil type. These include all calcic and calcaric soil units, with high Ca contents. Dystric soil units have a low base saturation (FAO/UNESCO, 1971—1981), and generally no free carbonates. Within the major soil groups of Fluvisols and Luvisols there are also sufficient data to present the estimates for the individual soil types. For the major soils Podzoluvisols, Ferralsols, Kastanozems, Nitosols, Andosols and Rankers there are no data for gypsum; the gypsum content was assumed to be negligible on the basis of the prevalent process of soil formation and/or soil classification (FAO/UNESCO, 1971—1981; Driessen and Dudal, 1991). For Lithosols, the CaCO content was estimated to  be 34% on the basis of five samples, and the gypsum content 5% on the basis of only two samples. As shallow soils formed in limestone are classified as Rendzinas, strongly calcareous Lithosols are considered to be exceptions. The fact that only in few locations Ca has been determined shows that the sample may not be a representative. Therefore, Lithosols are presented as soils with nil Ca. Similarly, for Greyzems, the CaCO  content was 8% on the basis of four samples of orthic Greyzems. However, on the basis of soil classification (FAO/UNESCO, 1971—1981; Driessen and Dudal, 1991) the CaCO and gypsum contents were assumed to be 0.  For Histosols, the estimate for CaCO content was made  on the basis of two samples. Here, the Ca content was

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assumed to be 0 on the basis of FAO/UNESCO (1971—1981). The soil Ca contents were distributed according to the 1°;1° soil database of Zobler (1986). This soil database presented the dominant soil types for each 1° grid cell. This generalisation leads to a number of major uncertainties, as discussed by Bouwman et al. (1993). The paucity of data on Ca content is apparent from the preceding description of the inventory construction. For many major soil groups the Ca data are not presented by soil type, which implies another generalisation. Another uncertainty in the transport of Ca is the particle size distribution. Small particles such as clay cannot be easily airborne as they are usually in aggregates. Sand particles are not easily lifted because of their size. Transport distance of the particles depends on the size; clay is transported over longer distances than sand. Differential composition with particle size is rather uncertain: Simonson (1995) found that Ca content may strongly vary with the size of the particles, whereas Schu¨tz and Rahn (1982) found no enrichment of Ca on mobilised dust from the North African desert soils. 2.3. The long-range transport model The long-range transport model used in this study is a Lagrangian statistical trajectory model covering the EMEP modelling domain of Europe which can be used in either single or multi-layer mode. The model, TRACK (TRajectory model with Atmospheric Chemical Kinetics), was devised for studying acid deposition in the UK and includes coupled chemistry of SO , DMS, NO and  V NH and their derivative species. TRACK has been de scribed in more detail by Lee et al. (1997a), in an investigation of sources of sulphur deposition to the UK. The basic formulation of the model is vc dc Q"E #D (c )! Q Q!K c Q Q H Q Q h dt

(1)

where c is the concentration of species s, E the emission Q Q rate of species s, D (c ) the rate of change of species s as Q H a result of the chemical reactions of itself and other species j, v the dry deposition velocity of species s, Q h vertical depth over which dry deposition takes place, and K the washout (wet deposition) coefficient of Q species s. For this present application, the model was used simply as a dispersion tool, treating Ca as an inert particle, in single-layer mode. There is experimental and observational evidence that Ca particles released as CaCO  undergo transformation in the atmosphere to CaSO  from SO emitted from man-made sources (Buttler, 1988;  Hoornaert et al., 1996). However, this is not relevant to the fate and distribution of Ca, such that this chemistry is ignored in the model. Of far more importance

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D.S. Lee et al. / Atmospheric Environment 33 (1999) 2241–2256

Table 2 The median of the Ca content of the sample population in the form of calcium carbonate and gypsum for the topsoils (0-30 cm unless soil is shallower, such as Lithosols and Rankers) for 106 soil types from FAO/UNESCO (1971—1981) occuring in the Zobler (1986) soil database, the basis for the estimate and an indication of the number of samples used for the estimate. Data are based on Batjes (1997) No. of Soil types

