Annex I Critical loads and levels

Critical loads (as defined in article 1) of acidity for ecosystems are determined in accordance with the Convention's Manual on Methodologies and Criteria for Modelling and Mapping Critical Loads and Levels and Air Pollution Effects, Risks and Trends. They are the maximum amount of acidifying deposition an ecosystem can tolerate in the long term without being damaged. Critical loads of acidity in terms of nitrogen take account of within-ecosystem nitrogen removal processes (e.g. uptake by plants). Critical loads of acidity in terms of sulphur are loads that – in the long term – will not cause adverse effects to the structure and functions of ecosystems. A combined sulphur and nitrogen critical load of acidity considers nitrogen only when the nitrogen deposition is greater than the ecosystem nitrogen removal processes, such as uptake by vegetation. All critical loads reported by Parties, and approved by the Executive Body, are summarized for use in the integrated assessment modelling employed to provide guidance for setting the emission reduction commitments in annex II.

In Canada, critical acid deposition loads and geographical areas where they are exceeded are determined and mapped for lakes and upland forest ecosystems using scientific methodologies and criteria similar to those in the Convention's Manual on Methodologies and Criteria for Modelling and Mapping Critical Loads and Levels and Air Pollution Effects, Risks and Trends. Critical load values for total sulphur plus nitrogen and exceedance levels have been mapped across Canada (south of 60° N latitude) and are expressed in acid equivalents per hectare per year (eq/ha/yr) (2004 Canadian Acid Deposition Science Assessment; 2008 Canadian Council of Ministers of the Environment). The province of Alberta has also adapted the generic critical load classification systems used for soils in Europe for potential acidity to define soils as highly sensitive, moderately sensitive and not sensitive to acidic deposition. Critical, target and monitoring loads are defined for each soil class and management actions are prescribed as per the Alberta Acid Deposition Management Framework, as appropriate.

These loads and effects are used in integrated assessment activities, including providing data for international efforts to assess ecosystem response to loading of acidifying compounds, and provide guidance for setting the emission reduction commitments for Canada in annex II.

For the United States of America, the effects of acidification are evaluated through an assessment of the sensitivity and response of ecosystems to the loading of acidifying compounds, using peer-reviewed scientific methodologies and criteria, and accounting for the uncertainties associated with nitrogen cycling processes within ecosystems. Adverse impacts on vegetation and ecosystems are then considered in establishing secondary national ambient air quality standards for NO_x and SO₂. Integrated assessment modelling and the air quality standards are used in providing guidance for setting the emission reduction commitments for the United States of America in annex II.

Critical loads (as defined in article 1) of nutrient nitrogen (eutrophication) for ecosystems are determined in accordance with the Convention's Manual on Methodologies and Criteria for Modelling and Mapping Critical Loads and Levels and Air Pollution Effects, Risks and Trends. They are the maximum amount of eutrophying nitrogen deposition that – in the long term – will not cause adverse effects to the structure and functions of ecosystems. All critical loads reported by Parties are summarized for use in the integrated assessment modelling employed to provide guidance for setting the emission reduction commitments in annex II.

For the United States of America, the effects of nutrient nitrogen (eutrophication) for ecosystems are evaluated through an assessment of the sensitivity and response of ecosystems to the loading of nitrogen compounds, using peer-reviewed scientific methodologies and criteria, and accounting for uncertainties associated with nitrogen cycling within ecosystems. Adverse impacts on vegetation and ecosystems are then considered in establishing secondary national ambient air quality standards for NO_x. Integrated assessment modelling and the air quality standards are used in providing guidance for setting the emission reduction commitments for the United States of America in annex II.

Critical levels (as defined in article 1) of ozone are determined to protect plants in accordance with the Convention’s Manual on Methodologies and Criteria for Modelling and Mapping Critical Loads and Levels and Air Pollution Effects, Risks and Trends. They are expressed in terms of the cumulative value of either stomatal fluxes or concentrations at the top of the canopy. Critical levels are preferably based on stomatal fluxes, as these are considered more biologically relevant since they take into account the modifying effect of climate, soil and plant factors on the uptake of ozone by vegetation.

Critical levels of ozone have been derived for a number of species of crops, (semi-)natural vegetation and forest trees. The critical levels selected are related to the most important environmental effects, e.g., loss of security of food supplies, loss of carbon storage in the living biomass of trees and additional adverse effects on forest and (semi-)natural ecosystems.

The critical level of ozone for human health is determined in accordance with the World Health Organization (WHO) air quality guidelines to protect human health from a wide range of health effects, including increased risk of premature death and morbidity.

For Canada, it is understood that there is no lower threshold for human health effects from ozone. That is, adverse effects have been observed at all ozone concentrations experienced in Canada. The Canadian Ambient Air Quality Standard for ozone was set to aid management efforts nationally, and by jurisdictions, to significantly reduce the effects on human health and the environment.

For the United States of America, critical levels are established in the form of primary and secondary national ambient air quality standards for ozone in order to protect public health with an adequate margin of safety and to protect public welfare, including vegetation, from any known or expected adverse effects. Integrated assessment modelling and the air quality standards are used in providing guidance for setting the emission reduction commitments for the United States of America in annex II.

The critical level of PM for human health is determined in accordance with the WHO air quality guidelines as the mass concentration of PM_2.5. Attainment of the guideline level is expected to effectively reduce health risks. The long-term PM_2.5 concentration, expressed as an annual average, is proportional to the risk to health, including reduction of life expectancy. This indicator is used in integrated modelling to provide guidance for emission reduction. In addition to the annual guideline level, a short-term (24-hour mean) guideline level is defined to protect against peaks of pollution which have significant impact on morbidity or mortality.

For Canada, it is understood that there is no lower threshold for human health effects from PM. That is, adverse effects have been observed at all concentrations of PM experienced in Canada. The Canadian national standard for PM was set to aid management efforts nationally, and by jurisdictions, to significantly reduce the effects on human health and the environment.

For the United States of America, critical levels are established in the form of primary and secondary national ambient air quality standards for PM in order to protect public health with an adequate margin of safety, and to protect public welfare (including visibility and man-made materials) from any known or expected adverse effects. Integrated assessment modelling and the air quality standards are used in providing guidance for setting the emission reduction commitments for the United States of America in annex II.