Reliability of Ammonia Emission Estimates and Abatement Efficiencies

This group addressed the key concerns relating to ammonia (NH₃) emission estimates:

• The current status of uncertainty analysis of national NH₃ inventories

• The discrepancies between atmospheric measurements and models, and the most likely explanations

• The extent to which the uncertainty in regional NH₃ emissions can be distinguished between the absolute magnitude and the trends in emissions

• The agreement on the assessment of key NH₃ mitigation methods as good, promising and unsuitable in relation to current UNECE guidance

• The potential for ‘soft ’ approaches to NH₃ abatement

• A forward look for mitigation measures and the extent to which costs are expected to reduce in the future as methods become more widely adopted

Few countries have considered uncertainty in detail. Results available indicate national estimates may be accurate to within ±20%. For those countries which have created inventories using emission factors (EFs) measured elsewhere, the uncertainty may be much greater, perhaps c. 100%. The greatest uncertainty is likely to be for emission estimates for regions within countries. Sensitivity analysis of the UK Inventory showed that activity data, information on a range of relevant farming practices, were the inputs to which the system was most dependent. Cattle diets, especially those which are grass-based, were considered particularly uncertain.

The UK and DK reported good agreement between modelled and measured NH₃ concentrations. However, these countries use a finer grid size in their models than the NL. A detailed discussion of the Dutch ‘ammonia gap ’ suggested that the EFs used in the inventory were accurate. The discrepancy was considered to be due to either overestimation of abatement efficiencies or overestimation of dry deposition velocities. Adjustment of either could eliminate the gap but it was not yet known which was responsible.

The abatement efficiencies quoted in the UNECE Guidance document were considered robust. While means will not reflect the variability in the data, quoting ranges may create uncertainty over which point in the range is most appropriate to use. Since the data were obtained almost exclusively from Northern and North-western Europe, the efficiencies should not be assumed to be applicable across the whole UNECE area. Only a brief statement is given in the Guidance on the impacts of reducing emissions of NH3 following spreading on losses of other N pollutants, because nitrate leaching and nitrous oxide emissions are very specific to the site and season of manure application.

‘Soft’ approaches to NH3 abatement are those implemented using existing facilities and equipment (e.g. applying manure during weather conditions associated with little emission). While these offer an economically attractive method of reducing NH3 emissions, it is often difficult to know their uptake by farmers and their efficiency, and therefore to convince environmental authorities of their efficacy.

Experience from the adoption of abatement technologies in other areas, suggests that ex ante cost assessments tend to over-estimate the cost of implementation. However, taking emerging technologies into the industry can lead to a reduction in abatement efficiency. A number of emerging abatement options are discussed in the full report together with a summary of other developments that may have an impact on NH₃ emissions.