ENVIRONMENTAL POLICY ANALYSIS
AIR The Use of Critical Loads in Environmental Policy Making: A Critical Appraisal R I C H A R D A. S K E F F I N G T O N National Power, Plc Windmill Hill Business Park Whitehill Way, Swindon Wilts SN5 6PB UK
Critical loads are extensively used as scientific underpinning for air pollution control policies in Europe. They are intended to be thresholds below which pollutant deposition does not damage ecosystems. European policy makers have proposed that critical loads for acid deposition should not be exceeded anywhere in Europe by the year 2015. The way in which critical loads are currently being used in developing policy is outlined here, and the theoretical soundness of critical load calculation is examined, using the Steady-State Mass Balance Method for estimation of the critical load for acid deposition on soils as an example. The formidable difficulties and uncertainties in critical load calculation tend to be glossed over in dialogue with policy makers, who have developed misconceptions as a result. Critical loads are best viewed as highly uncertain estimates of relative risk, which themselves incorporate political choices, rather than precise damage thresholds determined by an objective, scientific process. Pollution-control science should go beyond critical loads to the prediction and communication of pollution control policies' effects on organisms and ecosystems.
© 1999 American Chemical Society
The Critical Load (CL) Concept is now widely used in developing emission control policies in Europe. The United Nations Economic Commission for Europe (UNECE) has made it the basis for decision making underlying the control of transboundary air pollution; it h a s used it to develop legislation on controlling S0 2 , NOx, and NH 3 emissions; and it is considering it as a scientific basis for control of heavy metals. The European Union (EU) is using the CL concept to develop strategies for the control of acidification and 0 3 —indeed, the EU's Fifth Environmental Action Programme states that the longterm objective of its policy on acidifying emissions is "no exceedance anywhere of CLs," and the European Parliament has demanded that this objective be attained by the year 2015 (i). The United States, although a member of UNECE, has not made significant use of CLs in policy development. After a brief description of CLs and how they are being used in European policy making, one of the major methods of CL calculation is evaluated in some detail, and simulations of a model ecosystem are used to attempt to clarify their ecological meaning and to consider what reliance can be put on the results. What Is a Critical Load? The formal definition of a CL is "a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge" (2). The underlying notion is that of a threshold. If deposition is below the threshold, there are no problems. If it is above the threshold, harm is being done to the environment. The CL is also intended to represent the long-term capacity of ecosystems to absorb pollutants. This usually involves the assumption that ecosystems (at the CL) are in a steady state. It is vital to appreciate this if CLs and their limitations are to be understood. Use of CLs in Europe The use of CLs in developing policies for the control of acidifying pollutants in Europe has become quite complex and sophisticated and is continually evolving. This brief description can only introduce the principles; see references (2-5) for more detailed accounts. The process was established under the UNECE Convention on Long-Range Transboundary Air Pollution. Most European countries have a "National Focal Centre" that calculates and evaluates CL data and submits them to the Co-ordination Centre for Effects (CCE) at the Rijksinstituut voor Volksgezandheid en Milieuhygiene (RIVM) in The Netherlands (3, 4). Much national CL data are produced on a 1-km grid: CCE is responsible inter alia JUNE 1, 1999/ENVIRONMENTAL SCIENCE & TECHNOLOGY/ NEWS « 2 4 5 A
FIGURE 1
The critical loads approach in Europe (a) Critical loads data for Europe as of August 1998. The quantity plotted is the 5th percentile CL for acidity due to sulfur (S) deposition in the absence of nitrogen (N) deposition (CLmax(S)), expressed in eq ha -1 yr 1 . Effectively, this is the CLfor acid deposition that will "protect" 95% of ecosystems in each grid square. Note the generally high CLs in the limestone-dominated areas of southern Europe; low or very low CLs in the base-poor soils of Fenno-Scandia, and Scotland; and the influence of specific geological situations on the values of individual squares, for example, areas of base-poor sands and sandstones in eastern England and northern Germany. Data reproduced by permission of the Secretariat of the UNECE Convention on Long-RangeTransboundary Air Pollution, Geneva. (b) Modeled deposition (dry and wet) of oxidized, non-sea salt Sin 1990. This is an important year as it is being used as a base case in current negotiations. The scale and units are the same as in Figure 1a. Deposition follows the distribution of S02 emissions modified by the prevailing westerly wind, with a peak in the "Black Triangle" of Central Europe. Based on of the EMEP Model developed atthe Norwegian Meteorological Institute, Oslo, as implemented in RAINS 7.2, International Institute for Applied Systems Analysis, Laxenburg, Austria. (c) Degree of exceedance of CLs for acid deposition in 1990 by S deposition alone, as calculated by RAINS 7.2. This is simply the difference between Figures 1a and lb—the CL is not exceeded in the white areas. The pollcy aim is to reduce this exceedance, preferably to zero. In some areas, notably Central Europe, deposition must be reduced very considerably to achieve this. The realworld implementation of emission control in Europe takes into account reduced and oxidized N deposition and inputs the CLs in the form of ecosystem protection isolines (see text), but the principle is the same as illustrated here.
