EFFECTIVE ACIDITY MODELING TO ESTABLISH ACIDIC DEPOSITION OBJECTIVES AND MANAGE EMISSIONS

A mesoscale deposition model has been modified to include a simple che mical mechanism for six sulphur and nitrogen compounds for the predict ion of the spatial distribution of effective acidity. Effective acidit y is defined as the direct mineral acidity plus the hydrogen ions gene rated 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 relati onship between emissions from sources and ambient deposition levels, i nformation required by both the objective setting process and the mana gement of source emissions to meet the objectives. Model predictions o f effective acidity resulting from wet deposition agree within 30% of that calculated from observations. Model outputs indicate that a maxim um effective acidity of 0.182 keg H+ per hectare per year (keq ha(-1) yr(-1)) is experienced in northeastern Alberta, a region containing se nsitive 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 Albe rta reduces this maximum to 0.146 and significantly shrinks the area w ith effective acidify in excess of 0.14 keq ha(-1) yr(-1).