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Spatial differentiation in LCA impact assessment

4. Acidification

• 4.1 Introduction
• 4.2 Classification
• 4.3 EDIP97 characterisation factors
• 4.4 EDIP2003 characterisation factors
• 4.5 Site-generic characterisation
• 4.6 Site-dependent characterisation
• 4.7 Normalisation
• 4.8 Interpretation
• 4.9 Example

Background information for this chapter can be found in:

• Chapter 4 of the "Environmental assessment of products. Volume 2: Scientific background" by Hauschild and Wenzel (1998a).
• Chapter 3 of the "Background for spatial differentiation in life cycle impact assessment – EDIP2003 methodology" by Potting and Hauschild (2005).

4.1 Introduction

Releases of nitrogen (NO_x and NH₃) and sulphur (SO₂) to air account in most countries for more than 95% of the total acidifying emissions. On a national level, acidifying emissions thus consist mainly of nitrogen and sulphur. In the life cycle inventory of a product, however, other substances may dominate the total mass of acidifying emissions.

Acidifying emissions are usually dispersed and converted before they are deposited on terrestrial and aquatic systems. The scale of the deposition area depends on the characteristics of the substance and on regional atmospheric conditions, but the main acidifying substances are transported over several hundred to thousand kilometres. The deposition of acidifying substances may lead to an increase of acidity (i.e. decrease of pH) in the water or soil matrix. This phenomenon occurs when the base cation of the acid is leaving the system, while the hydrogen ion is left behind. Natural weathering of minerals, nitrification, fixation of nitrogen in biomass, and fixation or precipitation of compounds of e.g. phosphor in the soil matrix are the main processes to avoid leaching.

Increase of acidity in for instance terrestrial systems leads to increased weathering of (essential) minerals. Weathering of minerals can to some extent neutralise acidifying depositions, though it also leads to an imbalance of nutrients. When the pH falls to a critical level, toxic aluminium becomes mobile in harmful amounts. The aluminium affects the hair roots and thereby nutrition and water uptake of vegetation. The resulting decrease in health lowers the ability of trees and other vegetation to cope with stress. The aluminium ions are also toxic to aquatic life in freshwater systems.

4.2 Classification

For a substance to be considered a contributor to acidification it must cause release of hydrogen ions in the environment and the base anions which accompany the hydrogen ions must be leached from the system.

The number of substances that may contribute to acidification is not large, and in practice Table 4.1 contains all relevant substances that contribute to acidification. Note that emission of organic acids is not regarded as a contribution to acidification because the base anion is generally degraded rather than leached.

4.3 EDIP97 characterisation factors

Presently, typical characterisation factors for acidification are based on the potential of substances to release hydrogen ions (i.e., the theoretical maximum acidification). The potential of a substance to release hydrogen ions is expressed as the equivalent emission of sulphur (SO₂). One mole of oxidised sulphur can produce two moles of hydrogen ion. The EDIP97 factors are listed in Table 23.5 in Wenzel et al. (1997).

There are several problems with characterisation factors based on substance potentials to release hydrogen ions. This approach does not take into account that:

• The geographical region of release and regional meteorological conditions determine the relevant deposition pattern of an emission. For every process in the life cycle of a product, the acidifying emission deposits on a large area containing very many ecosystems. So while geographically close sources have strongly overlapping deposition areas, this is not the case for sources which lie several hundred kilometres from each other.
• The other way around, the extent to which an ecosystem already receives acidifying depositions from other sources (background deposition) depends on its location in relation to major industrialised and inhabited areas. Most ecosystems receive acidifying depositions from very many sources, which usually makes the contribution from a single source to the total deposition very small.
• Ecosystems differ in their natural capacity to avoid leaching of base cat-ions and/or to neutralise acidity by weathering of minerals, and the already operative "background" deposition to an ecosystem determines to what extent its capacity is used and additional deposition is harmful.

As a result, the theoretical maximum capacity of an acidifying substance to release hydrogen ions is usually not determining the acidification impact. Specifically the acidifying impact from nitrogen emissions is overrated compared to sulphur when the potential to release hydrogen is used as impact indicator, and the final acidifying impact depends on the geographic location where an emission is released.

