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Vii. Exceedance Calculations DRAFT DOCUMENT (February 2015) Prepared by M. Posch (CCE) To be reviewed by ICP M&M NFCs VII. EXCEEDANCE CALCULATIONS Updated by Max Posch, CCE, from the initial text of the Mapping Manual 2004 Updated: 16 February 2015 Please refer to this document as: CLRTAP, 2015. Exceedance calculations, Chapter VII of Manual on methodologies and criteria for modelling and mapping critical loads and levels and air pollution effects, risks and trends. UNECE Convention on Long-range Transboundary Air Pollution; accessed on [date of consultation] on Web at www.icpmapping.org .” Updated: 16 February 2015 Chapter VII – Exceedance calculations Page VII - 3 TABLE OF CONTENT VII. EXCEEDANCE CALCULATIONS .............................................................................. 1 VII.1 INTRODUCTION .................................................................................................... 5 VII.2 BASIC DEFINITIONS ............................................................................................. 5 VII.3 CONDITIONAL CRITICAL LOADS OF N AND S ................................................... 6 VII.4 TWO POLLUTANTS .............................................................................................. 7 VII.5 THE AVERAGE ACCUMULATED EXCEEDANCE (AAE) ................................... 10 VII.6 SURFACE WATERS ............................................................................................ 11 VII.6.1 The SSWC model ....................................................................................... 11 VII.6.2 The empirical diatom model ........................................................................ 11 VII.6.3 The FAB model ........................................................................................... 11 VII.7 REFERENCES ..................................................................................................... 11 Updated: 16 February 2015 Chapter VII – Exceedance calculations Page VII - 5 VII.1 INTRODUCTION In this Chapter the calculation of In Section VII.4 the exceedance of critical exceedances, i.e. the comparison of critical loads is defined that involves two pollutants loads/levels with depositions/ simultaneously, in Section VII.5 concentrations, is described. In Section exceedances are regionalised by defining VII.2 the basic definition of an exceedance the Average Accumulated Exceedance is given, including some historical remarks (AAE), and in Section VII.6 the on the origin and use of the word exceedances of surface water critical loads ‘exceedance’. In Section VII.3 the concept are considered. Most of the material of a conditional critical load of S and N is presented here is based on Posch et al. introduced, which allows treating these two (1997, 1999, 2001). acidifying pollutants separately, and thus also makes exceedance calculations straightforward. VII.2 BASIC DEFINITIONS The word 'exceedance' is defined as "the where Xdep is the deposition of pollutant X. amount by which something, especially a In the case of the critical level, the pollutant, exceeds a standard or comparison is with the respective permissible measurement" (The American concentration quantity. If the critical load is Heritage Dictionary of the English greater than or equal to the deposition, one Language, Fourth Edition 2000) and is a says that it is not exceeded or there is non- generally accepted term within the air exceedance of the critical load. pollution discipline. Interestingly, the Oxford An exceedance defined by eq. VII.1 can English Dictionary (OED) database has an obtain positive, negative or zero value. example of 'exceedance' from 1836 Since it is in most cases sufficient to know (Quinion 2004) – 36 years before Robert that there is non-exceedance, without Angus Smith is credited with coining the being interested in the magnitude of non- term 'acid rain' (Smith 1872). However, the exceedance, the exceedance can be also term 'acid rain' (in French) had already defined as: been used in 1845 by Ducros in a scientific (VII.2) journal article (Ducros 1845). Critical loads and levels are derived to Ex (X ) = max { ,0 X − CL (X )} characterise the vulnerability of ecosystem dep dep X − CL (X ) X > CL (X ) (parts, components) in terms of a = dep dep ≤ deposition or concentration. If the critical 0 X dep CL (X ) load of pollutant X at a given location is smaller than the deposition of X at that An example of the application of this basic location, the critical load is said to be equation is the exceedance of the critical exceeded, and the difference is called load of nutrient N (see Chapter V), which is exceedance 1. In mathematical terms, the given by: = − exceedance Ex of the critical load CL (X) is (VII.3) Ex nut (N dep ) N dep CL nut (N) given as: = − (VII.1) Ex (X dep ) X dep CL (X ) 1 When comparing deposition(s) to target load (functions), one does not talk about the exceedance, but the ‘non-achievement’ of a target load. Updated: 16 February 2015 Page VII - 6 Chapter VII – Exceedance