Roy Wichink Kruit1, Jan Aben1, Wilco de Vries1, Ferd Sauter1, Eric van der Swaluw1, Margreet van Zanten1, Addo van Pul1
In this study, the effectiveness of two decades of policy measures to reduce ammonia emissions in the Netherlands is evaluated. It is shown that the ammonia concentration is more suitable for monitoring policy measures than the ammonium aerosol concentration or the wet deposition of NHx. This study shows that the decrease in ammonia emissions due to policy measures between 1990 and present did not result in a proportional decrease of the ammonia concentration in the air. The less effectively declining ammonia concentrations can largely be explained by the change in atmospheric chemical and meteorological conditions. The large decline in oxidized sulfur and nitrogen concentrations has led to reduced formation of sulfuric and nitric acid and consequently reduced formation of ammonium salts. In this way, relatively more ammonia remained in the atmosphere. Simultaneously, the absorbing surface became less acid, which resulted in less deposition of ammonia and more ammonia remaining in the atmosphere. Meteorology has a significant effect on the year-to-year variation in ammonia concentrations, but does not significantly affect the trend in the ammonia concentrations over the years. It is likely, however, that ammonia concentrations will increase due to climate change, e.g., dryer and warmer springs/summers.