for producing CL maps on the "EMEP" (European Monitoring and Evaluation Programme) grid of approximately 150 x 150 km, which is used for pollutant dispersion modeling in Europe. This scaling involves some difficult choices. Both sulfur (S) and nitrogen (N) contribute to acidification; it is thus not possible to define a unique CL for either. Instead, a "critical load function" is generated for each ecosystem, specifying the different combinations of S and N deposition, which just meet the CL; S and N deposition are not equivalent because there are ecosystem processes such as denitriflcation, which can specifically neutralize N deposition. Large areas such 3.S EMEP grid squares will have many ecosystems and a corresponding number of critical load functions than 50 000 in some cases {4) The critics! load functions are weighted by ecosystem area and ranked to form a set of cumulative distribution functions at various Dercentiles To avoid outliers and small areas of verv sensitive ecosystems having a disproportionate effect the cumulative distribution function representing a low percentile usually 5% is used for further work This is known as an "ecosystem protection isoline " The ultimate aim then of policv is to reduce S and/or N deposition to reach the isoline in every grid square in the most cost effective manner 2 4 6 A • JUNE 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
Currently, acid deposition exceeds CLs over much of Europe (see Figure 1 above): Varrous policy measures are therefore aimed at reducing deposition. In several grid squares, the calculated CL is close to the amount of "background" S and N deposition from natural sources, and in others, the CL will not be attained even with the maximum, technically feasible, end-of-pipe emission reduction measures over the whole of Europe (6). Policy development has thus concentrated on reducing the gap between deposition and CLs in as cost-effective a fashion as possible, normally with the aid of integrated assessment models (7). This "gap closure" is an example of a target load method, in which the CL is replaced bv a target selected by using political criteria, such as attainability or cost Even gap closure is expensive; it was estimated in 1997 (6) that it would cost the EU countries $40 billion annually for a 50% gap closure to reduce the area of CL exceedance in each EMEP grid square by 50% relative to 1990 by the year This brief description will doubtless raise many questions in readers' minds, but the main purpose of this paper is to consider the validity of the CL calculation. Figure 1(a) is a CL plot derived from more than 1.3 million values—how have they been calculated, and what is their ecological meaning?
CL calculation example To demonstrate how CLs are calculated, I describe here the Steady-State Mass Balance (SSMB) Method for the CL of sulfur deposition to soils. The SSMB has the largest influence on mapped CLs for acid deposition (5); other current methods are very similar in concept (3). The current practice of this method is described in references (3-5) and the scientific basis of the method in references {8-10). The aim of the SSMB method is to balance all the significant long-term sources of acidity against all the significant long-term sinks of acidity or sources of alkalinity, which amounts to the same thing. Calculation of the mass balance for acidity requires consideration of mass balances for S species, nitrogen species, and base cations. The mass balance for S is ''le
=
^dep ~~ ^ad ~~ ^u ~ S ( - S r e - S p r ,
(1)
where S is the flux of sulfur and the subscripts represent: le (leaching); dep (deposition); ad (adsorption—typically onto sesquioxides or similar soil minerals); u (uptake—into plants); i (immobilization— into soil organic matter); re (reduction); and pr (precipitation—as a chemical compound). Uptake, immobilization, precipitation, and reduction of S are considered insignificant in forest soils. Some of this could be challenged: For instance, S uptake into plants is perhaps 10% of base cation uptake; the soil organic S pool is normally quite large, and fluxes into and out of it are not well known; soils that are sufficiently anaerobic to reduce S are common in the landscape as a whole (11). Sulfate adsorption is an admittedly significant process, but because soils are considered to have a finite capacity, it too is ignored, since the system is assumed to be in steady state at the CL. All S leaching is assumed to be in the form of S0 4 2 —hence the S mass balance reduces simply to S0 4
le
= S dep .