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

Orthic Acrisols Ferric Acrisols Humic Acrisols Plinthic Acrisols Gleyic Acrisols Eutric Cambisols Dystric Cambisols Humic Cambisols Leyic Cambisols Gelic Cambisols Calcic Cambisols Chromic Cambisols Vertic Cambisols Ferralic Cambisols Haplic Chernozems Calcic Chernozems Luvic Chernozems Glossic Chernozems Eutric Podzoluvisols Dystric Podzoluvisols Gleyic Podzoluvisols Rendzinas Orthic Ferralsols Xanthic Ferralsols Rhodic Ferralsols Humic Ferralsols Acric Ferralsols Plinthic Ferralsols Eutric Gleysols Calcaric Gleysols Dystric Gleysols Mollic Gleysols Humic Gleysols Plinthic Gleysols Gelic Gleysols Haplic Phaeozems Calcaric Phaeozems Luvic Phaeozems Gleyic Phaeozems Lithosols Eutric Fluvisols Calcaric Fluvisols Dystric Fluvisols Thionic Fluvisols Haplic Kastanozems Calcic Kastanozems Luvic Kastanozems Orthic Luvisols Chromic Luvisols Calcic Luvisols Vertic Luvisols Ferric Luvisols

Basis for estimate

Ca content (%)

No. of samples for CaCO


No. of samples for CaSO ) 2H O
 

g g g g g g t g g g t g g g g g g g g g g g g g g g g g g t t g g g g g t g g a t t t t t t g t t t t t

0 0 0 0 0 0.8 0 0.8 0.8 0.8 3.0 0.8 0.8 0.8 1.4 1.4 1.4 1.4 0 0 0 4.7 0 0 0 0 0 0 0 4.6 0 0 0 0 0 0 1.7 0 0 0 0.7 4.0 0 0.7 0 1.7 0.2 0 0.2 4.9 0.4 0

m m m m m v h v v v v v v v v v v v 1 1 1 h m m m m m m h m 1 h h h h v v v v — v v m v h m h h v v m m

1 1 1 1 1 m m m m m 1 m m m 1 1 1 1 — — — 1 — — — — — — m 1 — m m m m 1 1 1 1 — 1 h — 1 — — — 1 1 — — — (continued on next page).

D.S. Lee et al. / Atmospheric Environment 33 (1999) 2241–2256

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Table 2. (continued) No. of Soil types

Basis for estimate

Ca content (%)

53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106

t a t a a g a g a a a g g g g g g g g g g t t a a g g g g g g g a t t g t g g g g g g g g g g g g g g g g g

0 0 0 0 0 0.2 0 0.2 0 0 0 0 0 0 — 0 o 0 0 0 0 0.1 0.9 0 0 0.2 0.2 0.2 0 0 0 0 0 0.4 0.8 0 0 0 0 0 0 3.7 3.7 3.7 3.7 7.8 7.8 7.8 7.8 7.8 5.7 5.7 5.7 5.7

Albic Luvisols Plinthic Luvisols Gleyic Luvisols Orthic Greyzems Gleyic Greyzems Eutric Nitosols Dystric Nitosols Humic Nitosols Eutric Histosols Dystric Histosols Gelic Histosols Orthic Podzols Leptic Podzols Ferric Podzols Humic Podzols Placid Podzols Gleyic Podzols Cambic Arenosols Luvic Arenosols Ferralic Arenosols Albic Arenosols Eutric Regosols Calcaric Regosols Dystric Regosols Gelic Regosols Orthic Solonetz Mollic Solonetz Gleyic Solonetz Ochric Andosols Mollic Andosols Humic Andosols Vitric Andosols Rankers Pellic Vertisols Chromic Vertisols Eutric Planosols Dystric Planosols Mollic Planosols Humic Planosols Solodic Planosols Gelic Planosols Haplic Xerosols Calcic Xerosols Gypsic Xerosols Luvic Xerosols Haplic Yermosols Calcic Yermosols Gypsic Yermosols Luvic Yermosols Takyric Yermosols Orthic Solonchaks Mollic Solonchaks Takyric Solonchaks Gleyic Solonchaks

No. of samples for CaCO
 m — m — — m — m — — — h h h h h h h h h h m h — — v v v h h h h — v v v m v v v v v v v v v v v v v v v v v

No. of samples for CaSO ) 2H O
  — — 1 — — — — — — — — m m m m m m 1 1 1 1 — m — — 1 1 1 — — — — — h m m m m m m m m m m m h h h h h v v v v

g is the mean for soil group.; t the mean for soil type used: a the assumed value on the basis of expert judgement.
1"1—5; m"5—15; h"15—30; v"'30 different profile data included in calculation.  Mean of all eutric and thionic Fluvisols.

