Empirical critical loads for nitrogen are provided as a range for each habitat e.g. 5-10 kg N ha-1 yr-1. Modifying factors have sometimes been used to determine what critical load value should be applied with the range based on the sensitivity and specific location of features - for example bogs in areas with high precipitation have been considered less sensitive. However, UK evidence included in the 2022 review of critical loads for nitrogen (Bobbink et al., 2022; Rowe & Hina, 2023) showed ecosystem impacts at low deposition rates – generally at the lowest point of the given critical load ranges. The UK Conservation Agencies and Regulators therefore require the minimum value of the critical load range for the most sensitive habitat type present on the site to be used during the screening / Likely Significant Effect stages of air quality assessments. This enables assessment of exceedance of a threshold that reflects sensitive examples of the habitat. If exceedances of any thresholds are indicated during the detailed modelling stage, modifying factors can be considered based on the evidence provided in Bobbink et al. 2022 and Rowe & Hina, 2023 reports or other evidence sources for review by the relevant regulator during detailed assessment stages / Appropriate Assessment.
Roland Bobbink, Christin Loran, Hilde Tomassen. 2022. Review and revision of empirical critical loads of nitrogen for Europe
Rowe EC, Sawicka K, Hina NS, Carnell E, Martín Hernandez C, Vieno M, Tomlinson S, & Jones L(2023) Air Pollution Trends Report 2023: Critical load and critical level exceedances in the UK. Reportto Defra under Contract AQ0849, UKCEH project 07617.