Base Cations

Introduction

Base cation emission estimates for the UK are presented in this chapter. The emission estimates cover the period 1990-2000 for Calcium (Ca), Magnesium (Mg), Sodium (Na) and Potassium (K). These estimates are very uncertain.

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 SO₄^2- 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 SO_x and NO_x. Acidifying emissions of SO_x and NO_x 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.

The NAEI has attempted to estimate emissions from the following sources:

· Stationary combustion of fossil fuels: mainly in the fly ash from solid fuel combustion

· Mineral extraction processes: e.g. limestone quarrying

· Processes in the mineral products industry: e.g. cement manufacture and concrete batching

· Industrial processes using limestone, dolomite and soda lime:

- iron and steel manufacture

- glass manufacture

· Agricultural use: e.g. liming of soils and dust due to cultivation.

· Construction and demolition activities

· Mobile sources: mostly in the form of dust resuspension by traffic and exhaust emissions of potassium from lead replacement petrol (LRP).

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 mm) and will be deposited close to their source. The quantities of these minerals extracted in the UK are given in the Minerals Yearbook (1990 – 2000).

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) or AEAT estimates of PM₁₀ composition.

Industrial processes using limestone, dolomite and soda ash

Processes involving limestone, dolomite and soda ash include iron and steel production and glass manufacturing. Emissions of base cations from the iron and steel industry and the glass industry are based on the PM₁₀ inventory combined with emission factors for cations taken from Lee and Pacyna (1999) or based on AEAT estimates of PM₁₀ composition.

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) and 2000 given by DUKES.

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

	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000
Comb. in Energy Prod
Public Power	3211	3187	2989	2488	2210	1681	1548	1040	1077	844	952
Petroleum Refining Plants	6	7	7	7	8	7	7	7	7	6	4
Other Comb. & Trans.	14	13	9	6	3	1	1	1	1	1	1
Comb. in Comm./Inst/ Res
Residential Plant	1679	1801	1605	1640	1264	850	883	854	850	941	640
Comm., Public & Agri.	147	138	116	102	90	66	72	80	52	46	38
Combustion in Industry
Iron & Steel Combustion	3	3	3	4	4	3	2	2	2	3	1
Glass Production	109	101	108	110	106	106	80	75	58	45	41
Other Comb. in Industry	1542	1509	1559	1529	1477	1530	1428	1447	1336	1227	1051
Production Processes	4424	4389	4091	4209	4655	4287	3985	4087	4086	3889	3786
Shipping	8	8	8	8	7	8	9	11	10	7	6
Other Transp & Mach.	0	0	0	0	1	1	1	0	0	0	0
Waste Treatment & Disp.	0	0	0	0	0	0	0	0	1	1	1
Non livestock agriculture	269	84	84	84	76	76	177	177	42	34	34
Total	11413	11242	10579	10187	9899	8615	8192	7783	7524	7045	6556

Figure 7.1 UK Emissions of Calcium

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

	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000
Comb. in Energy Prod
Public Power	1035	1029	966	804	715	543	500	337	349	273	308
Other Comb. & Trans.	4	4	3	2	1	0	0	0	0	0	0
Comb. in Comm./Inst/ Res
Residential Plant	543	583	519	531	409	274	285	276	275	304	206
Comm., Public & Agri.	46	44	36	32	28	20	22	12	16	14	12
Combustion in Industry
Iron & Steel Combustion	3	3	3	3	3	3	3	3	3	3	3
Processes in Industry	53	45	43	38	45	48	44	44	44	42	42
Glass Production	55	53	54	54	54	59	40	40	30	23	22
Other Comb. in Industry	225	245	280	258	241	225	191	174	134	123	87
Production Processes	551	546	532	523	517	376	354	360	329	286	277
Waste Treatment & Disp.	0	0	0	0	0	0	0	0	0	0	0
Non livestock agriculture	0	0	0	0	0	0	0	0	0	0	0
Total	2517	2553	2436	2245	2012	1549	1439	1244	1180	1069	958

Figure 7.2 UK Emissions of Magnesium

Table 7.3 Summary of Sodium Emissions in the UK (tonnes)

	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000
Comb. in Energy Prod
Public Power	1032	1024	959	798	708	539	497	333	345	271	305
Petroleum Refining Plants	4	5	5	5	5	5	5	5	5	4	3
Other Comb. & Trans.	4	4	3	2	1	0	0	0	0	0	0
Comb. in Comm./Inst/ Res
Residential Plant	542	581	519	530	410	278	289	280	278	307	211
Comm., Public & Agri.	50	47	40	36	32	24	26	14	19	17	14
Combustion in Industry
Iron & Steel Combustion	308	298	288	287	295	302	317	322	314	300	268
Processes in Industry	18	15	14	12	15	15	14	14	14	13	14
Glass Production	113	102	111	113	108	101	86	78	64	52	46
Other Comb. in Industry	229	250	284	262	245	227	193	175	135	122	87
Production Processes	127	144	135	128	148	152	167	152	103	124	130
Other Transp & Mach.	6	5	5	6	5	6	6	7	7	5	4
Non livestock agriculture	0	0	0	0	0	0	0	0	0	0	0
Waste Treatment & Disp.	0	0	0	0	0	0	0	0	0	0	0
Total	2433	2475	2363	2179	1972	1651	1599	1380	1286	1216	1083

Figure 7.3 UK Emissions of Sodium

Table 7.4 Summary of Potassium Emissions in the UK

	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000
Comb. in Energy Prod
Public Power	855	850	798	664	590	449	413	278	288	226	254
Other Comb. & Trans.	0	3	2	1	1	0	0	0	0	0	0
Comb. in Comm./Inst/ Res
Residential Plant	482	515	468	477	376	265	274	267	266	290	209
Comm., Public & Agri.	49	48	42	38	35	29	20	21	25	24	22
Combustion in Industry
Iron & Steel Combustion	409	395	383	380	392	401	420	428	418	398	357
Processes in Industry	56	47	44	39	46	48	45	44	44	41	43
Glass Production	31	30	31	31	31	35	22	20	15	12	10
Other Comb. in Industry	145	204	233	215	202	189	161	146	114	103	74
Production Processes	122	119	122	126	127	126	130	127	125	106	113
Non livestock agriculture	0	0	0	0	0	0	0	0	0	0	0
Waste Treatment & Disp.	0	0	0	0	0	0	0	0	0	0	0
Total	2150	2212	2123	1971	1800	1542	1486	1331	1295	1200	1083

Figure 7.4 UK Emissions of Potassium

Accuracy of Emission Estimates Of Base Cations

Quantitative estimates of the uncertainties in emission inventories have been based on calculations made using a direct simulation technique, which corresponds to the IPCC Tier 2 approach recommended for greenhouse gases and also the methodology proposed in draft guidance produced by the UN ECE Taskforce on Emission Inventories. This work is described in detail by Passant (2002b). The estimates are shown in Table 7.5.

Table 7.5 Uncertainty of the Emission Inventories for Base Cations

Pollutant	Estimated Uncertainty %
Calcium	-50% to +100%
Magnesium	-40% to +80%
Potassium	-60% to +200%
Sodium	-40% to +100%

Inventories for base cations have been significantly revised since the previous version of the NAEI, but many of the emission estimates are still subject to significant uncertainty. This is because they are based on emission estimates for PM₁₀ (which are themselves highly uncertain), coupled with estimates of the chemical composition of the PM₁₀ which add further uncertainty.