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is the parameterisation of the deposition processes of Ca particles from the atmosphere which is described below. The industrial emissions inventory for Ca is essentially for small particles which pass through electrostatic precipitators of controlled sources (Lee and Pacyna, 1998). Uncontrolled sources make up the largest fraction of emissions from small combustion boilers. Electrostatic precipitators are very effective (of the order of 95—98% efficient) for particles with an aerodynamic diameter of '10 lm, but below this size, they are less effective (Pacyna, 1986). Calcium is an involatile element (Meij, 1994), and therefore found in the coarse phase (1—10 lm), and we have assumed that the bulk of the material has a diameter of 2 lm, which is consistent with observations of the ambient aerosol size and composition distant from source areas (Bergin et al., 1995; Hoornaert et al., 1996). For particles originating from soils in arid regions, a simplified system of that proposed by Tegen and Lacis (1996) was used. Tegen and Lacis (1996) used a size spectrum comprising 8 size bins, of which the bulk of the mass loading was found in a size range of approximately 1—6 lm (diameter). Given that these source areas are distant from the UK (the principal region of interest in this work), a single dry deposition velocity of 0.2 cm s\ was used. For wet deposition, a scavenging coefficient of 1.3;10\ s\ was used as a basis prior to scaling by precipitation amount and direction. This value was based upon the value given by Derwent et al. (1988), which is representative of sub- and super-micron particles. The ‘‘constant drizzle’’ parameters, K , for each species Q s (taken from Derwent et al., 1988; Metcalfe et al., 1989), are assumed to give valid mean values of wet deposition where the annual precipitation is equal to a standard value, RM (calculated as the mean over the UK land mass; in TRACK RM is 1000 mm yr\). In TRACK v1.0, the wet deposition rate, ¼, for a given species at a point, g, along a trajectory is

variable by wind direction. Values are ascribed to each grid square of the annual precipitation, the wind frequency for each of the 12 angular sectors and the fraction of the time, r , that it rains when the wind is blowing from F each sector (i.e. r is the conditional probability of rain F given that the wind is in sector h). This revised formulation of the scavenging coefficients used in TRACK v1.2 accounts for their variation from place to place and the directionality of precipitation amount. In addition, the orographic enhancement of wet deposition as a result of the seeder—feeder mechanism (Bergeron, 1950) has been included in a more rigorous manner than previously parameterised. Orographic precipitation, by direction, has been modelled using the model of Weston and Roy (1994). The non-orographic precipitation has been calculated by subtraction of coastal gradients in precipitation from the total precipitation, yielding the non-orographic and orographic components. The directionality of non-orographic precipitation was calculated using meteorological data from the ISMCS (1995) database at 47 locations across the UK. These components were then used in a modified expression for the wet scavenging coefficients, replacing eq. (2):






fM fL R E F EF(1!S )# EFS ¼ "K c FfM E EQF Q QE RM fL E E E

(4)

where S is the fraction of precipitation that is nonorographic, and f a precipitation factor in orographic and non-orographic cases denoted by superscripts o and n, respectively, for direction h. The factor F is applied to the scavenging coefficient on the orographic component of precipitation in order to represent the higher concentrations of pollutants found in cloud water during seeder—feeder events. More detail of this parameterisation is given by Lee et al. (1997b).

