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Guide to Critical Loads and Critical Levels

Critical Loads and Levels are generally defined as: “ a quantitative estimate of 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” (Nilsson & Grennfelt, 1988). It is important to distinguish between a critical load and a critical level. The critical load relates to the quantity of pollutant deposited from air to the ground, whereas the critical level is the gaseous concentration of a pollutant in the air. For terrestrial ecosystems typical biological criteria based on no adverse effect on growth, soil stability, and groundwater quality have been used, whilst for surface waters, stream and lake fauna have been used.

Critical Loads:
The threshold level for the deposition of a pollutant above which harmful indirect effects can be shown on a habitat or species, according to current knowledge. Additional deposition above the Critical Load is termed Critical Load Exceedance. Pollutants concerned with critical loads in the APIS database are:

• Nitrogen deposition (kg Nitrogen ha^-1 year^-1)
• Acid deposition (keq ha^-1 yr^-1)

Critical Levels:
The threshold level for the atmospheric concentration of a pollutant above which harmful direct effects can be shown on a habitat or species, according to current knowledge. Pollutant air concentrations above the Critical Level are termed Critical Level Exceedances. Pollutants that are concerned with critical levels in the APIS database are:

• Ammonia (µg m^-3)
• Sulphur Dioxide (µg m^-3)
• Nitrogen Oxides (µg m^-3)
• Ozone (ppb hours)

There are currently no critical loads or levels for Halogens, Heavy Metals, POPs, VOCs or Dusts.

Critical Load Mapping
There are three approaches generally used in the calculation and mapping of critical loads. The simplest level, Level 0, involves using existing data to classify ecosystems into different levels of sensitivity. This type of approach is common when calculating critical loads over large areas for which there is sparse data. A more complex and data intensive method is to use a mass balance approach in which inputs of acidity are balanced against ecosystem sinks and outputs. This approach has been used for selected woodland ecosystems. Finally in order to consider temporal aspects of critical loads exceedance and recovery, the use of dynamic models has been used in level 2 assessments. Most typically these are undertaken on a site specific basis. Ranges rather than fixed values are used to allow for natural variation, uncertainties about deposition values and temporal variability of available data.

Uncertainties?
Despite the usefulness of critical loads and levels, the concept is still a relatively new approach in environmental science and therefore contains assumptions, parameters and data which need wider testing to validate the methods of assessment. It is important to understand that different sources of information are used to set different critical load or critical level values. For SO₂, the values are primarily derived from field observations, while for O₃, the primary source is exposure response studies in field chambers, and for NO_x and NH₃ the primary source is experimental studies. The empirical critical loads of total N deposition for vegetation were derived primarily from field experiments and observations. It is important to note that there is uncertainty attached to all these values, which has implications for interpretation of data showing areas where they are exceeded.

In addition to noting the limitations to the methods of calculating critical loads it is important to flag some of the assumptions made in mapping critical loads for both freshwater and terrestrial ecosystems. For example, in the soils work no account is taken of the soil variability within each 1km square, as the classes were assigned for the dominant soil type present. For freshwaters each 10x 10 km grid square is represented by a single spot sample from one water body within the square. Whilst a screening procedure to identify the most sensitive water body in the square was used it is highly unlikely that the results of a single sample represent the spatial and temporal variation in water chemistry (and hence critical load) within the square. Similarly in calculating exceedance spatial (and temporal) variation in deposition inputs can considerably change the estimates of ecosystems potentially at risk of damage. Currently deposition is calculated on a 5 x 5 km grid and a mean value for the square is given. In reality the most sensitive ecosystems are in upland areas where altitude will vary considerably leading to significant changes in pollutant deposition across these grid squares. These gradients in deposition are currently masked and not accounted for in exceedance maps. All of these simplifying procedures have direct implications on the use and interpretation of the maps particular at small scales which are often the focus of concern for nature conservation.

To improve the utility of critical loads and their exceedance there is a growing need to build on our knowledge of the dose-response relationships for different species. Secondly there is a need to consider the growing role of nitrogen as sulphur emissions decline. Currently the co-effects of N deposition with sulphur and the moderating effects of base cation deposition are not taken into account. In dealing with nitrogen alone there are several important gaps in our knowledge, the most important of which suggested by Bobbink and Roelofs (1995b) are the: effects of enhanced N deposition on soil fauna, effects on forest floor vegetation and insufficient knowledge of the different effects of NO_x and NH₃ to produce separate critical loads.

Using critical loads/levels to assess risk at the ‘site level’

APIS provides a Simple Site-Based Assessment tool which can be used to assist the user in obtaining a broad indication of the likely pollutant impact at a specific location (Search by Location).

The Simple Site-Based Assessment provides a quick tool to screen-out queries where there is clearly little risk of air pollution impact on a habitat or species at a specified location. Where this method suggests likely significant pollutant impact, a detailed site-based assessment should be conducted.

The Simple Site-Based Assessment is based on national maps of air pollutant exposure and Critical Loads or Critical Levels. The maps are those documented in the NEGTAP report (www.edinburgh.ceh.ac.uk/negtap). In this section, atmospheric deposition is compared with the most relevant critical loads, while air concentrations are compared with critical levels. In many cases the Critical Loads and Levels applied do not vary spatially, but are linked to a specific habitat type. This process of using nationally available mapped data and habitat specific values is subject to a series of uncertainties. These include:

• Maps of pollutant air concentrations and deposition are generated by a combination of models and measurements (see www.edinburgh.ceh.ac.uk/negtap). If your queried location is close to known large emission sources, then this tool should be used with caution as it may underestimate deposition or concentrations.
• Maps of pollutant concentration and deposition are mostly available at a 5 km grid resolution. The exception to this is for ozone exposure which is mapped at a 1 km grid resolution. For many pollutants there is real sub-grid variability which is not revealed in the 1 km or 5 km averages. The uncertainties are particularly large for the concentrations of primary pollutants e.g. NH₃ , NO_x and SO₂.
• The critical loads data for acidity are linked to mapped soils data. The critical load is based on the dominant soil type in a 1km grid square may not represent small areas of a square which may be more sensitive.

Full details on modelling and mapping critical load (and levels) can be found by visiting the UK National Focal Centre: www.critloads.ceh.ac.uk

References:
Nilsson J. and Grennfelt P. (Eds) (1988): Critical Loads for Sulphur and Nitrogen. Miljorapport 1988:15. Nordic Council of Ministers, Copenhagen.

Bobbink R. and Roelofs J.G.M. (1995b): Nitrogen critical loads for natural and semi-natural ecosystems: the empirical approach. Water, Air and Soil Pollution 85, 2413-2418.

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