(2)
This illustrates a consequence of the steadystate assumption; the S mass balance does not necessarily represent the behavior of catchments now. It is intended as a worst case, and in the short term, S0 4 2 ^ leaching could be much less than deposition, as it is in the Southern Appalachians of the United States, for instance (12). The mass balance for N is N d e p + Nflx = Nj + N u + N d e + N a d + N flre + N e r o s + N vol + N le ,
(3)
where N is the flux of nitrogen species, and the suffixes represent: dep (atmospheric deposition of combined N); fix (N fixation—reduction of atmospheric N 2 to NH 3 and incorporation into other N species); i (immobilization into soil organic matter); u (uptake by plants into perennial tissues—the N required for incremental growth); de (denitrification—
reduction of N0 3 ~ to N 2 or N 2 0, and release to the atmosphere); ad (adsorption—typically of NH 4 + onto clays); fire (N loss during combustion—in accidental or deliberate fires); eros (erosion—probably of particulate N); vol (volatilization—of NH 3 , from alkaline soils); and le (leaching—of N0 3 ", NH 4 + , and dissolved organic N). Space precludes a detailed discussion of the individual N sinks, which can be found in references (13-15), but usually only N d e p , N„ N u , Nle, and N d e play a significant part in calculations. The mass balance for base cations is BCle + BCU = BC dep + BCW ,
(4)
where BC is the flux of base cations (Na+ + K+ + Ca2+ + Mg2"1"). The suffixes represent: le (leaching); u (uptake by plants into perennial tissues); dep (deposition); and w (weathering—i.e., release of base cations from soil minerals or rock minerals)—all expressed in equivalence units, for example, keq h a 1 yr"1. The complete mass balance at steady state is thus N
de P + S dep + BCle + BCU = BCW + BC dep + N, + N u + N d e + S0 4 2 " l e + N le .
(5)
To calculate a CL for soils, the leaching terms have to be constrained. An infinite CL would result if the soil were allowed to export any excess acidity into groundwater or surface water. This constraint is achieved through setting a limit on the acid neutralizing capacity (ANC) that the soil can export. From the definition of ANC (16), it can be expressed as ANCl0 = BCle + NH 4 + le - S0 4 2 " l e - N0 3 " l e - Cl"le ,(6) where ANCle is the flux of acid neutralizing capacity in leachate, expressed in equivalence units, for example, keq ha - 1 yr"1. It is further assumed that chloride inputs equal chloride outputs and that all N leaching from terrestrial catchments is in the form of N0 3 " (this may need some re-evaluation as recent work (e.g., 17) has highlighted the importance of dissolved organic N in catchment outputs). Combining equations (5) and (6) then generates ^dep + N d e p - BC dep + CI d e p = BCW - BCU + N; + N u + N d e - ANCle .