4.4 EDIP2003 characterisation factors

As argued in the previous section, the potential to release hydrogen is a poor measure to express the acidifying impact of an emission. In EDIP2003, the RAINS model¹ is used to establish acidification factors which overcome most of the identified problems. Site-generic factors have been established (see Table 4.1), as well as site-dependent factors for 44 European countries or regions (see Annex 4.1 to this chapter). The acidification factors relate an emission by its region of release to the acidifying impact on its deposition areas.

The RAINS model (version 7.2) estimates dispersion and deposition of nitrogen and sulphur compounds on grid elements (150 km resolution), resulting from the emissions from 44 countries or regions in Europe. The grid consists of 612 elements covering all 44 European regions, including the European part of the former Soviet Union. Total deposition for one grid element is computed by adding up the contributions from every region and the background contribution for that grid element. The dispersion and deposition estimates are done with source-receptor matrices based on the EMEP model - a Lagrangian or trajectory model. In this model, an air parcel is followed on its way through the atmosphere along its (horizontal) travel during 96 hours preceding their arrival at a specified grid element.

Figure 4.1 Two dimensional trajectories of atmospheric motion of an air parcel (Alcamo et al. 1990).

The soil capacity to compensate for acid deposition is described by the critical acid load. Critical load functions for acidification of forest soils, heath land, grassland, peatland and freshwater have been estimated for the grid elements, and cumulative distribution curves for ecosystem sensitivities have been compiled in the RAINS model for all ecosystems within each grid element (for some grid elements over 30.000 ecosystems have been registered).

The RAINS model calculates the site-dependent characterisation factor for a country by looking at a fixed, but marginal emission of the substance from this country (e.g. 1 ton NO_x) on top of the actual emissions from all countries together. The impact from the resulting deposition increase is the additional area of ecosystem becoming exposed above the critical acidification load. For each grid element, the impact increment is determined from the cumulative distribution curve of unprotected ecosystems in the grid element. The impact increments for all grid elements within the deposition area are summed and expressed as the total area of ecosystem becoming unprotected, i.e. exceeding its critical load, as consequence of the emission.

A more detailed description of the RAINS model and its use for calculation of site-dependent characterisation factors can be found in Potting and Hauschild, 2005.

The application of the EDIP2003 site-generic acidification factors is similar to the application of EDIP97 factors which are also site-generic (see next section).

Application of the site-dependent acidification factors is also straightforward (see Section 4.6). Typical life cycle inventories already provide the only additional information which is required for site-dependent characterisation, namely the geographical region where the emission takes place.

The use of site-dependent acidification factors adds a resolving power of a thousand between highest and lowest ratings, while combined uncertainties in the RAINS model are cancelled out to a large extent in the characterisation factors due to the large area of ecosystems they cover.

The dependence on the background situation of the receiving environment means that the potential for acidification must be expected to vary with the total emission level and hence in time. To allow assessment of this variation, the characterisation factors are also calculated for the predicted emission levels for 2010 as shown in Annex 4.1. The factors based on the 1990 emissions are chosen as the default EDIP2003 characterisation factors but the factors for 2010 allow temporal differentiation for those emissions of the product system that will take place in the future (e.g. from the late use stage of long-lived products or from the disposal stage). Compared to the spatially determined variation between countries, the temporal variation within countries, determined in this way, is modest.

What do the impacts express?
The site-generic as well as the site-dependent EDIP2003 acidification potentials of an emission from a functional unit are expressed as the area of ecosystem within the full deposition area which is brought to exceed the critical load of acidification as a consequence of the emission (area of unprotected ecosystem = m² UES/f.u.).

In comparison, the EDIP97 acidification potential is expressed as the emission of SO₂ that would lead to the same potential release of protons in the environment (g SO₂-eq/f.u.).

4.5 Site-generic characterisation

The site-generic acidification factors are established as the European average over the 15 EU member countries in EU15 plus Switzerland and Norway, weighted by the national emissions in Table 4.1.

The site-generic acidifying impact of a product can be calculated according to the following formula:

(4.1)

Where: sg-EP(ac) = The site-generic acidification impact, or area of ecosystem that becomes unprotected by the emission from the product system (in 0.01 m²/f.u.).

sg-CF(ac)_s = The site-generic characterisation factor for acidification from Table 4.1 that relates emission of substance (s) to the acidifying impact on its site-generic deposition area (in 0.01 m²/g).

E_s = The emission of substance (s) (in g/f.u).

