Introduction

Base cation emission estimates for the UK are presented here for the first time. The emission estimates cover the period 1990-1999 for Calcium (Ca), Magnesium (Mg), Sodium (Na) and Potassium (K). These estimates are very uncertain and the methodology used for making these emission estimates will be built on for future versions of the NAEI.

A base cation is essentially a positively charged ion from group 1 or 2 of the periodic table (the alkali metals or alkaline earth metals). The most environmentally abundant of these are Na, K, Ca and Mg. Base cations are important in the environment because their deposition has an impact on the surface pH. The deposition of base cations increases the alkalinity of the surface, the effect in the environment is to buffer or neutralise the effects of the acidity generated by S an N deposition (which in their mobile anionic form as SO42- and NO3- leach Ca and Mg from the soil). Therefore the primary purpose of these emission estimates is to use them to generate spatially resolved emission maps, which enable deposition maps to be calculated.

Background

A "critical load" approach is often taken to predict the maximum levels of acidity or alkalinity that an ecosystem can tolerate. The base cations (Na+, K+, Ca2+, Mg2+) are known to be present in ambient air and in precipitation. The deposition of these base cations to ecosystems will offset the acidifying inputs from the deposition of SOx and NOx. Acidifying emissions of SOx and NOx have already been quantified in the NAEI, and the effects of these acidifying emissions can now be complemented with estimates of emissions of important base cations.

The Review Group on Acid Rain (1997) reported on the decline in base cation deposition that has been observed in Europe and N America since the early 1970’s and how such a decline may offset the benefits of reductions in SO2 emissions. Interest in the deposition and acid neutralising effects of base cations is mainly confined to Ca, K and Mg. It has long been assumed that the major source of these base cations in air is dust from soil erosion, but patterns of concentrations in air and precipitation also suggest significant emissions from urban and industrial sources. The concentrations of Ca, K and Mg in air and in precipitation measured at three rural sites in the UK declined dramatically between 1970 and 1991 (Lee and Pacyna, 1999). It has been suggested that the decrease in base cation deposition which has been observed is due to the reduction in emissions from urban and industrial sources. Concentrations of Na in air and rain have shown much smaller decreases over this period, consistent with its mainly marine origin as sea-salt.

Emissions of base cations have not previously been estimated by the NAEI. This year, the NAEI has attempted to estimate emissions from the following sources:

There are likely to be base cation emissions from other sources, for example incineration. Currently, these are not included in the estimates as such sources are likely to be much smaller than the sources listed above.

Stationary combustion of fossil fuels

The base cations emitted from stationary combustion arise from the trace concentrations of the cations found in the fuels. The base cations will enter the atmosphere contained in the primary particulate matter (PM) which is emitted from the combustion source. Calcium has been found in large amounts in the fine particle size fraction collected from combustion sources.

The NAEI currently estimates PM10 emissions from large combustion plant for power generation using total PM emissions data submitted by the operators to the Environment Agency and the Scottish Environmental Protection Agency. Where reported data are incomplete, PM emission factors for the appropriate fuel are derived and combined with the amount of fuel used by the combustion plant to estimate the total mass of PM emitted.

The mass content of cations in coal has been estimated from the Turner-Fairbank Highway Research Centre (US Transport Department) figures for fly ash from bituminous coal. Data regarding the composition of fuel oil is given in the Marine Exhaust Research Programme.

Mineral extraction processes

Limestone quarrying is a major source of atmospheric emissions of base cations, principally calcium. Quarrying of dolomite (CaCO3 MgCO3), rock salt (NaCl) and potash (KCl) are the principle sources of magnesium, sodium and potassium respectively.

The NAEI currently estimates PM10 emissions from quarrying using USEPA emission factors combined with UK mineral statistics on the production of each type of aggregate. The dust emitted from limestone quarrying will be mainly particles of limestone (CaCO3) itself. These particulates will be mainly in the coarse particle size range (>2.5 m m) and will be deposited close to their source. The quantities of these minerals extracted in the UK are given in the Minerals Yearbook (1990 – 1999).

 

Processes in the mineral products industry

Emissions of calcium from the mineral products industry are estimated from total PM10 emissions using emission factors from Lee and Pacyna (1999). The current NAEI inventory for PM10 emissions from cement manufacture is based on USEPA emission factors and cement and lime production statistics.

Industrial processes using limestone, dolomite and soda ash

Processes involving limestone, dolomite and soda ash include iron and steel production and glass manufacturing. A PM10 inventory is estimated by the NAEI for these processes using overall PM emission factors for the process, taken from the USEPA AP-42, and combined with UK production statistics.

Emissions from the iron and steel industry are based on the PM10 inventory with an emission factor for calcium given in Lee and Pacyna (1999). The emission factor for magnesium is calculated from the relative quantities of limestone, dolomite and chalk used by the iron and steel industry and the relative molecular masses of calcium and magnesium.

However, unlike the mineral extraction and products industry considered above, the particulates emitted will contain a variety of elements, including metals, depending on the nature of the process and raw materials used. There are currently no data detailing the chemical composition of PM10 emissions from the glass industry, and therefore initial estimates of the emissions of Ca and Mg are derived from the total quantities of raw minerals used in the process (data from UK Minerals Yearbook). This method will be improved further when more data become available. The main sodium compound used in industry is sodium carbonate (Na2CO3), or soda ash. Sodium emissions are calculated from values for total soda ash consumption by the glass making industry currently estimated in the NAEI.

