Ammonia deposition measured with Conditional Time-Averaged Gradient (COTAG) systems in the Netherlands : methodological advances and results for 2012-2020

Reliable measurements are required to develop nitrogen policy in the Netherlands. This includes measurements of how much nitrogen is deposited on plants and soil (deposition). Excessive nitrogen deposition is causing some plant and animal species to disappear. Nitrogen deposition consists of the dry and wet deposition of ammonia and nitrogen oxides. In wet deposition, these substances end up on the ground through rain. In dry deposition, they fall directly from the air onto plants and soil. Air vortices (turbulence) then cause the substances to move to and from the ground. The more air vortices there are and the greater their intensity, the more dry deposition there can be. In the Netherlands, dry deposition of ammonia makes up around half of all nitrogen deposition. RIVM determines the dry deposition of ammonia using the Conditional Time Averaged Gradient (COTAG) method. COTAG masts measure the concentrations of ammonia at two heights above the ground and the difference between them. This difference is then combined with the amount of turbulence. The masts use this method to measure the dry deposition of ammonia in a month. This makes the COTAG method suitable for measuring the dry deposition of ammonia in various locations over a longer period of time. RIVM has developed the COTAG method further and described it in more detail. Previously, RIVM reported the results of the COTAG masts with an upper and lower limit. For the sake of usability of the measurement results, it is more convenient to report a single deposition value every month. The new method makes this possible. The uncertainties inherent in this method have been identified as well. In this report, RIVM describes the results of the COTAG measurements taken between 2011 and 2020 in three Natura2000 areas: Bargerveen from 2012, Oostelijke Vechtplassen from 2014, and Hoge Veluwe from 2017. There are now six COTAG masts in the Netherlands, with four more to be added in the years ahead.