The spatially determined variation which potentially lies hidden within the site-generic acidification impact, can be estimated from the standard deviation given in Table 4.1 for each substance.

Table 4.1. Equivalency factors for site-generic, and for site-dependent characterisation (in 0.01 m² unprotected ecosystem/g)

*Phosphate will normally bind to the soil matrix and then phosphoric acid will not contribute to acidification

**The unit of sd-CF(ac)i(H⁺) in Annex 4.1 is m²/g, whereas the unit for the factors of the other substances is 0.01 m²/g

4.6 Site-dependent characterisation

The acidifying impact from a product system is often determined by one or a few processes. To avoid unnecessary work, applications where a site-dependent assessment is desired, may therefore start with calculation of the site-generic acidifying impact of the product as described in the previous section. This site-generic impact can be used to select the processes with the dominating contributions (step 1), and then to adjust their site-generic impacts with the relevant site-dependent acidification factors (step 2 and 3). This procedure can be seen as a sensitivity analysis-based reduction of those uncertainties in the site-generic impact which are caused by refraining from site-dependent characterisation.

Step 1
The site-generic acidifying impact of a product, as calculated in the previous section, is broken down into the contributions from the separate processes. These contributions are then ranked from the largest to the smallest contribution, and the process with the largest acidifying contribution is selected.

Step 2
The site-generic acidifying impact of the product calculated in step 1 is reduced with the contribution of the process selected in step 1. Next, the site-dependent impact from the emissions of this process is calculated with the relevant site-dependent acidification factors in Annex 4.1.

(4.2)

Where: sd-EP(ac)_p= The site-dependent acidifying impact or area of ecosystem that becomes unprotected by the selected process (p) (in m²/f.u.). sd-CF(ac)_s,i= The site-dependent characterisation factor for acidification from Annex 4.1 (default 1990 factors) that relates the emission of substance (s) in country or region (i) where the selected process (p) is located to the acidifying impact on its deposition area (in m²/g). Emissions from an unknown region or from non-European regions can as a first approach be represented by the site-generic factors.

E_s,p = The emission of substance (s) from the selected process (p) (in g/f.u).

The geographic region in which the emissions take place determines the relevant factors. The impact of emissions from unknown but probably European regions should be calculated with the site-generic acidification factors. The information about the spatial variation in these factors (see Table 4.1) should be taken into account in the next step. As a first approach, also the emissions from a non-European or unknown region can be calculated with the site-generic acidification factors from Table 4.1. The standard deviations in Table 4.1 give a range of potential spatial variation for the application of the site-generic factor within Europe. Given the size of the variation in emissions and sensitivities within Europe, the site-dependent factor is expected to lie within this range for most regions also in the rest of the world. Expert judgement may be used in the interpretation to assess whether the factor for emissions from processes in non-European regions should be found in the lower or upper end of the range.

Step 3
The site-dependent contributions from the process selected in step 1 are added to the adjusted site-generic contribution from step 2. Step 2 is repeated until the site-dependent contribution from the selected processes is so large that the residual spatially determined variation in the acidification score can no longer influence the conclusion of the study (e.g. when the site-dependent share is larger than 95% of the total impact score).

4.7 Normalisation

The EDIP2003 person equivalent for acidification is 2.2?10³ m2/pers/yr.

Following the EDIP97 approach, the normalisation reference for acidification is based on the impact caused by the actual emission levels for 1990 (see Hauschild and Wenzel, 1998c and Stranddorf et al., 2005). Applying the EDIP2003 characterisation factors for acidification, the total area of unprotected ecosystem in Europe is 82?106 ha or 82?1010 m². The person equivalent is calculated as an average European impact per person assuming a total European population of 3.70?108 persons.

4.8 Interpretation

The EDIP2003 acidification impact potentials are improved in two aspects compared to the impact potentials calculated using the EDIP97 characterisation factors; the environmental relevance is increased, and spatial variation in the sensitivity of the receiving environment can be taken into account.

Environmental relevance
The environmental relevance is increased because the exposure of the sensitive parts of the environment as well as the variation in sensitivity of these ecosystems are included in the underlying model, which now covers most of the causality chain towards the protection area: Ecosystem health. This is particularly important because it increases consistency with weighting factors based on the environmental relevance. The EDIP default weighting factors for acidification are based on political reduction targets. These targets are also aimed partly at protecting ecosystem health. In comparison, the EDIP97 factors only cover the potential for release of protons.

