Ammonia Exchange at the Forest-Atmosphere Interface: Influence of Turbulent and Biophysical Factors Across Temporal and Spatial Scales

Atmospheric ammonia (NH₃) is a major pollutant in natural ecosystems, and at high concentrations it can lead to soil acidification, eutrophication and biodiversity loss. Quantifying NH₃ deposition is essential for assessing ecosystem exposure, but the exchange dynamics between forests and the atmosphere remain poorly understood due to measurement challenges and the complexity of turbulence above forest canopies. In this thesis, we address this knowledge gap by presenting and analyzing two high-resolution datasets of NH₃-exchange over coniferous forests (Douglas fir and Scots pine) in the Netherlands, spanning two and one year respectively. The results show that turbulent exchange is enhanced above forest canopies due to the presence of the roughness sublayer, dependent on canopy architecture and meteorological conditions. Canopy surface conditions, such as stomatal opening, ecosystem activity, and wetness, further affect NH₃ exchange. Based on these findings, we propose a new framework highlighting the roles of NH₃ pollution level, ecosystem state, vegetation type, and turbulent transport in regulating NH₃ exchange.

The candidate of the PhD defence "Ammonia Exchange at the Forest-Atmosphere Interface: Influence of Turbulent and Biophysical Factors Across Temporal and Spatial Scales".