3. Results R E ¼ "K c EQF Q QE RM

(2)

where the annual rainfall rate at that receptor, R , has E been divided by its mean value at a receptor, to give a relative weighting by direction only. The above equation is used to calculate ¼ at all points along the EQF trajectory, with the total wet deposition rate at the receptor being the sum of all incoming trajectories, weighted by the wind frequency for 12 angular sectors p F ¼ " p ¼ (3) EQ F EQF F TRACK v1.2 now has a revised treatHment of wet deposition than that reported by Lee et al. (1997a) who used the above scheme, in that scavenging ratios are now

3.1. Results of the combined emissions estimations The global database of mineral particles from Tegen and Fung (1994, 1995) was designed for use in global climate models in studies of radiative forcing. Thus, a resolution of 1.125°;1.125° is more than sufficient resolution for such models. For the model domain here, it is rather coarse, and lacking in detail. The reason for adopting these data is that considerable thought and effort has gone into deriving the fluxes, and they are the only available published flux estimates that can be utilised. The impact of the coarse resolution is that the main source region identified are the arid zones within the EMEP domain, i.e. North Africa including the northern Sahara, and Kazakhstan in the vicinity of the Caspian

D.S. Lee et al. / Atmospheric Environment 33 (1999) 2241–2256

2249

Fig. 2. Calcium emissions from industrial and natural (desert) sources (t yr\).

Sea. Combined with the data on Ca composition (as defined above), the total flux was calculated to be approximately 6 Mt Ca yr\ from these natural soil sources. This is approximately an order of magnitude greater than the estimated industrial emissions of Ca across Europe (Lee and Pacyna, 1998), and is shown in combination with the industrial emissions in Fig. 2.

3.2. Results of model runs A number of model runs were undertaken to identify the contributions of Ca sources to deposition to the United Kingdom. A description of the conditions of these computations and their results is given in Table 3.

Table 3 Modelled source contributions of deposition to the United Kingdom Industrial emissions

‘‘Natural’’ emissions

All sources, low estimate: 745 kt All sources, high estimate: 800 kt All sources, low estimate: 745 kt All sources, low estimate: 800 kt — Cement low: Cement high: Iron and steel low: Iron and steel high: Power (point) low: Power (point) high: Power (area)

6 Mt 6 Mt — — 6 Mt — — — — — — —

Dry deposition (kt Ca yr\) 3.7 3.7 3.7 3.7 0 0.07 0.09 0.002 0.004 0.10 0.11 3.5

Wet deposition (kt Ca yr\)

Total deposition (kt Ca yr\)

26.1 26.3 25.6 25.8 0.5 0.51 0.64 0.018 0.026 0.70 0.72 24.4

29.8 30 29.3 29.5 0.5 0.57 0.73 0.02 0.03 0.80 0.83 27.9

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The total deposition (i.e. wet and dry deposition calculated to the UK land area) for all sources was approximately 30 kt Ca yr\. The calculated uncertainties on this is very small as deposition is dominated by Ca emissions from domestic and small boilers (28 kt Ca yr\) for which no uncertainty estimate was made. The deposition arising from identified point sources (cement works, iron and steel works and coal-fired power stations) was rather small, approximately 1 kt Ca yr\. The deposition of Ca in the UK arising from natural soil emissions in arid regions was also rather small, approximately 0.5 kt Ca yr\. The dominant source, power generation from small boilers, contributes some 93% of the modelled deposition. The measured wet deposition distribution across the UK is shown in Fig. 3 (Fowler, pers. comm.; RGAR, 1997). The general distribution is common to most maps of wet deposition of inorganic species for the UK, following that of precipitation amount. The modelled distribution is shown in Fig. 4. It should be noted that the grey scales in these figures are the same but the numerical scales different. The different numerical scales were chosen to show that the general distribution of wet deposition is well represented, although the magnitude is not. The spatial distribution of modelled dry deposition is shown in Fig. 5. The only published distribution of dry deposited Ca to the UK was made by inference, using the wet deposition field and scavenging ratios of Ca (CLAG, 1994). The modelled wet and dry deposition of Ca over the EMEP domain is shown in Figs 6 and 7. Modelled wet

Fig. 4. Modelled wet deposition of Ca (kg ha\ yr\).

Fig. 5. Modelled dry deposition of Ca (kg ha\ yr\).