(7)
This is the steady-state mass balance equation or simple mass balance equation, so called because it is recognized that there are a number of simplifying assumptions (5). It is customary to use "nonmarine" deposition fluxes in these calculations (symbolized by *). This makes little difference to equation (7), as marine (S dep + Cr d e p ) ~ marine BC dep . CLs for S and N can be derived from this equation, given that values are assigned to all the other parameters, and an "acceptable" value of ANC,e is defined. JUNE 1, 1999/ ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS » 2 4 7 A
There are several ways of calculating an acceptable ANCle, but by far the most widely used is the critical base cation/aluminum ratio. This states that the molar ratio of (K+ + Ca2+ + Mg2*) to Al in the soil solution must exceed a certain value (usually 1.0). This criterion is derived from work on toxicity of Al3+ to tree roots, showing that Ca2+ (and with rather less certainty, Mg2+ and K+) exerts some protection against Al toxicity {18). Sodium is considered to offer no protection, with the three protective base cations (K+ + Ca2+ + Mg2*) being abbreviated to Be. This ratio has attracted criticism on the grounds that laboratory experiments with seedlings should be extrapolated very cautiously to mature plants in the field (e.g., 9); that it ignores basic plant physiology [19); that surveys have shown no relationship between the ratio and forest vitality; and that it takes no account of nutrient uptake from the humus layer where Al is likely to be complexed and be nontoxic (9). Nevertheless, Cronan and Grigal {20) concluded that it was reasonable to use the Ca2+/Al3+ ratio as a risk parameter, with a ratio below 1 indicating a 50% risk of adverse impacts on tree growth and nutrition. Another (equivalent) formulation of ANC is used to calculate the critical ANC leaching: ANCle = - Alle - Hle = -Q x ([Alle] + [Hle]) , (8) where quantities in square brackets are concentrations as opposed to fluxes; Alle and Hle are the leaching of ionized monomeric aluminum and H+, respectively, expressed in equivalence units, and Q is the effective precipitation (precipitation-evaporation). Base cation concentrations are calculated from the base cation mass balance, and application of the required Bc/Al ratio gives the Al term in equation (8) directly. The H+ term has to be calculated by assuming equilibrium with aluminium hydroxide (usually called "gibbsite"). Reference (5) gives different apparent equilibrium constants depending on the organic matter content of the soil. Many authors consider that the activity of Al in the surface horizons of acidic soils is not generally determined by equilibrium witii aluminum hydroxide (e.g., 21), and gibbsite itself is a rare mineral in temperate soils, but gibbsite equilibrium is the only relationship currently available for modeling purposes. Assuming that the N sink terms in equation (7) can account for all N deposition, the CL for S deposition alone (known as CL^^S) is where Kgibb is the gibbsite equilibrium constant. ^^max l^v
=
"*- dep — ^ ' dep
+
BCW-BCU+(1.5X + O^y ni;y
gC
BcdeD + Bc w - Bcu
^(BcMU
dep + Bcw -
)
Bc
u
)I/3
The SSMB equation thus provides an apparently objective method of estimating CLs with a clear theoretical foundation. When applying it in practice, 2 4 8 A • JUNE 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
however, various problems occur and decisions have to be made. These are discussed below. Are CLs thresholds? Critical loads are presented as thresholds (3,4). If deposition is below the threshold, there are no problems; if it is above the threshold, harm is being done to the environment (at least for long-term problems at steady state). Specifying thresholds for the natural environment is very problematic, however, for several reasons. The first is that sharp thresholds are rather rare. The natural environment is characterized by variation in many dimensions. Variation in pollutant response exists • among species, • among individuals in their response to pollutants, • due to interactions with physical conditions (e.g., a species under stress because it is at the extreme of its climatic range may be more sensitive to pollutants), • due to changes in physical conditions with time, • due to competition between species, and • due to human activity. Then there is random variation. All such variation implies that living organisms rarely show a sharply defined response at certain pollutant concentrations or depositions. In the field, the response always occurs over a range, sometimes narrow, but often rather wide. Such dose-response relationships are probably easier to establish for aquatic than for terrestrial organisms. For example, the experimental evidence that underlies the choice of a Bc/Al ratio of 1 shows a response over 4 orders of magnitude—from 0.01 to 100 (8). Perhaps more relevant for determining a threshold, the lowest Bc/Al ratio with no growth reduction was 0.6, and the highest with some growth reduction was 20. This range would produce a huge range in CLs if fed into equation (9). The second reason follows from the difficulty of dealing with diversity and variability. Any damage threshold will depend on which species is chosen for protection and how damage is defined. Applying a uniform criterion, such as a Bc/Al ratio of 1.0, effectively imposes a uniform environmental quality standard on soils. It implies that soil acidification is acceptable as long as Norway spruce, for which Bc/Al = 1 is appropriate, can continue to grow at a reasonable rate. In some areas, this will be an undemanding target, and attainment of the standard would allow loss of more sensitive species that currently exist or have existed in the past. In other areas, the quality standard may represent conditions that may never have existed, such as soils with a Bc/Al ratio
