Effective acidity modelling to establish acidic deposition objectives and manage emissions

A mesoscale deposition model has been modified to include a simple chemical mechanism for six sulphur and nitrogen compounds for the prediction of the spatial distribution of effective acidity. Effective acidity is defined as the direct mineral acidity plus the hydrogen ions generated as a result of receptor-processing of acid-producing substances in deposition, minus the hydrogen ions lost by receptor-processing of acid-consuming substances. This model provides the quantitative relationship between emissions from sources and ambient deposition levels, information required by both the objective setting process and the management of source emissions to meet the objectives. Model predictions of effective acidity resulting from wet deposition agree within 30% of that calculated from observations. Model outputs indicate that a maximum effective acidity of 0.182 keq H+ per hectare per year (keq ha−1 yr−1 is experienced in northeastern Alberta, a region containing sensitive soils. This value lies within the range of 0.1–0.3 keq ha−1 yr−1 which has been suggested as an acidic deposition objective for the protection of the sensitive soils. A scenario with a 40% reduction in emissions of SO2 and NOx from a single source in northeastern Alberta reduces this maximum to 0.146 and significantly shrinks the area with effective acidity in excess of 0.14 keq ha−1 yr−1.