calculations It should be noted that exceedances differ The exceedance of a critical load is often fundamentally from critical loads, as misinterpreted as the amount of excess exceedances are time-dependent. One can leaching, i.e. the amount leached above speak of the critical load of X for an the critical/acceptable leaching. This is in ecosystem, but not of the exceedance of it. general not the case as exemplified by the For exceedances the time (or the exceedance of the critical load of nutrient deposition) for which they have been N. The excess leaching due to the calculated has to be reported, since – deposition Ndep , Exle , is given as: especially in integrated assessment – it is (VII.4) Ex (N ) = N − N exceedances due to (past or future) le dep le ,accle anthropogenic depositions that are of Inserting the mass balance of N and the interest. deposition-dependent denitrification one Of course, the time-invariance of critical obtains for the excess leaching (eqs.V.2– loads and levels has its limitations, V.5): certainly when considering a geological Ex (N ) = 1( − f )⋅(N − CL N)( ) (VII.5) le dep de dep nut time frame. But also during shorter time = 1( − f )⋅ Ex (N ) periods, such as decades or centuries, one de nut dep can anticipate changes in the magnitude of which shows that a deposition reduction of critical loads due to global (climate) 1 eq/ha/yr reduces the leaching of N by change, which influences the processes only 1–fde eq/ha/yr. Only in the simplest from which critical loads are derived. An case, in which all terms of the mass early example of a study of the (first-order) balances are independent of depositions, influence of temperature and precipitation does the change in leaching equal the changes on critical loads of acidity and change in deposition. nutrient N in Europe can be found in Posch (2002). VII.3 CONDITIONAL CRITICAL LOADS OF N AND S The non-uniqueness of the critical loads of sulphur deposition Sdep can be derived from S and N acidity makes both their imple- the critical load function. We call it the mentation into integrated assessment conditional critical load of nitrogen , models and their communication to CL (N|S dep ), and it is computed as: decision makers more difficult. However, if (VII.6) one is interested in reductions of only one of the two pollutants, a unique critical load CL N)( if S ≥ CL S)( CL N |( S ) = min dep max can be derived, and thus also a unique dep − α ⋅ < CL max N)( S dep if S dep CL max S)( exceedance according to eq. VII.1 can be calculated. with CL N)( − CL N )( If emission reductions deal with nitrogen (VII.7) α = max min only, a unique critical load of N for a fixed CL max s)( In Figure VII.1a the procedure for calculating CL (N|S dep ) is depicted graphically. Updated: 16 February 2015 Chapter VII – Exceedance calculations Page VII - 7 (a) (b) dep dep S S S1 CL(S|N1) CL(S|N2) S2 Ndep Ndep CL(N|S ) CL(N|S ) N N 1 2 1 2 Figure VII.1: Examples of computing (a) conditional critical loads of N for different S deposition values S1 and S2, and (b) conditional critical loads of S for different N deposition values N1 and N2. In an analogous manner a conditional When using conditional critical loads, the critical load of sulphur , CL (S|N dep ), for a following caveats should be kept in mind: fixed nitrogen deposition Ndep can be (a) A conditional critical load can be computed as: considered a true critical load only when the (VII.8) chosen deposition of the other pollutant is kept constant. 0 if N ≥ CL N)( dep max (b) If the conditional critical loads of CL N)( − N = max dep < < CL |( NS dep ) if CL min N)( N dep CL max N)( both pollutants are considered α CL S)( if N ≤ CL N)( max dep min simultaneously, care has to be exercised. It is not necessary to reduce the exceedances of both, but only one of them to reach non- α where is given by eq. VII.7. The exceedance for both pollutants; procedure for calculating CL (S|N dep ) is recalculating the conditional critical load of depicted graphically in Figure VII.1b. Setting the other pollutant results (in general) in Ndep = CL nut (N), the resulting conditional non-exceedance. However, if S > critical load has been termed minimum dep CL max (S) or Ndep > CL max (N), depositions critical load of sulphur : C Lmin (S) = have to be reduced at least to their CL (S|CL nut (N)). respective maximum critical load values, irrespective of the conditional critical loads. VII.4 TWO POLLUTANTS As shown in Chapter V, there is no unique (current) deposition of N and S. By critical load of N and S in the case of reducing Ndep substantially, one reaches acidity or in the case of a critical load the point Z 1 and thus non-exceedance function derived from multiple criteria without reducing Sdep ; on the other hand related to N and S. Consequently, there is one can reach non-exceedance by only no unique exceedance, although non- reducing Sdep (by a smaller amount) until exceedance is easily defined (as long as its reaching Z 3; finally, with a reduction of both amount is not important).
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