Fig. 3. Measured wet deposition of Ca (kg ha\ yr\).

deposition in Fig. 6 is at its greatest in the southern European countries, parts of Russia, Ukraine, Georgia, Armenia and Azerbaijan. The spatial pattern of deposition is the combined result of high emission densities

D.S. Lee et al. / Atmospheric Environment 33 (1999) 2241–2256

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Fig. 6. Modelled wet deposition of Ca (kg ha\ yr\).

Fig. 7. Modelled dry deposition of Ca (kg ha\ yr\).

in, for example, Russia and areas of large precipitation amounts such Georgia, Armenia and Azerbaijan, and the Balkans. Measurements of wet deposition of ions in precipitation has been mapped from various countries’

precipitation composition network data by van Leeuwen et al. (1995a, b). A map of Ca deposition in Europe is presented by van Leeuwen et al. (1995a). The modelled wet deposition of Ca has not been compared with the

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D.S. Lee et al. / Atmospheric Environment 33 (1999) 2241–2256

mapped measurements in a rigorous manner on the European scale, but the general spatial pattern (if not the magnitude) is consistent with that presented by van Leeuwen et al. (1995a). An attempt has been made to map dry deposition of Ca on a European scale by Erisman and Draaijers (1995) using precipitation composition measurements, scavenging ratios and an assumed dry deposition velocity which is inherently a very uncertain procedure. There is little agreement between the spatial pattern shown in Fig. 7 and that given by Erisman and Draaijers (1995). In terms of the estimated sources of Ca in the modelling, and their distribution (see Figs 1 and 2), Fig. 7 can be readily understood. However, it is probably no more uncertain than the inferred dry deposition distribution of Erisman and Draaijers (1995).

4. Discussion The modelled deposition to the UK of Ca has been shown to account for approximately 30 kt of the estimated annual deposition of 103 kt yr\ (RGAR, 1997). Thus, it could be concluded that the discrepancy between modelled and measured deposition arises from one of the following: uncertainties in the emissions used; incomplete estimation of sources; uncertainties in the model parameters; uncertainties in the measurements. Uncertainties in the industrial source estimates used have been discussed by Lee and Pacyna (1998). Point source emissions uncertainties were calculated on the basis of process-specific emission factors, the contribution of different processes in western and eastern European countries, and the Ca content of emitted particles. However, the uncertainties for emissions from the dominant source, power generation from unabated small sources was far more difficult to calculate because of a lack of data. Given the large contribution of this source to European industrial emissions and modelled deposition to the UK, further work on emission factors is required before a reliable emission estimation can be made. An incomplete estimation of sources is known to be a significant factor in the discrepancy between measured

and modelled deposition. Inorganic Ca in soil (CaCO ,  CaSO ) has been shown to be a substantial mass fraction.  The inventory of mineral particle emissions used in this work (from Tegen and Fung, 1994, 1995) was made for global studies of their effect on radiative forcing, and by necessity, the resolution is fairly crude. Thus, the uplifting of mineral aerosols within the EMEP domain covers only the northern Sahara and the arid regions of Kazakhstan. Of these two areas, the northern Sahara is the dominant, estimated to generate approximately 5 Mt Ca yr\. Semb et al. (1995) estimated emissions of Ca from the Sahara to be 1.8—10 Mt yr\, based upon an estimate of dust emissions of 60—200 Mt yr\ (Junge, 1979) and the observations of Arimoto et al. (1995) that aerosols apparently from this region had a Ca content of 3—5%. An average emission from these data would suggest a total emission from the Sahara of approximately 10 Mt Ca yr\. This is in reasonable agreement with the estimate presented here for the northern Sahara, although the Ca content of soils has been estimated in this work to be higher: of the order of 3—15%. Thus, in this exercise, Ca from soils per se has not been modelled, rather Ca from specific arid regions. It is therefore likely that a substantial part of the difference between the measurements and the modelled Ca estimate to the UK are from agricultural soils that have not been included in the emission source estimations. Sources other than those identified can be of importance; for example, it has been suggested that biomass combustion may be important in some areas (Antilla, 1990). The choice of model parameters also has an effect on modelled deposition rates. In order to test the sensitivity of the model to this, dry and wet removal rates were varied by a factor of 0.5 and 2. The results are given in Table 4. This analysis is important as it gives some uncertainty bounds on the modelled deposition rates using the same emission scenario. Varying the removal terms by factors of 2 gives a total UK Ca deposition budget a range of approximately 24—36 Kt yr\. The approach taken to parameterise dry deposition is relatively crude, but a more sophisticated treatment is not justified by the data available on particle sizes emitted from the sources under consideration.

