Acid equivalent index - Aeq

Details

Last Updated on Wednesday, 05 September 2012 15:51

Associated topic: acidification

Emissions monitoring period: since 1980

Data source: CITEPA / SECTEN format - April 2012

Origin of the indicator

The acid equivalent index (Aeq) has been developed to measure the overall quantity of air emissions that contribute, to a varying extent over time and depending on geographical scales, to acidification on land, in water and in the air.

The index is calculated on the basis of potential fixation of the H⁺ ion. Only sulphur dioxide (SO₂), nitrogen oxides (NO_x) and ammonia (NH₃) are taken into account, as a quick calculation shows that because emission levels of the other acidifying substances, such as hydrochloric acid (HCl), are low compared to the three substances mentioned above, they contribute only marginally to acidification. The following weighting coefficients are used in the calculations: 0.0313 for SO₂, 0.0217 for NO_x and 0.0588 for NH₃.

Emissions and trends

Measurement unit: kt (kilotonne)
N.B. : emissions include those from combustion and from processes.

Comments

The graphs below show that the relative proportions of the pollutants included in the index (Aeq) have changed over time. In 1980, the majority of Aeq emissions were of SO₂ (54%), but in 2010, SO₂ emissions only accounted for 12% of the Aeq index value. Conversely, NH₃ accounted for only 23% of acid equivalent emissions in 1980, but this had climbed to 54% in 2010.
The changing share of each pollutant in the index is partly due to the fact that emissions of the different pollutants concerned declined unevenly during the monitoring period, and partly to the weighting coefficient used for each pollutant (NH₃ has the highest weighting coefficient).