Predicting atmospheric ammonia dispersion and potential ecological effects using monitored emission rates from an intensive laying hen facility in Ireland.

Abstract Agriculture is responsible for 98% of atmospheric ammonia (NH3) in Ireland, of which pigs and poultry produce 7%;with laying hens specifically contributing 0.6%. Though a small proportion of the national NH3 total emissions, the ecological impacts on sensitive sites attributed to laying hen farms can be substantial. NH3 emission monitoring was conducted in Spring (February to March) and Summer (July to August) 2016 to account for seasonal variation. The total average emission and ventilation rate was 0.25 g bird-1 day-1 and 931 cm3 s-1 bird-1. This is lower than the previously used emission factor for the Irish national inventory of 0.5 g bird-1 day-1, but broadly similar to factors reported in the United Kingdom (UK) and the European Union (EU). Dispersion modelling using monitored data indicated potentially acute effects within 84 m, critical level exceedance within 312 m and exceedance of 0.3 kg N ha-1 year-1 deposition within 2.9 – 5.2 km. The sensitivity of the model was tested using SCAIL-Agriculture emission and ventilation rates which showed P-values for one tailed critical level below 0.01 for all models, indicating that when normalised the maximum extents modelled by AERMOD were significantly different. This analysis showed emission rate having more influence than ventilation rate. Both parameters combined had the greatest increase in dispersion extent, on average 55.8 % greater than the use of monitored rates. A deposition rate of 0.3 kg N ha-1 year-1 was reached within 5.1 – 7.7 km when using SCAIL-Agriculture rates, indicating that the use of SCAIL-Agriculture recommended emission and ventilation rates would have been sufficiently precautionary to assess negative ecological effects on a Natura 2000 site under the Habitats Directive (92/43/EEC). In relation to Appropriate Assessment (AA) screening, the use of any contribution from a source within a set distance may be an appropriate full AA trigger.