DOI: 10.5285/1f71b91f-32e2-4958-aab0-9baaa4234781
title: Average Accumulated Exceedance (AAE) of critical loads of acidity and nutrient nitrogen by acid and nitrogen deposition for the UK, 2018-2020

description: This data resource consists of two files: (a) 1x1 km resolution Average Accumulated Exceedance (AAE) data summarising the exceedances of acidity critical loads for eight habitats; (b) 1x1 km resolution summarising the exceedances of nutrient nitrogen critical loads for thirteen habitats. The data provide information on the amount of excess acid or nitrogen deposition above the critical load values set to protect acid- and nitrogen-sensitive habitats in the UK. The AAE has been calculated using UK 1x1 km Concentration Based Estimated Deposition (CBED) data for 2018-20 (see related data collection). The data were generated under Defra-funded work to assess the potential areas of acid and nitrogen sensitive habitats at risk of adverse impacts from excess atmospheric acid and nitrogen deposition. Reducing the area and amount of critical load exceedance continues to be a driver of Government policy on reducing emissions of acidic and nitrogen-containing air pollutants (sulphur dioxide, nitrogen oxides and ammonia).
description: Average Accumulated Exceedance (AAE) is calculated at 1x1 km resolution and summarises the exceedance of critical loads of acidity, and of nutrient nitrogen, for multiple habitats. It is derived from the exceedance (i.e. the amount of excess acid or nitrogen deposition above the critical load) for each habitat, the exceeded habitat area and the total acid- or nitrogen-sensitive habitat area in each 1x1 km grid square.

For all habitats (except managed woodland) nutrient nitrogen, empirical critical loads have been allocated values based on habitat type and have been defined under the Coordination Centre for Effects (CCE) working group as part of the United Nations Economic Commission for Europe (UNECE) Convention on Long-range Transboundary Air Pollution UNECE. (https://wge-cce.org/Publications/Other_CCE_Reports/Review_and_revision_of_empirical_critical_loads_and_dose_response_relationships_2011) . For managed woodlands the steady state mass-balance approach is used where the long-term inputs and outputs of nitrogen from the system are calculated based on nitrogen uptake (removal by harvesting of trees), nitrogen immobilisation, denitrification, and nitrogen leaching. Acidity critical loads are based on the mineralogy and chemistry of the dominant soil type in each 1km grid square together with habitat-specific data. Critical load values are calculated depending on soil type - mineral soils or peat soils.

Critical loads for acidity and nutrient nitrogen are compared with deposition data over the UK using three-year rolling mean 1x1 km Concentration Based Estimated Deposition. By overlaying CBED data of acid or nitrogen deposition on the critical loads data for acidity and nitrogen, exceedance maps can be generated. Simple statistics are further generated to quantify the area of sensitive habitats associated with critical load exceedances. Exceedances are calculated separately for each habitat type using habitat-specific deposition values: mean deposition to moorland (ie, low-growing vegetation) is applied to the coastal, grassland, heathland and montane habitats; mean deposition to forests is applied to all woodland habitats. Critical loads for terrestrial habitats are mapped at 1km resolution and national deposition data are currently available on a 5km grid. Exceedances for these habitats are calculated at 1km resolution by assuming that deposition values remain constant across each 5km grid square. Exceedances for freshwaters use catchment-weighted deposition values.

Further details are given in the supporting documentation which gives reference to the full online methodological report.