Two options to explain the ammonia gap in The Netherlands

Abstract

Abstract This paper addresses two hypothesis that try to explain the difference observed between the estimated NH 3 emission levels in The Netherlands and those indicated by atmospheric measurements, the so called ‘ammonia gap’: the role of SO 2 emissions regulating ambient NH 3 concentrations through co-deposition, and long-term NH 3 emissions after slurry injection. It was found that throughfall measurements of NH 4 + could not be used as indicator for changes in NH 3 emissions. The throughfall deposition of NH 4 + is in close equilibrium to SO 4 2− and NO 3 − and is thus regulated by the equilibrium of ambient NH 3 and NH 4 + in wet deposition and canopy water layers. When SO 2 emissions decrease, the amount of available SO 4 2− decreases, which imposes a limit on the deposition of (NH 4 ) 2 SO 4 . Long-term emissions of NH 3 after application of manure were monitored using a new technique, which continuously measures the concentration of NH 3 in a cross-section of the emission plume downwind of the source. The emissions could be registered for 3 weeks after application of manure. The results indicate that the long-term emissions only contribute 1–2% to the total emission level. Both the effect of SO 2 on the NH 3 deposition levels and the long-term emission fluxes are not enough to explain the observed ammonia gap. It seems that several counteracting effects, some of them emerging from the new emission reduction regulations, contribute to the ammonia gap. An integrated approach to abate ammonia emissions is, therefore, needed. The implementation and regulation of production ceilings for reactive nitrogen might be a good option.