Table 4 Sensitivity analysis of particle removal parameters Wet removal (s\)

Dry removal (m s\)

UK dry budget (kt yr\)

UK wet budget (kt yr\)

UK total budget (kt yr\)

1.3;10\ 1.3;10\ 1.3;10\ 2.6;10\ 0.6;10\

0.002 0.004 0.001 0.002 0.002

3.7 6.9 2.0 2.6 5.2

26.3 24.1 27.6 33.2 18.8

30.0 31.0 29.6 35.8 24.0

Parameters routinely used in model runs given in Table 3.

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The measurements with which the model output is compared are also subject to uncertainties. Analytical uncertainties using ion chromatography for Ca are of the order of 20—30% for individual samples (K. Vincent, pers. comm.). In order to determine Ca concentrations with better precision and accuracy in precipitation samples, it would be necessary to acidify the samples. Wet deposition is calculated in the UK by enhancement of concentrations by an empirical method to account for orographic enhancement of wet deposition from the seeder—feeder process (Dore et al., 1992). A parameterisation of this process is incorporated into the model, further uncertainties are introduced as wet deposition estimates to a particular 20 km;20 km grid square of complex terrain have been estimated to be as high as $80% for sulphate (Smith et al., 1995). Calcium deposition maps (including Fig. 3) are enhanced to account for the seeder—feeder process, and a review of the observations of field campaigns and long term studies (Lee et al., 1997b) supports the use of the same factor as that used for other ions such as sulphate, nitrate and ammonium. However, whether the measurements represent the spatial variability of ion concentrations in precipitation adequately is a further question. While some Ca is found in the fine phase of the atmospheric aerosol, much of the Ca is found in the coarse phase (Eaton et al., 1978; Lannefors et al., 1983; O®blad and Selin, 1986; Tanaka et al., 1980; Willison et al., 1985; Semb et al., 1995). In urban areas, it has been found that Ca concentrations in precipitation are generally higher than those found in adjacent rural areas (Gatz, 1991), which has been shown to be true for at least one large conurbation in the UK (Lee and Longhurst, 1992b). In a comparison of data from bulk and wet-only collectors in the centre of a large conurbation, Lee and Longhurst (1992b) found that 46% of the Ca deposition was the result of dry deposition to the bulk collector funnel, which was attributed to local sources of large Ca particles from the urban environment. Other sources have been detected in rural areas: for example, a distinct enhancement of precipitation concentrations by a factor of 3 in the vicinity of a limestone quarry over background concentrations was shown by Raper and Lee (1996). It was not identified whether this effect arose from a dry deposition sampling artefact as only bulk precipitation collectors were used, but the spatial extent of the effect was approximately 20 km. There are also further possibilities for contamination in agricultural regions. Thus, the measurements of wet deposition in the UK are rather uncertain, potentially arising from dry deposition artefacts, analytical uncertainties and uncertainties in deposition to complex terrain. Dry deposition of Ca to the UK has been estimated elsewhere from precipitation concentrations and is known to be very uncertain (CLAG, 1994), which will certainly be the case when significant dry deposition artefacts to collector funnels occurs. Owing to these particular uncertainties, it

