Potential limitations of ammonia flux data beyond 72h after field application of slurries 

Ammonia volatilization from animal slurry applied to fields is a major source of air pollution in Europe. The ALFAM2 model  was developed for estimating ammonia emission from such sources and is used in research, as well as  for inventory calculations. Until now, the focus has been on emissions up to 72 hours after application (ALFAM2 model version 1.2), as it was generally assumed that over 90% of total emissions from the applied nitrogen occur during this period. The recently updated ALFAM2 model (version 2.3) now includes emission data up to 168 h after application, which has led the model to indicate substantial emissions between 72 and 168 h leading to a significant increase (<40 %) in total emissions. However, ammonia fluxes from field applied slurry beyond 72 h after application are small and difficult to quantify accurately. A reassessment of the model input data for this period is required to determine whether the measured fluxes can still be causally attributed to the applied slurry and whether they differ significantly from the detection limit. We have examined the values included in the ALFAM2 database with regard to these questions, which has revealed patterns that lead to potential biases of the ALFAM-2 model results.