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 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.

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.

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:

• 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.

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.

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).

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.

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.

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).

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.

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.

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.