Being defined so early in the cause-effect chain, the EDIP97 impacts in principle do not exclude any damage caused by the like the damage to man-made materials. For the EDIP2003 characterisation factors, damage to natural ecosystems is chosen as the most sensitive end point (and as the end point that current regulation is focused on), and therefore damage to man-made materials is not explicitly addressed by these factors (although it will at least partly be represented). If there should be a wish to explicitly include acidification damage to man-made materials, these must thus be calculated separately using e.g. the EDIP97 factors.

Spatial variation
The spatial variation in sensitivity to exposure for acidification is large due to differences in background exposure of ecosystems and their natural resilience to acidifying impacts. The variation in sensitivity between European regions shows a factor 103 of difference between least and the most sensitive emission countries when expressed on a national scale. This variation is hidden when the EDIP97 characterisation factors or similar site-generic factors are used for characterisation.

4.9 Example

Applying the EDIP2003 factors, characterisation is performed on the inventory presented in Section 1.6.

Site-generic characterisation
As described in Section 4.5, first, the site-generic impacts are calculated. The acidification impacts shown in Table 4.2 are determined using the site-generic factors from Table 4.1.

Table 4.2. Site-generic acidification impacts for one supporting block made from plastic or zinc expressed as area of unprotected ecosystem (UES) per functional unit.

Using site-generic characterisation factors, the largest acidification impacts are found to be caused by the zinc supporting block. However, the potential spatial variation is so large (as revealed by the spatially determined standard deviation) that the conclusion is highly uncertain. Therefore, a site-dependent characterisation is performed for those processes which contribute most to the site-generic acidification impacts in order to reduce the spatially determined uncertainty and strengthen the conclusion.

Site-dependent characterisation
Table 4.2 shows that the predominant contributions to the site-generic acidification impact are caused by emissions of SO₂ and NO_x. For the zinc component, the main sources for both substances are identified as the production of zinc from ore which takes place in Bulgaria, the casting of the component which takes place in Yugoslavia, and that part of the transport of the component, which takes place by truck through Germany (data not shown). For the plastic component, the main sources for both SO₂ and NO_x are found to be the production of plastic polymer in Italy, the flow injection moulding of the supporting block in Denmark and the transportation of the component by truck, mainly through Germany (idem). The emissions from these processes contribute between 91 and 99% the full site-generic impacts of Table 4.2 (data not shown).

In the calculation of the site-dependent impacts for these key processes, the relevant factors from Annex 4.1 are applied. The results are shown in Table 4.3.

Table 4.3. Site-dependent acidification impacts for key processes from either product system.

The site-generic impacts from the key processes are subtracted from the original site-generic impacts in Table 4.2 and the site-dependent impacts from the key processes calculated in Table 4.3 are added. The acidification impacts thus corrected are found in Table 4.4, and the difference to the original site-generic impacts of Table 4.2 is illustrated in Figure 4.2.

Table 4.4. Acidification impacts from either product system with site-dependent characterisation of key process emissions

Around 95% of the resulting impact is calculated using site-dependent characterisation factors for both the zinc-based and the plastic-based component. Even if the site-dependent characterisation was performed for all the remaining processes in the product system, the result can thus not change significantly, given their modest share in the total and the standard deviation. The spatially conditioned potential for variation of the impact has largely been cancelled.

As seen from Figure 4.2, the inclusion of spatial differentiation at the level of country of emission reverses the dominance. When the major part of the spatial variation in the dispersion patterns and sensitivity of the exposed environment is removed, the acidification impact from the plastic component is larger than the acidification impact from the zinc component.

Figure 4.2 Site-generic and site-dependent acidification impacts from the two product systems. For the site-dependent impacts, the site-dependent characterisation factors have only been applied for the key processes as described above.

Annex 4.1: Site-dependent characterisation factors for acidification

(*) The mean and standard deviations relate to E15+Norway+Switzerland and are for nitrogen and sulphur weighed with the national emissions of these countries

Footnotes

1 RAINS is an integrated assessment model that combines information on national emission levels with information on long range atmospheric transport in order to estimate patterns of deposition and concentration for comparison with critical loads and thresholds for acidification, terrestrial eutrophication-via-air and tropospheric ozone formation.

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Version 1.0 january 2006, © Danish Environmental Protection Agency