 

Soil liming and cultivation in agriculture

The practice of soil liming in agriculture will lead to the emission of Ca as the lime is applied to the ground. Statistics are available on the quantity of limestone used each year for liming (UK Minerals Yearbook) and an emission is estimated using an emission factor for non-metallic particles given by the USEPA.

The average quantities of re-suspended dust, as a result of land cultivation, may be estimated from data reported in the MAFF Report CSG 15 (2000). Emissions are estimated from the average chemical abundance of each cation in UK soil (Lindsay, 1979).

 

Construction activities

The NAEI currently uses a USEPA emission factor combined with UK construction activity statistics to estimate fugitive emissions of PM10 from these processes. A modified PM10 emission factor based on the fraction of total aggregate used in construction (UK Minerals Yearbook) that is limestone, dolomite or chalk, is used to estimate the base cation emissions.

 

Mobile sources

Emissions of base cations from mobile sources will mainly arise from the resuspension of road dust by traffic. Recently, Nicholson (2000) has made an estimate of the total PM10 emission from UK roads. Using this information with data on the average chemical composition of road dust (Sloss and Smith, 2000) Na, K and Ca emissions have been estimated. There are insignificant quantities of Mg in road dust.

Potassium compounds are the primary additives in Lead Replacement Petrol (LRP). LRP has been available since Autumn 1999 and is the main source of potassium emissions from vehicle exhausts. Emissions have been estimated from UK LRP sales in 1999 (calculated as a fraction of leaded petrol sales) given by DUKES.

Table 7.1 Summary of Calcium Emissions in the UK (tonnes)

 

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

Stationary Combustion

                   

Domestic Coal

634

655

593

563

397

258

278

272

292

357

Coal Power Plant

819

813

763

636

601

575

530

449

462

387

Coal (Misc.)

347

376

406

379

349

315

271

253

193

184

Fuel Oil (all sources)

50

49

45

43

38

35

34

50

44

35

Mineral Extraction

129

134

133

130

143

158

141

136

146

146

Mineral Products

                   

Cement & concrete batching

1097

903

821

831

954

939

958

1002

1020

977

Industrial Processes

                   

Iron and Steel

125

120

116

116

120

123

128

130

126

120

Glass manufacture

1

2

2

2

2

2

1

1

1

2

Agriculture

                   

Cultivation

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

Construction

16

16

16

15

15

16

14

15

17

17

Mobile

                   

Road dust

1000

1000

1000

1000

1000

1000

1000

1000

1000

1000

Total

4218

4068

3895

3715

3619

3421

3355

3308

3301

3225

Table 7.2 Summary of Magnesium Emissions in the UK (tonnes)

 

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

Stationary Combustion

                   

Domestic Coal

50

51

46

44

31

20

22

22

23

28

Coal Power Plant

107

107

100

83

74

56

51

35

36

28

Coal (Misc.)

28

30

32

30

28

25

21

22

15

 

Mineral Extraction

12

12

12

11

11

11

10

11

10

9

Industrial Processes

                   

Iron and Steel

18

18

16

15

16

16

15

18

17

16

Glass manufacture

0.2

0.3

0.3

0.3

0.3

0.2

0.2

0.3

0.3

0.3

Agriculture

                   

Cultivation

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

Construction

1.3

1.6

1.5

1.4

1.3

1.3

1.4

1.4

1.5

1.4

Mobile

                   

Road dust

100

100

100

100

100

100

100

100

100

100

Total

317

320

308

285

262

230

221

210

203

183

 

 

Table 7.3 Summary of Sodium Emissions in the UK 1990 – 1999

 

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

Stationary Combustion

                   

Domestic Coal

40

41

37

35

25

16

17

17

18

22

Coal Power Plant

86

85

80

67

59

45

41

28

29

23

Coal (Misc.)

22

24

26

24

22

20

17

18

12

12

Mineral Extraction

3

5

4

3

5

5

6

5

2

4

Industrial Processes

                   

Glass manufacture

3

2

3

3

2

2

3

3

3

2

Agriculture

                   

Cultivation

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

Mobile

                   

Road dust

100

100

100

100

100

100

100

100

100

100

Total

251

252

246

229

208

183

178

166

162

159

Table 7.4 Summary of Potassium Emissions in the UK 1990 - 1999

 

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

Stationary Combustion

                   

Domestic Coal

30

31

28

26

19

12

13

13

14

17

Coal Power Plant

64

64

60

50

44

34

31

21

22

17

Coal (Misc.)

17

21

19

18

17

15

13

13

9

9

Mineral Extraction

2

2

3

3

3

3

3

3

3

2

Agriculture

                   

Cultivation

0.046

0.046

0.046

0.046

0.046

0.046

0.046

0.046

0.046

0.046

Mobile

                   

Road dust

1100

1100

1100

1100

1100

1100

1100

1100

1100

1100

Exhaust

                 

0.016

Total

1213

1218

1210

1197

1183

1164

1160

1150

1148

1145

 

Accuracy of Emission Estimates Of Base Cations

The estimation of base cation emissions is a recent development and although the available data has been considered in some detail, it is expected that there will be a number of revisions and refinements to these emission estimates in future years. Data on sources which have not been included here are expected to become available, and data currently used may become superseded.

Of the data presented here, it is the fugitive emissions which are particularly high in uncertainty. This is due to the availability of few measurements and the variable nature of the source. Emissions from other sources are better characterised due to the controlled way in which measurements can be made. As a guide, the current uncertainties associated with the emission estimates are considered to be a factor of 5.