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OPTIONAL INDICATORS FOR MONITORING AIR POLLUTION IMPACTS REFERRED TO IN ARTICLE 9 (a) for freshwater ecosystems: establishing the extent of biological damage, including sensitive receptors (microphytes, macrophytes and diatoms), and loss of fish stock or invertebrates: the key indicator acid neutralising capacity (ANC) and the supporting indicators acidity (pH), dissolved sulphate (SO4), nitrate (NO3) and dissolved organic carbon: frequency of sampling: from yearly (in lake autumn turnover) to monthly (streams).

(b) for terrestrial ecosystems: assessing the soil acidity, soil nutrients loss, nitrogen status and balance as well as biodiversity loss:

(i) the key indicator soil acidity: exchangeable fractions of base cations (base saturation) and exchangeable aluminium in soils: frequency of sampling: every 10 years; supporting indicators: pH, sulphate, nitrate, base cations, aluminium concentrations in soil solution: frequency of sampling: every year (where relevant);

(ii) the key indicator soil nitrate leaching (NO3,leach): frequency of sampling: every year;

(iii) the key indicator carbon-nitrogen ratio (C/N) and the supporting indicator of total nitrogen in soil (Ntot): frequency of sampling: every 10 years;

(iv) the key indicator nutrient balance in foliage (N/P,N/K, N/Mg): frequency of sampling: every four years.

(c) for terrestrial ecosystems: assessing ozone damage to vegetation growth and biodiversity:

(i) the key indicator vegetation growth and foliar damage and the supporting indicator carbon flux (Cflux): frequency of sampling: every year;

(ii) the key indicator exceedance of flux-based critical levels: frequency of sampling: every year during the growing season.