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is suggested that the combined wet and dry deposition estimates to the UK (89 and 14 kt Ca yr\, respectively) probably represent the upper limits. The principal interest in base cations and Ca deposition is their contribution to offsetting deposited acidity from other S and N compounds. Calculating the inputs of acidity from N compounds is not straightforward because of the interactions of deposited N species with their receptor; i.e. uptake by vegetation, denitrification, immobilisation in the soil etc. (Galloway, 1995; Bull et al., 1995; Hornung et al., 1995). Deposited acidity from S species is somewhat simpler to calculate and it has been assumed in UK critical loads work that 1 mol of deposited S results in the release of two moles of H> (CLAG, 1994). So, for the purposes of illustration, ignoring the acidifying effect of N species, it is possible to calculate the deposited acidity from S species to the UK and the mitigating effect of Ca deposition on a country-budget basis. The UK mean wet and dry deposition for the period 1992—1994 was 341 kt S yr\ (RGAR, 1997), and 30 kt Ca yr\ (this work). This results in an annual input of 21.3 Gmol H> from deposited S, and 2.6 Gmol of Ca, resulting in a net deposition of 16 Gmol of H>. The modelled input of Ca from industrial sources results in deposition of approximately 30 kt Ca yr\ to the UK, mitigating 7% of the acidity deposited from the S species. These calculations are, inevitably rather crude and do not account for location within the UK, nor acidifying effects of N species, but provide an indication of the mitigating effect of deposited Ca from industrial sources. If industrial emission sources of Ca are correlated with fossil fuel combustion and thus sources of SO , it is  possible that emissions of Ca will also decline. Thus, in regions where atmospheric inputs of Ca are of ecological significance, reduced inputs of acidic S species cannot result in the expected ecological recovery because of reduced atmospheric inputs of Ca. In order to investigate this possibility, better quantification of emission sources is required. A further question arises from the speciation of the Ca particles: it is likely that these are in different forms, e.g. oxides, sulphates, chlorides and silicates. Some of these species will have only a low ability to neutralise acidic species. Furthermore, some species such as CaCO can be converted to CaSO in the presence of   SO (Buttler, 1988) and there is some evidence that this  occurs during atmospheric transport (Hoornaert et al., 1996).

5. Conclusions E Small particle emissions of Ca from industrial sources and natural soils from arid regions have been modelled in a long range transport model to determine the contributions of these sources to deposition in the UK and Europe.

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E Emissions of Ca have been estimated to be 0.75—0.8 Mt yr\ from industrial sources and 6 Mt from soils in arid regions. Emissions of Ca from agricultural soils across Europe have not been calculated. E The total deposition to the UK land surface is estimated to be approximately 100 kt Ca yr\ from measurements of wet deposition and dry deposition inferred from precipitation measurements and scavenging ratios. Modelling the above emission sources accounts for approximately one third (30 kt Ca yr\) of the measured deposition. Of these sources, longrange transport of soil particles from arid regions constitutes only 0.5 kt Ca yr\, point sources of cement manufacturing, iron and steel production and power generation from coal combustion contributed 1 kt Ca yr\. The overwhelming contribution was from power production from small sources not equipped with abatement technology. E The uncertainties in the emission, modelling parameterisations and measurements have been discussed. The measurements of Ca deposition are less certain than those of the other ions, such as sulphate, nitrate and ammonium, because of analytical constraints and the potential for dry deposition artefacts. However, it is quite clear that one large source, i.e. agricultural soil emissions, has not been accounted for. Under the assumption that the estimation of industrial sources is of the right order, then it is speculated that two thirds of the UK deposition of Ca may be the result of the wind-blown soil. E The conclusions drawn from this work must be rather tentative, as the emissions used in the modelling are rather uncertain, and known to be incomplete. E It is estimated that on a national scale, deposition of Ca from industrial sources offset approximately 7% of the deposited acidity arising from S species. E It is recommended that more work is undertaken on the emissions factors for small unabated combustion sources of particles and their Ca content, and that an inventory of agricultural soil emissions is made on a European scale. An agricultural soils inventory would allow the testing of whether the magnitude of Ca deposition that soil emissions are inferred to result in is of the right order.

Acknowledgements The modelling work presented here was funded under contract N/01/00031/00/00 by the UK Department of Trade and Industry. We are grateful for the encouragement and enthusiasm of Drs Trevor Morris and Chris Franklin. We would also like to thank: Dr Jaap van Woerden of the National Institute of Public Health and the Environment (RIVM), Netherlands, for kindly providing the population database prior to general release;

Prof. David Fowler of the Institute of Terrestrial Ecology, Edinburgh Research Station, for providing the figure of Ca wet deposition data; and Dr Keith Bull, of the Institute of Terrestrial Ecology, Monk’s Wood, for useful discussions on base cations in relation to critical loads.

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